10 mpg 3.2 - can’t find where the fuel is going
 

Got into this fuel puzzle the end of ‘24. Interruptions occurred since then forcing me onto other matters including a few fixes found along this fuel hunt path. Getting back into it now and starting a fresh thread. This has been more twisted a puzzle to solve than any I’ve ever faced.

Background:

Engine - ‘85 3.2 in an ‘80 chassis. City mpg was 18 prior to engine rebuild. After rebuild, it’s now 10. Highway remains a respectable 27. These results have been tested a number of times with the same results. My fault when rebuilding? Everything was done by the book. Rebuilt a 3.3 before this so this 3.2 was done with a measure of prior experience. Odo is good—matches state posted mile markers. Not a leadfoot as is easy to suspect. Have filled tank consistently at the same Shell station’s nozzle until deciding to change the fuel brand to see if gas was the cause. Wasn’t. Engine runs well… Immediate start, idles evenly, no hesitation throughout RPM range with throttle eased on, or hammered WO. Pulls strong.

Mods done with rebuild:

964 cams, 40-40 grind, 1.45mm timing.
SSI exhaust
Danks 2-in-1-out muffler

What critical measures & machining 3.2 rebuild called for was done by Porsche specialists: Total Engine Concepts and Xtreme Cylinder Heads.

Checks & tests done to date. (Not necessarily listed in sequence until after 5.8.25 notes begin):

Fuel leaks - None
Air filter - Clean
Sweeper inside AFM was tested - Consistent across entire sweep.
Adjusted AFM spring tension - Advised not to do this - Returned it to original position.
Set ECU fuel dial from zero to position #3 - Advised not to do this - Returned it to zero.
Tire pressure - OK
Wheel alignment - OK
Slipping clutch - No. Acceleration is too crisp. New clutch & pressure plate was installed.
Vacuum leaks - Brake booster system. Booster rebuilt, new check valve, new hoses front & rear. Still 10 mpg.
ECU chip date checked - Correct
Individual exhaust down pipes temps checked - OK
Free roll of car - OK when I checked it. Then on occasion the parking brake would stick. Reverse gear slightly and they released. Pulled rear disks and found signs of pads dragging against the hub. Thought this was it. Wasn’t. Rebuilt parking brakes and still 10 city.
Dragging caliper pads - No
Compression tested - OK
Hand turn engine with plugs out - OK
Spark plugs checked - cleaned - OK
Spark plugs replaced - W6DPO to Iridiums - slight mpg improvement but negligible.
Fuel pressure - OK
Fuel flow - Not checked correctly but there’s good flow
Exhaust manifold tightness to heads - Tight
Tested fuel injectors. The ones installed (Lucas) and spares (Original Bosch.) Done by Mr. Injector - Both sets "passed."
ECU - tested - “No faults”
Resistance tests:

Ignition wires - OK
Cap & rotor - OK
Fuel injection cables - Redone prior to rebuild so proven to get 18 city mpg - OK

Distrib - Well lubricated - OK
Grounds - All tight
Replaced Lucas injectors with original Bosch - Still 10 mpg
"Cylinder balance" - Pulled injector leads one at a time - does affect O2 as it should.

Checks / tests done after 5.8.25 (these are sequential):

964 cams continue being questioned - specs on stock 3.2 and 964 cams as provided by Webcam are compared. Post #32
Fuel pressure regulator and dampener vacuum hoses checked after engine run - both dry
Analyzing exhaust gas composition recommended - Checked around and shops don't do this any longer (in FL.) Tool, new condition = too costly to invest in. Used = unappealing as gear needs to be well cared for to operate accurately, and all the parts need to be included.
Rear end lifted to check rear hubs for any rock (bearings) and spin - No rock either side. Both sides spin equally in neutral and in gear.
Removed brake pads on RR and repeated spin test in neutral and in gear. Slightly more resistance---attribute to smaller OD of hub compared to tire OD.
Gear box oil - full. Looks good.
AFM - inside disk turned & set at 3 notches clockwise / increasing tension on "barn door" to lean fuel - wideband confirms leaning. Engine not running well now. Returned to original position. Post #69
Ohm test throttle position switches - Idle and WOT both OK.
Throttle butterfly position when gas pedal is floored = fully open. No slack in throttle cable - post #72
Idle & full throttle position switches ohms tested - OK - post #72
WOT signal checked - points close at 69-70% throttle - post #78
Odometers, trip & main tested for accuracy - OK - post # 93
AFM intake air temp sensor - OK - post #96
Cylinder head #3 temp sensor - QUESTIONABLE - Not related to fuel per Sal in post #98
Speed & reference sensors resistance and dynamic - OK - post #100 - Done per Specialized ECU procedure
Spark plugs pulled again to look at color - OK - post #103
Looking for friction... soapbox brake release test - OK - post # 130
Looking for friction... brake release test on each individual wheel - OK - post
Looking for friction... each wheel corner inspected with a checklist - chafing of a-arms against rotor shields found. Not calling this friction related to mpg puzzle - post #162
Looking for friction... individual brake release test - all corners OK - post # 168
Timing check - timing advances as rpm increases - looks OK to me but I don't know timing - post # 168
Suspected worn tie rod found - passenger side - "suspected" as it has yet to be removed and proven - play is 3.75 mm - not severe enough to cause handling issue. Not calliong this the cause of mpg puzzle - post # 169
Because of suspected tie rod... toe is checked - post # 169
ECU - although previously tested by ProGrama with "No faults" found, ECU remained suspect as it was NOT with car prior to engine rebuild. A loaner ECU known to be good was borrowed from "Inaminit" / Dean and swapped in for my wonky "10 mpg" ECU. City miles were driven. Result: 17.64 city mpg - post #173

PROBLEM SOLVED:

By Systems Consulting, 310-545-3716. They replace my wonky ECU with a restored unit of theirs. Result:

City = 19.001 mpg
Highway = 28.18 mpg

Post #270

Wideband

SSIs came with single bung on 1-2-3 side. I added second bung on 4-5-6 side. 2nd bung is positioned so both are symmetrical from collective and angles match. Bosch 4.9 sensors are connected to Innovate LC-2 controllers. Running LogWorks for charting O2 streaming data. Interesting way to “see” into the engine… but no smoking gun… yet.

EDIT: After dealing with Innovate's LC-2 system including their 4.9 O2 sensor hardware over a number of months, I found the system problematic from the get go. But issues were corrected by investing a great deal of time debugging. Finally, it became so glitchy that it was not worth dealing with---last issue seemed internal to the controllers. Innovate tech support offered no assistance each time I called them. My take is the hardware is made cheaply and tech support knows little about the workings of their product. LogWorks software worked flawlessly. It's a free download, installs in seconds, is quick & easy to configure, and simple to operate. Unfortunately LogWorks appears to be dedicated to Innovate's systems exclusively---don't quote me on this.

ECU

ECU now in the car is NOT the ECU that was in the car when getting 18 city mpg. Why this new ECU is installed is a long story and not relative. Getting the prior ECU is now not possible. I did send the currently installed ECU out for inspection. “No faults” were found.

EDIT: Turns out it was a bad ECU. To their credit, my bad ECU came from Vertex.

O2 Logs

Red dot-dash line is 1.0 Lambda = 14.7 AFR. Blue and green trace lines represent each side of the engine.

http://forums.pelicanparts.com/uploa...1746560347.jpg

Above, city drive. Not hard on the throttle here. According to log data, engine is running lean yet this does not match fuel consumption. Because of this, wideband system accuracy is questioned. System is checked / tested including:

Controllers - swapped side to side
O2 sensors - swapped side to side
O2 sensors replaced with new sensors, calibrated per Innovate specs
Sensor cables replaced with new cables
Reloaded LogWorks software

With each change of gear and software, car was city-driven and new logs recorded. Log data did not change.
Conclusion: wideband system works as it should.

http://forums.pelicanparts.com/uploa...1746560895.jpg

Steady cruise reading. 3,000 rpm in 4th gear.


http://forums.pelicanparts.com/uploa...1746560955.jpg

WOT.


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I can’t give you a definite answer, but can give a few thoughts. Around town, you are stopping and going, on the freeway, once you get to steady speed, you keep it there.

So, is there something that is causing a bunch of fuel to “dump” into the system when first applying the pedal? Maybe a distributor issue not properly advancing or retarding until it spins up fast enough?

Good luck, it will be interesting to see what the cause is.

Just a thought: are you sure the correct cams are installed and timed correctly?

One track day at Charlotte Superspeedway, I was told to fill up before the day of track fun. I filled up agin before going back to my hotel, and did the calculation of about 8 MPG. There is a LOT of WOT on track. I can't imagine you can do anything like that on the street.

I know you have been chasing that issue for a while. The fuel injectors should not dump enough fuel to get that poor of mileage without a lot of air to go with it.

Have you had the oil tested for fuel contamination? Even if there is gas in the oil, one tank of gas would have the oil system purging oil from overflow. Maybe some is getting into the oil however.

Have you tried a exhaust gas sniffer at the tail pipe to read the A/F ratio?

We all want to know what your solution is.

What DME chip are you using, stock?

I'm assuming you are not running the narrowband o2 sensor since you state only 2 bungs and both have wideband sensors for logging.

AFR during city driving looks very lean. Cruise AFR looks normalish, maybe a little lean. WOT looks to be expected.

What is the history on the air flow meter (AFM)? Is it the same one that was on the car prior to rebuild? Any indication it was ever opened up previously? When i bought my car, i found the clock spring in the AFM was adjusted to make the car run lean, no doubt to get it to pass emissions. Its possible your rebuild fixed a few issues a misadjusted AFM was hiding, so now the car is running leaner as a result. At WOT the car defaults to a preset fuel map which looks to be reflected in your datalog.

Interesting you are getting such mad mileage. Is their any other indication of this? Rich smelling exhaust? Sooty tail pipe / bumper? Smoke from tailpipe during idle / accelerating?

How was fuel pressure checked? Does fuel pressure match the normal spec at idle? (i forgot what it is). How about when you pull the vacuum line off?

I had a bad fuel damper. Started with bad fuel mileage. Progressed to rough idle and then smoking( running rich). car relies on vacuum at lower rpm to pull fuel. Vacuum is highest at idle and lower at higher rpm. Could explain the mileage difference when car is operating at lower rpm(higher vacuum)during city driving. If you have a small tear in the diaphragm in the fuel damper, it will pull unmetered fuel into the intake manifold . Just a guess on your issue. maybe test the fuel damper?

Need a friend with a known good 3.2 that passes smog and swap both DME and AFM from that car to yours. I don't think you have adequately ruled those out at this time. I like the fuel damper idea too. Don't remember from the other thread if that was looked at it not.

This is wild—my '88 consistently gets over 20 mpg.

In my experience, it’s rarely a coincidence when something changes. It almost always traces back to the last thing that was altered or disturbed.

Something worth checking: I recently learned about a potential vacuum leak source. The thermoswitch on the back of the throttle body controls the ambient air valve that draws "warm air" from the oil tank into the throttle to prevent icing in cold temps. If this system fails, it can also lead to increased oil consumption.

I plan to eliminate that system entirely. It might be worth ruling it out on your end too—though I doubt it would account for such a drastic drop in fuel economy.

http://forums.pelicanparts.com/porsche-911-technical-forum/987569-ambient-air-valve.html

ive had an injector stick open then it would dump fuel into the cylinder bore whe parked , and blow white smoke upon startup. it wasn't caused by a new pope ;-) I think you would have caught that if it was the cause.

I was just reading anbother post referring to the coil, and it went into depth explainng how the coil charged and how it has less time to charge at higher RPMs , the factory engineers probably did not make bad decisions on the ratio of the windings etc but some coils ou there seem to perhaps a bit falsely adverticse that swapping to their new fangled coil with a different ration , perhaps a higher output voltage , imporoves the spark.

it seemed to suggest that installing a "hotter" coil could actually cause the transistor in the ECU or I shoudl say MCE to degrade and that the programming may be adapted change this charge rate. I wondered if perhaps a higher RPMs if there was misfiing going on that wasnt; that noticible?

it made a valid point that at high rpms the time for the coil to charge for it's next spark is shortened and that would make the spark weaker.. and I assume a coil with a different winding ratio may influence this.

I dont have the theory to back all that up nor the experience. im as confuesed as anyone about your real cause of the poor mileage. Nothing I said above was said with a wealth of experience under my belt so take it with a pinch of salt.

i wonderd if you had updated to a coil with a different winding ratio and if swapping to the OEM one might affect the mileage?

obviously driving habits are an influence in mileage so in the city if you are using wide open throttle well That could be quite habitual for some and "never" for others. eg some will go wide open throttle when the next light is red, because they are enjooying doing so mostly,

Others think ahead and want o save fuel and may think well im going fast enough to coast to the light before it changes so why would I apply any more throttle? Ill lay off and allow my momentup to get me there in time for the light ot change because anything other is just a waste of gas.
some may even change their pace to avoid stopping and starting as each start uses fuel , truckers are masters at this. they will percieve the other light to predict more closely . when its going to change.

no matter of your personal habits you are probably still comparing oranges to oranges because any changes will reflect the same driving habits.

Given all the data you've collected without any obvious clues, and given that the ECU is not the same, if you have the ability I'd try to borrow another ECU to swap and test the car with that.

Rutager... Thought the same about steady throttle and variable and the answer being in there. Installed wideband and the city traces don't show any sign of full dumping. Distrib is in good order. Well lubricated. Just looked inside to confirm.



EM... 964 cams from Webcam were installed with rebuild. 40/40 grind. 2.45 Overlap matched perfectly both sides. I may have said 1.45 earlier---will look and correct if I did. According to Webcam, Cathy as I recall, these cams are suited to a 3.2. She advised a more aggressive overlap than stock 2.25. I've discussed mods done to engine with one who knows much more about air-fuel than I. Cams, SSIs, Dansk muf are not the cause---there's something else.



Glen... Have not had oil tested. Did change crank oil not long ago and it looked as I think it should. No sniffer. With wideband system tested and parts and software replaced to pizz away money, the data did not change no matter what was done so have to believe wideband data is accurate.



Jon... chip is stock and has been confirmed correct for '85 3.2. ECU was also tested and "No Faults" found. (I still question this testing.) AFM has been opened by me... to check sweep track. Is marked but test of sweep proved it working across its full range. I also messed with the spring tension but put that back to where it was. Fuel setting was also altered from 0 to 3 and returned to 0.

Your idea of the rebuild fixing things and now the AFM being out of sync---I trust you mean runner richer rather than "leaner." If this were the case, would the wideband logs not reveal this?

Soot in the tail pipe was the first indication for this journey. Then the fuel gauge didn't look right. Fill at the pump confirmed. This journey then began. Now, no soot. Never been any smoke no matter the throttle. Gas smell there used to be with nose stuck in pipe. With the testing, adjustments here and there, it's impossible to know why the soot is gone. I keep records but keeping track of each small detail doesn't happen. I genuinely thought wideband was the holy grail here. So far it has added to the mystery as there's no sign of when the fuel is dumping.

Fuel pressure was checked at the 1-2-3 rail port. Was good as I recall. Flow was also checked but out of the rail rather than after the pressure relief valve as was suggested it should be.

Vacuum... remove oil cap and engine suffers. Disconnect brake booster line and engine suffers.




I appreciate all the ideas / input. Whatever is going on, the answer(s) will be found. There's no giving up here.
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See if the vacuum hoses attached to the pressure reg and the damper have fuel in them, as previously mentioned. They are there to route fuel to the intake in case a diaphram leaks. You can attach a longer hose to each nipple and run the engine to see.

The puzzling thing is your saying bad MPG, but your data log looks like its running lean during city driving and normal at steady cruise.

Your saying no soot on the tailpipe or bumper so the only other place for it to go is in your oil. How many miles are we talking since this has been going on? Oil may smell like fuel if its bad enough, or send it in to be tested. The low tech way is to compare viscosity with a new sample of the same oil. Put a small amount on a piece of glass and hold it at an angle, both samples should run the same amount over time. Look up lube oil viscosity race test to see what i mean.

With the data log you posted, the reason i question the air flow meter is the test you did only checks the carbon track for wear. If the carbon track is worn, you will experience bucking or hesitation when accelerating.

What you cannot test on the air flow meter is how far the barn door is opening at a set engine load. This is what influences your air fuel ratio at partial throttle conditions. Adjusting the clock spring changes the tension on the door. Less tension on spring means the door opens further than normal, so it will run richer. More tension on spring means the door opens less, so leaner mixture. The clock spring can really only be set using a flow bench and to my knowledge ive never seen official numbers on where to set it at posted. The clock spring in the AFM never needs to be adjusted, unless you know the full history of your AFM i would still question it.


More info on the AFM and the clock spring;
https://forums.pelicanparts.com/porsche-911-technical-forum/835870-afm-mixture-am-i-lean.html

https://forums.pelicanparts.com/porsche-911-technical-forum/1105174-air-fuel-meter-adjusting-screw.html

Actually, the AFM can be tweaked using a CO tester, e.g. from Snap-ON, very easily with good results, nothing complicated here!
I've tweaked numerous 911 3.2 AFMs to pass emissions tests, and then re-adjusted for optimum performance.
Followup testing an be done using a wideband O2 sensor under various driving conditions.

Now when to get to a 993 and later Porsches with MAF sensors, tweaking AFRs are basically impossible.

86... are you referring to the dampener on the 4-5-6 rail? If so, to test for a tear, is that the check for fuel in the vacuum line connected to it?



917... I like that idea. No one I know locally has a Motronic ECU / AFM. You are correct about neither being ruled out. Would cost me $460 to "rent" an ECU from Specialized. ECU is a point of special interest given it's history as I noted in the first post. AFM... will learn exactly what adjusting the spring tension and wiper arm adjustments do. Then see if anything about those areas relates to this.



Merk...

http://forums.pelicanparts.com/uploa...1746691015.jpg

I believe those caps are tight but will check them today. That switch has been capped for many years---when engine was running 18 city mpg.



Wrench... Blaster coil went in with the rebuild. I checked specs before doing this and on paper, Blaster matches the Bosch (black) original coil. I got a used black Bosch from eBay on Sal's recommendation to swap it for the Blaster. eBay coil was no good. Am not to fond of eBay. Doing that again... little appeal. But, Blaster coil, like the ECU, were not with the engine when it was doing 18 city. So Blaster is added to the culpable list together with the ECU. I will look for a used black Bosch on Pelican.

With city mpg as it is, am driving very conservatively. I do look ahead, coasting when possible. And timing traffic lights. Were the engine doing 18 city, I might get 20 in let's call it Lindbergh mode! Something to test after the current fuel puzzle is solved.


Otter... yeah, I may get to renting one from Specialized. Don't know anyone local to do this with. Am hesitating going the rent-an-ECU program on the basis that the ECU has been tested. Doesn't guarantee it being 100% good. ECU is on the "culpable list" together with the Blaster coil.
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John... I'll do that.



Jon... There's indeed a conflict between the city fuel guzzle according to the run & pump fill results (together with what I see when looking at the fuel gauge)---and then wideband data NOT showing any out of place guzzling. Wideband, as I've mentioned, has been tested and parts replaced to prove it being faulty. But couldn't make that happen no matter what was done. Conclusion... guzzle exists and wideband is accurately not showing where/when the fuel is being guzzled. Simple answer: fuel is leaking at a rate of 1/2 a tank per 200 city miles. That's not happening---I've checked and rechecked given this being an obvious conclusion. This guzzle & wideband conflict, I thought there was a clue in this but found nothing. Instead, it's a head-spin that leads away from from getting on with hunting down the solution.

Sweep... acceleration, easy or slammed WO, is without a moment of hesitation.

Links... I've not yet looked at them but will. Thanks.

Sal advised me on adjusting the position of the sweep arm. Am not going to do anything now with either the spring tension or arm until I know exactly what I'm doing. I did adjust the spring tension very early in this expedition but returned it to its original position. This was done prior to having the wideband installed so don't have any data relative to that adjustment. Now, with the wideband, it would be interesting to see what comes of any AFM adjustments.


Based on everyone's input to date...

Hit list:

• Vacuum line connected to dampener - check for fuel in line.
• Learn Motronic in order to intelligently adjust AFM - book ordered.
• Culpables = ECU - although tested and "no faults" found, this unit was not installed when engine was delivering 18 city. Coil - Blaster, although specs as promoted match original Bosch coil specs, Blaster was not with engine when it did 18 city. Undecided how to attend these items.
• Intelligently adjust AFM spring tension and log wideband results for each move.
• AFM - Look into Bavarian Restoration for AFM resto - email sent - 5.9.25 response = $365 return shipped domestic. 45 day turnaround ("lead time"). Greg is founder/owner. No retail shop. He does this part time at home.

Source notes on dampener if needed - tested 5.8.25, not needed.

Pelican - $665
Adapt in Switzerland - $322 - uses Bosch module rather than vacuum connection
Amazon - $50 - China
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From my experience, a torn diaphragm in the fuel pressure regulator often leads to hot start issues—but definitely something worth checking. We've seen several of these regulators fail over the years.

I've bought parts from Adapt before—great folks to deal with—but I haven’t tested one of their regulators on a 3.2 yet.

As for the thermoswitch, those caps won’t cause a vacuum leak on their own since they don’t connect to the intake path. The switch simply opens or closes the passage between the two nipples based on throttle body temperature. It appears that you've disabled/removed that entire system already.

I picked up a pair of ignition coils from a 964 on eBay. Since 964s are twin-plug, the coils come in pairs. They use the same coil design as the 944 and 3.2 Carrera, though the 964 versions are typically from newer model years.

John... after running engine for a while just now, disconnected both the fuel pressure reg and damper vacuum hoses. Put paper towel on ends with hose ends facing downwards... both are dry. Also a visual inspection into both ends. Dry.
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in my 88 ford van with a bosche ICU I wonderd if I could replace the electrolytinc capacitors so I opened the cover, had a look and found that some had indeed leaked their oil.. when I looked at the board I could see a slight stain or different colored patch near some capacitors.

the caps can sometimes plump up or explode themselves, they are filled with stuff you dont want in your eyes so careful if you do clip them, they can squirt. I dotn see much harm in opening the container for a peek but Im not sure what that one will look like physically. if you ddi see what I saw a bit of discoleration near them then its definitely a sign of bad caps.

I bought new caps, accidentally clipped the end of a transistor so I just orderd a reconditioned ECU not wanting to change the transistor which would require a bit more , but I guess it's somethign to look for IF you can see and access the capacitors. they aren't expensive or very hard to swap if you are ok witha little soldering.

what I did ws clip the cans just above the board leaving wire stubs, then coiled the wires of a new cap around a pin so they look like a spring then crimp on the stubs and solder, it meant I dint have to dig in further to get to the backside of the board where there is heat transfer putty.

electrolytics have their capacitance in microfarads and working voltage on the side and a line , the line indicates negative and polarity IS important with electrolytic caps.so take pics first.
its not uncommon for the electrolytic caps to fale with age, the results would be a bit of a gamble.
If you own an ESR meter you can check them in place without unsoldering. caps are only a dollar or two apiece.but obvioiusly if you go mucking int here you run some risk of doing damage.

if you touch the board particularily the Eprom you can wreck it by discharging static, they are sensitive to that so good to ground yourself with a wrist strap

likely you could send it for rebuild or buy a reconditioned one.

what actual symptom would be the result of failing capacitors? not sure. in the case of my van it was still running with csome physically leaky caps. I assume it could do strange things maybe mess up the fuel map or something?
nothing definitive here for sure.. just food for thougt...

This is definitely an interesting one. Sorry you are dealing with it.

It seems that you have verified the AFR data your getting. Any excessive fuel getting into the combustion chambers would show up in this data. (correct?)

This would mean it has to be external of the engine to me. Maybe you could pump smoke into the fuel line at the tank, blocking it off at the engine and verify no leaks? Or pressurize it and do a leak down type of test?

Best of luck figuring this out. I definitely will be looking forward to you fixing it and learning along the way.

Foster.