1988 Idle Mixture Adjustment - Out of range
 

Hi All,

I'm looking for some advice, over the last month my 3.2 Carrera has developed a poor idle that hunts from 800 to 1300 rpm, the engine sounds like its fitted with a lumpy lumpy cam.

I believe the hunting is related to the idle fuel mixtures, however, when attempting to adjust the mixture I can achieve no better than 12.9 afr (CO meter not readily available, that is the AFM bypass is completely removed!

First thing inspected is the CHT which reads 900ohms at 90 deg c (using the cluster oil temp gauge), according to my readings this is outside the sensors range.

Does the CHT have such a significant input on fuel mixtures that it result in the scenario described above?

Thanks in advance,

Rob

At 100 C it should be 160-210 ohms which might explain your rich condition.

To eliminate the CHT as a potential problem given the present issue, just bypass
it by using a paper clip inserted into the CHT connector.

Hi All,

The other week I was able to replace the CHT sensor with a new part, there was no change in idle mixture. I also bridge the harness side with a paper clip, as with the new and old sensor the maximum lambda value I could achieve was 0.85 (without access to a gas analyser I am using a wideband sensor targeting 0.95 lambda).

Because my car has an aftermarket cat converter there is no access port, for this reason I must remove the Motronic narrowband sensor and fit the wideband in its place. I'd like to understand if anyone believes this could impact my ability to set the mixture? My understanding is that the O2 sensor must be disconnected and for this reason my practise would not influence the results.

What else should I consider that would cause the idle mixture to run rich: AFM, ECU and fuel pressure?

Thanks,

Rob

Check the Fuel Pressure with Fuel Pressure Gauge.

Place it on the test port and remove the vacuum on the Fuel Pressure Regulator it should read 25 bars while running the car.

Replace the vacuum and it should drop to 2 bars.

If not, then culprit could be a failed Fuel Pressure Regulator, a biotch to replace but should do the trick.

If readings are correct then you need to move to the Injectors for proper flow and pattern if it streams out the injector is shot. Have a pro-shop test the flow to be absolutely certain. If this passes mustard, then it could be as you suspected a timing cam related issue. As I understand it, over time, on a long mileage cars, the chain could stretch and timing will be compromised.

The other thing is on the last rebuild, the shop may have not timed it properly, it is art to be able to time our 911s properly. So this is the absolute last thing.

BTW, my buddy's 86 is going thru this exact same thing, we tried everything and our next diagnostic is fuel pressure test then inspect the injector wiring to 4 pin connector at the fire wall for any corrosion under neath the connector. We changed out the CHTS and removed the AFR adjustment screw AFR floating between 12.9-13.5, NOT GOOD, changed DME, AFM, O2S, checked for exhaust leaks, CAN NOT use spray ether at suspect areas method because of the rich condition. Maybe we need to use a smoker to find a potential vacuum leask. But we suspect something electrical if not fuel pressure related.

Jim

It certainly wont be a vacuum leak, that would result in a lean condition.

As with you, once I hit 0.85 lambda the AFM screw will not lean out the mixture any further. I'l give the fuel pressure a test, do you know what the fitting is on the fuel rail? I will also check the AFM to ensure it is tracking smoothly, from what I understand they tend to fail where the trace has worn through and there are discontinuities rather than the resistance changing significantly.

The test port is on the injector fuel rail near injector 1, take care the cap has a ball that will pop out.

I doubt it is the AFM, trace, we replace with a known good AFM and it was still rich.

Check also the vacuum lines from the Throttle Body "T" to the Fuel Pressure Regulator and Dampener, if compromised that will cause the FPR to fail.

I am having the exact same problem with my 84'. I have tested and replaced man parts. Still lumpy at cold start, then it smooths out. I am curious in what you find. Good luck.

Anthony

Out of Range Rich Condition FIXED
 

OP, sorry for the hi-jack.

Update:

Buddy's 86 3.2

What he did since last Cali Smog Check, 2 years ago:

Changed: Fuel Pump, Fuel Pressure Regulator and Fake 911 Injectors with 4 port sprayer $27 courtesy Amazon (WTD?) I wanted to kick him in the balls but felt sorry for him. We had fun though :-P

Symptoms:

AFR out of range, at Operating Temps = 12.9-13.5 AFR, AFM adjuster taken out, AFM verified working, richend to 10.1, so it does adjust and verified with a known good AFM

Lumpy idle 100-150 RPMs ( minor but irratating ) Based idle is low, probably due to rich mixture.

Diagnosed:

CHTS = Proper OHMs Cold to Hot
Exhaust = Checked for Leaks, none to be found
Vacuum Leaks = really could not test here so we used a CIGAR INTO VACUUM LINE at the cruise control outlet method. Found a crack in the rubber attached to the brass vacuum line from Large Elbow at Throttle Body to Brake booster hose. ( NOTE THIS DOES NOT AFFECT RICH CONDITION )
4 pin Injector Connector at Firewall next to LHS rear Strut mount = No discernable corrosion found ( Cali Car )
Fuel Pressure Test: Running 2.2 Bars (a bit high), Running Vacuum on top of FPR removed 2.7 bars ( again a bit high but within tolerances per Bentley ) Car Off 2.5 Bars ( proper )
Fuel Pump Test = 2.7 bars ( maybe the reason running shows 2.2 )

Fix:

Replaced old FPR, same fuel pressure readings thinking towards fuel pump but decided to check the injectors. Upon removal we discovered all injectors were not seated properly and to our amazement the nozzle tips were very, very different in format with 4 pin holes at the tip, nothing like the OEM.

We really did not expect anything and were ready to tackle the fuel pump but then, BINGO!

We dialed out the AFM 7.5 turn from closed ( full rich ) and then started the car, the AFR read 18.2-18-5 immediately and idle though rough, sounded much better :-D, we proceeded to dial in to richen her to 14.2 average (14.1-14.3, because of the lumpiness) at Operating Temps. Base idle set. Hooked up the O2S 14.7 Stoic! HOT DOG!. Test drove, proper warmup idle then to smooth idle, smooth off the line and shifting. No unburnt fuel smell, pinging or back fires.

We also fixed the vacuum leak to insure we don't go too lean.

He is ready to smog now and get off the Gross Polluter :-/ List.

Moral of the Story: DO NOT BUY FAKE INJECTORS, $27 is a great price for a junker but not a 911.

Once he passes, we will place the old working fuel pump to see if we can get the fuel pressure numbers down. If not then as long as it is within tolerances he will be good to go.

Hope this helps some of you. SmileWavy

Great job, but don't screw around with the fuel pump. The pressures are perfect. Plus that won't change anything and you guys will be wasting your time.

Glad to hear it worked out for your buddy Draco.

I'm going to check the fuel pressure next, might contact Len at Autosport Engineering for a test hose.

Steve: Since your reading this thread, do you have any idea what that electrical port (test port/) below and attached to the intake manifold cross over is for? I believe its three pins when you pull out the plug.

The port is to center the ICV for the throttle body adjustment (setting idle speed) and has one output from the DME that signals mixture adjustment (not used)

Thanks Doc! Will let him know :D


Oh yeah, like that annoying song, " He's.....so Happy..... "

"hose" well, if you're in a pinch, you can always get the gauge set from HB for $20 and go to your FLAPS and pick up 3/8" X 9" fuel line hose ( HB kit has the clamps and fitting for the gauge, you just use the clamps for the fuel test port and gauge. Take care not to drop the ball inside the port ).

If I may, that 3 pin test port is no longer used by Porsche mechanics and most if not almost all have actually removed it. Tony Callas at Callas Rensport removed mine.

Jumpering the B&C will permit you to set the proper base idle after it reaches operating temps assuming AFR is correct of course.

Jim

Disconnect the O2 sensor, does the problem get corrected?

A shorted O2 sensor will ground the O2 signal line and the DME will see this as a 'false' lean condition and then the DME will richen the mixture a lot to try and correct the false lean condition.

Simply disconnect the O2 to rule it out.

As already mention I agree 100% on ruling out the CHT sensor. When you unplug the CHT sensor what happens to your mixture? It should get significantly richer with it unplugged. And if as already mentioned you jumper the CHT sensor pins then the mixture should get significantly leaner, does it?

As for hunting, this is a characteristic of the 3.2L when it runs rich, the hunting is a result of the rich mixture.

Draco,

I run 4 hole bosch injectors 0280155715 in my 3.2L but these are hi-impedance injectors and they won't work correctly unless you know where and how to model injectors in the DME. The DME has tables and scalars for modeling injectors.

Hi-impedance injectors have much longer injector dead times (the amount of time it takes an injector to turn fully on). So you must correctly model them, for that matter every injector has specs and diffrent dead times and you MUST know the specs to model them.

But I agree that you should never just get a set of injectors and hope for the best. You need to be 100% certain the injectors have the same specs as the stock ones.

That funny 3 pin port near the TB it's a special serial data stream port. The DME serializes several internal variables (InjTime, RPM, IAT, CHT, ...) the port was used by specail factory computer. The data stream speed is non-standard so you'd need special hardware to process the data. It's a RS232 data stream but at a non standard speed.

Update,

Buddy failed Cali Smog but barely, at least he is off the Gross Polluter List, all numbers were good except 25MPH smog run, limit was 685 he was at 711. Cat must have been compromised by all the heavy richness/unburnt fuel as suspected by the Smog Tech. But car runs great he says :-/ just the cat was not doing its job.

Dr Sal, I agree, if the 4 port sprayer was properly calibrated I am sure they would have worked but in his case he had no idea or way of checking, He assumed it was plug and play as advertised, NOT. In any case, we solved it by process of elimination but now to find a new cat or try to clean the dirty one??? Almost there.

BTW, OP does not have a O2S.

This should ne easy. Have him sign up for a track day, that'll clean out the cat all right. Every time I was out there the insides of my exhaust tips where back to shiny metal afterwards :D

LOL, yeah, I told him to drive at high RPMs on the freeway for over an hour before testing to help clean that cat out. But maybe he didn't. Who knows. Well at least his rich condition is fixed.