Sorry - Yet another CIS troubleshooting thread...
 

Thanks in advance for the help. I thought I was relatively adept with CIS troubleshooting (lots of previous experience on old Audi stuff, but apparently not serving me well on this one). I'm hoping the CIS gurus here and point me in the right direction.

Our patient is a '75 Targa with a newly rebuilt (by me) 2.7L in stock trim; stock CIS P&C's, stock cams, freshly-redone heads, and stock CIS. Part numbers for the major CIS components are:

WUR: 0 438 140 009 (with vacuum port)
Air flow sensor: 0 438 120 004
Fuel distributor: 0 438 100 006

There is no idle air control valve or enrichment valve on the throttle body. The bypass air valve is still present, connected, and pops open ~550 mm Hg of vacuum. I flushed the tank/lines/fuel pump during the rebuild and installed a fresh fuel filter. Injectors are "reconditioned" by a professional injector service and tested for drips and spray pattern (more on this later). The EGR valve is present but "isolated" so that it will not open.

Initial symptoms: The thing was "jerky" at light-load/part-throttle conditions from ~2200-3000 rpm. It also "popped" out the exhaust when lifted completely off the throttle at >2800 rpm. This started as a minor problem ~50 miles after initial start-up and progressively got worse over the last several weeks.

Since most CIS problems are really ignition, I started there. I already had fresh wires, plugs, distributor cap, etc from the rebuild. Vacuum retard and mechanical advance working correctly. Pertronix installed and working correctly. I verified no cracks in the distributor cap.

I did determine that the distributor had excessive axial play (wasn't that way at start-up) and pulled the distributor. I pulled the distributor apart and shimmed the main shaft to remove the end play. I measured shaft run-out once reinstalled in the distributor and found no radial play at the rotor.

Fixing the distributor end play removed the exhaust popping and improved the jerking, but did not eliminate it.

Initial idle mixture was set with the "lift the air plate" method; it will idle smooth, and timing is set at 5 ATDC (stock). Oregon DEQ tells me that idle mixture is at 2.3% CO. I did richen it a tad without luck.

I rounded up the following diagram for the WUR:

http://forums.pelicanparts.com/uploa...1384142218.jpg

Here are my test pressures with my CIS test gear:

System pressure: 4.8 bar
Cold control pressure (15°C): 1.4 bar w/o vacuum
Cold control pressure (15°C): 1.8 bar w/ 400 mm Hg vacuum
Warm control pressure: 3.1 bar w/o vacuum
Warm control pressure: 3.5 bar w/ 400 mm Hg vacuum
Leakdown: dropping below 1 bar in 10 minutes

Near as I can, everything is where is should be (except the leakdown). I have not yet pulled injectors to personally check for leakdown and spray pattern. I have not verified fuel pump delivery volume yet. Interestingly enough, there are no hot starting problems (the normal reason for needing good pressure retention on leakdown). I'm guessing that my leakdown issue is across the WUR.

I pressurized the air box (vacuum cleaner in reverse with air box on the car) to check for excessive leaks. I find one "slow" leak on one seam; am guessing there may be others.

After much research, I ran across another CIS thread where it was advised to temporarily disconnect/plug the vac line to the WUR; I tried that and the majority of the jerking at light load disappeared.

Three questions:

#1 - WHY does disconnecting the vac line to the WUR mostly solve this problem?

(My best theory is that there is a vac leak in the air box and disconnecting the vac line to the WUR enrichens the mixture across the lower operating range to counter this.)

#2 - What else (not mentioned above) should I be doing to troubleshoot this?

#3 - What else do you want to know?

Thanks for the help.

I believe you are on track for the vacuum hose removal enrichening to compensate for false air.
Think about how small a volume of air the vacuum method is pushing and then reverse that flow with an engine running, trying to get as much air as possible.
A small pressure leak is a huge vacuum leak.
Replace the airbox....

It seems your system pressure is at the low end of the admissible range.
I would try to get it to 5.0 and do a run.

I also had some jerkiness at partload (2-3K) and increasing sys press from 4.8 to 5.0 (and thoroughly cleaning the return line path) seemed to fix it.

However, a vacuum leak may also cause those symptoms...

timmy2 - Thanks for the reality check on what a leak means. I'll definitely pull the air box and investigate further.

PJR - For reference, I found the info for this WUR in this post:

http://forums.pelicanparts.com/3543078-post7.html

I believe Mr. Sims pulled this info from his Porsche spec book, and this would indicate that 4.8 bar is right in the middle of the range. However, I can see how increasing the system pressure might rebalance things. I'll give that a try if the air box fixes don't improve things.

Yes, you're right. Minimum is 4.5, but you can try it as it's very simple...

I don't think I saw a mixture reading (CO%). A too rich mixture can cause the symptoms you mentioned.

Keep in mind that sometimes people richen the mixture to make up for vacuum leaks.

Check the fuel lines. If they have been replaced by ones that have too thin a wall, you can get surging @ low speeds.

And raise the fuel pressure as others have said. 5.0 seems sufficient.

Fanaudical,
Do you want to borrow my Gunson CO meter?
I'm down the road a bit from you in West Salem.
PM me and we can figure something out. :)

^ start here ..

When I was having issues it was with the WUR which was totally screwed. I followed the recommendations from the members here and its all working.

My system pressure was up around 7 bar if I remember correctly. See below this is the thread that finally got me running well.

1977 911s Fuel Pressures

I had CO measured at ~2.3% during a recent emissions check (but did increase mixture to ~2.5%, which did seem to make the thing run better).

It sounds like the most prudent thing to do is to remove the air box and make sure all the leaks can be sealed (or replace the air box if not). I will do that in conjunction with bumping up the system pressure. When I get to that point, I will give timmy2 a PM and make sure I've got the CO meter available (thanks for the offer!).

No problem, happy to lend a hand or tool when needed.

Paul - Sorry, missed your post. Can you please explain to me how a too-rich idle mixture might cause the problems mentioned? I wasn't aware that position of the idle adjustment screw affected how much fuel was metered when the air flow plate is raised off rest past an idle condition.

I have replaced almost all my plastic fuel lines, but did use the "good stuff" - Cohline fuel line for CIS that I purchased through BoxsterGT here on the forums. I did investigate this one, and have not run across anyone with surging issues traceable to this specific fuel line type.

CIS troubleshooting.......
 

fanaudical,

I've been following your post since the very beginning and you are doing good. So I just stayed back and watch this developed. You need to do a pressure/vac test to determine if your engine is getting unmetered air. Without testing and verifying the integrity of your vacuum system, you are guessing and hoping that there was none (air leak). It maybe that's the case, but without testing your air box and other areas affected by unmetered air diagnosing your problem would be tricky (difficult).

BTW, do you have a pop-off valve? Keep us posted.

Tony

very good job. nice to have someone that is clear with their results and to the point and thorough with their testing.


first thing i noticed is even though your WUR may be in spec, it is only getting a .4bar change in pressure with and w/out vac. personally, i would like to see about .8bar. with 2.8bar with no vac and 3.6 with vac.

have a smoke test done. it may be worth while to test your air box properly, or just get another one.

what is the timing at full advance? set it for full advance, not idle.
i dont know if i should say this or not.. timing can cause the popping off throttle. with not enough advance and the retard connected, it can pop.
personally i like the retard at idle. it sounds better and it is out so fast its like another advance, also, i think you can run a little richer mixture because of the retard at idle.

is the WUR connected to the correct vac port on the TB? this should be manifold vacuum. if you look at the ports and look inside the TB, you can figure out which ones are above the plate and which on es are below.

did you replace the boots on the intake runners? are they secure? what about inectotrs sleeves and Orings?

how about injectors? do a flow test and compare the volume of the injectors. put all 6 in containers, turn the key on and lift the plate. the more fuel the better the test. this also check the FD and lines.

Thanks for the kind words. Here are some answers to the questions:

Yes, I do have a pop-off valve installed. (Wish I had found Peter Z's treatise on the evils of the pop-off valve before doing it.)

I have done the "reverse-flow vacuum cleaner pressure test". Injector sleeves, o-rings, and runner boots are new. Injector sleeves and seals are tight (no bubbles). Runner boots are tight. Pop-off valve is tight. I did detect a minor (one bubble per 10 seconds) on one accessible seam of the air box. I'm guessing that there are more leaks in the seams I can't reach. However, it does seem to me that the system should be able to compensate for small leaks like that.

Interesting note regarding the WUR pressure change with vac/no-vac; I didn't catch that the difference was only about 1/2 of what is indicated by spec. (I applied vac with my hand-held vac pump for the test, so should be fairly accurate.) I'll have to investigate that.

The WUR is connected to manifold vacuum, but the vac line to the WUR is also tied into the decel valve. An open decel valve will limit vacuum available to the WUR when it opens.

New question: Is there any possibility my decel valve is opening too late? How much vacuum should be applied to the decel valve to get it to open?

To be honest, I have not yet set timing at full advance. I don't have a lot of time on this engine yet and am somewhat timid about running it up to 6000 rpm and holding it there with no load. (I guess I'll just have to get over that.) I have checked timing at lower RPM's and it seems to follow the published ignition advance curve to 3000 RPM.

Seems my next course of action (in order) should be:

[ ] Verify timing at full advance

[ ] Retest the pressure change from the WUR with vac/no-vac. Check that there isn't a malfunction in the the WUR.

[ ] Injector flow and spray pattern test

[ ] Pull the air box and properly test all seams for leaks

Any and all comments appreciated; thanks for the help.

you dont have to hold it and you dont have to go to 6k. rev the engine and you will see the advance stop coming in. rev it and check it. set it at idle, then rev and check. you will be OK.

dont sweat the decel (DV). it should be closed at idle, when vac is highest, then when you let off the gas, the higher vac will open it. i am sure there is a spec but you can use your handvac and check the idle vac. then pump it up to see when it opens. should be just a little higher than idle vac. thats all i did with mine. just made sure it was closed and it would open but most important,it did not leak.
i ended up removing my DV when i put a 3.0 throttle body on my 2.7. i think its a noce little mod, not much mentioned on here about it. kinda suprised.

I haven't had a lot of time to play with this. (Been wrenching on the wife's daily driver all weekend; I'm never buying another car that needs timing belt service where the first step is to remove the front of the car ever again.)

I did check timing at full advance (got the car warm, disconnected/plugged vacuum module, quickly revved it, watched "peak" advance) and it looks like I'm getting 30° BTDC. I also tried with vacuum module connected and also see a maximum of 30° BTDC.

Went to check Wayne's book; it indicates 38° BTDC (I'm assuming maximum) with hose connected. My other manual references 32-38° BTDC with hose disconnected. Here's a advance curve I pulled from another thread (this appears applicable to a 2.7L in stock config with CIS):

http://forums.pelicanparts.com/uploa...1384740729.jpg

Anybody have definitive timing advice that works well for a stock 2.7L? Looks like I can advance timing another 2-4° safely and reset idle/mixture and see how it works.

One Possibility
 

Fan, I have followed this thread closely as it caught my eye due to a similar
problem I had. I could not push past 3200 RPM in any gear, under load and with
the help of many of the forum members that have supported you, they coached me to
a solution. I can see where you have verified your motor to "Spec" and for
most DIY'ers, beyond. I have one thought that you may want to consider and it
is based on the KISS principal: The Bosch CIS fuel distribution works with
differential fuel pressures, the system pressure and the regulated pressure
provided by the WUR meet in the fuel distributor (FD) to ensure accurate fuel
flow. In my 77 911S, the WUR did not provide enough pressure to balance the
system pressure in the FD and thus, a restricted or limited fuel flow. It flowed fuel
with a manual movement of the control arm, but the movement needed with air
flow was restricted due to a low differential pressure. With all the
testing and replacements you have done, have you gone inside the FD to verify
there is no obstruction or blockage in the metering system? I know it is a
challenge and something I would not want to do, but from my vantage point, I
would take a look in that direction to verify proper flow. I do not recall seeing a
lot of "How To" information on verifying the FD, but from my limited experience
and seeing all you have done with pressures, timing and electrical system components,
the only thing left to verify is FD performance. I am sure the
"Experts" on this forum will have more comments or thoughts, but the FD is what
I would consider/pursue.

All good points made here. If in fact the FD is your culprit I'd recommend giving these guys a shout:
CISFLOWTECH
There are very few qualified repair people for the Bosch units. They're not particularly complex - either in concept or construction, just precise and finicky. I'm a die-hard DIY'er, but sometimes you gotta know when to hold 'em and when to fold 'em.
They will also e-mail you a very thorough manual with all the pressure testing data you need, or if you PM me I can send you what I received from them.
-C

I've seen this same issue before and it was the pertronix ignition unit. Did you substitute points just to eliminate the possibility ? Did you use a scope to check ignition under load? As you said ignition is the problem most of the time. It only takes a minute to test a set of points.