Running rich, engine stumble when changing gears - Need ideas please

[jeford]

OK - I've been unsuccessfully chasing this down for a while now, and would really appreciate any ideas or advice. Sorry for the long post...

Car: 87, 944 non-turbo, ~ 150k, all stock except cat has been removed.

Symptoms: Rough idle / poor performance when cold. Once up to operating temp - performance is better. There is a pronounced engine stumble when changing to higher gears (cold or warm, but gets less noticeable once warm). I can push in the clutch and let the RPM drop - not changing gears - release the clutch and do not detect the miss. If I gear down there is no miss. Occasionally there is a miss when under load in high gear (ie: ~2500 rpm + acceleration), and I can see a slight miss slowly when rev'ing from under the hood - again around 2500 (I can see the engine shake slightly). It is running rich (smell when crusing - no smoke). WOT seems perfect.

Things checked (multiple times ): AFM flap voltage - Works correctly; AFM air temp sender - within specs; TPS - was out of adjustment (no WOT) but is now correct; coil - switch with known good; Aux air valve - cleaned and checked. Engine is in proper time.

Also - check temp senders and TPS back to DME - all wiring seems good.

Things replaced: Fuel pressure regulator and damper; DME temp sender (not gauge); both reference senders; Plugs and wires; Dist cap and rotor button; O2 sensor; DME relay; had injectors cleaned and inspected by Witchhunter - all good; all small and large vacuum hoses (ie: to/from aux air valve; air/oil separator hoses, and anything else under the intake - as well as those going to fuel rail and climate control) were replaced; air/oil separator seals. Replaced all with Porsche or Bosch parts.

I don't think I have a vac leak, as I can remove the oil filler cap at idle and the engine begins to stumble a bit.

I'm going to check fuel pressure as soon as I get an adapter to the rail. As well as replace the fuel filter and check compression.

Unless it is something mechanical (maybe lifter?) or something in the DME; I just don't know what else to check... Ideas? Thanks in advance!

[John_AZ]

You have done a lot of work to get to the problem. +1

Engine Stumble.
I did not see that you have taken off the cap on the AFM to check the "tracks" for wear.
Air Flow Meter (AFM) Operation and Testing

You have checked or changed most of the parts I would have suggested.

When you do the compression test, also do a leak down test. You may have a burnt valve.

Have you adjusted the DME FQS fuel setting at any time?
Adjusting the Fuel-Quality Switch (FQS)

Finally, the DME may be bad.

GL
John

[jeford]

Thanks John... I did pop the AFM cap. The tracks had marks, but the the voltage ramped up as indicated on that test (I tested in car - with switch on but not running). I also moved the wiper down a bit to slightly move off the worn areas. Finally cleaned with electrical cleaner, dried and resealed. I checked the fuel quality switch - it was in the stock position. I experimented with the other settings, but didn't see/feel any improvement.

As per a mechanical condition. If a lifer - say on an intake valve -were not adjusting correctly would such a condition exist? I do hear some lifter lash, but not excessive. I know this is a long shot, but when I had the head off a few months ago due to a timing belt failure - I didn't like the way one of the lifers felt. I put it back in as everything seemed to be ok before the belt broke. It has been running well for months after the valve job.

On the bright side; once I find this just about every other common problem point has been addressed!

Thanks again!

[John_AZ]

The recent topic on lifters concluded that if a lifter does not return if pushed in, the lifter is bad. Semi rock hard and rock hard lifters are good. They are pumped up. Soft lifters are good if the piston returns (usually can get pumped up----may be noisy). New lifters purchased are in the rock hard category.

Dissecting 944S lifters

When the head was off were the valve quides OK.

Lifters should not cause the "rich" condition. If the lifter noise goes away it should be OK. Seafoam is used by a few members to clean out the lifters.

Compression test and leakdown test. Remove the DME/fuel pump relay when you do the compression test.
Compression-leakdown test

Have you cleaned the ICV/ISV with Gumout fuel injector and intake cleaner?
Idle Control Valve Testing - Porsche 951 - www.texasblake.com

The throttle body can be rebuilt with a kit from Arnnworx.
http://www.arnnworx.com/944_tb_kit.htm

GL
John

[Slam]

How about your grounds? A fresh ground strap from the battery to the engine can work wonders. If there's too much resistance in any of the circuits, the engine will run rough.

Just a thought - as John says, you've done everything else I'd think of.

[HondaDustR]

Yeah, sounds like some electrical issue is a possibility. My idle stumbles as the cooling fans come on (doesn't run that great while they are running, either), or if I try to roll the electric windows up when they are already all the way up . Strangely, nothing else really affects it, and all the grounds have been cleaned. I even replaced the battery cables with 0 gage car stereo wire. Oh, wait...I remember I saw the three primary supply wires to the fuse box looked kind of corroded inside near the battery. Forgot about that project...

[jeford]

Thanks everyone... I had one springy lifter and the others were solid. I assumed it would pump up, but it is good to know that a lifter shouldn't cause it to be rich. One starts thinking weird things when they have reached the end of so many dead end roads...

I researched the leak down jig - looks easy to make so I'll invest in that next week.

I did remove, clean and lube the ICV, but didn't do the electrical test as per the link. I removed it from the circuit and plugged the hoses. I'll try the electrical test as well.

I did check the engine grounds (removed and cleaned them - forgot to mention that), but I didn't try adding a ground strap. That is cheap and easy - so its worth a shot. I also remove the fuse box and disassembled / cleaned all contacts as I was having some intermittent issues with circuits not working. Cured that, and the sun roof started working after - I assumed it was those plastic gears!

If I finally come down to DME - is there anyone who can test and repair if bad, or does it make more sense to buy a used one? I have access to some fairly sofisticated solder equipment, and I've been toying with the idea of just re-flowing everything. However, I would like to have someone who could exercise it properly with diagnostic equipment - if there is such a place. Also - I don't like the idea of tinkering with something like the ECU unless I have a spare. I've been watching e-bay for a good deal on the ECU and an AFM (just to have even if mine are working well).

Thanks again for everyone's advice - This issue isn't all that bad, as the car is drivable, and once warm "almost" not noticeable. However, I have so much time invested that it has become a matter of principle that I find it and fix it!

[John_AZ]

Here are a couple of DIY DME resolder links:

Early DME repair write-up with pics. - Rennlist Discussion Forums

DME Repaired w/pics - Rennlist Discussion Forums

A couple of places will check the DME if you send it to them.
There are also shops that will repair your old one.
Usually-------getting a used working DME is the normal way to go.
Links available if you decide to.

GL
John