Tyler Rose

Hi all,

Hopefully everyone is safe, healthy, staying inside and keeping busy with their cars during this time.

I am reigniting my thread in the general 944 forum (https://rennlist.com/forums/924-931-...ion-issue.html) with more focus.

I am having an issue leading to rich running, misfiring and under boost, early cutoff and black smoke.

On start, the fuel trim appear at or near stoich. My wideband meter indicates 14.7-15 (with some lean spikes,) then 13-15 seconds later, the mixture hits 10 and below. Then under boost, the cycling valve appears to be dumping at a point above/around 4k RPM and 7+ PSI, violently cutting out and blowing black smoke.

What is interesting is that the idle gets noticeably rougher 13-15 seconds after start whether hot or cold, however, it violently fires on maybe two cylinders when cold starting (see here: https://www.dropbox.com/s/p7erffv2it..._0937.MOV?dl=0.)

I am leaning towards this being a sensor-related issue, due to the consistency in the start cycle and am trying to pinpoint what exactly the DME begins reading for mixture at that 15 second mark. My car does have a mysterious DME and KLR chip (soon to be replaced,) however this was not an issue prior.

My indy has tested the FPR, replaced the DME temp sensor and installed rebuilt injectors, both yielding nothing, and contradicting my belief to not throw money at a car without a tested fix. I have cleaned the AFM wiper arm, cleaned/adjusted the throttle body. Plugs, wires, distributor, rotor and coil are all new.

Any insight is much appreciated.

Tyler

moalaska

Is your DME original? I've had all sorts of stumbling/rich/lean weird things happening on multiple DME's that just started having problems. Eventually found a good used one and it went away. Are you running stock injectors?

moalaska

I'm a little rusty I haven't messed with my car in about a year, but you could try and unplug your o2 sensor and see what it does. Check your tps microswitch that it's working and adjusted too. You just have to start working through things testing them. I think Clark's garage has troubleshooting checklist. Having a multimeter and knowing how to use it will be your best friend.

Tyler Rose

The injectors are stock.

The TPS tests fine at the DME and KLR. Still runs bad with O2 sensor unplugged. I’m going to clean up the bell housing grounds and check all the vacuum lines under the intake manifold tomorrow, as I have it off the car.

moalaska

Get a multimeter, and take about an hour of your time. pull your carpet and dme, and the dme connector.


start working through some diagnostic checks.

Voltage supply to dme: ignition: on pin 18 to ground and pin 35 to ground should be about 12 volts

Grounds: ignition off: check continuity, 5 to ground, 10 to ground, 16 to ground, 17 to ground, 19 to ground

full throttle switch: 3 to ground continuity, should read no continuity if throttle closed or partially open
same test, throttle open full, should have continuity

tps idle: 2 to ground continuity, throttle closed, no continuity, open partially or fully, should have continuity

air temperature sensor: ignition off: ohms between pin 6 and 22 , 20 degrees C 2000-3000 ohms, 80 deg C 250-400 ohms

idle speed control valve: pins 34 and 33 with ignition off, should read 40 ohms

o2 sensor: pin 28 and ground with ignition off, should read continuity

There are lots of other tests you can do, but I would work through those and it will help eliminate wiring or sensor problems. If you find any abnormalities post it.

moalaska

Also are your plugs gapped properly and in good shape? condition of your plug wires? and check all your vacuum hoses, and your J boot coming out of turbo, and intercooler rubber hoses. check for leaks around your intake manifold gaskets. if you have a boost leak after whatever your metering device is, your afr's will be off.

Droops83

Have you or your shop checked fuel pressure and volume? I know you said that they tested the fuel pressure regulator, but I'd be suspicious of something like a bad FPR or kinked fuel return line causing excessive fuel pressure.

The oxygen sensor of an older car like the 944 tends to take a bit longer than 15 seconds to begin generating a significant enough voltage signal to affect the AFR----though it is very easy to unplug the factory O2 sensor to eliminate this as an issue.

If it is neither of the above, I would try swapping the AFM and DME with units that are known to be good; maybe a fellow 951 owner nearby would be willing to lend these to you.

There is also the possibility of a wiring harness issue causing all of this----if none of the above helps, it is time to check the relevant circuits at the DME as @moalaska suggests, but focus on the circuits that actually affect AFR, such as the coolant temperature sensor, AFM signal circuit, etc. An oscilloscope to check the fuel injector waveforms would also be ideal.

Good luck!

Droops83

Yes, but none of these will cause the pig-rich condition that the original poster is experiencing. Something is way off to cause such a dramatically rich condition.

moalaska

Its a rich and misfiring issue. Are you running stock afm or do you have some sort of mass airflow sensor?

Tyler Rose

I went through these tests last month and didn't seem to get anywhere. I'll give them a try again tomorrow with the specifications you gave and see.

Plugs are brand new and gapped to factory spec. Wires are brand new. J boot and intercooler hoses are new and I am going to clean up the mating points as well as change the manifold gaskets while I have it off.
My instinct told me it was a boost/vac leak, however, the fact that the car operates fine for the first 15 seconds after starting is pointing towards an electrical/DME related issue.

AFM is stock and original TMU. I have cleaned the wiper arm, as well as tested it within spec.

Droops83

A vacuum or boost leak will not cause this! I am a professional Porsche technician with 15 years of experience if that means anything. A large vacuum leak will cause LEAN running at idle. Don't waste your time taking off the intake manifold.The fact that your engine begins to run pig rich after 15 seconds of idling points to either to excessive fuel pressure or something being way off in terms of the DME or sensors/wiring that affect air/fuel mixture.

Do you have a copy of the factory 944 Turbo "DME Test Plan" manual? That is a good place to start.

moalaska

I gave him the procedures for simple tests from DME manual, the rest require an oscilloscope. I never told anyone to take off an intake manifold, only to check for leaks around intake manifold. When you are dealing with 30 year old stuff, sometimes it's not 1 problem, but a couple things making it more hard to to diagnose.

Droops83

I hear you about the multiple issues/old car thing. Tyler's issue is something causing a dramatic increase in the amount of fuel injected into the cylinder, so he should focus solely on things that would specifically cause that.

Tyler, is your factory oxygen sensor connected? If so, unplug it and see if that changes anything. I have never seen a bad O2 sensor cause such a dramatic rich condition, but it could be shorted or something.

Again, what is the fuel pressure?

Another relatively simple check is the grounds on the bellhousing. I have seen loose/corroded sensor grounds cause some strange running issues, though not specifically a very rich condition.

After that, you can do some simple resistance checks with an ohmmeter---focus on the engine coolant temp sensor (NTC II in the manual), and check resistance between the DME plug and sensor connector. If the NTC II wiring has very high resistance or is open, it will cause a very rich condition.

Tyler Rose

My reasoning for attributing it to a vacuum leak (and please correct me if I'm wrong) is the fact that the car is misfiring, so my wideband sensor is detecting the excessive fuel in the exhaust.

I removed the intake manifold to repair a broken vac hose, and inspect the Venturi, as well as hoses going to and from the CV and ISV. Not much to my surprise, the Venturi tube was loose in its fitting, and the other hoses were original.

The O2 sensor is plugged in, and unplugging it does not change anything. I was not provided with the fuel pressure. I tried to get to the grounds today, but even with the manifold off, struggled. They look dirty, but not dirty enough to create this issue. Any tips to get to them?

The coolant temp sensor was replaced twice (second time on warranty) in the past three months by Rennwerke. I will double check the resistance tomorrow.

Tyler Rose

Update -

Interestingly enough, I may have uncovered the issue today, but need some confirmation that it connects to the issues I'm having. After reinstalling my intake manifold, I started the car once and it went through the usual cycle, however, the second time starting it, I jumped the DME pins for the ISV. The car then seemed to be running normal.

My question would be - how does this pertain to the boost issue? What would cause the KLR to dump boost if there is an issue with the ISV? The sequence of events for the two problems seems to show a connection. 1. After replacing my turbocharger, due to FOD, at my indy, the car comes back idling around 1100RPM. They could not confirm if one of the techs had adjusted it higher so I took it with a grain of salt.
2. Car was driving down the road and boosting fine for 2 or so weeks, then 45 minutes or so into a drive, the car violently dumps boost sporadically depending on gear.
3. The next day, the car appears to run richer and misfires at idle. Now it take 5 minutes of horrendous idling for it to clear up, and once warmed up, it hits an idle mixture of 12.5:1-ish. It still seems to struggle returning to idle (lean surge sensation.)

Could the ISV be the culprit?

Extra notes to add - my idle vacuum is around 12 at cold and maybe 15 when warm.

I need some resolution as it is getting nicer out, and I've put maybe 1500 miles on my car since I bought it.