aks944

Hi All,

Having studied every Porsche forum and having suffered endless failures to find the problem i'm at my wits end with my 87 944 S. Symptoms as follows:

- Extremely rough idle or wont idle most of the time, often stalls as soon as the car is started unless accelerator is applied
- Can be coaxed to run and drive if the revs are kept up. At high revs it pulls strongly at times but sometimes its impossible to get it into 3,000 plus RPM without stalling out first
- Any load like going up a small hill will cause it to sputter and it will backfire into the intake or even afterfire into the exhaust
- Switching ignition off and restarting the car seems to reset things a bit and the car will run again for a few minutes, albeit poorly and with an extremely rough idle

Done so far:

- Compression tested OK with all cylinders between 180 to 185 psi warm
- Replaced plugs and leads
- Replaced TPS
- Replaced engine Temp sensor
- Injectors cleaned and flow tested
- New fuel pressure regulator
- Checked vacuum lines, all appear ok
- New dipstick o-rings
- Replaced throttle body O-rings. Appears to be no more vacuum leaks
- Recently new\reconditioned AFM installed by previous owner
- Recently new\reconditioned DME installed by the previous owner

Any thoughts or tips?

Thanks!

V2Rocket

other possible leaks might be the giant plugs on the underside of the intake manifold from the casting process, they can even fall out.
or the AOS (which has 2 o rings to the block, one between the lower and upper halves, and 1 under the oil cap) could be leaking too.

how old is the O2 sensor? the 944S relies heavily on the O2 sensor and the idle control valve to idle and run at part throttle, very emissions oriented.

aks944

Thanks, I previously removed and resealed the AOS. I could be wrong but i don't believe even the largest vacuum leak could cause running this bad. These cars can still run with the oil cap removed. My car barely runs under any conditions.

No O2 sensor on this car. Its a UK spec (Euro).

Maybe i should be looking into the DME side of things, although it is fairly new that does not mean it can not fail?

Swenny

And any previous work that was done by the P.O. should be suspect and tested,repaired or replaced. A faulty DME will drive you crazy.

Jfrahm

It's good that the plugs were replaced. After it's run rough for a time, can you right away pull them out and see if they show signs of running rich?

aks944

After running for a while the plugs were black and sooty, with some soot coming out of the exhaust.

I replaced the plugs again and backed off the AFM spring to lean it out and got a nice flesh colour on the plugs after running the car.

Unfortunately rich or lean the car still runs extremely poorly and with very similar symptoms.

Swenny

Have you checked the fuel system? Bad fuel,pump,screens,filters,injectors etc.

aks944

One other strange symptom. Not sure if its related to this issue is that the tach bounces to 6000-7000 rpm when cranking. Is this normal? When the car is actually running the tach works fine. Does the tach get its readings from the Crankshaft Position Sensor? Could it be a duff crank sensor?

aks944

Have ordered a fuel pressure gauge, will be getting it in the next few days and testing fuel pressure and fuel flow. Will update with results.

Has had a few different tanks of fuel through it so not a bad batch of fuel.

Injectors have been cleaned and tested and flow matched.

V2Rocket

is the Hall sensor (back side of camshaft gear cover, black plastic connector, above #1 exhaust manifold) intact/functional?
without it, IIRC the DME defaults to limp home mode, 10 degrees timing.

T&T Racing

With "sooty" spark plugs, looks like FRP failef closed but fuel pressure test will confirm positively or negatively.

aks944

Hall sensor is intact. It has resistance of around 1 mega ohm between the two outside poles and has 12v going to it.

Not sure if it actually functions as i have no idea how to test it properly. Is it possible without an ocilliscope?

I also noticed there's a knock sensor under the intake manifold, would that be worth testing for correct resistance?

aks944

Ok, ill confirm that as soon as the fuel pressure gauge arrives.

CyCloNe!

DME Temp sensor (creates starting and running issues and stalling out) and ISV (if bad and/or stuck open makes stalling, hard starts need acceleration and if open can cause stalling and rough driving) come to mind or a vacuum leak. TPS won’t generally effect your idle especially if bolted up and set correctly. You can usually disconnect it and the car will still run. If FPR failed check your oil it will stink of fuel and be thinned out. Usually it will cause fowled plugs, hard starting, back firing and eventally a no run condition (due to fowling).

aks944

UPDATE: Fixed! Was a Vacuum leak which has now been corrected and the car runs perfectly. Thanks for all the helpful suggestions.