My 87 944S Runs Rough at Low RPM, Stalls at Idle, Help!
#1
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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!
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!
#2
Rainman
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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.
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.
#3
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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.
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.
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?
#5
Addict
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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?
#6
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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.
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#8
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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?
Last edited by aks944; 01-22-2020 at 02:37 PM. Reason: For completeness.
#9
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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.
#10
Rainman
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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.
without it, IIRC the DME defaults to limp home mode, 10 degrees timing.
#12
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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?
#13
#14
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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).