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My 87 944S Runs Rough at Low RPM wont idle (Read 445 times)
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My 87 944S Runs Rough at Low RPM wont idle
Jan 22nd, 2020 at 1:53pm
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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?

« Last Edit: Jan 22nd, 2020 at 5:02pm by Buddy »  
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Re: My 87 944S Runs Rough at Low RPM wont idle
Reply #1 - Feb 16th, 2020 at 3:53am
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I recently finished a full rebuild of my '87 S engine and went through a similar problem. I had an inconsistent idle and a bad TPS, but once I replaced my TPS it became impossible to start it. There were several things it could have been POST TPS, including something as simple as a malfunctioning idle control valve on my intake or an ignition coil issue. But, on closer inspection, I discovered my particular issue was due to VERY loose timing and balance shaft belts that caused both the cam and balance shaft sprockets to be 2ish teeth off (thankfully they were just loose from breaking in the cams for a few minutes, so no damage was done).


Check your throttle body setup. If there's resistance, or when you close it it slaps shut, making no contact with the throttle stop stud, you need to clean and lightly lubricate the spring, loosen that 7 or 8mm locking nut that holds that little stud in place, take a flathead, screw it in until it touches the throttle cam and then apply an extra little turn. That will make sure the throttle isn't slamming shut when you aren't on the accelerator. If that doesn't help, then it's time for some work.

The easy work:

I would replace your O2 sensor first. If it starts okay, it's not the ignition coil I don't think. If no change, try replacing the distributor rotor. If neither work, replace your idle control valve, which is a very common culprit but requires a bit more work than the O2 sensor.

The harder work:

A poor timing setup is very dangerous and can destroy your valves, lifters, and pistons if you're not careful. I would suggest taking the airbox off to get to the upper plastic belt cover and check the belt tension. If the timing belt feels tight, you should be good, and if the balance shaft belt feels tightish, but with some play, that's ideal too. If the belts feel loose, take the distributor cover off, crank the car to top dead center, and check to make sure all of your notches line up. If they don't, you gotta do the timing belt and balance belts. I'd replace them if you don't know how old they are using the 944Online tensioning kit sold on the site here specifically for the S. Timing belt should be read 40 lbs on the clicker (with 90 degrees twist of flexibility using your fingers) and 27 lbs on the balance belt (180 degrees of twist). Clark's Garage has a plethora of tutorial info for the timing belt tensioner and how to line up the balance shaft sprockets.

Happy wrenching  Cool

1987 944 S Guards Red
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