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944 s2, idling rough, loss of power, cutting out - then all ok again!! help!


Old 12-13-2009, 04:01 PM
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Default 944 s2, idling rough, loss of power, cutting out - then all ok again!! help!

Hi All,

Apologies if i'm being dim, but here's my story from today..
so my 1991 944s2 has been sitting for a week, today was very cold (just above 2 degrees in Scotland!), normally this isn't a problem, however, today i start the car and...

1) it doesn't idle smoothly
2) the exhaust seems to be blowing a lot
3) I drive off and things just aren't smooth, accelerating seems sluggish and the car seems to be lacking it's normal power.. it seems almost as if it isn't firing on all cyclinders.

I get to my destination and park up for an hour. When i restart the car, it seems to be idling a bit more smoothly but not perfect. as I drive home and the car seems almost perfect again, idling ok, accelrating ok, no loss of power, then..... at 60 mph it just cuts out. It failed to start a couple of times, leaving me stranded on a busy road. I eventually get it started and crawl to a safe place to park. It cuts out again but restarts. I then manage to drive home and all seemed ok. as if the problem had gone away... (but i bet it hasn't)

Am thinking DME relay??
but a friend suggested it could simply be water in the HT leads?

Anyone got any other ideas or knows what my problem could be?

Thanks for helping a novice out!

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Old 12-13-2009, 08:05 PM
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O2 sensor? - would cause poor running / loss of power
DME Temp Sender? Would cause the car to default to rich
Hall Sender? would cause rough idling
DME / Fuel Pump relay? could be the cause of the 'no start'. typically happens when the car has been runnign and the relay gets hot
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Old 12-14-2009, 12:52 AM
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Man, am I interested to hear what cures this!!
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Old 12-14-2009, 01:16 AM
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well previous to today, had it been running perfectly?
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Old 12-14-2009, 01:17 AM
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Fuel pump on its way to the graveyard, or something (water?) plugging the outlet of the tank.
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Old 12-14-2009, 01:52 AM
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A spare DME Relay should always be carried in the glove box as cheap insurance or the 3 wire jumper. Look under "D" for DME and "S" for Speed Sensor.


When was the last time the fuel filter was changed? When the car won't start, look at the TACH Needle to see if it moves slightly about 1/16 inch. No needle bounce the speed sensor is not making good contact. Check the connection in the top back side of the engine and wiggle the wire. Then try and start your car.


Last edited by Grandpa#3; 12-14-2009 at 01:58 AM. Reason: add
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Old 12-14-2009, 09:05 AM
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I chased a similar problem for more than a year. On my 944S (2.5 L version of the same engine) I replaced everything mentioned above, I even had the car on a dyno to try to isolated the issue. My issue was a bad crank sensor wire. The problem was found by wiggling the crank sensor wire while someone else tried to start the engine. Good luck.
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Old 12-14-2009, 09:52 AM
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All this guessing will get expensive if you don't narrow it down some with basic tests. Testing for a good strong spark when you are having the problem, for example, would be a good first step.

It could be water in the fuel as one person suggested. On the S2 it could also be the ignition module. I have seen this sort of bad running caused both by fuel and by spark so checking for a good, strong spark with a spark checker is important. Just looking to see if you can make a tiny spark does not tell you much.

When I got a tank of watery gas, after 50 miles or so I could barely keep my truck running. Thankfully it got a lot better with some fuel additives and fresh gas, but it took quite a few gallons of good gas to make it go away completely.

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Old 12-14-2009, 02:13 PM
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In addition to the above comments, I would add the following:

#1 The airflow sensor is mounted upside down on the 944S2. If you follow the wiring harness to the plug in the sensor, you can disconnect it by pressing on the snap ring in the plug. Clean it with dielectric grease and reconnect. See if this solves your problem. I've had similar symptoms disappear by disconnecting and reconnecting the plug. I've also seen people replace the entire sensor only to find this problem to reoccur until they reconnected the plug.

#2 The ignition control module is specific to the 16V cars and creates an erratic 'bucking and shutting down' behaviour when it starts to go bad. Doesn't cost much and is easy to replace. I'd give it a shot.
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Old 12-14-2009, 02:16 PM
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You would be amazed at how a bad crank sensor wire can affect a car. I was chasing some of the same problems, and by accident move the wire and they (almost) all went away. If the wire is bad, or is shorted, the car will not run, and will cut out.
Good place to start checking.
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Old 12-14-2009, 03:06 PM
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Many thanks for all of your replies so far.

Car has been running fine up until now - in fact it has covered three 300 mile trips in as many weeks without missing a beat and is my daily drive.

I've just been out now to see if i can replicate the problem, needless to say it started first time and idled quite happily with no misfires, a quick drive round the block and no drama's.

It's due an MOT this week so will get the professionals to give it a once over see if they can spot anything that i can't!

Will keep you posted if/when I learn the root cause!
Whats worse - a problem that you can fix ($$$), or a problem that goes away by itself (but may return at any time)!
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Old 12-14-2009, 09:54 PM
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My 89 S2 does somethg similiar. It will start surging at idle, nottoo bad, but noticable, and sometimes while accelerating, I will feel it doing the same thing. It doesn't last long, and it only does it ever so often. It isn't bad enough to worry about yet, but it sounds like it may be close to the same problem you are having. I hope you can track it down, good luck.
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Old 12-14-2009, 10:54 PM
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Heh, boy. Ordinarily I'd recommend measuring everything you can - get your hands on the DME-Diagnosis / Troubleshooting Guide for 944S / 944S2 in the FSM and run through it and measure everything, measure vacuum, anything you can. Do that - but this is an intermitant problem so you may not find it. But you'd be surprised how good you'll feel knowing what's good. You might be interested in my similar, many years-long issue for ideas:


I finally replaced the AFM and it hasn't happened again but I haven't driven it when real cold ever since since I got the C4.
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Old 12-14-2009, 11:06 PM
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Mine does almost the same thing... I know its a fuel leak but cant find it...
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