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very rough warm-up 900 1994

posted by  janglim on  Tue Jun 23 08:09 CST 2009
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last visit: Tue Sep 9 11:55 CST 2014
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I'm having trouble diagnosing a rough-running / no power condition on my '94 940T. Here are the symptoms: At cold start (actually luke-warm start, 60 degree F ambient) it will start after 10-15 sec cranking but will not idle without throttle manipulation. I can hold it at a steady high idle for several seconds before the rpms drop rapidly to 0. I can overcome the stumble with more throttle manipulation, but the same stumble starts several seconds later. After a couple minutes of this, the idle stability improves somewhat (seems to be a noticeable step in stability, not a gradual improvement) to where I can hold a consistent high idle, but it will still stall with no throttle. Even as the idle stability improves, I still have no power. Under load it will not accept any more than a touch of throttle without bucking and stumbling. The threshold for this may be the point at which the TPS signals opening throttle - that's how little throttle it will accept. If I keep it running in this condition and driving slowly around the block, it will eventually "snap out" of this state and run normally. The takes between 5 minutes and 25 minutes. I have been through this cycle about 5 times now and it seems to be taking progressively longer to get to the strong running point.
After reaching the running point, there are no problems at warm start. The condition returns after the car sits for 5 or 6 hours.
What I've tried: No OBD codes set. I replaced spark plugs and rotor. Plugs were light gray and dry at electrode with no deposits. I've checked the ground straps and all vacuum hoses. Several months ago I had similar symptoms that were apparently solved with a new fuel filter. All sensors were replaced a little over 2 years ago when I bought the car with a blown head gasket.
I guess I could try another fuel filter, but the relationship to the warm-up period with an abrupt change to normal running makes me wonder. Would a bad coolant temp sensor give this behavior? How about fuel pressure regulator? (I don't have tools on hand to check pressure but could work on that if it is likely.) Any other thoughts? -thanks







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    very rough warm-up 900 1994

    posted by  mmal231294 on  Wed Jun 24 02:21 CST 2009
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    last visit: Fri Mar 18 12:34 CST 2011
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    The FPR is very easy to test, use the "see how quick it fills a bell jar" test described in the FAQ's. That is a fairly common failure. The temp sensor(ECT) could be failing but it will make the vehicle run rich and if that were the cause of what you are describing you would certainly be smelling the rich exhaust and perhaps seeing black smoke. It almost sounds more like your catalytic convertor has broken apart internally and piece is intermitently clogging/restricting flow. You test this, there is a small round plug upstream of the cat which you can pop off when the car is running like crap and see if smooths out. Also when was the last time you cleaned your Throttle body?








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      very rough warm-up 900 1994

      posted by  janglim on  Wed Jun 24 17:31 CST 2009
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      last visit: Tue Sep 9 11:55 CST 2014
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      Thanks,

      I found the fpr flow test on the faq. I'll give that a try.

      I haven't noticed signs of running rich, and yesterday the rough running came and went intermittently on a 45 minute highway drive - so maybe the problem is not tied to the warm-up period like I thought it was.

      The cat is a two year old dynaflo, but it should be easy enough to open the port and rule that out.

      I cleaned the throttle body and IAC motor 2 or 3 months ago when I had similar symptoms. I'll clean the TB again as soon as I get time though - it's nice to rule things out.

      joe








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        very rough warm-up 900 1994

        posted by  janglim on  Sun Jun 28 12:58 CST 2009
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        last visit: Tue Sep 9 11:55 CST 2014
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        I just did fpr test looking for flow through the fuel return line. I get no return at all, not even damp in the line.

        I also did two faq tests for fuel pump operation: jumpering fuse 1 to near and far contacts of fuse 11 (pre-pump operates, but main does not), and also jumpering teminal 30 to 87/2 on the fuel injection relay socket (pre-pump runs, main does not).

        I confirmed 12v across the main pump harness at the pump during the second test above, and also started the car with the main pump harness disconnected. The car ran with the same symptoms I've been getting.

        As soon as my work location is in shade I'll remove the main pump and try to bench test, but it is my most likely culprit so far.

        Joe








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          very rough warm-up 900 1994

          posted by  mmal231294 on  Thu Jul 2 07:02 CST 2009
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          last visit: Fri Mar 18 12:34 CST 2011
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          By George I think you got it! Nice Job.








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            very rough warm-up - solved 900 1994

            posted by  janglim on  Sun Jul 5 06:53 CST 2009
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            last visit: Tue Sep 9 11:55 CST 2014
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            No so fast. The problem seems to be solved, but not through my diagnosis of a bad pump. Instead I got a reminder to thoroughly rule out easier fixes.

            When I was checking the pump, I reseated the power and ground connectors to the pump several times, but still got no action from the pump. Since i could measure 12v accross the connectors I figured the pump was bad.

            Once it was out I started testing the pump. It seemed intermittant so I was ready to order a pump and go to bed but decided to try a bit more. With better jumpers to the battery and decent alligator clips on the pump terminals, the pump ran fine - and with lots of starting and stopping I couldn't find any dead spots on the winding like I've had with electric fans.

            I stuck it back in (twice because I forgot the check valve the first time) and it run fine since. That was a week ago.

            I can't quite figure how a bad electrical connection would have a fairly predictable cyclic action (bad during cold start, going away at some point during warm up), particularly in an area that doesn't get much heat. But for now it seems fixed.

            thanks

            joe







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