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unpredictable intermittent engine hesitation

posted by  samsdad on  Wed Jun 29 20:04 CST 2011
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last visit: Fri Jul 1 14:03 CST 2011
[RELATED]

1994 Volvo 940 Turbo. Automatic transmission. 132,000 miles. Now and then, upon start up, engine can’t rev up much beyond 2,000 RPM. Car can back out of garage, but in drive, can’t go forward with usual speed. Step on gas and at about 2,000 RPM, cars slows and struggles – but doesn’t stall out. Ease off on the gas and it seems to engage transmission better at a lower RPM, but still no usual speed. Ease down and up on gas and car picks up some speed but again, engine can’t go much over 2,000 RPM. Go a few hundred yards like this, get annoyed and turn off the engine and let it rest. Start up again and sometimes it runs with no problems. Other times, have to stop/rest/start a few times. It usually recovers.

The problem has always been upon start up, not when I’m driving at regular speeds and engine/transmission are engaged and working fine.

However, a few days ago, on a thruway, the problem surfaced while driving at 60 mph. Car lost ability to accelerate – it slowed down and the engine couldn’t run beyond 2,000 RMP. Pulled onto the shoulder. Tried the stop rest start routine but no luck. It never recovered. The only thing it could do was limp along on the shoulder to the first exit I could find.

Got it towed to my mechanic a day later, but upon start up, runs fine.

Problem started in cold weather. Getting more frequent as weather warms up. Much of the time, it starts and runs fine. But other times and with no warning or indication, the problem – call it hesitation, sluggishness, limping out – appears. Engine light never goes on. How does a mechanic diagnose this problem if he can’t experience it first hand? Help!!!!!








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    unpredictable intermittent engine hesitation

    posted by  Will740turbo on  Thu Jun 30 02:01 CST 2011
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    last visit: Wed Oct 22 16:09 CST 2014
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    Hiyo.

    I wish I could say "it's obviously a blah blah blah" but I can't.

    My suggestion is to read through the FAQ and see if anything pops out at you. Maybe you could make a list first of all the things that you notice.

    When you push the gas down, does it hesitate, then increase engine speed? Or does it start accelerating immediately, then slowly bog down as it approaches 2000?

    The best thing you can do is to document the heck out of everything the next time the issue occurs. Noises and such first, then things like the weather, if the car's warmed up, how long since it had been driven before that. Let it run for a while. It's not going to blow up from running rich or lean, and if it finally stalls out and won't restart, then you have a "non-intermittent" problem ;)
    Listen under the hood and note any noises you hear. It's a 940, so you should be able to check the fuel pressure at the rail through the schrader valve. Don't use a tire pressure gauge, there must be a specific gauge.

    You can also probe the O2 sensor with the car limping along and see if the voltage is stuck on one side or other of its range.

    You can also -I know this is safe for the LH2.4 models, I ASSUME it is safe for the 3.1 version or whatever they used in a 94 940- unplug the AMM and see if the car improves. If it runs better (through the whole power band) then that means the AMM is probably bad.

    You should also check for vacuum leaks, though I can't think of anything that would stick in cold weather.

    The engine coolant sensor comes to mind - there are 2 of them. You can check to see if it is in the right Ohm range and also to see if the wiring is intact. I believe it's in the runner near the car's firewall.

    Then, compare all these values (write them down) to the car when it's acting normally. That should give you a good starting place to track down the issue. If it frequently happens on moist/rainy days, it could be a cracked distributor cap or some kind of wiring short.

    Good Luck!

    Then compare all these







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