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Old 11-08-2020, 12:41 PM   #1
masaidwakeupson
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Default Rough idle and misfire

Picked up a '91 940 Turbo a few months back.
When I got the car it was missing at idle. Wasn't a huge concern at the time.
When over 1000 RPM the motor ran just fine.
So, the car sat for a few months until I found the time to start giving it some attention.

I've replaced spark plugs and wires, distributor cap and rotor, fuel injectors, fuel filter, fuel pressure regulator, MAF sensor (+some other non-issue suspension components).
The car is still idling very rough, missing, and running very rich (very strong fuel smell).
I'm going to check and clean the PCV box and hoses and the IAC today and see if that helps.

I've checked for vacuum leaks as well. Found a pretty massive leave at the intake manifold gasket and replaced it. I've also replaced some dry rotted vacuum hoses and checked for additional leaks using carb cleaner. Couldn't find any more obvious leaks.

Is there anything that I'm missing that is notoriously known for causing similar idle issues?
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Old 11-08-2020, 04:49 PM   #2
Mr. V
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I don't see that you cleaned the throttle body and throttle position switch and also properly adjusted the switch position.
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Old 11-08-2020, 11:16 PM   #3
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^^^ What he said.
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Old 11-09-2020, 12:06 AM   #4
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TPS won't cause misfires.

Air leaks are the most likely cause of a poor idle on and old 940T. I use a an expensive smoke machine to find induction leaks. You can spray around with brake cleaner and see if the idle changes.

Common areas of air leaks are the idle air hoses, intercooler hoses, and the turbo inlet hose.
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Old 11-09-2020, 07:44 PM   #5
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I pulled the throttle body and IAC, cleaned them both and the rough idle issue remains unfortunately and the hunt continues.
I've got a new PCV box on the way but in the meantime I wanted to change the o2 sensor.
Any advice on how to remove that sucker?
I've tried from above and below and can't seem to get the leverage to crack it loose. I'd rather not drop the down pipe if I don't need to.
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Old 12-06-2020, 02:51 PM   #6
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Well, I finally found the time to swap the o2 sensor and it had no effect on the hunting idle and misfires.
It actually made the stuttering on acceleration much worse. It's pretty rough and somewhat terrifying accelerating through intersections from a stop. The car just slowly creeps and hesitates to move forward.

I can't find any vacuum leaks but it sounds like that may be issue I'm chasing. I've tried carb cleaner on visible hoses and junctions. even tested the brake booster for leaks.\
I might need to see getting a smoke machine and testing that way.

Anything else I could try in the meantime ?
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Old 12-09-2020, 03:19 PM   #7
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bad main fuel pump caused hesitation on my 940..
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Old 12-10-2020, 02:36 PM   #8
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Get to know the condition of the engine with a compression test. To see what cylinder is dropping out or if it's a random miss disconnect a plug wire one at a time. Use a well insulated glove. The cylinder that doesn't make any difference is the problem one. If they all make a difference then it could be a random missfire from a vacuum leak.
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Old 12-10-2020, 05:51 PM   #9
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Disconnect a FUEL INJECTOR one at a time for a safer cylinder drop test for you and your kitty.

A slighty clogged injector is a possibility. Or just the mixture going too rich/lean.

Maybe check the valve clearances and do a leak down test.
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Old 12-10-2020, 06:00 PM   #10
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My buddy is texting me right now that he's got the same car, '91 940 B230FT, one of the injector connectors was backed out, causing same symptoms. Clicked it back into the connector, fixed.
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Old 12-17-2020, 12:08 PM   #11
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Well, I'm pretty sure I found the issue.
Disconnected fuel injectors as suggested and found that cylinder 4 isn't firing at all.
Pulled #4 fuel injector plug and there was absolutely no change to the idle of the car. Zero.
Cleaned up the connectors with some electrical cleaner and reinstalled, again no change.

I have a hunch that this may account for the rich fuel smell when the car runs so I think this cylinder is getting fuel and only missing spark.
I'll have some time with car again in a few days and I'll test for spark on #4 and go from there.

I'm not an auto mechanic by trade or by hobby but I've taken a large interest in these older Volvos and I'm learning a lot about them thanks to you guys.
So thanks again everyone.
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Old 12-17-2020, 12:52 PM   #12
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I'd do a quick compression test at this point.
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