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AWD - Still have stumble/rough issues

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Still have stumble/rough issues 850 1995

Hi All,

I'm still having issues with my 850. It wasn't fixed, or it came back. I'm looking for any further suggestions.

First, here is the original thread. (I tried to reply, but it was read-only.)
href="http://www.brickboard.com/AWD/volvo/1427501/850/intermittent_stumbleroughstall.html">Intermittent stumble/rough/stall

To summarize, I have a '95 850 Turbo sedan, and I'll be driving along just fine, and it will start running rough and lose power, run that way for 1 to 5 seconds, then continue on just fine.

If I'm driving down the freeway, constant speed, flat road, the car will start to do this, and I feel it surging, slowing down. The tachometer is rock steady, so I'm pretty sure it's not on the primary ignition side.

It was giving me 1-2-1 error codes, which is the MAF sensor. I changed that, and it seems to have not fixed it. In retrospect, that may have been a red herring. Most of the time when it stumbles, it does not set any codes. It's only if I'm at low speed and the engine stumbles to the point that it does (or almost) stops running completely after 10+ seconds of hiccuping around that it sets the code. If it's shorter, or at a higher engine speed, it does not set a code.

Seems to make no difference if tank is full or empty.

Makes no difference if it's hot or cold (or wet or dry) outside.

Yesterday, when it was stumbling at idle, I opened the throttle wide, (pedal to the floor,) and it made no difference. It kept stumbling, until it stopped stumbling and behaved; I parked. When I subsequently started it again (10+ hours later) it stumbled a little bit, then after less than a mile, it stopped stumbling and was fine.

I changed the spark plugs. They had 30,000 miles on them - Bosch Platinum. All were identical, with a very light, light grey color. Put in a set of Champion copper. No change, same occasional behavior.

The behavior is very binary - either it's running fine or it's stumbling, like flipping a switch.

I'm suspecting an intermittent electrical issue, but can't figure out where.

If it was the fuel pump, it would just cut out and stop. If it was a partial blockage, it would tend to be worse at speed. (or would it?) Yes, it would. Even though the pump is at constant volume, the pressure regulator would end up closing a little more to maintain pressure, and it should idle fine, just have issues at WOT. I also don't think it would be intermittent like this - it might be intermittent run or not-run, but not intermittent stumbling. Wish I could monitor fuel pressure while driving.

Thus, I'm trying to think of what sensor might be going open circuit to cause this behavior, but not set a trouble code.

It also doesn't seem to be related to movement. Going over bumps or a bumpy road does not trigger or fix this. Nor cornering vs. straight. It will, however, start or stop while driving straight and flat down the freeway.

It's not a cam or a timing belt issue - that wouldn't be intermittent.

Vacuum leak wouldn't be intermittent - it would tend to make it run badly at idle, and be fine at speed.


Any suggestions?

Thanks in advance!

Roger

'95 854T 235k miles
'87 245 M47 278k miles






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New Still have stumble/rough issues [850][1995]
posted by  roger  on Sat Sep 18 18:00 CST 2010 >

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