Hi All,

I'm still fighting an intermittent rough running issue. It's been going on for almost a year, and I still haven't found the problem. 1995 850 Turbo Sedan, 245k miles.

Here is where I have addressed it before here on the Brickboard:

http://www.brickboard.com/AWD/volvo/1427501/850/intermittent_stumbleroughstall.html

http://www.brickboard.com/AWD/volvo/1443084/850/still_stumblerough_issues.html

To summarize: Sometimes it drives just fine. Might drive fine for days, maybe even weeks. Then sometimes it just misses a little, driving down the road. Sometimes it cuts out completely for 1-2 seconds, then picks up and keeps going. Sometimes I will be driving along slowly, and it will just die. It may start right up before I coast to a stop, or it may take several attempts to start up again.

Usually this sets no trouble codes. If it is running really extremely badly, it might set a 1-2-1 code (MAF sensor) - it has done this only twice in a long string of issues. I replaced the MAF sensor, with no change.

Since the cam position sensor can cause issues without setting trouble codes, I tested it. Tested fine, (backprobing the connnector,) but I broke the connector, so I replaced it anyway. Made no difference.

At one point a vacuum line broke and made it run really poorly. It was one of the lines going back to the turbocharger. Fixed that, and back to the status-quo. I then went on a very aggressive vacuum leak hunt, but didn't find anything significant. I replaced all hoses that looked even remotely old, and any vacuum connectors in other-than-new condition. I've tried checking with a butane lighter, and tried listening with a hose, all to no avail.

Fuel pressure is on spec, and it modulates properly in response to vacuum, so fuel pressure regulator appears ok. With pressure on spec, this also indicates fuel pump etc. is also fine. I was able to watch the fuel pressure during a period of poor running, and fuel pressure was normal. To double check, I pulled and inspected the fuel pump and the inside of the tank - all looked clean, proper, and normal, no contamination, no split hoses, no leaks, no foreign material, etc.

It also makes no difference how much fuel there is in the tank - it has happened anywhere from 100% full to empty.

It also makes no difference hot or cold. It's had problems in both. It's not warmup related. No correlation to outside air temp either, happens both in hot weather and cold.

Last night, I was testing, and I realized the "mild rough running" sounds exactly the same as when I pull the lead from one spark plug. The idle air control valve then opens a little to maintain engine speed, making it difficult to detect. Pulling spark plug wires did set a trouble code, so if it is misfiring on a cylinder, it's not sufficient to drive emissions out of spec.

I have inspected and change spark plugs multiple times. They are consistently all identical, a nice tan color, although a little darker than I would like to see. (I think I'm burning a little oil, leaking through the turbo shaft seal.) There is definitely not sufficient buildup on them to cause a misfire.

I tried putting the timing light on the common spark plug lead from the coil to the distributor. I suspect it may not be firing all the time, but I have not been able to catch it when the car is running rough to confirm.

Sometimes when it runs poorly, I can do a Wide Open Throttle acceleration, and it *seems* to clear it, at least for a little while. This may be coincidence.

Thinking back, this may have started after I pulled the intake manifold to replace the knock sensors. I don't know if improper knock sensor installation would do this or not; they were installed and torqued per directions. I am also pondering if I generated a vacuum leak when I had it off, but have not found it yet.

I'm getting ready to pull the intake manifold again, and reassemble with significant quanties of gasket glue, and replacing every rubber vacuum elbow and connector, even if they only have 10k on them, just to do them all.


This car is frustrating me. Even though it has not stranded me, it feels unreliable, and I am hesitant to take it for a long trip. (Long = more than AAA will tow me home for free.) The hardest part is that it's intermittent. It runs fine some days, and poorly others.


Do you have any other suggestions? Anything else I might look at and test? Any other test procedures I might try? Thanks in advance for any suggestions!

Regards,

Roger

'95 850 Turbo Sedan, 245k miles, "problem child"
'87 245 M47, 288k miles, "old reliable"