1997 960 with 124K mi. Car is definitely mis-firing and I can feel it at idle; however, it is not readily noticeable at higher rpms/highway speeds.

The car is consistently throwing two codes, usually ~20-30 mi. after I reset them.

If I check it immediately after it codes, it will be a P0172 (rich fuel mixture). If I let it code and keep driving for awhile, I get the second code which is P0303 (misfire cyl. # 3).

Here's what I've done so far in trying to track down the cause. Each step has resulted in no change in the car missing and I continue to get the same two codes:

Compression test on # 3 = 220 lbs
Checked spark at plug gap on #3 and #4, neither looked bad
Swapped the #3 and #4 coil packs
Installed all new plugs
Removed the #3 injector, moved the #6 injector to #3 position, installed a spare parts injector into #6 position.

I was going to change out the power stage (I have another post asking which one controls which set of cylinders), but since I keep getting the P0172 code first it seems likely to be a fuel issue.

The latest test was to hook up to an OBDII that streams data and one of the O2 sensors is responding in real time while the other, sensor #2, is flat lined (maybe saturated with fuel?) Anyway, I can't see how a bad O2 sensor would a) not throw a code itself and b) affect on cyl. #3. I tend top think its behavior is due to the excessive fuel mixture.

Any suggestions as to what to try next would be appreciated!

 

I have a 1997 965 and I am just throwing a couple of things out here, mine has not had this problem. Engine Coolant sensor, also the O2 sensor in my mind is a candidate. If I had more experience on this kind of problem I could be more help.

Good Luck

 

As for the missing, check out this post: https://volvoforums.com/forum/showpo...2&postcount=15


The other is likely a vacuum leak somewhere.

 

Test your intake gasket for leaks using starting fluid or propane, get it in and around that cylinder. I bet you have a bad gasket.

And something I have never been able to figure out is an intake gasket leak would make one think the mixture would be lean, but my experience has been the computer program over compensates and runs a rich mixture. I wonder if the engine is harder to start when hot, or the ambient air temp is warm. That would be another symptom.

DanR '94 964 356,000 miles (122,000 on the new engine)

 

Thanks guys for the input. The wiring harness didn't look bad, but I didn't look carefully at the connector ends for #3. I'll check both that and also for a vacuum leak when I get home tonight. The car always starts right up, whether hot or cold.

If the problem were due a bad 02 sensor (I've contemplated this since one of them is flat lining in the real time data stream), why would I consistently get a code only on #3 and why no O2 sensor code? The only test would be O2 sensor replacement and I just can't justify that until I've exhausted other diagnostic options...

 

Update:

Checked the wires where they insert into the backside of the coil pack connector- looks fine.

Checked for intake air leak w/ some carb cleaner- no noticeable increase in rpms... (my Volvo mechanic friend said when this happens it is almost always cyl. # 1.)

Pulled all coil packs again and blew them out + chambers with compressed air to make sure there was not some moisture that was causing the problem- no difference.

So, looks like I'm taking it to my buddy's shop tomorrow morning to see if he can diagnose the problem. How frustrating...

 

Please keep us updated.