Interesting issue with our 95 850 Turbo with 260K. If you accelerate normally or put the turbo under a light/moderate load, the car accelerates and runs smoothly. If you try to accelerate briskly or put it under a heavier load (i.e. quickly pulling out into traffic, going up a hill or passing someone) it will buck, cough, and jerk like it is backfiring or mising on two or more cylinders. Back off a bit on it and it is fine. Press the issue and it does it again until you back off. Only does it under heavier load or throttle. It will run up to 80+ smoothly if you just accelerate moderately, but really acts up if you try to move with some urgency. Weirdest part is NO CODES whatsoever any time it does it.

Car just had a fresh tune (plus, wires, filters, timing belt and all accessories) about 8000 miles ago.

Any ideas?

Glenn

 

"Car just had a fresh tune (plus, wires, filters, timing belt and all accessories) about 8000 miles ago." Usual suspects covered... fuel pressure check? vacuum?
maf clean, connections on the cam and crank sensors in good shape?
Outside of that possible downstream exhaust, clogged cat?

 

Two major things are affected by "going uphill" so to speak providing the car does not downshift. When you do that, cylinder pressure is high, and the intake pressure is also high.

So in that case the fuel pressure needs to adjust way up higher, and it may be you aren't getting enough fuel pressure at that moment. You can test this by fitting a fuel gauge and then taking manual control of the fuel pressure regulator.

The more common problem when cylinder pressure is high is that ignition failures occur. The mixture becomes more difficult, because the air and fuel in the combustion chamber is an insulator. At high cylinder pressure dicey ignition systems will fail to work, but then return to working normally when cylinder pressure comes back down. These are cases usually where the spark goes "somewhere else" and not across the gap of the spark plug.

The recent tuneup is a curve ball if this is the problem. How could this fail in 8000 miles? The plugs could be gapped too wide. they might be platinum plugs which are extremely difficult to fire. The wires you bought may be not very good and already breaking down. The rotor and cap may not have been replaced. The coil may be going bad.


For some reason Volvos have a really low voltage ignition system and really small spark plug gaps. Not sure why. I've never seen such a low energy ignition system on anything.

 

Well, to tackle this I am going to do the following......change the plugs. If that does not help, we have a 98 s70 I'll try the coil, cap, MAF and wires from. If that does not help, I'll move on to fuel pressure. If that does not help, I'll scream bloody murder here for advice before I shoot the car!! Sonce it is not my daily driver right now I have a little time to play with it.

Glenn

 

Plugs first. Thats the most common cause.

Do you have a boost gauge? Wondering if your hitting fuel cut off?

 

Yeah, I have a boost gauge. It shows boost when it happens, but I am not pushing it super hard.....maybe half to 2/3rds throttle. What would cause fuel cut-off at other than full throtte/boost?

 

14.5psi

 

New plugs seem to have fixed it. However the plug hole for #4 plug was full of oil (to the top of the plug when the wire was pulled off).

 

Very cool!! as for the oil check to make sure your oil cap is good, if not replace the seal.

 

Yep....Did that yesterday.

 

Overboosting can cause those issues as Boxpin suggested. You may want to check your wastegate actuator just to be safe.

 

Will do....Thanks Robert!