Just purchased a 96 Volvo 850 R.. when it boosts up it hits a certain rpm and breaks up/ back fires falls on it's face throws a flashing random misfire code. It has a tune on it but I feel something else is at work considering the car has been running the tune for quite some time. Any help from an experienced person would be greatly appreciated (no "Parrots" please) I've owned many Volvos but not the 850 R sedan with a tune.

 

I would start with the basics. Usually misfires are caused by ignition problems. However with a turbo car you could have a hole in an intercooler hose - boost get to the point where it pushes the hole open, and engine will go rich and lose power.

 

So, I had this same problem on my modded 1996 Volvo 850R wagon, it was a problem with the spark plug gaping. The gap was too big so the turbo would just blow the spark out around 5k-5500rpm. It caused the car to jerk forward and back and not upshift. Hope this helps!

edit: fixed a word

 

These cars have a shockingly, to me, small spark plug gap. They operate with comparatively low voltage ignition, and I treat them accordingly. It's definitely low voltage in practice, whether it's incapable or not. The shop manual has some good diagnostics for much of the system, but I don't know what that extra electronic stuff is at the coil. I don't know a means to test it. but I will say I haven't seen a bad one.

Good comment above about the rubber and boost. I have sure seen that. So it'll be useful to determine if it breaks up at high rpm in neutral. You won't have any boost pressure in neutral.

With a simple timing light and old fashioned dwell meter operating as a tach, I think you can determine very firmly whether the tune is actually the problem. If the PCM is telling the coil to fire, at the appropriate time, then you don't touch that. You may have to learn some interpretation (I mean, it ain't gonna have a setting for 5 cylinders) but anybody with any sense would be okay there.

 

Let's not over complicate things. What is being described is most likely related to worn out plugs or a leak in the turbo boost system. This model used plug wires, not coil packs.

 

yep. My suggestion would be to do a "stage 0" - cap/rotor/wires/plugs (gapped to .028) to baseline the car. Don't bother with a fancy platinum/iridium plug - these cars run best on old school copper core plugs (OEM, Bosch, Champions etc in the correct heat range). You can check the wires for date codes if you are curious as to how new/old they are. It is possible to remove the distributor cap without removing the air box (if you don't value the skin on your knuckles) but getting that out of the way makes it really easy. Plus with the airbox out, you can inspect the tubing to the turbo/boost controller etc.

Finding an airleak may require a smoke test - but not a bad thing for a new to you car, particularly if you find the PCV is creating some positive crankcase pressure since that can blow off older vacuum lines or a cap on the vacuum tree.

 

I think it goes beyond "don't bother". If you use a non-copper plug it's going to be a huge huge mistake. Don't do it.

These cars have a very low-voltage ignition system from what I can see.