850 Bad Misfire

TurboBrick850 · Post by **TurboBrick850** » 17 Aug 2022, 16:14

I have a 1997 Volvo 850 GLT that has a 2.3L and a manual transmission swapped into it. I've done some upgrades too it and am running a tune on it. I also decided to go coil on plug with a custom wire harness and coils for a v70 I drove it for a few days with little issues until one day I was downshifting into second gear, and I heard a loud pop/bang. The car started to backfire very loud, and my brake pedal went hard. I put it into first to get it out of the road and it struggled and misfired when I pushed down the gas pedal but when I released it the car would start to accelerate by itself. I looked over the engine and didn't see and physical damage and there were no knocking noises, so I think the internals were unharmed. When trying to start it again, the car would idle for about 5-10 seconds and misfire very loud then die. The misfiring noise sounded like it was coming from the intake manifold but I'm not 100% sure. We pulled the plugs to find that 3 of them were wet. We replaced the plugs, but nothing changed. The coils are not burnt and seem fine. I did pull my O2 sensor to find that it seemed damage. I have a wideband to replace it with, but I was running it on the stock O2 just to get it to run. If more information is needed or you have any questions, just let me know.

Post by **scot850** » 17 Aug 2022, 19:00

When it comes to the level of customization you have done it makes it much harder for us to help. I would suggest first checking your fuel pressure.

As I don't know the extent of your tune I don't know what your psi is on your turbo. Are you using a TCV for turbo control from an 850 or V/S/C70?

Next I would be looking at all your vacuum lines as well as your intercooler pipes for them being loose or split. You may have to remove them to be able to do a full and proper inspection.

Neil.

fitz548 · Post by **fitz548** » 21 Aug 2022, 08:43

Hello,

I had this same issue also running a tuned ecu on my 1998 v70xc with 2.4L Turbo(13G) 11psi of boost i don't have the coil on plug like what you have though. the thing that was causing the misfires on my car was the crankshaft position sensor once i replaced that everything cleared up

Oly850 · Post by **Oly850** » 01 Sep 2022, 16:43

On other suggestion is that you have an enormous vacuum leak. The brake booster failing tells the tale.

Look at your vacuum hose routing diagram and fine the leaking culprit. You can also use brake cleaner spray to see where a leak might be. Give a spritz with the engine running and if the engine rpm increases, you found the leak.

Thomas

Post by **manovlov** » 02 Sep 2022, 03:52

@Oly850,
As a newbie in mechanic, i don't understand the ink between the brake booster (that could explain the brake issue) and the engine misfire.

@turbobrick850 : you've written " I also decided to go coil on plug with a custom wire harness and coils for a v70". Can you tell us more ? Advice given seem to me good track to follow. But i would control also coils... Couldn't it be an issue between ECU and Coils ?

manov

Post by **Chuck W** » 02 Sep 2022, 06:44

A lot of difficult to read text there...

I've done the COP conversion on both of our cars. What method/tune did you use? VAST? You don't *need* a WBO2 for the car to run fine. I've got over 10K on the conversion on my car. Only the last couple thousand has been on a WBO2.

What were you doing when the problem occurred? You say "downshifting into 2nd", were you driving it hard?

3 wet plugs means no spark on those, most likely. It's going to backfire and run like crap on 2 cylinders. It should be throwing all sorts of ECU DTCs.

Recheck your COP wiring for faults/breaks. I had issues with my install on my wife's '97 with a couple of bad connections.

Did you modify the ECU for the conversion? You could have bad solder joints on those tiny resistors.

It's possible the backfire has blown off a vacuum hose, or even popped an intercooler coupler. An extreme loss of vacuum could case the hard brake pedal.