Does this sound like a burnt valve?

Jazzer · Post by **Jazzer** » 20 Sep 2016, 19:31

Hi guys,

96 850 turbo platinum edition; I had to gun it to merge into traffic last night, and immediately afterward I got a case of putt-putt and cel. Code came up as misfire on cylinder 4. Rpm's did not go above 4500, so I was a bit surprised. The plugs and wires are old, like 1999 old (I just ordered new plugs and wires 2 days ago of course). Here's the kicker: it shakes like a mother at a stoplight, but runs smooth as ever above 1000rpm. No misfire. Any suggestions? I'll do a compression test in the next day or so.

Bob

Post by **MadeInJapan** » 20 Sep 2016, 20:35

I don't think you would get a smooth ride at any speed if it were a burnt valve. Instead, maybe you tore through a motor mount- possibly the upper torque mount above the engine or the lower tranny to engine mount. I still am not able to explain the sudden misfire. I hope you ordered the correct bougicord wires.

Jazzer · Post by **Jazzer** » 20 Sep 2016, 21:07

I did order Bougicord wires and Volvo plugs, as well as a Bosch distributor. I'm still getting the misfire, but it only kicks in after coming to a stop light. I'll clear the code on the highway and will be smooth sailing, but starts to shudder as I slow down. The upper torque mount is brand new oem, but the lower mounts must be shot since it sounds like a squeaky mattress!

Post by **rspi** » 20 Sep 2016, 21:40

Huh? No misfire while driving? Then it's not a burnt valve. Valves don't unburn because you are driving. I will say this, when these cars are misfiring on only 1 cylinder, they do drive pretty smooth when above 1500 RPM's. If you do have a burnt valve and decide to continue to drive the car, disconnect the fuel injector to the bad cylinder.

Jazzer · Post by **Jazzer** » 21 Sep 2016, 06:01

I guess the compression test will tell the tale; once I've cleared the code while driving it will stay clear until I exit the turnpike, then the cel comes on as I slow to a stop. I'm hoping it's not a burnt valve, obviously...

Post by **erikv11** » 21 Sep 2016, 07:47

That does not sound like a burnt valve.

Check all vacuum lines, and especially the intercooler hoses and connections, for leaks.

Post by **Clemens** » 21 Sep 2016, 09:34

Maybe one of the intercoolerhoses got knocked loose a bit. The non metered air could cause this behaviour. Or something with the idle conrol (therefore good on the freeway, bad at idle). Good luck

Jazzer · Post by **Jazzer** » 21 Sep 2016, 14:42

So, would a vacuum problem also cause the misfire? All the injectors seem to be working ok at idle (listened with screwdriver trick)...

Post by **erikv11** » 21 Sep 2016, 15:25

Rough running especially at idle is classic for a split or torn hose. The misfire would be unusual for a vacuum leak, you are correct about that, unless it is a big one (e.g. a split intercooler hose). The car is 20 years old, unless all the vac lines are suspect unless they have all been refreshed, I would definitely check them out carefully. TBH I like to just replace them all whenever I get a P80 car.

Jazzer · Post by **Jazzer** » 21 Sep 2016, 19:13

I've got the plugs, wires, cap and rotor, and smaller vacuum hoses ready to install; just waiting for a day off to do it all! Though right now the turbo is running quite nicely, with plenty of boost (stock boost, but it's there). Would this be the case with a torn or broken intercooler hose?