Missfire caused by comprssion loss ?? 1998 s70

peterz · Post by **peterz** » 12 Mar 2007, 13:30

Got error code P0300, multiple missfire. Mechanic says 2 cylinders show low compression and he needs to inspect cylinders (head off). Does this make sense to get an electrical error code for a mechanical problem? Would a burnt exhaust valve or failed head gasket cause two cylinders to show low pressure?

Thanks,

Pete

Post by **pfeener** » 12 Mar 2007, 14:50

I'm skeptical of what your mechanic is telling you. I would not do the head removal until your sure that's what's causing your issue, because it'll run you $2,000 to find out if he's right. The PO300 code doesn't make a lot of sense. If it was a specific cylinder misfiring you should get a code for just that cylinder or cylinders (PO301 for #1 cylinder etc.) and I don't believe a low compression condition will give you a 300 code, but someone else may have better information. PO30X codes are typically cured by changing one or all of the following: plugs, wires, cap, rotot)

Questions:
How is the car running? If you have 2 cylinders not firing it should run like crap.
What are the actual compression test results by cylinder. It should be around 180 - 190 on a good cylinder. If it's under 125 or so then you'll need to look into it. If it's zero in one or more cylinders, you typiclly would have a bent or burned valve.

peterz · Post by **peterz** » 12 Mar 2007, 15:08

pfeener wrote:Questions:
How is the car running? If you have 2 cylinders not firing it should run like crap.
What are the actual compression test results by cylinder. It should be around 180 - 190 on a good cylinder. If it's under 125 or so then you'll need to look into it. If it's zero in one or more cylinders, you typiclly would have a bent or burned valve.

Car does run very rough during idle. Smooths out at speed. Just checked again and my Haynes manual does suggest that low compression can be a cause for missing "through driving range", but I can't sense missing after speeding up. I don't usually drive car so don't know what it's like normally. Besides p0300, also got two individual codes, p0301 and 302 I think. Mechanic said compression is low on adjacent cylinders. Said likely head gasket blown. But what's funny about their $1,100 quote is they claim that head needs machining before installing new gasket. I thought machining was only done if not within tolerance.

Post by **pfeener** » 12 Mar 2007, 18:44

If you're going to the trouble of removing the head, I would have the head redone which could include milling the head to ensure it's flat. With your additional info, it's sounding more like a head gasket.