missfire codes
 

I have 3 cylinders that supposedly are misfiring. Full tune up was done, volvo parts used. CEL is still on and codes say 3 misfires. Car runs fine. whats up with it? Any help?

Did the codes get erased after the tune-up? If not, erase them. If yes, give more information: year of the car (this matters- 98 uses a totally different engine management system from 99 and 00), which cylinders are misfiring, any driveability problems?

First off, thanks for your response time. I have a 98' s70. She only has 52k miles and was driven to store and back by mother in-law. The codes were reset after a full tune up with volvo parts. After 5 min drive CEL came on with same codes: p300 misfire cylinders #2, 3, and 5 (p302, p303, p305) The car runs fine. Additionally, ignition coil was replaced, exhaust restriction checked, crank sensor replaced, injection serviced, and even swapped injectors around and still had same code read-out.

If three cylinders are misfiring the car would not run at all would it?

Any misfire would cause the car to run rough. Were the wires replaced as well as the distributer and cap? If those were done, check fuel pressure. Replace the fuel filter and check the pressure.

Bad vacuum leak could cause this, or anything else causing it to run lean. Grab a spray bottle of water, and spray down the intake manifold while the car is running. Pay particular attention to the mounting flange, the injector ports, and the vacuum nipples. Check all vacuum hoses and lines. Get a can of Mass air flow sensor cleaner- yes, it's a special cleaner, and don't use anything else, and clean the MAF with it. Since the car isn't misfiring constantly on those cylinders (it wouldn't "run fine" if 3 cylinders were dead!) I suspect a slightly lean operating condition. If none of the above yield any fruit, next stop is a compression check.

Also: Ignition coil and crank position sensor were likely totally unnecessary. Those rarely fail on these cars, can both be tested, and would keep the car from starting if they were bad. If a shop did that, I consider it irresponsible and would find someone else- throwing parts at a problem is never a good sign for a professional. If you did it yourself, hey- we all learn somewhere, and I recommend getting a good repair manual that has comprehensive troubleshooting information including component tests. Alldata is great, and they have a service for DIYers (alldatadiy.com) that lets you get individual- car subscriptions on a yearly basis for a reasonable cost, and they include all factory service info, all technical service bulletins, and a lot more.

Cap and wires were replaced during tune up, and fuel pressure was checked to be good.

Sounds like it's time to do a compression test.

Yes- check compression. It could be low on those cylinders. If it is, the next step is a leak- down test to find out where the compression is going. If compression is fine, check for vacuum leaks as I pointed out above. Good luck!

Was the car running fine before the tune up.

The car runs fine. It just doesn't like to start.

yes the car was running fine before the tune up, and after it as well

Here's what I'm thinking; you picked up this little cream-puff with very low miles on the clock for its age. I think, because this car mostly sat idle, the valves are gummed up and occasionally sticking, causing the very transient misfires.

I would add some top-end lube (e.g. marvell mystery oil) to the oil and fuel for a period of time while driving the piss out of it every chance i got. This will do two things:

1. Put a smile on your face.. :D
2. and suspect your very rare misfires will dissipate too..:)

Good luck!

You can also try some Lucas Fuel system/injector cleaner in the tank. When you add the bottle, add it 1/2 way during your fill-up to mix it in real good, then drive the car until it's almost empty.