Now that ignition systems are distributorless, the next step is wireless! *NM*

Hi all,
3.0 Audi V6. 128000 miles. CEL. Changed coil, swapped plug and injector. No change. Still #3. Has slight lifter noise on start up and runs a little rough on start up. Takes a few days to a week to set code P0300 & P0303. Daily check of readiness and pending codes show nothing. O2 sensors and MAF check OK. It's also using some coolant but no oily coolant or foamy oil.
Any ideas?

Ed B

Quote
Ed B
Hi all,
3.0 Audi V6. 128000 miles. CEL. Changed coil, swapped plug and injector. No change. Still #3. Has slight lifter noise on start up and runs a little rough on start up. Takes a few days to a week to set code P0300 & P0303. Daily check of readiness and pending codes show nothing. O2 sensors and MAF check OK. It's also using some coolant but no oily coolant or foamy oil.
Any ideas?

Ed B

the misfiring cylinder. What did the plug look like when removed from the misfiring cylinder? If 'clean' this is a good sign of coolant in the chamber. (The coolant vaporizes in the heat and 'steam cleans' the plug and in fact the entire combustion chamber of deposits.)

You can have the oil analyzed for any presence of coolant related materials.

If the cylinder is not getting any coolant a persistent misfire in one cylinder can be due to a burned valve. In this case the 'lifter noise' is not lifter related at all but the noise that is the combustion chamber gases leaking past the burned valve.

A compression test might be called for. You already know there's a weak cylinder from the misfire code but you want to try to get a determination if is due to a cylinder that is down on compression or something else, though that something else: plug, coil, fuel injector; has already been pretty much eliminated by swapping the suspected ones with ones that are apparently working ok.

And in this case a leak down test might be of some value since you can then check for any escaping air at the intake and exhaust sides of the engine along with at the oil filler tube and even the coolant tank filler opening.

Sincerely,

MarcW.

Thanks for the thought on the head gasket. Yes, the plug was clean, but not a lot more than the others. It runs well enough so I didn't think a compression test would show much. I'll check it and leak down, too. I'll let you know in a week or so.

Ed

allowing coolant into that misfiring cylinder is greatly reduced.

You should eliminate other possible causes of the persistent misfire and that involves as Boxsterra touched upon the coil connector or possibly some other wiring.

If you are unable to locate the cause though that still leaves the possibility of a burned valve, or less likely but still something to be considered a broken valve spring or a collapsed lifter.

A compression test with a leak down test on the weak cylinder is probably still a worthwhile effort. The cause is sufficient to cause a persistent misfire so a compression test/leak down test could be worthwhile to determine if the problem is with the intake valves or exhaust valves or at the piston/ring/cylinder interface.

Depends upon how much you can do yourself. If you have to pay you might just have this done as part of the diagnostics by a professional to id the source of the misfire. Regardless of what you find he's probably going to want to do the compression/leak down test.

Sincerely,

MarcW.

Car has been sitting for a week. Started it to put it in the garage to check compression. Ran a little rough as usual upon starting. #3 plug darker than the rest. Compression in #3 was 137. Others varied from 120 to 136. #3 plug looked damp. Resistance varied, 1500/2500 ohms. Cleaned plug. Resistance infinity. Coolant measured 1500 ohms. Looks like a head gasket!

Ed B

I've seen bent clips on ignition coils causing them to not snap into place properly. Or it could be a real wiring problem. The latter can sometimes be diagnosed by measuring continuity for each of the wires from end-to-end while wiggling the wires. Also check for shorts to ground/B+.

I find that these types of problems are much more often something external to the engine than internal.

The next step is to get rid of the fuel. I hear from a reliable source that the e-Boxster is a go.