engine misfire

[kgriff]

We have a 2007 GTI with the 2.0. The car developed a misfire while normal driving. The fuel injectors and coil packs were changed and it didn't make a difference. I just did a compression check and cylinders 1,3 and 4 were all about 165 to 170 psi cylinder 2 was 30 psi. before the head comes off should we look at anything else that could be causing the lower compression? I probably didn't need to but I did put a little bit of oil in the cylinder 2 and it made no difference in compression.

 

[Fred930]

If you have (or can borrow) a leakdown tester (which uses shop air to pressure the cylinder via the spark plug hole with each cylinder at top dead center - all valves closed), you will be able to listen for where the air is leaking out.

If hissing is heard at the tailpipe, you've got a open (burned) exhaust valve ; if hissing is heard when you open the oil filler cap there is ring blow-by ; if hissing is heard from the air cleaner - an intake valve is stuck open.

If you're really lucky, the intake valve pockets might simply be all gunked up, and a piece of carbon crap is slightly holding open a #2 intake valve. Should that be the case, you might be able to clean it without having to pull the head (by removing the intake manifold).

But this is just a wild guess. There are guys on this forum who work on VW's for a living and much better qualified than me to comment if the carbon buildup can get that bad, and (more importantly) if it can be cleaned without running all the nasty crap down into the cylinder through the partially open valve.

A leakdown tester is a pretty inexpensive (maybe $35-$50) but handy piece of test equipment.

 

[kgriff]

hooked the leakdown tester up and the air is coming out the exhaust. I guess at this point the head has to come off.

 

[Fred930]

Bummer ....

But just to verify there is no misunderstanding -- when checking each cylinder for percent leakdown, you must manually rotate the crankshaft to insure that particular cylinder being tested is at TDC with its valves closed. It might be necessary to hold the crank to prevent the engine from turning (and opening a valve) due to air pressure being forced into the spark plug hole. You should normally see a leakdown (air pressure loss) of just a few percent between the two gauges (certainly less than 10).

Check all 4 cylinders - they should have about the same reading - what numbers did you get? That comparison will insure your technique and gauge are working correctly. If only #2 is showing a high leakdown, then I guess you're screwed.

 

[kgriff]

the valve cover is off cam lobes off the followers for #2 and the air is just blowing through. if time permits we will have the head off this weekend and see what we have.