Ok, I didn't want to post on this, because I feel like I should be able to figure this out...but I'm not sure what to do next. After my long thread about putting a new head on my mom's car and getting it all back together - I have the car up and running.

The car, however, runs slightly rough and has a misfire on cylinder 4. You can't feel the misfire or tell it's there, and it doesn't throw any codes. If I didn't have my vag-com to watch the misfire count, I'd not even know it was a misfire.

This misfire is ONLY on cylinder 4. It seems to be fairly random where it misfires on idle vs. revving to about 1500-2000 RPM. It seems to lean towards misfiring on idle though.

The only codes that have been thrown are for the SAI pump - flow insufficient. I know that on the passenger's side, I didn't get the valve on just right and it was making a tapping noise like an exhaust leak - so I fixed that and I'm thinking that MAYBE I'm having the same issue on the driver's side head. I took it off tonight, repositioned it, got 2 of the 3 bolts tightened, and dropped my wrench somewhere where I can't see it...so I called it a night.

I have swapped the coil packs for cylinder 4 & 5 and no change. I put a brand new spark plug in Cylinder 4 and no change. I changed the fuel injector from cylinder 4 to cylinder 5, no change. I swapped the wires for the power output stages, and no change. I also checked vag-com engine measurements #90,91, 92 and it appears that the cam adjustment is always between 0 and 1% +/-. It's consistently and appears to be ONLY cylinder #4. Even revving to 2000 RPM or so doesn't get other cylinders to misfire.

I'm thinking that getting the combi valve on correctly (not sure it was incorrect to begin with) will solve the problem - but I'm not sure why that would effect just 1 cylinder on the bank.

If that doesn't fix it, at this point, I'm thinking I have to check compression next (ironically I checked the passenger side before I put it all back together and it was great - but didn't do the driver's side for whatever reason).

Any thoughts on what else I should check that would cause just one cylinder to misfire??

 

You may need an ignition module amplifier {4A0 905 351A}. Pull the air-box engine cover. There are two ignition control modules on top of the air-box. The one that controls cylinder 4 is closest to the radiator with brown harness terminals. Switch the harness's on the modules. If your Vag-Com tells you that the misfire switched banks to the corresponding pot, then it is time to get new ignition control modules. Some dealers do not stock them and charge an arm and a leg. However, Deutsche Parts brings in some well made Hamburg Tech units for about $60.00. Hope this helps. Internet diagnosis is usually 50/50 at best.



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Full disclosure; I have no affiliation with the above vendor.

 

Before you buy an IMA swap IMAs?. If the misfire moves to 1, you might have a bad IMA.

 

Thanks for responding - I tried that. Some reason I've heard those called "power output stages" ..unless I have something confused. I swapped the wirest to the two on the top of the air box and no change, still #4.

 

Power Output Stage (aka POS) is the correct term.

 

Hate to say it but do a leak down test again. If you swapped all electrical and fuel devices and the problem is still with the cylinder, it may be a valve not seating 100%

Also carbon buildup (not likely here) can cause the same issue.

Check resistance from injector connector through the harness to the ECM

The only other thing is the output from the ECM. Maybe it is getting flaky.

Don't you have the same car? swap ECM's.

 

Good thinking on my having the same car - I will try this.

Stupid question - ECM...is this the same as the ECU? Just different nomenclature? We're talking the computer under the windshield?

This head has NO carbon in it...it was a reman'd unit and clean as could be. I checked the valves with alcohol and food coloring...no leaks were found. I will retest compression also.

My other thought is that maybe I should change the pack on the cylinder before #4 in the firing order. I've seen things happen where one pack was faltering and it was off just a little - so the cylinder that it was on didn't register a misfire, but the next 1, 2 or 3 cylinders did. I think this is unlikely, because I would expect more than one cylinder to be off if this is the case.

Also, I noticed that the O2 sensors were not reading the same numbers as I flipped through the measuring blocks. Maybe I saw things incorrectly, but it looked like one read 7-8 (not sure of the measurements) and the other was 14-16. Shouldn't these two be the same? It didn't occur to me to look more closely, and I won't have time to check it out tonight, but I will tomorrow.

Thanks...

 

ECM = engine control module per Bently manual. So yes the one under the windshield.

Misfire from low fuel will cause the O2 sensors to read differently.

 

Ok,
Haven't tried the ECM swap yet...I am just hesitant to buy into it yet...

I swapped the coil pack again last night w/ Cylinder 1, which I thought would be a waste of time and it was. I did finally get the combi valve on so that I'm pretty confident it's sealed, and I took the car for a spin and no codes are thrown. I did a compression check on cylinder 4 and got 175 PSI - so that's good.

The car picks up great, but idles really rough. The entire car shakes. The misfire seems to occur in relation to idle. I don't know how to explain it yet - but it seems that if I am idling and I rev to 1500-2000 rpm, it will misfire as I press the peddle. If I hold it at RPM, no misfire. If I let off the pedal, it will misfire as it returns to idle.

I did some searching and I'm wondering if I have to run the idle adaptation in the Vag-Com??

I wanna say it was idling around 800 RPM...but I didn't look closely, it seemed normal and steady to me so I didn't pay close attention.

There's no real sputtering, no exhaust noise, etc. It's actually a small problem, but it just is really rough when it idles and I've got that one misfire. I can't figure out why it's cyl. 4 and no others. I've had 10-15 misfires on cylinder 4 and none on any of the others...

I seem to have the two SAI codes popping up here and there, but i was working on the valves and I think that's what it's from (started with the hose disconnected to listen to the sound when I finally got it on). I cleared the codes and took it for a spin and they didn't seem to come back yet...

 

So you have a lean condition on #4 cylinder with good compression, may be gasoline(fuel) or vacuum leaking ( pcv system, throttle body, intake manifold gasket and other vacuum hose or plastic line etc) and you have to test fuel volume and fuel pressure as well. x19