Random misfire, cyl 4
 

So, after the tank of fuel that I set aside, I pulled into the fuel station, filled the tank, and went in to pay. After I got outside, I climbed into my car excited to resume hypermiling and didn’t get 200m before I was interrupted by a shudder and then a misfire on cyl 4... car had been running fine since the ignition module/coil pack fiasco with 130k on the odometer...

I moved the spark plug around but the miss stayed on 4, so I’m down to replacing the ignition module/coil pack again(I used a used one with 190k), or beginning the arduous task of removing the fuel rail/injectors and seeing where the issue is...

The misfire is constant, whether at idle or st 6600 RPM/full throttle

Cranking the car over with injectors off, it sounds even, like compression is good.. dealing with compression loss on my Redline has taught me what they sound like when a cylinder loses compression...

I bought a pint of fuel system cleaner and am hoping that fixes things but am not optimistic... and with the snow and cold I’m currently averaging 20MPG... or at least that’s what the fuel gauge and trip meter point to...

Hmm lots of directions to go. Do you have an SGII or similar instrumentation? Something with some live data would be good. Misfire counters can be fooled.

You can do a quick and dirty cylinder contribution test by unplugging your injectors one by one. Similar to the cranking sound test you did, listening to the rpm drop with each injector could help you narrow it down.

Since you say it feels uniform from idle to redline, I don't think you'll find anything here, but you might take a spray bottle with water and spray around the intake, listening for a change in idle quality and/or if any water is being sucked in. For good measure, spray anywhere there might be engine vacuum.

Don't forget a basic visual check: look for anywhere wiring might be compromised, and if your car has a mass air sensor, pull it and see what the elements inside look like.

I hadn’t thought to just unplug an injector, great tip!

I had sprayed a cleaner around the intake gasket near cyl 4 with no change in idle, so I doubt a vacuum leak exists... the car also runs speed density fuel injection so no MAF sensor to keep clean... sadly I can’t just swap coils around, as they are part of a big “cassette” that sits over my spark plug wells...

I looked at the plug, and while it didn’t look the best (it was the factory original with 130k) I moved it around and the miss stayed on 4, ruling out the plug...

THE cars changed its behavior on me... I dunno if it was like this before but it’s definitely running like it’s jumped time... idles like crap(shaking like an old diesel), only runs decent at light throttle(bogs at heavy throttle) and won’t go past 5k RPM, even in neutral, foot resting against firewall...

Hmm...is there enough wire running to the coil packs to allow you to pull the whole thing, stick a spark plug in and see if you have spark? A test lead with alligator clips could ground the spark plug body if necessary.

If you have spark, you might want to look at the EGR system...if it's stuck open, it can cause some interesting issues. But usually to all cylinders...

So I’m thinking that the lack of upper RPM power was due to the cat loading up with fuel... I’ve thus far disconnected injector 4 to stop dumping fuel into the exhaust and what results is the same dead miss on 4, but without the crippling loss of high RPM power... power is linear with RPM and throttle, but fuel consumption is still higher than normal and it *might* make 85hp lol...

It still cranks over even enough for me to not suspect a compression issue, I’m guessing it’s either fuel or spark...

I’ve got a spare intake manifold readied and a spare fuel rail/injector set cleaned up and ready to try... they haven’t been professionally rebuilt but I did clean the crap outta them with carb/choke cleaner and used new o-rings... the other car ran fine when parked, hoping they work...

My theory is that my problem is fuel because if a coil has failed, TWO cylinders would be dead, not just one... maybe the ignition module isn’t wanting to fire #4?

So I’ve got the misfire narrowed down to spark... I changed the injectors(and their o-rings), fuel rail, injector harness, and the intake manifold(with all new gaskets)

The misfire stayed on 4...

I swapped the ignition module from the fiancé’s 2006 L61 powed car, and the misfire remained, but when I swapped over its entire ignition coil/module assembly, the misfire went away and the car ran great(actually with the ported manifold it has noticeably more power above, say, 2500 RPM)

I just did realize that the boots can be taken off of the coil cassette... I’m wondering, what if it’s just a faulty spark plug boot? If one of the two coils in the casette had failed, logic says that two cylinders would lose spark, not just one... when I get back to my car, I’ll swap boots from one coil group to the other and see if the misfire moves

I'm biased in assuming most misfires are spark related since I've yet to run into a fuel problem. Usually I start on Youtube when diagnosing an issue. The more common problems are likely to be well represented there.

Here's what I found on a quick Youtube search for a similar car:

https://www.youtube.com/watch?v=CN1UF1lMKeM

My boots are dry, however... no valve cover leaks to be found in my L61...

Maybe the spring inside mine has broken?

Wish you were closer, I have spares.