Help with misfire at idle, extremely low compression

[Trevor Minton]

Hi everyone. For starters, I’m brand new here. Desperately seeking advice for a persistent and now-mysterious issue that I thought I’d have fixed by now with all the work I’m about to run you through. This will be a long post, apologies in advance, but I want to provide all the details in hopes that someone is patient enough to check this out and offer some opinions.

Car is a 2004 Mini Cooper S, 153,000 miles. I’m the original owner and it’s my son’s daily driver now. This is the supercharged 4-cylinder, not turbo. The engine is the same as what was in Neons and I think several Mitsubishis in the 90s.

We had a code for cylinder 2 misfire. I DIY most of the work on this car, and I started with thinking about the coil pack and wires and spark plugs, but they were all too new to think it was those (20,000 miles on coil pack and wires, 5,000 miles on the plugs). I did swap wires (no change, misfire stayed on 2) and tried new plugs even though the ones in there were so new. No change. I also bought a new coil pack and new wires to narrow it down for certain but there was no change with that either (so I sent those new ones back).

The misfire is only at idle, I think. They confirmed the misfire is only at idle. I can hear it missing on cold starts and idling at stop lights. Once in awhile, but rarely, it will miss and die on a cold start. It starts back up easily and runs fine. Under power, everything feels great, no loss of power, and it doesn’t sound or feel like it’s missing at all once on the throttle. I took it to my mechanic to help with diagnosis.

The shop did a smoke test that found a vacuum leak at the intake manifold. They pulled the injectors and replaced the seals/o-rings, and bench-tested the injectors. All of them checked out and were re-installed. They got a borescope through the intake and discovered a burned valve on cylinder 2. I took the car home, pulled the head, and got it to a machinist for a valve job. The machinist actually thought the valve was maybe “on its way to being burned” but said he’d seen much worse. But, I got my head back from him looking beautiful, and got it re-installed with a new head gasket of course. While the head was out, I did a lot of other work – new oil pan gasket, new oil filter housing gasket, new o-ring for a leaking crank sensor. Also put in new pulleys for the crank, the supercharger (15% reduction), the idler, and a new tensioner assembly. Finally, I replaced the original timing chain, the chain guides and the chain tensioner.

I put everything back together and it started right up, and took it for a drive. The drive was fantastic, the new supercharger pulley provided more boost that made the car feel better than it did when new. The idle didn’t sound perfect, but it was okay. However, the SES lit up within a day, and the misfire code was back. I took it back to my mechanic to continue helping me troubleshoot.

First, they checked all my work and gave me an A+ which felt good (I’m all over DIY but I’m very, very new to working on my cars. Learning a lot, and there is a ton I definitely do not know yet). Smoke test found no vacuum leaks. This time they checked the compression, and that read 60 psi on all four cylinders (!!)

This stumped them, and me. They felt like the car shouldn’t even start, but it does without any trouble. It just idles rough, with the slightly audible miss. Now, because of the supercharger, the normal compression is already kind of low – it is supposed to be around 90 psi. So 60 is still really low even for this.

So with all these details of what I changed, I’m not actually sure any of it matters – I’m right back where I started. Cylinder 2 misfire, regardless of the valve job.

Last week, I discovered that I put on a slightly too-long belt. I have a 2% larger crank pulley on it and along with the 15% reduction supercharger pulley, I needed a shorter belt by 15mm. I put that on last weekend and the idle was better – sounded more normal, less rough and rumbling, but I can still hear it missing. And the code is still there. It’s died on cold starts twice this week.

So that’s the long, long story, which probably could have been as short as “I have a misfire” since all this additional work hasn’t changed a thing. Thanks for reading this and if anyone has any ideas for me, I’m all ears.

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