So my r53 started idleing rough. And threw a code. I changed the plugs, no change, wires, and #4 cylinder fuel injector. All with no change. Compression test shows 145 in all cylinders. Fuel mileage is still at 29 and performance off the line is unaffected. What could possibly cause this. Seems as if I have checked everything.

 

When you replaced the wires, the pins on the coil were fine?

 

Yep. Even tried a different coil.

 

Some other options: http://www.obd-codes.com/p0304. It does seem like you've covered the bases.

What plugs, wires, fuel injector did you use to replace? You cleared the code after doing all of the above and it just comes back and idles rough again?

 

Have you done a leakdown test? Compression tests only tell a part of the story.

 

Leak down next on my list. I don't have a leakdown tester yet. I tried after market map and mapt sensors. Car rejected them and barley ran. Put the old ones back in and the car came back to life and drove me to work. No signs at high rpm. I wonder if I put the sensors in the wrong spot as they look the same but the new ones don't have the same mini markings. I have a set of old o2 sensors that I swapped 150k miles ago that were still working. See if that is it. I don't understand how a sensor could cause only a cylinder 4 misfire. If I pull cylinder 4 plug when it is idleing it slows gets rougher and throws the misfire code immediately and flashes like it is suppose to. So it seems my misfire is small or intermittent. But you can defiantly hear and feel the pop in the exhaust at idle. Anything over 1500 rpm and it is not noticeable behind the wheel or with the hand in the exhaust. It may be time to take it in and have a mechanic hook up the fancy computer. After 250k miles I have been able to diagnose and fix everything myself. This one hmm. Could be a small leak in cylinder 4 that the leak down will show that is not represented in compression test. I am almost ready to just buy a new to me low mileage gen 1 r53. My car gave me a good run. But I was in a wreck 2 years ago and fixed in mechanically but never got the body work done. Thinks to think about.

 

So you swapped the #4 fuel injector? Said it was an injector code... Or was it a misprint...meant to say #4 misfire?

With 250k...kinda wondering what the valve train looks like...the cam and rollers at higher miles on some cars are a bit worn...not too hard to yank the valve cover and look...
Something I might consider...cost is a couple $ valve cover gasket and a few minutes...
No real SPECFIC reason why I'm thinking this way...just a bit if a hunch...

 

Yeah...reversing the map and t-map results in a car than can't idle and needs to be revved to run....

 

Not much to report back except the misfire is still happening. Fuel mileage dumps around town. and climbs back quick on freeway. I'm taking my car in next week to the dealer to have the power steering pump inspected so I can get a refund for my replacement last year. Also will have them plug in the big computer and run a diagnostic. I want to keep the car running. But have started to shop for a low mileage replacement. My car looks like a POS and now runs like a POS. might as well drive it until it brakes and do a leisurely shop for a red MCS with no sunroof sub 100K.

 

I put back in the map, and mapT. This time I swapped them from before and the car ran normal. I plugged in my WiFi code reader and watched the parameters on all the sensors. They are doing what they are suppose to. So I must have a small valve leak that will only get worse over time. I will not have the dealer do diagnostics. As they want $160 for one hr. And will probably come to the same conclusion. That 160 could go towards a new head. So at a crossroads. New head $1000. Body work $2000. 8 hrs of my labor. To swap. I also have a evap leak and now my steering position sensor is going out. Maybe $200 for those parts used.