M54 suddenly running rough

paramax55 · #1 06-12-2023, 07:08 AM

I have an e53 X5, manual transmission, with an m54 3.0. It does all highway driving, very little in the city. It has been idling smooth as silk. You have to look at the tachometer to make sure it is running. Yesterday, I took the wife to dinner. The car started and ran like normal. We did about 20 minutes of highway driving and, when we stopped at a traffic light, the thing felt like a Harley. So much so that the wife asked "what's going on with this thing?" And she isn't one to notice car problems.

It has not completely dropped a cylinder. It's running rough, but all cylinders are trying to fire. It's not throwing any codes related to running rough. I cleared the codes and they didn't re-appear while I was messing with it. It has had an intermittent cam sensor code for several years and it has a new sensor for that. But cam sensors make it run or not run. If you crack the gas pedal, and bring it up to 1,000 rpm or so, it's much better, but still there, so it's not related to the idle control.

Does this sound like a coil issue? I have no problem buying a set of coils, but I hate throwing money at "guesses" with vehicles.

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Effduration · #2 06-12-2023, 07:16 AM

I would try and start it with MAF unplugged...If it runs/idles better, it is likely a vacuum leak. look at intake boots first, but a vacuum leak can come from any where. A smoke test is a good idea if you suspect a vacuum leak.

If no change with MAF unplugged, I would suspect a bad coil. While its running, unplug coils one at a time and see which one DOES NOT cause the X5 to run rougher...that's your coil...or spark plug...

paramax55 · #3 06-12-2023, 07:21 AM

I didn't think about unplugging the MAF. I'll try that and the coils. Thanks.

X5chemist · #4 06-12-2023, 07:35 AM

You have may to keep it running until codes pop up. Or you find something obvious.
How old are components? 100k mile plugs are a myth. I've yet to see a set get that many miles and run good. My range is 50-75k before a change. Mine has Iridium tip upgrades and new coils. The NGK coils run great.

paramax55 · #5 06-12-2023, 07:44 AM

It has old plugs. It has 206,000 miles on it. I've never changed the plugs myself. Whatever it is, it happened within a 10-20 minute period. It was running fine at one stop sign and it wasn't at the next traffic light. If it is a plug problem, it would have to be something like a cracked ceramic. A coil wire can break inside the potting and give a weird/weak/intermittent contact and that would happen quickly. Of course I have all sorts of free time this week, so it's a perfect time for this to happen (not).

andrewwynn · #6 06-12-2023, 09:27 AM

Failed coil will give the symptom. If you get a code on a particular cylinder swap a couple coils to see if it migrates.

Wife's car did this. Ended up changing two coils. It still ran a little rough until I ran a bottle of injector cleaner.

paramax55 · #7 06-12-2023, 09:34 AM

I'm expecting a failed coil. I'm going to get to it in a few hours and try to find the one that makes it run "less worse" when I unplug it. 5 of them should make it run like complete garbage AND down a cylinder. One if them should just make it run like it's down a cylinder. But it's going to be a completely subjective observation.

Effduration · #8 06-12-2023, 10:11 AM

Quote:

Originally Posted by paramax55

I'm expecting a failed coil. I'm going to get to it in a few hours and try to find the one that makes it run "less worse" when I unplug it. 5 of them should make it run like complete garbage AND down a cylinder. One if them should just make it run like it's down a cylinder. But it's going to be a completely subjective observation.

I think we're saying something similar, but there might be no change if you unplug the coil that is causing your issue.

With that mileage it could be a couple of things. coils, plugs, vac leak, etc.

andrewwynn · #9 06-12-2023, 10:22 AM

Did you recently gas up? I've had my car go to limp mode immediately after a fill up I thought I may have put in regular or the station pumped the wrong octane. A load of bag gas will give the symptoms

paramax55 · #10 06-12-2023, 10:45 AM

I did recently fill up, but that was over 100 miles ago. If I fill up amd make one trip to work and back, it's over 100 miles. So there would have been plenty of time for bad gas to mix in. But that's a good "outside the box" thought.

I know that a vacuum leak could cause this, but I would expect a more gradual progression. This was like flipping a switch over a 10 minute period.

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