nrubenstein

So, cylinder #1 has started backfiring constantly, which is rather awkward. I've tried the quick and obvious steps of changing the plugs/wires, and have swapped injectors around. Unfortunately, the problem is clearly at #1.

While I had the plug out, I scoped it and didn't see any obvious issues (and the plug that came out looked fine too).

Any thoughts on what this could be?

It began on a throttle lift when the car was being driven moderately aggressively, but not at high revs are high speeds.

Next step is to leakdown the cylinder, but I'm kind of at a loss here. The car seems to run fine on five cylinders with the injector disconnected, but that's of little use beyond diagnostics.

billswim

What makes you think the problem is cyl #1?

nrubenstein

Because if I disconnect the injector, it stops backfiring and just runs on 5.

Stethoscope shows the noise as in the intake manifold.

nrubenstein

Hmm. 1-2% leakdown.

I remain super puzzled.

nrubenstein

Next best guess is that I have is a bad lifter. (That's what I get for replacing all of them, I guess...)

Hope it's the intake lifter, although I have a sneaking suspicion that it's probably exhaust side.

billswim

I wouldn't guess its a lifter. You'd hear the mechanical noise very definitively at 1/2 engine speed.

No check engine light? If it's backfiring out the tail pipe when the fuel injector is plugged in then you've got a lot of unburned fuel coming getting through the cat, which doesn't make a lot of sense either. Check the plug wire connections at both the cap and plug end. The ends are replaceable and I've seen them fail.

nrubenstein

Intake backfire, NOT exhaust backfire.

CEL is present. Flashing if the injector is plugged in. Just misfire codes, though, which is not that helpful.

Coils are new (replaced while diagnosing).
Spark plugs are new (replaced while diagnosing).
Cap and rotor are new (replaced while diagnosing).
Injectors have been swapped around and it stays at #1.
Leak down is 1-2%, so it's not a burnt valve.
Rotor is pointing at the right location at cylinder 1 TDC, so timing appears to be good.

A lifter is just a guess, but I'm very, very low on guesses here.

ACEparts_com

I know you've done a leak down (filling the cylinder with compressed air).

Have you also done a compression test? Just guessing but that might give a better indication of lifter / valve spring / valve sticking issue (possibly?!)

nrubenstein

Not yet. It's on the to-do list. I'm trying to avoid towing it out to my usual work area (a friend's garage, but where it would be stored outside), so I'm working with fewer tools than I am accustomed to.

OverBoosted28

Is this on the TT?

nrubenstein

Well, I probably have better places to discuss BMW problems than a Porsche forum.

So, yes.

billswim

OK, if its backfiring into the intake, either the charge isn't getting into the combustion chamber or the spark is happening while the valve is still open. If your lifter collapsed, the valve would just open late and close early and make a lot of noise. I'd look at the intake manifold for a leak. It'd cause a lean condition and possibly a misfire. Check the base gasket there. Stock motor, or modified?

billswim

I'm guessing he's asking you this because you said the coils have been replaced. The TT only has one coil.

nrubenstein

I've checked for leaks on the manifold runner and base. There is nothing obvious. And for it to backfire on every opportunity, there would need to be a monster leak.

Motor is stock.

nrubenstein

Meant to type coil wires.