Misfire questions

[monkeyodeath]

This is probably a stab in the dark, but I'm curious if anyone can help.

I have a persistent misfire that causes my engine to run rich and have "puh-puh" sounds coming from the exhaust. Idle is about 600 rpm or so. This is a 325i.

I've replaced all the ignition components, adjusted the valves with the spring tool, and have the timing spot on. The ICV, DME, and AFM have been checked.

I've smoke tested the intake at 3psi and found nothing. I replaced the intake manifold gaskets and bitch tube o-rings just in case, and closely inspected the intake elbow pipe for leaks.

The misfire doesn't really start until after about 10 seconds after the engine starts. It's not a consistent misfire, or at least it doesn't seem to be - the stumbles are kind of erratic. The car seems to misfire more consistently at idle than it does when throttle is applied.

Hooking up a vacuum gauge, the reading starts at about 20 psi or so, then after 10 seconds or so of idling, begins to fall to 15 and move erratically, covering a range of 3 psi or so.

I think that the misfire is coming from the #2 cylinder - I tried pulling that plug from the distributor and noticed that doing that didn't affect the idle/roughness as much as removing another plug.

My plan at this point is to get a leakdown tester and see what that uncovers.

Anyone have any other ideas as to what this could be? I'm pulling my hair out trying to figure out what's up.

Sorry if these questions seem dumb - I'm fairly new to working on engines and am trying to learn as much as I can, but since I lack experience I'm not always sure I know what I'm talking about.

My plan is to eventually take it to a mechanic if I can't sort things out, but I'd really like to track the issue down myself if possible.

[downforce22]

cams could cause a rough idle.. but if you know it is stock maybe cylinder 2 is not getting a consistent spark?

[monkeyodeath]

Yeah, cam is stock, at least I'm pretty sure. Has a big "K" stamped on it. What's a good way to check for consistent spark?

[jlevie]

Yeah, cam is stock, at least I'm pretty sure. Has a big "K" stamped on it. What's a good way to check for consistent spark?

The best way to check spark is with an ignition o'scope. But given the way the M20 ignition works, a spark problem on only one cylinder will be a problems with ignition wires, plugs, distributor rotor/cap, injectors, valves, or rings. The first step in diagnosis would be to get compression and leak down numbers.

[monkeyodeath]

One other thing - I have a couple of AFM units, one that came with the car and another I snagged from a wrecked car at the junkyard. Is there supposed to be some kind of cap over the mixture screw in the AFM? Could that be an issue here?

[monkeyodeath]

Yeah, leakdown test is pretty good, actually. I bet this would be a kickass motor if I could ever get it to work right! I think I'm going to start replacing the injectors one by one to see if that helps.

[monkeyodeath]

This thing is driving me insane, it makes no sense. I'm totally out of ideas. Anyone have anything else I can try?

[kharv]

Any updates???

[EatsHondas]

The first step in diagnosis would be to get compression and leak down numbers.

This gentleman knows his stuff. Don't throw parts at it blindly.

[MoreMayhem]

I'm having a similar problem, except my "problem cylinder" is #5. Have done most of the things OP has tried with similar poor results.
I'm not understanding how injectors, valves, or rings can cause a weak or no spark condition. I understand how they could cause a plug to become fouled quickly, but how/why would the ECU know there are problems with those items and send less/no spark to the offending cylinder?

[DanHatesCars]

I have the same symptoms as you, erratic iidle after a few seconds, it feels good under load, and the put-put exhaust noise. I can't seem to figure it out. I go to a pretty reputable bmw repair shop and my mechanic keeps insisting its normal which is driving me crazy. Any insight or updates would be awesome.

[digger]

I'm having a similar problem, except my "problem cylinder" is #5. Have done most of the things OP has tried with similar poor results.
I'm not understanding how injectors, valves, or rings can cause a weak or no spark condition. I understand how they could cause a plug to become fouled quickly, but how/why would the ECU know there are problems with those items and send less/no spark to the offending cylinder?

how do you know its a weak spark?

i think what was meant earlier that an injector, bad compression ring can cause or lead to a misfire, rather than bad spark

[MoreMayhem]

how do you know its a weak spark?

i think what was meant earlier that an injector, bad compression ring can cause or lead to a misfire, rather than bad spark

That makes sense to me.

In my situation, I used a spark testing device on each spark plug lead. #5 shows no sparking, while all the other leads make a good 'snap' noise and display a good spark on the tester. I tried different leads on output #5 of the distributior and the problem stays with #5.
I've ordered a new cap and rotor, even though what is on the car is only 3 years/ 25k miles old. Really hope it is the solution.

[MoreMayhem]

New Bosch cap and rotor fixed my problem. Even though what was on there previously was not very old, it was either bad out of the box & I didn't notice right away or it went bad quickly.

[BMWManiac]

Might be a good time to renew all the injectors too....get them flow tested, new o-rings....often a neglected part with a motor that is original.