WTBuy (or WTBorrow): AFM for a M42 (see inside for why!)

[jquirit]

Okay, while driving home Thursday my car started doing a weird hesitation whenever accelerating moderately and from stops. While at a constant speed and gentle acceleration (grandma mode!), it doesn't hesitate. So I shut her down when I got home and started her right up. She idles for a few seconds as if fine, but as soon as the ECU goes into closed-loop mode... boom, very rough idle.

I let her sit overnight and tried cranking her when she's cold. She starts up fine, idles fine, and stays that way until she warms up (about a minute or two), then once the ECU goes into closed-loop mode she starts sputtering and hesitating.

I stomp-tested and there's no codes and the CEL isn't on. At this point I'm guessing AFM giving the ECU some erroneous information. I discounted a vacuum leak since it's not doing it during warm-up, but is that a bad assumption?

Anyway, I'm looking for a known-good AFM that I can swap to see if the issue stays or not. Anybody got one I can buy/borrow to troubleshoot this issue?

[xwill112x]

I have one i'll sell you. PM me

[e30austin]

Member "Ryann" has one in Roseburg, IIRC. Might try LuckyHenriksen, he may have one also.

Good luck!

[bmwm42]

Tps

[Ryann]

In my experience the DME will usually throw a 1215 code if the AFM is the culprit.

[jquirit]

In my experience the DME will usually throw a 1215 code if the AFM is the culprit.

Hmmm, guess I'll still poke around to figure out what it might be. Would the CPS/TPS also throw a code if it's wigging out? Would a vacuum leak also show itself on cold start-up?

[bmwm42]

i doubt its an vac leak especially if this started out of now where, my guess is the tps sensor as its pretty typical when its shits itself that the engine stumbles hard under wot or hard accel. ryan is correct in the 1215 code but that also could be a vac leak as well.

[jquirit]

So, I'm now wondering what is wrong. Went about checking all the vacuum hoses (all fine), reseated all the sensor connections, replaced the TPS sensor. No fixie. So, I start dickin' around the engine while it's running rough and I'm hearing this slight puffing sound. I pull the oil fill cap, and the freakin' valve cover is pressurized. Whenever the car stumbles, there's a noticeable blast of air that comes out from the now open valve cover.

WTF?

[Ryann]

Do you have access to a compression tester or better yet a vacuum gauge?

[jquirit]

Nope, I can ask around to see if someone has one to borrow. So what do you think shit the bed?

[jquirit]

Okay, found a coworker that has a compression test gauge, but no such luck finding a vacuum gauge. Unfortunately I won't be able to get it until Thursday night so I won't have more information until then.

After reading into this more (why the valve cover would be seeing pressure) it seems I see where you're (Ryann) going with this. It could be blow-by on the piston rings (which would be identified by low compression in a cylinder). It also could be a valve seal leak (which I imagine is why you're asking for a vacuum gauge).

This irks me. I've taken good care of this car and to have something like this happens (especially now) is just really poor timing (doesn't help that the furnace in my house died this weekend and I had to shell out more money to have that fixed). Anybody have any estimates/experience on how much it might take to repair a bad head and/or a bad bottom end on a M42 if it turns out to be either of the two? Would it be better (read- cheaper) off just getting a motor swap?

Or does someone want a fairly clean 318iS body (Lucky and Abe has seen it) with an iffy motor for their swap project? :)

[PNWDan]

Im not familiar with M42s but its common to have pressure in the valve cover on some cars. It's part of the emissions system. Its also common to have some blow by in all engines. This is why they must have a vent or be part of a closed loop system like the ccv or pcv. Pulling the fill cap while running will also throw a CEL in some cars. I would pull your plugs and verify they look consistent and o would check your ignition components.

[Ryann]

^Positive crankcase pressure is normal yes but it seems like the OP knows what normal is and thinks that his is excessive. He'll have a chance to examine the plugs when he runs his compression test.

which I imagine is why you're asking for a vacuum gauge

A vacuum gauge can help diagnose a bunch of different things, but yeah that's kinda where I was going with that. A usable vac. gauge can be had for $10-20 and may help quickly resolve your issue.

Don't give up!:)

[jquirit]

^Positive crankcase pressure is normal yes but it seems like the OP knows what normal is and thinks that his is excessive. He'll have a chance to examine the plugs when he runs his compression test.

The reason why I'm thinkin' it that it's excessive is that you get a very noticeable blast of air coming out of the valve cover when it does misfire/stumble. It's not a constant thing. When it's idling along, it's about what I'd normally expect for a typical engine.

Perhaps I'm misdiagnosing the issue?

Then again, my specialty is electronics and I understand the basic concepts of an operating engine... troubleshooting it is a whole 'nother shooting match.

A vacuum gauge can help diagnose a bunch of different things, but yeah that's kinda where I was going with that. A usable vac. gauge can be had for $10-20 and may help quickly resolve your issue.

Don't give up!:)

I'll have to hit up the auto parts store to pick one up. Reading up on them, the vacuum gauge sounds like a great little tool to have when diagnosing issues. Where do you recommend to tap into to hook it up?

Thanks for the reassurance. Just gettin' hit with multiple bills for stuff breakin' at the same time really put me into a funk. Now that I've gotten some time to think it through and step back, things are now "okay."

:)

[Ryann]

The best place to connect a vacuum gauge on an m42 is the brake booster hose.