M42 Head Gasket Blown, Project Thread

[bmwman91]

something something aftermarket ECU you'll wonder why you never did it earlier. easier to troubleshoot and you can retune it youself and not rely on anyone else

Ha, yes that is the planned next step. It'll likely be an AEM Infinity 506. But I'd like to drive and enjoy this thing for at least a few months first!

[digger]

something something aftermarket ECU you'll wonder why you never did it earlier. easier to troubleshoot and you can retune it youself and not rely on anyone else

[bmwman91]

OK, I have finally found the cause of my high idle. After replacing the ICV (got one from roguetoaster which was in better shape than my old one) and swapping in a clean throttle body I got a hold of, there was no difference at all. I also tried swapping a spare ECU in to no avail. What DID work was swapping a slightly older revision of my custom tune in. Although it ran rich (old tune was for 24# injectors, current ones are 28#), it did idle at the correct RPM. The latest tune from Sssquid is a little bit "experimental" I think, so they will get me a corrected one soon, since I just emailed them with my findings. It must be that some bytes related to the ICV PWM signal got nuked since it was indeed the case that the ECU was opening it too much (air flow signal should be 1-1.5V at idle normally, and I was seeing 2.25V meaning that too much air was being allowed in to the engine NOT as a vacuum leak).

Anyway, I promise I will get all sorts of pictures up at some point relatively soon lol. I've been busy with work and dicking around with getting this thing running correctly lol.

[bmwman91]

The flapper was moving around when I shook it, so it at least was not stuck. There did seem to be some hysteresis in the motion though as I varied the input voltage up and down. I took the thing apart yesterday, which requires destroying it, and it is a very simple device. There was a little rust inside and a tiny bit of build-up in places that carb cleaner may not have been able to get to, so perhaps that was causing it to get stuck further open than it should.

Anyway, once I get the one I bought from you we'll see if it fixes the issue. Fingers crossed!

[ForcedFirebird]

Yeah the IAC is just and electric motor and the ECU uses PWM to modulate the opening. Typically with key on, you can feel the IAC vibrating. When off or disconnected, if you shake it (in a twist motion) you should feel the flapper rocking back/forth and hitting the return spring(s).

Now you have me curious, never took one apart to see the mystery smoke inside.

[bmwman91]

Update. Yeah the TB is a non issue. The one thing that I had assumed could not be the issue, turned out to be the issue lol. It is the ICV.

The car was running fine on it 7 months ago, and I have no idea how sitting for that long could have caused it to die. I did blast it out with brake cleaner, but I doubt that hurt it. Basically, it operates smoothly and does not stick. When I hook it up to my adjustable power supply, the following seems to be the case:
1.7-2V: vane starts to move
3.5V: vane is fully closed
7V: vane is fully open

I am not sure what the normal behavior is supposed to be, but it sort of seems like maybe the return spring is weakened or broken. Either that or there is something going on with the ECU or tune. I got the oscilloscope on the ICV terminals while it was running and the PWM signal is present, so it is not like the signal is stuck ON. I'll try a used ICV to see if that helps, or maybe suck it up and buy a new one.

[bmwman91]

I see. Yeah I had PMed Jordan to ask about the gray coating and he said that it was basically insignificant. So, maybe I do indeed have a stuck ICV, or some sort of big vacuum leak elsewhere. Time to poke around in there some more I guess!

[ForcedFirebird]

Oscillations indicate a leak between the AFM (MAF) and throttle body. Being the size the BMW IAC is, those little cracks are tiny in comparison, so doubtful that is the issue.

When the ECU has unmetered air going around it, it gets "confused" and rolls the idle trying to make corrections for the "mystery" air that is causing a high idle. So it dips too much, then rises too much, essentially trying to chase it's own tail.

[bmwman91]

So I got it fired up today. Mechanically, it runs correctly and does not make any odd noises or give any other indication of a major problem. However, it is not currently running correctly. When I started it, it immediately wanted to idle at ~2000RPM. After doing that for a couple of minutes, it would drop to ~1500RPM for a moment, rev back to ~2000RPM and then just keep quickly oscillating like that.

I think that I have ruled-out the AFM (MAF conversion) and TPS since disconnecting those did not really make a difference (the engine would stumble briefly and then resume the previous behavior). I had checked the ICV a month or so ago and it was moving freely when 12V was applied, so I don't think it is stuck open, and I will check again tomorrow. When the engine is revving up to 2000RPM, the AFM output does rise to ~2.25V, which means that metered air is getting in somehow...so it is not a vacuum leak at least. I think that the only sources of a leak like that could be either the ICV, or the TB itself.

Back when I got a spare TB and cleaned it, I used acetone. Some of you may have noticed that the M42 TB (and maybe other engines too?) have a slick gray coating that looks like it was dripped or smeared into the bore. Whatever it is, acetone dissolves it, and when I was done cleaning the TB I could see slivers of light around the perimeter of the throttle plates when I held it up to a light. So, I guess the gray stuff was (maybe?) a sealant intended to produce a zero-clearance fit between the throttle plates and bores. When I noticed the newly created gaps, I wondered if it would be enough to cause enough air to leak by that it might cause issues at idle, but the gap around the perimeter was only ~0.002" (0.05mm) based on checking with some feeler gauges so I figured it would be fine.

I believe that the engine changed from a steady 2000RPM idle to oscillating due to either a switch from open-loop to closed-loop idle control, or some other switch in the software based on coolant temperature. I only ran the engine for 2-3 minutes since I don't want to get it fully warmed up until I am ready to drive it to get the piston rings seated.

Anyway, has anyone else had experiences with leaky throttle bodies? Is a gap that small enough to get the engine to idle at 2000RPM?

[bmwman91]

I'd guess E85 because alcohol is a decent solvent for petroleum gunk, and it is a hell of a lot less expensive per gallon than lab-grade alcohol. Granted, the Lucas cleaner is not all that cheap, and I am not sure what the mix ratio is, but I trust Sssquid to know what they are doing. I have spoken to a few more people I know who are into cars and apparently I am not the only one who has had injectors glue themselves shut after sitting around for too long. One theory is that it is actually a small formation of surface rust due to ambient humidity that causes them to stick. In my case I doubt it since they were in hot-barred plastic bags, but whatever. I am not all that worried.

[digger]

curious why E85 was used?

[bmwman91]

Well, the injectors are going off to Sssquid for refurb. Their customer service is awesome and they are going to get them flowing for me at no charge, despite these being stuck probably being my fault for letting them sit for so long.

I did try to free them myself with no luck. Applying 12 & 20V pulses, and smacking them on the workbench, did not help. I also clamped some 15mm PCV hose into the outlet side, poured in some injector cleaner and then pressurized it to 45PSI with my air compressor while pulsing & smacking them, but even trying to back-flow them like that made no difference. Whatever is in E85 and/or the Lucas stuff certianly seems to turn to cement after a few months! I don't want to risk really messing these up, so I will let the professionals work on them. Considering this project started in November...what is another week of delay, really? Plus, now I know that every single other sensor and actuator is working properly since I tested it all lol.

I promise to get pics up at some point. I am taking them as I go, but don't have time to edit & upload right now.

[ForcedFirebird]

Personally, I fond the "snake oils" useless. The tough part is the same chemicals that wash away varnish, also corrodes metals.

[bmwman91]

Ha yeah, I tend to overthink stuff since I am not putting food on the table doing any of this as a service. It would be a different story if I was trying to turn products and services around for customers.

Hopefully these suckers are not going to be too gooped up since they were just cleaned recently. The last thing put through them was an E85 + Lucas fuel injector cleaner mix.

[ForcedFirebird]

4a@20vdc isn't going top hurt them. My supply is 8a max with internal breaker and just manually tapping the leads on the power supply works just fine. Being in a shop environment, often no time for fancy-pants shenanigans - gotta get it done and roll the next one in lol. As you said, though, it's worth hitting them with voltage while it's still all together. Just keep in mind, they will probably need to be run for a while to clear completely. Have an 88 Trans AM GTA here with a brand new motor (built 18yr ago) that's been sitting since. I used this method to get the injectors free and the car started right up for a few days in a row. The project was set aside for a few days to tend to something alese, and they were stuck yet again. Like the m42, the TPI L98 v8 manifold requires plenum and runner removal to get to the injectors - and the allen key bolts are buried. Grrr....

You have to remember the injectors are steel inside, and are just a seat/needle solenoid (much like a seat/needle on a carb bowl float, but spring loaded). Since there's not a whole lot of contact area from the needle to seat, they can easily stick together with varnished fuel, dried cleaners, and even plain old rust. We built the first injector cleaning/testing station many years ago and learnt the hard way that they need to be cycled in oil after cleaning to keep them fresh. One of the very first sets of injector ruined in the machine was a set of Dekka 85lb/hr - and those years ago, they were top $$.