New Motor In: Lousy Idle / Can't Start When Hot

[bmwman91]

OK, so I got the MM motor in and hooked up. The first start went fine and it fired up with no drama, although the idle was sort of lousy. I got a CEL, but it would clear if I gave it some gas. The motor was running on all cylinders and I got it warmed up enough to bleed the coolant.

That's all good, so I turned it off so I could check the engine fault code. It gave back 1222...O2 sensor. Well, I am running a wide band setup and it sat unpowered for 4 months, so MAYBE it needs to be re-programmed and re-calibratred. The only other thing in there that is different is my MAF conversion, which was tested on my sister's E36 318iS.

Anyway, I decided to take it out for a spin since it seemed to run fine, except at idle. I start it, and the engine dies about 1 second later. OK...I start it again, and it runs for about 1/2 a second and then dies. No drama, no sputtering, just turns off. If I give it gas, it will run, but it feels like it is on 3 cylinders / not very strong. Hmmph.

So, any ideas? Everything is good mechanically, and I am 99% sure that there ar eno vacuum leaks. The two items on my list to try first-thing are to re-calibrate the wide band O2 sensor, and to swap in the stock AFM. If it is not one of those two things though, I am at somewhat of a loss. Ideas? This isn't due to an accidentally connected plug up on the firewall either...what else could cause hot-start failure? I will check it in the morning since I am too tired and it is too dark to mess with right now.

Thanks guys!

[bmwman91]

OK, small update.

- Recalibrating the LC-1 wide-band O2 sensor helped a little, but didn't fix it.
- Installing the stock AFM helped quite a bit, but didn't fix it.

The idle is still stumbly and rough. On warm starts, it stalls immediately upon starting. If I keep on the gas for ~10 seconds, it will idle thereafter, but poorly, and it will randomly stall sometimes. The initial start-and-die seems to be linked to the O2 sensor. If I keep my foot on the gas for ~10 seconds (time it takes for the WBO2 to warm up & start outputting) and the WBO2 status light indicates full function, it will idle poorly and stall occasionally. All of this indicates a big vacuum leak to me.

Other than that though, it runs great. I put 65 miles on it on mixed city/highway driving and the new engine sure runs strong. I never revved it over 5000RPM, but even the lower part of the power band feels good. It felt better and better the more I drove it, presumably since it is still breaking in. Per MM's instructions, I am changing the oil and will put 500-1000 miles on another fill of crude "break in" oil, and then switch to Mobil1 15W-50.

Once the engine cools off, I will drain the oil and disassemble the intake to check for leaks.

[bmwman91]

After some fiddling around, I got things worked out. There was a big vacuum leak around the upper intake manifold gasket. You know how there is that steel bracket on the front of the head that the upper manifold bolts to? Well it was offset such that the threaded dowel in the upper manifold was interfering with it and could not sit flat on the gasket.

So I adjusted some things, cut a new gasket out of a gasket sheet, cut some paper gaskets for the throttle body and checked all vacuum lines. No leaks! After that the car ran great...for about 7 minutes. Then it all went to hell and started running on 3 cylinders and I am at my wit's end now. That's the topic of another thread, unfortunately.