M50TU 1221/1222/1264 codes, surging idle, no leaks...

[kronus]

No leaks I can *find* find, that is.

Just put a bog standard M50TU into a 318is, and having trouble getting it running right.

It is throwing a 1221 O2 sensor code, although internet suggests that this can be basically anything, and it is having a surging idle issue.

Did the following diagnostics..
- pulled booster vacuum line, plugged, retested, no change
- pulled purge valve vacuum line, plugged, retested, no change
- smoke test - no smoke out of anywhere in the intake plumbing (injector seals, intake manifold gaskets, ICV hoses and plugs, TB, CCV hoses, FPR line, purge valve line)

Ideas on what could be going wrong? I have a BAR appointment on Tuesday morning and would like to not miss it.


monday update

well, tuesday morning 2AM update

spent the whole day fucking with it

learned several things

a) I'm not getting -5inhg vacuum at idle, I was just reading vacuum from a shitty place (CCV vac line to ICV-manifold tube). reading at the booster nipple, I get -20inhg

b) I have really good leakdown or I'm bad at using a leakdown tester. readings were 0% 0% 3% 1% 0% 0%.

c) I still have no idea what the hell is happening


installed a new O2 sensor, and traced out the O2 sensor wiring to the DME. I was suspicious of the O2 heater, but after searching, it seems to be a crapshoot whether it's on or off at cold idle.

jumpered the O2 heater relay, finally started getting different codes - 1264 (O2 heater circuit) and 1222 (some lambda exceeding possible trim issue)

tried two ECUs - a stock 413 silver label and a 413 silver label with a SSSQuid stage 1 chip, with similar results.


the current failure symptom is - after running for a few minutes from a cold start, the motor coughs and pops the 1221 CEL. Idle is wavery throughout.

looks like I'm not going to BAR tomorrow :(

[sublimation]

I had this code and my MAF was bad.

[kronus]

updated with troubleshooting results, but I don't feel any closer to an answer.

[pandaboo911]

I cleaned the MAF with sensor cleaner before I gave it to you. It was working fine when I pulled it from my car.

Unplug it and see if the car runs better


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[kronus]

I think the MAF is fine - I tried another one and the car ran way worse.

It does have a more even idle with it disconnected, though, but AIUI when that's done the DME goes into open loop and ignores the O2 sensor, the MAF and the coolant temp sensor.

[pandaboo911]

M50TU 1221/1222/1264 codes, surging idle, no leaks...

Check voltage at the MAF with zero flow. Should be .4-.5 volts.

Check your plugs to see if it's rich or lean.
Lean could be low fuel pressure, bad fuel pump, clogged filter(likely), clogged injectors, blockage in fuel feed line, stuck open icv

Rich could be stuck/leaking injector(unlikely) bad MAF(likely), bad fuel pressure regulator, blockage in fuel return line


Did you ever open the valvetrain? The guy who did that motor could have timed it incorrectly

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[kronus]

thanks! I will look at all those things. fuel is a not unlikely culprit seeing as the car sat for a few years.

AFAIK the guy didn't pull the motor apart at all, but I don't know for sure.

if he did there is also the possibility of the vanos being mistimed but I have no idea if the guy removed it

[pandaboo911]

M50TU 1221/1222/1264 codes, surging idle, no leaks...

Well if the car sat there could be bad gas that has emulsified stuck in a line. Did you install a new fuel pump? Try blowing compressed air through the lines.

I'm assuming you put a new filter too right?

If a problem persists even though all the sensors work it's usually fuel or vacuum related since those won't show up on the ecu.

Could be a bad knock sensor too and the car is running in limp mode

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[kronus]

I drained the gas and replaced the filter during the build.

Reset the ECU and put in a different MAF as a guess while I read about MAF testing. No difference.

[kronus]

plugs look ok, maybe a bit lean

wouldn't a knock sensor failure have its own code?

dropped in a different (used) set of m50 injectors, same deal.


to recap, the symptoms

once car is hot, idling for less than a minute will lead to it violently stumbling once, throwing the 1221 O2 sensor code and having a rougher idle thereafter.

No code is thrown during driving.

Power feels normal until code is thrown, there is no misfire or hesitation.

The idle is normal until code is thrown.


things I've tried with no effect

replaced oxygen sensor with a new one

replaced MAF with a different used one

replaced ICV with a cleaned, working one

replaced injectors with known-good ones

replaced coils with known-good ones

replaced spark plugs with new ones

replaced fuel filter with a new one

replaced TPS with a known good one

checked manifold vacuum; -20inhg (in spec)

leakdown test; 0% 0% 3% 1% 0% 0%

smoke test; no leaks

reset ECU; no change

replaced ECU with stock-tune 413; no change


remaining diagnostics

meter out the TPS

meter out the MAF

monitor fuel line pressure

[kronus]

fuel pressure is 3.2 bar (46psi) at idle, which is on the high side of spec but normal.