Sudden AMM Failure?

Completed a +t on a 1990 240 last weekend. Green giants, 012 AMM, BPR7es Plugs, 16t, TLAO chips, IPD upgrade fuel pump, 5 PSI to start. Lh3.1 converted to lh2.4. Car previously ran very solid and never had problems. Started up first time and ran great for the past week. AFR idle - 14.5. AFR WOT- 11.6. Idle was very steady. Last night, the idle AFR went down to 11.7 and smelled very rich, slow +-50 rpm pulse. AFR cruise and WOT were still normal though. Soon after, O2 sensor code and check engine light. While trying to get home started bogging. No throttle response. Guessing this is limp mode. Pulled over and wouldn’t start and smelling like a lawn mower. Guessing the rich condition fouled the o2 and threw the code but does anyone have experience with a sudden AMM failure without a code or similar symptoms?

Most die from the self cleaning mode. Every time the car is shut off the ecu sends current to the amm sensor wires raising there temperature which cleans them. Sometimes they don’t survive.

thevinman99 said:

Lh3.1 converted to lh2.4.

Click to expand...

How did you convert? Swap the whole wiring harness? Swap the MAF?
LH2.4 uses a "hot-wire" AMM, with ECU controlled burnoff (after engine shutdown); LH3.1 uses a "thin film" AMM, with no burnoff needed. There are a couple extra wires going to the LH2.4 AMM for the burnoff control.

What are you using for an air cleaner? If it's a u-oil-it type, the oil can contaminate the AMM more rapidly.

I'd start by getting a can of AMM/MAF cleaner and see if the performance recovers after cleaning.

Differences are only in the TPS and AMM. Read the diagrams and converted the wiring over. Ecu and Ezk needed 2 wires to be changed. WOT blue white wire needed to be cut. Made new ground. I think if I botched it up it wouldn?t have ran at all let alone as good as it was. Using a new K and N washable 3.5? cone on the end of the AMM. Got the AMM from someone who pulled it from a junkyard, said they?ve sold many 012s without complaint. Is there a way to test the burnoff if the car doesn?t start?

You probably bought an intermittent AMM. I have a pile of them out in my shop. It is a typical failure mode with all the different hot wire AMMs.

In my experience it's not usually the kind of thing that fails progressively. Is the air filter just mounted on the end of the AMM? Bosch says there should be a short length of straight tubing before the AMM and air inlet, probably to "straighten" the air being metered and reduce turbulence.

Update

Okay here?’s an update. New 3 bar FPR because I smelled gas in the vacuum line for the old one, primed fuel pump, cleaned / checked .028 plug gap, started and stalled seconds later. Pulled the AMM connector off and it started and idled fine and held consistent RPM for 5 minutes. This must indicate that the AMM is bad? If there was a big vacuum leak the idle should hunt? Although I have chips mapped for the 012, I ordered new coupler and filter for the 016 and put it on, reset the ECU before starting. Same problem with the 016. Had to hold high rpm to stay running but would immediately die afterward... I probably shouldn?’t have reconnected the AMM while the car was running... I redid my AMM ground connection. When I moved the wires to the passenger side, I spliced into the ground for the headlights on the fender, but was informed that the AMM needs to retain the same intake manifold ?“signal ground?”. So I spliced into the original brown ground wire stub where the old lh3.1 AMM was on the driver side that connects to the manifold. Same problem. Is it possible this bad ground fried both AMMs? Asking every nearby red block owner for a spare amm to test now.

And interesring I didn?t know about the turbulence problem. I read a thread where their amm only worked on the hot side of the turbo for some reason.