LH2.4 - Suspect bad AMM - what's your experience?

What's your experience with bad AMMs that DON'T throw codes? What sort of drivability issues did you notice?

1992 240 running LH2.4, -016 AMM, and it's a +T (13c, ~7psi, just a daily driver). Was a LH3.1 car but was converted many years ago.


Problems started a couple months ago. Wouldn't start after it rained. But after much cranking, it would fire up, and run normally. It probably happened at least 4-6 times. Through trial and error, I found that cranking it for just a couple of seconds, many times, tended to work better than cranking continuously for 10sec+. But I never found a certain cause.

Checked connections in the engine bay. Plugs / cap / rotor / wires all look good....wires were replaced w/Volvo brand within the past couple years when I did all of it at once. I swapped the coil, tried a different ignition module, and changed the plugs (NGK copper). I thought I found the problem when #4 plug wire was barely attached because the thread-on end was unthreaded from the plug and not secure. Swapped plugs, secured all plug wires, no change.

I used dielectric grease on the battery terminals and the plug for the ign. module & AMM. Didn't find any obviously corroded connections. Didn't notice any change.

The no-start after rain seems to have gone away, but it's more like it changed to a different type of issue. The car became noticeably sluggish in the middle of the power band and throttle response has become poor. It's very apparent during normal driving when shifting at ~3,000-3,500 RPM, the RPM drops to 2,000 or below, and it just doesn't accelerate. It makes noise....turbo spools....goes into boost....but it feels like there's a soft rev limiter. It's as if it pulled a ton of timing. This is a B230F block, 13c, 3" downpipe into 2.5" straight-through exhaust. I expect it to accelerate from < 2,000. Something is clearly wrong.

I unplugged the knock sensor and drove it...no change. I plugged in a spare knock sensor and secured it away from the block....no change. Confirmed there was a code thrown for missing signal from knock sensor while it was unplugged. Code did not return after I drove it with the spare knock sensor plugged in. I should test the spare knock sensor...I just assumed it was good.

I tried a spare AMM (history unknown). It drove MUCH worse, and threw a code. Swapped back, and the car drives the same, and the code doesn't come back related to the AMM.

Fuel pumps run. I checked the fuses and I can feel the main pump when it's running. I've not yet disconnected the main pump to verify that the fuel flow looks strong. The engine seems to run closer to normal at higher RPM (4k+). It never stutters or falls flat. The symptom is poor acceleration and throttle response, but it remains smooth through the power band. I've known fuel delivery issues to give a big stumble or stutter. Experienced that in our 24hr lemons 240 race car a few times.

I've run it below a half tank and filled it up full. I don't suspect the fuel pumps or that the in-tank has a torn hose. I have not pulled the in-tank pump.

One thought was maybe the timing belt jumped a tooth. I've never driven with the cam timing off that far....but I know it would shift the power band significantly. That's on the to-do list to check this weekend.

The car runs the same cold vs. fully warmed up. I haven't checked, nor suspected the coolant temp sensor.

When it's run poorly, I could do a few pulls under load and then observed the turbo/manifold glowing at the inlet. It makes me suspect a problem with timing and/or air-fuel mixture. Going to pull the plugs this weekend and check them.

From my reading, it seems that diagnosing an AMM problem comes down to swapping in a known-good one....but I don't have a known good one at the moment.

The ECU is chipped...have thought about swapping the stock chip back in. I don't have spares for the ECU or ignition box to try.

The car starts fine and idles normal. Doesn't throw codes (except for idle air valve, which has been disconnected for years).

Any comments, questions, thoughts?

Towery. I have a known good one I can send you if you're so inclined and I'm just down in Raleigh.

The only other thing I could offer would be... does it run better with the AMM unplugged? But it sounds like you may have a hot wire issue.

Also, check your O2 sensor. Mine was doing odd stuff and it ended up being an O2 sensor clog from it running hella rich.

I doubt the chip would cause that issue.

For sure I would check the timing belt along with the cam itself to make sure that when the gear is at Zero the cam is also (broken roll pin). As I have seen a cam snap this and then spin and stick just a few deg later.

I wouldn't rule out a bad ECT, Removing it and checking it hot and cold might help you know for sure if it is in spec.

I have a known working AMM here I could send you for testing as long as you promise to send it back if the results are the same.

Do you have a wideband? sounds like not, but knowing if the AFR's are just huge rich when it is bogging might help a little.

Have you tryed a known good RPM sensor? I have had some odd issues that were more than just a start no start issue when my rpm sensor was going out.

02 sensor is also a fairly common issue point, myself I have prefered the NTK branded 02's as of late.

I have an lh2.4 car that seemed to never wanna start in the rain and also didn’t wanna start on warm starts after sitting 30 minutes.

$14 amazon crank sensor has fixed that entirely.

I have had 2 bad AMMs on lh2.4 240s. Both of which caused the car to be nearly undrivable. Trash idle after 30 seconds, bogged down with barley any throttle.

You can ohm out the air meter to see if the impedance is out of spec. the Bentley has a spec in there. I will edit when I find it. Also check that the burn off function is working. Because if it isn't then that can make weird stuff happen. Also a fuel pressure check will give you good info. Sometimes when the pressure regulator is failing the diaphragm is sticking and won't respond to the vacuum change and can also make the engine too rich.

Ok on LH2.4 you measure between terminals 2 and 3 on the sensor. It should measure between 2.5 to 4 ohms at ambient temp. Outside of those limits replace it.

To test burn off they want you to have the engine warmed up. Then back probe with a voltmeter on the sensor connector terminal 4 and ground. Start it up again and rev to 2500 rpm and return to idle. Then shut off. After about four seconds the meter should deflect for about a second. If it doesn't deflect and the system relay is working replace the ecu.

I had a really strange no-start on mine way back. All the usual stuff checked out. During my diagnostics, I had an injector unplugged, forgot to plug it in, went to crank and it lit right off, though only on 3 holes. Plugged the 4th back in, smooth. Shut off, crank, no start, unhook the injector, lit right off. Ended up being the RSR couldn't handle the current for 4 injectors during startup but 3 was fine. Swapped in a known good, completely resolved it. Everything else checked out.

Still worth checking the timing as well.

I think I can borrow an AMM from a friend that's local, but thanks for the offers. Car starts and idles decently with the AMM unplugged, but overall runs worse. Had to be very gentle with throttle position to accelerate (as one might expect). With the AMM plugged back in, it definitely ran better and accelerated better.

PromiseRing said:

I have had 2 bad AMMs on lh2.4 240s. Both of which caused the car to be nearly undrivable. Trash idle after 30 seconds, bogged down with barley any throttle.

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That's exactly how mine was with the known-bad AMM. I drove around the block and barely made it back, it would hardly go at all. Do you remember if yours threw a code? Mine certainly did.

Thanks Dave, I'll try bench testing the AMM this weekend. I've got a bentley here, and a cheap meter somewhere that might still work. Might be time for a new/nice one.

I've got a spare crank position sensor, so I'll swap that in and see if there's any change. The harness on it is pretty shredded so I'd taped it up and kept it as a spare, but didn't really suspect it could cause issues like I'm having. Suspected it during the wet start problem, though....so maybe there's a connection.

Fuel pressure regulator...forgot to mention, I replaced the vacuum line and confirmed no fuel leaking out. Didn't go any further, but might try swapping it.

No wideband in this car. The narrowband O2 was replaced by me at some point, but I suppose I should get under the car and see if it's still connected. I checked to make sure it was tight and the wire wasn't obviously severed.

Filled up with 12.05 gallons of premium tonight after.....183 miles. ~15.2mpg. I saw similar mileage in the previous half a tank, but this confirms mileage is bad. Usually I'll get at least 19+, mostly short trips and a heavy foot. Low 20s when I mix in some highway.

Hmm it was quite a long time ago but I think it might have thrown some type of mixture code.
Never threw an AMM code IIRC.

I knew nothing of lh2.4 at the time and I found a local guy with an 89-93 240, swapped AMMs, and that was my issue.

I have had the crank sensor act weird under high moisture conditions. One that I had caused random intermittent no starts till it finally failed completely and I figured it out.

With the fuel pressure the last bad regulator we had in a friends car had just a stuck diaphragm no fuel leakage. But the stuck diaphragm made the fail pressure the same as pump pressure flooding the engine. The main symptom though was stalling and hard starts especially when warm. So that probably isn't the issue here.

Don't forget about that intake hose on the turbo being sucked shut or restricted under quickly changing airflow.

I had a situation for maybe a year and a half where my daily would ALWAYS crank on the third time. Like clock work. It was sort of funny. Swapped AMM's one day just to see if it made any difference and boom lights on the first try. It never once threw a bad code or otherwise drove poorly.

I really think, at this point, we're all dealing with like 20+ year old AMM's that were never really super duper reliable to start with so they're all going to act weird. It's honestly one of the nicest things about my microsquirt car and the MAP sensor. Vac leak, change something, whatever and the MAP sensor doesn't care. It'd be super easy for you to just dump the AMM and all of that mess and get a PNP microsquirt from Kenny.

The hard starting when damp is likely a CPS issue. The flat spots in the power band are classic failing AMM symptoms. When an AMM is responsible for flats spots, there is never a code triggered in my experience.

2manyturbos said:

The hard starting when damp is likely a CPS issue. The flat spots in the power band are classic failing AMM symptoms. When an AMM is responsible for flats spots, there is never a code triggered in my experience.

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Same here. I’ve also noticed a warm start issue with a fail(ing) CPS. It was very inconsistent.

Found and fixed a boost leak. No change.

Trying a spare AMM tomorrow I hope...will report back if/when I find the cause.