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Old 08-08-2021, 11:34 AM   #1
DavePolyakov
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Default Only runs without AMM plugged in

1990 740 b230ft. The car only runs with the AMM unplugged. I got a 3-inch AMM and larger injectors but I'm still having the same issues with this "new" used AMM.

If the car only runs with the AMM unplugged does that mean the problem is definitely the AMM and my replacement is also broken? Or is there possibly a problem elsewhere?
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Old 08-08-2021, 11:58 AM   #2
dl242gt
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That typically means the air mass sensor is dead. Even if you have the same issue with a second one. Then the second one is bad as well.
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Old 08-08-2021, 12:05 PM   #3
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This is what I thought but wanted to make sure. Guess I need to find another 012 AMM.
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That typically means the air mass sensor is dead. Even if you have the same issue with a second one. Then the second one is bad as well.
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Old 08-09-2021, 12:21 AM   #4
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I've since tested two other AMM's and the same issue. The car throws code 2-3-2, "Fuel trim (lambda control) too lean or too rich at idle".

The issue began when I was at 12 pounds of boost and the car shut off. It now will only run with the AMM unplugged. I'm not sure what I broke but probably not the AMM unless the three different units I tested were all bad. Could It possibly be my oxygen sensor?
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Old 08-09-2021, 03:12 AM   #5
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Quote:
Originally Posted by DavePolyakov View Post
I've since tested two other AMM's and the same issue. The car throws code 2-3-2, "Fuel trim (lambda control) too lean or too rich at idle".

The issue began when I was at 12 pounds of boost and the car shut off. It now will only run with the AMM unplugged. I'm not sure what I broke but probably not the AMM unless the three different units I tested were all bad. Could It possibly be my oxygen sensor?
From my experience a failing/failed o2 will cause a motor to run richer, but not enough to cause a stall or no-start.

Did you do a resistance check on your AMM's ? I'm not sure if this helps but here is a resistance check(of hot wire) from the Bentley Bible

3. On LH-2.2 and LH-2.4 systems, connect an ohmmeter to between terminals 2 (green-yellow wire) and 3 (white-red wire) of the air mass meter.

Air mass meter platinum wire resistance
* LH-2.0, 2.2 .................3.5-4 ohms
* LH 2.4.........................2.5-4 ohms


iirc the AMM numbering system starts from the firewall and goes forward( so #1 is near firewall, #4 is nearer radiator), maybe someone will chime in if this detail is incorrect.


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Old 08-09-2021, 05:25 AM   #6
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The bentley manual is wrong, there is no conclusive test for an AMM other than to swap a known working one
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Old 08-09-2021, 06:06 AM   #7
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I assume the system is LH2.4, correct?
Did you check the wiring to the AMM? The diagnostic system is very simple, it can not differentiate between a broken AMM and broken AMM-wiring/ connector pins.

I once had a similar error on a 740 (LH2.4) and it turned out there were corroded connectors out of sight (behind the strutt towers) that were the cause of the problem. it wouldn't surprise me if the 240's have similar large rectangular connectors to connect the engine bay loom to the loom inside the car.
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Old 08-09-2021, 02:54 PM   #8
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As suggested it's time to make sure the wiring is correct and not corroded anywhere. If that is the case and it's checking good. Then it may be time to try another fuel ecu. The code that it is setting is usually associated with a vacuum leak or some type of air leak. It is also the code that a bad ecu like the 561 ecu can set if the unit is one of the bad flaky ones.
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Old 08-09-2021, 03:35 PM   #9
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I verified the MAF is getting 12v and I tested the ground wire too, It measured 1.2 ohms when I tested using the negative terminal as the ground. The ohms between pin 2 and 3 on the AMM measure 3.9 ohms.

Does everything check out fine? I know the MAF could still be faulty though these numbers look ok.

My ignition ecu under the passenger kick panel is 3 517 370.

Quote:
Originally Posted by dl242gt View Post
As suggested it's time to make sure the wiring is correct and not corroded anywhere. If that is the case and it's checking good. Then it may be time to try another fuel ecu. The code that it is setting is usually associated with a vacuum leak or some type of air leak. It is also the code that a bad ecu like the 561 ecu can set if the unit is one of the bad flaky ones.
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Old 08-09-2021, 03:47 PM   #10
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I'd be looking for other air leaks. Did the IC split, a hose split that isn't obvious, something along those lines, especially since you mentioned it happened in boost. With the MAF unplugged, it goes way rich, so if you have unmetered air getting in, that added fuel may allow it to run.
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Old 08-09-2021, 03:50 PM   #11
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Kind of what I've been wondering, it seems like a coincidence the AMM would quit at 6k under full boost. If I remember I think there may have been a popping sound. I'll do a thorough check, it must be a pretty bad leak for the car to not even run?
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I'd be looking for other air leaks. Did the IC split, a hose split that isn't obvious, something along those lines, especially since you mentioned it happened in boost. With the MAF unplugged, it goes way rich, so if you have unmetered air getting in, that added fuel may allow it to run.
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Old 08-09-2021, 04:19 PM   #12
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I am such a fool.

The idle air control valve hose had slipped off of the intake manifold. I didn't notice it because it's under all the other hoses and hard to access. Now the car starts and runs.

It runs a little bit lopey at idle, is this normal while lh2.4 adjusts to larger injectors and AMM? It's just slightly rough, it's not horrible and smooths out with a few revs.

I currently do not have a wideband. Before I get one installed, would I probably be ok to run a medium boost level like ~10psi? I understand the computer takes 2 tanks of gas/ ~500ish miles and several cold starts before it fully relearns. Before I hit those milestones I'll drive gentlyish.
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Old 08-09-2021, 07:00 PM   #13
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If you've been trying to run it with that big of a leak, yeah, it will need to adapt a little, as it will with the AMM and injector swap. Also worth at least taking a peek at the simple stuff, plugs, all the basic maintenance if you don't know the status or it's been a while. All the usual stage 0 stuff.

Just remember, when diagnosing...KISS...keep it simple stupid. ;)
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Old 08-10-2021, 04:55 AM   #14
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Quote:
Originally Posted by hessam69 View Post
The bentley manual is wrong, there is no conclusive test for an AMM other than to swap a known working one
Personally I use the hot wire resistance check as a starting point, and I do I agree it is not a 'conclusive' test of an AMM.


Last edited by Otto Mattik; 08-10-2021 at 05:06 AM..
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Old 08-10-2021, 02:29 PM   #15
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Quote:
Originally Posted by DavePolyakov View Post
I am such a fool. The idle air control valve hose had slipped off
Boost is magic
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Old 08-10-2021, 07:41 PM   #16
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Quote:
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Boost is magic
Extra air in = HP gainz
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Old 08-10-2021, 07:48 PM   #17
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BTW, you can test the output of the MAF, should always be between 0 to 5 VDC, anything else is a problem.
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Old 08-10-2021, 08:33 PM   #18
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Very much so. I wonder if this loose hose connection was causing occasional break-ups when in boost. I want to let the ECU adjust for a few hundred miles before I start boosting again but I'm curious to see how it behaves.

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Boost is magic
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Old 08-13-2021, 02:16 PM   #19
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If you have access to an air compressor, build yourself a clamp that will allow you to pressure the intake system. Then set it to, say, 10+ psi and go around with a spray bottle with soapy water to see if you find a leak.

I can't find the instructions but this should give you an idea of what I am talking about
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