740 & 760 740 won't go much above idle. Code 223 IAC (solved)

edited June 16 in Troubleshooting

Yet another 740 issue. Love these cars but I need to get better at fault finding.


Basically the car starts immediately and has a high idle, around 1200. Try and touch the accelerator and as per the attached video she just starts surging to a few hundred RPM up and down (obviously not driveable, unless your happy with 5KPH and downhill). It's like as if the there's extra air going through the throttle body but no extra fuel.

- Replaced AMM, no change. Tried disconnected, slightly lower idle but same issue.

- Searched for vac leaks, nothing obvious.

- Disconnected TPS and IAC plugs to look for changes, nothing. Isolated the IAC VAC hose temporarily and goes back to a smooth 700 RPM idle, won't go above other than same surge.

- Fuel Pressure Regulator has no leaks at diaphragm

Car history was laid up by previous owner for several months outdoors and was apparently running fine before being parked (other than brakes keep locking up after car warms, but no calipers seized and no issues cold, most unusual).

So I'm thinking obviously 223 could cause the high idle and the IAC will be swapped out / cleaned when I get the chance..... But can 223 cause the issues I have?


  • What year model 740?

    Did you try a known working AMM? It seems like the ECU isn't getting the signal that the throttle is open

  • Oh yes, 89.

    Yes, I did find last night after Google efforts of a similar example on a 240 that was a failed ECU.

    The AMM was a brand new Chinese one. Was sure it was not that because issue was identical with new one and no real change after disconnection, whereby normally disconnected you can at least REV the motor even if it will fart and carry on.

  • Do an ECU swap, easy to do and will eliminate that if it is faulty

    The 1989 ECU's were known for failing, I have had one fail (no injector ground) so had fuel and spark but no injection

    Any LH2.4 ECU will work

  • SLT16SLT @SLT16 Gold Coast

    I've had a similar problem about half a year ago.

    My car was even worse - It was idling at 1600 RPM.

    Turns out the actual IAC was leaking. Took it out, cleaned it with carb cleaner and sealed it in tight (it spins so you can take it out of its body - this was too loose and it was spinning freely).

    My car is LH2.2, if you have LH2.4 then your IAC will be different and i'm not sure if you can do the same. It could also just be a faulty IAC, may not necessarily be leaking - just not cooperating with the ECU.

    As per the car surging, my issue with that was a faulty AMM.

    I wound't trust a brand new Chinese one - pile of junk.

    You better buy a rebuilt OEM Bosch or a good working condition 2nd hand one.

    You may also need to test your TPS. There are plenty of tutorials online, and all you need is a multi-meter.

    My bet would be on the ECU, AMM or TPS.

  • carnut222Greg S @carnut222 Daylesford VIC

    Yes, I'd look at throttle switch and connector. Confirm computer is getting the correct signals from the throttle switch as if it thinks throttle is closed all the time (defective switch, corroded connector or wiring, or bad earth - check all earths!) then it may freak out. How's the crank sensor? Make sure somebody hasn't stuffed around and gotten the throttle switch connector on the IAC and vice versa. I think they're both 3-pin connectors.

  • edited June 16

    SOLVED 😊

    Okay, so swapped out a 951 ECU (out of a running car) into the problematic 740 (which had an old pink label 561 ECU) and she started up perfectly and revs normally. Idle is even higher and a bit erratic, but I'll put that down to the IAC code being valid and unrelated to the issue I was having. So I'll give that a clean next.

    Anyone have a spare ECU to suit LH2.4 (is a 90 not 89 as I mentioned above but same anyway, memory is rooted)? Also, do we have any good rebuild options for these ECU's here in OZ?

    Also, RE the Chinese AMM I've heard one contributor say the tolerances of the AMM aren't particularly important since the O2 sensor completes the feedback loop and corrects for any discrepancies? As per the ECU it's getting extraordinary difficult to find even rebuilt units, and then the price is about what you'd expect to pay for a new unit. As time goes on it seems there will reach a time that if you want to keep your car running then the huge cost of an aftermarket engine control system might be necessary.

    Now for the really unusual brake issue; no calipers seized, brakes work perfectly when car is cold. Let it idle for 20 minutes and the whole system locks up so the car will only move a couple of inches at a time. I suspect the car was quite neglected by the previous owners daughter who had been driving this car and is long overdue a fluid swap, but surely even the most water logged fluid couldn't swell like that with heat produced at idle under the bonnet? I'm suspecting the ABS unit has some sort of internal fault that shows up when it gets to normal under bonnet temps, any ideas?

  • carnut222Greg S @carnut222 Daylesford VIC

    Re: the brakes, could it be some sort of internal fault in the booster? Maybe try disconnecting booster vacuum line and see if that alters the situation (obviously if you drive it that way the pedal will be very hard, so be careful!)

  • carnut222Greg S @carnut222 Daylesford VIC

    PS: I have a couple spare ECUs...may be pink label but they are functioning fine. PM me if you're interested and we can negotiate a price.

  • I'll give that a try. Can a faulty booster apply hard Brake pressure to the system when it gets warm? Out of all the booster issues I've had that's never been something I've experienced.

  • carnut222Greg S @carnut222 Daylesford VIC

    Not sure - just a theory. Only if one side of the diaphragm is defective then maybe it could apply uncontrolled vacuum? Seems like pedal would drop though so that's probably an incorrect theory to try. Easy to test though! :)

  • bgpzfm142bgpzfm142 @bgpzfm142 Launceston TAS

    Anyone have a spare ECU to suit LH2.4 (is a not, not 89 as I mentioned above, memory is rooted)? Also, do we have any good rebuild options for these ECU's here in OZ?

    Pink label 561 ECUs are problematical. The hybrids fail.

    This is the bad hybrid. (thanks to cleanflametrap for the pics)

    This is the good hybrid.

    Repair options in OZ? Google shows a number of places in OZ which may repair them, phoning and speaking with them will be necessary. A pink label 561 I extracted from a parts car has a sticker attached to it to indicate it had been repaired in the past. It may be less expensive to replace the dud pink label 561 with a 951 and be done with it.

    As for the suspected ABS issue - is there any change when you pull the fuse for the ABS system?

  • edited June 16

    Ahhh, great idea RE fuse. I'll try that when I'm near the car again. I was thinking that somehow heat was causing some sort of fluid / valve lock up in the unit, but just speculation.

    RE the 951 units, just finding these is a bit of challenge. Have these units ever been known to fail (other than rare cases?).

    "Cool drive" say they "may" be able to repair. $550 with 12 months warranty.

  • carnut222Greg S @carnut222 Daylesford VIC

    FYI one of the 561s I have available has the repair sticker/anti-tamper sticker...if you want that one I can do that.

  • Thanks, PM'd

  • Regarding the 740 brake issue I have had the same issue on my '90 740 T.

    New fluid, cleaned the sensors on the wheels (not really related though wheels were off to check pads and callipers), new master cylinder, when it gets proper hot all 4 lock up. Though not always, has not happened since driving to Berwick last year and around town it is fine. A trip city to Daylesford return last weekend was fine.

    The booster seems the most likely problem as there is little left that will impact all four wheels.

    Please report back if you find the problem.

  • Could it be the ABS module?

  • There's actually nothing wrong with the engine or brakes you're just experiencing the raw 30kw power of a 740!

    I think @dmc is on the right track with the booster issue.

    Maybe the diaphragm has swollen due to some oil from excessive blow by or failed pcv?

    Abs module would be unlikely to hold the brakes on and you would hear the 12v pump running

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