High idle and ECU replacement

[idle_man]

Hey there

I'm another Daewoo owner suffering from a high idle problem e.g. idling at 1.5K, 2K and sometimes even 3K. This time it's a Lanos SE 2002. Eventually the engine speed will reduce to near enough normal when at a standstill but not always. This is a serious problem since when this happens there is no engine braking so the stopping distance of the car is increased.

When I bought the car it was mostly fine except for two occasions when the high idle occurred but then disappeared at the next key on. However after taking it to a garage to get a full service and cam belt changed the problem became much more frequent.

I took the car back to the garage and they assured me that nothing involved in the servicing or cam belt replacement would have caused the high idle problem and seeing as it had happened a couple of times before I gave them the benefit of doubt. The garage said that there was a split in a hose and the IAC valve needed replacing. They even showed me plus I'd already read up on the problem and this sounded logical so I went ahead and had the IAC and hose replaced. After this was done the TPS low voltage DTC came on. I decided I could fix this myself so ordered a new one and replaced it myself. The DTC cleared and there are now no other DTCs either pending or active now but the idle problem still exists.

I contacted my nearest Daewoo dealership i.e. Chevrolet and they mentioned an ECU upgrade that resolves this known issue. However it turns out my vehicle's VIN is outside the range that the upgrade applies to suggesting that it's new enough to already have the upgrade included (November 2002). The garage have spent quite a lot of time on it even removing the entire throttle body and giving it a good clean but they are none the wiser. They tell me they suspect the ECU may need replacing which is something like £450 plus labour but they admitted that they cannot guarantee that this will fix the problem and if they order a new ECU I have to pay for it whether it fixes it or not.

How likely is it that the ECU does actually need replacing? I worked for five years at a company that designed and wrote ECU software (Pi Technology) and from my experience there's normally enough self diagnosis on an ECU to identify a problem with itself and therefore set the appropriate DTC (except of course for some sort of catastrophic failure in which I would have thought the car would not run at all).

How difficult would it be to replace the ECU myself if I were to find one? I'm sure there will be some issues with the immobiliser. Does anyone know exactly what is involved? Are there any other modules other than the immobiliser that may well have issues?

Does anyone have any ideas as to why a service would exacerbate the problem or is this really just a coincidence?

btw I've tried the idle learn procedure found elsewhere in this forum (key on 5 secs, off 10 secs etc), the fix involving disconnecting/reconnecting the IAC valve as well as reseting the ECU through removal of the battery. Any more help would be greatly appreciated.

[MMamdouh]

when i had high idle it was due to bad TPS... i know yours is new but it could be its wiring or so

are you sure that the IAC is not faulty??? i saw a friend's car with the rod moving inside and outside broken internally... the controller would look normal when it is off the TB but the rod was not actually connected to the motor thus their were no control from the ECU over the air flow from the idle port

i recon if it was an ECU issue it would have triggered a CEL... anyways replacing the ECU is a piece of cake... yours will be located either under the passenger seat or beside the blower fan found under the dash... remove the plastic panel at the right side and the ECU will be behind it

MMamdouh

[idle_man]

I'm pretty sure it's not the TPS. With a scan tool connected I've monitored the actual throttle position from the ECU and this changes correctly when I move the pedal up and down.

Also the IAC is brand new and I'm pretty sure it's fine. I guess it could be the wiring. I've tried moving the wiring loom about whilst it is idling high to see if it suddenly drops down thus indicating a bad connection but with out result. I think I probably need to do a more conclusive test than that though.

[MMamdouh]

the TPS motion could be normal but the value it sends to the ECU at idle can cause that

we had similar issue on the 2.0 nubira where at idle position the TPS gave higher than normal voltage to the ECU causing it to think that the gas pedal is partially pressed thus causing high idle

MMamdouh

[idle_man]

OK I see. Well with the foot off the pedal it was definitely reading 0%. However I don't remember if it was that whilst it was idling high. Definitely worth verifying.

[idle_man]

Have only just had the time to investigate further. The TPS is definitely working correctly and when the problem is occurring the actual throttle position is reading 0%. However I now have DTC P507 (Idle Speed RPM Higher Than Desired Idle Speed) set. Grrrr! Like it's telling me something I didn't already know.

[idle_man]

I take it back. The freeze frame data set by this DTC does give some useful information. The STFT and LTFT in bank 3 for fuel system 1 and 2 are both at -100.78%. Any ideas why this would occur? To me it suggests that not enough air is getting in. Or is this the cause of the high idle or just a side-affect of what ever is causing it?

[Trey05Woo]

You found your problem man... You have a vaccum leak aka extra inlet of intake air which is not monitored, your fuel trims are telling you so... Your next step would be to find the vaccum leak. On a Lanos very few things to check evap canister and hoses, intake manifold to head and egr feed gaskets.

If you take starting fluid, or carb cleaner and spray it around the suspected area it should affect idle either drop or or rise. usually the engine will stumble. How many miles are on the car?

[idle_man]

OK thanks for. I'll take a look with I've got some time again.
The car is currently just under 54K miles.

[Trey05Woo]

Did you perform the idle re-learn procedure? this must be done whenever the battery is disconnected, the ISC is replaced, a vaccum leak repaired, or the PCM replaced...

[idle_man]

I never did find a leak. Strange thing is the problem seems to have gone away. I'm thinking it could be temperature related now that it's got cooler as autumn sets in? Weird!

[Trey05Woo]

No your problem was the idle speed control all along. When you replace it and do not perform idle relearn the car will idle high, or start and stall until the pcm compensates. Still, after pcm compenstates, you will still suffer from surging at idle, higher than normal speed idle when cold, engine stalling when ac compressor starts, or turning the steering wheel full lock.

Perform the idle relearn and you should be good.

And keep an eye on your engine management none stable fuel trims signify a problem with air/fuel ratio. Dont be surprised if in the future you get a cel for 02 or something...