Poor Idle is driving me crazy!

[dogfriend]

Originally Posted by vertex

Is a vacuum leak the only possibility, or is there something else? I can try the propane approach one more time.

No, I don't believe it is the only possibility, just the most common one.
You could also have a sensor reporting the wrong data to the ECU , a wiring issue (as Patrick suggested) or a problem with the ECU itself.

[jayman]

I should mention that I've used the propane method in the past, on OLD vehicles. Eg, carburetor vehicles with no emissions or O2 feedback. The propane method works extremely well in that scenario

[vertex]

I will be dropping the car off in the morning at a local shop that has a smoke system. At least we can definitively rule in or out the leak theory. I don't think it is a wiring issue. The wiring buzzed out OK. I also put my scope right at the ECU and monitored the voltage. It definitely exceeds the 2.2 volts allowed. This could be because the idle is too high. As I mentioned the other day, I tried trimming the MAF output by partially blocking the MAF tube. I got the idle to behave much better (I thought it was fixed) but when I drove it, the mixture was too lean, giving other DTCs. I trimmed the MAF blockage back a little at a time. The idle problem got worse, before the DTCs went away.
I'm suspicious of the computer. The ECU did suffer some damage from the accident, the 5 volt output for the MAF and TPS etc. was wiped. I but on an external 5V regulator to get it working. Its not easy to get another ECU and try it because it needs to be programmed to match the rest of the computers in the car. That is very expensive, and would be the last step.

[dogfriend]

Originally Posted by vertex

I'm suspicious of the computer. The ECU did suffer some damage from the accident, the 5 volt output for the MAF and TPS etc. was wiped. I but on an external 5V regulator to get it working.

Doh! You never said that before. Now the ECU goes to the top of the list IMO. But rule out the vacuum leak anyway.

[qbee42]

Originally Posted by dogfriend

Doh! You never said that before. Now the ECU goes to the top of the list IMO. But rule out the vacuum leak anyway.

I always enjoy when posters dole out information a bit at a time. It makes diagnosis much more fun, almost like a good crime mystery.

Tom

[jayman]

Originally Posted by vertex

I'm suspicious of the computer. The ECU did suffer some damage from the accident, the 5 volt output for the MAF and TPS etc. was wiped. I but on an external 5V regulator to get it working.

I need a drink .... and my liver is already the size of a basketball

[Frayadjacent]

Have you tried disconnecting the MAF? That's the easiest way to troubleshoot the device. When disconnected, the engine ECU should switch to a default fuel map, and should run well.

If that happens, the MAF is bad. If it still idles like arse, it's something else.

[Patrick Wong]

Originally Posted by dogfriend

Doh! You never said that before. Now the ECU goes to the top of the list IMO...

+1

The OP earlier asked how to get his car to pass emissions control testing, knowing he has an impaired engine ECU. If the ECU can't provide +5VDC, how can it control idle speed?

This is almost like asking how to make a car run without a functioning transmission.

[dogfriend]

Thinking about this a little more: if you have supplied an external 5v reference, how do you know that its referenced correctly in the ECU? You could have an offset and that would throw off any sensors (e.g. MAF) that depend upon an accurate 5v reference.

[qbee42]

Originally Posted by dogfriend

Thinking about this a little more: if you have supplied an external 5v reference, how do you know that its referenced correctly in the ECU? You could have an offset and that would throw off any sensors (e.g. MAF) that depend upon an accurate 5v reference.

But they were good batteries when he put them in. Maybe the dropping resistor was too large?

Tom