Erratic high idle in gear

[95SleeperAcheiva]

I noticed this, it's more of an annoyance then anything.

After warm up and driving down a road, normally, I come to a stop light, idle holds around 1200 in gear. About 12-15 seconds go by before she drops to around 750 where she belongs.

In park, I goose her to 2500 and she comes to and holds the 1200 for 5-10 seconds then idles down.

Slow IAC? She runs fine otherwise.

oh '94 LQ1 cutlass

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[pocket-rocket]

As long as my car is moving the RPMs are held around 1200. It's probably normal, especially if the ECM is commanding it, although that 5-10 seconds doesn't sound right. I would clean the IAC and passage to rule that out.

[brian89gp]

My old car did the same thing, though I don't remember the delay being that big.

[Denis.V]

Put oil in the acelerator linckage system. Then, try again & compare.


Denis V.

[TGP37]

Put oil in the acelerator linckage system. Then, try again & compare.


Denis V.

Well, the engine is getting too much air somewhere if the idle is far too high. Determine if the PCM is commanding this high idle or if it is the result of component failure. I believe the PCM will throw a code if the idle speed is too far from commanded.

Did you get the car scanned? Some codes can be thrown but they do not illuminate the check engine light.

Check for any codes thrown.

Unplug and remove the IAC and inspect the plunger.

[95SleeperAcheiva]

Took the TB apart, cleaned everything, lubed it, replaced IAC. Cleaned MAF ever so gently. Checked for vacuum leaks, nothing.

Road test, same problem. Gave up.

2 hours later, went to store, came out of store, went to hit key, whole car went dead, no power. hmmmm.

checked battery connection, just touched the ground and bingo all power came back. Got home, ohmed cable. Now feeling stupid. Replaced ground cable, no problems, has even quick return idle. Guessing power flux was freaking out comp.

[TGP37]

Took the TB apart, cleaned everything, lubed it, replaced IAC. Cleaned MAF ever so gently. Checked for vacuum leaks, nothing.

Road test, same problem. Gave up.

2 hours later, went to store, came out of store, went to hit key, whole car went dead, no power. hmmmm.

checked battery connection, just touched the ground and bingo all power came back. Got home, ohmed cable. Now feeling stupid. Replaced ground cable, no problems, has even quick return idle. Guessing power flux was freaking out comp.

Glad you found the problem. Fuel injectors have a skewing table for under voltage scenarios as they open/close slower. And my guess was wrong, oh well.

I had a bad ground once. It heats up and as it does the voltage is reduced until it cooled. It was a hidden break in the line. It tested fine normally, ran fine at first but after 5-10 minutes......it stutters and dies. Would not restart until the cable cooled off. I was given a big clue when it occurred way faster on a cold snowy night (lights, heater, wipers, etc)

Changed the ground, trouble gone just like that. Now I have a multi point ground system...

I just don't understand why the alternator didn't prevent issues while running...It was good at the time.

[pocket-rocket]

I just don't understand why the alternator didn't prevent issues while running...It was good at the time.

Alternators provide "dirty" power. It isn't clean and buffered after the rectifiers so the power goes up and down very fast. The battery acts like a buffer to filter the crap out.

Think of a big boom boom stereo before and after installing one of those giant capacitors. The battery does along the same lines except a capacitor stores electricity where as a battery produces it (but can still filter like a capacitor).

[TGP37]

Alternators provide "dirty" power. It isn't clean and buffered after the rectifiers so the power goes up and down very fast. The battery acts like a buffer to filter the crap out.

Think of a big boom boom stereo before and after installing one of those giant capacitors. The battery does along the same lines except a capacitor stores electricity where as a battery produces it (but can still filter like a capacitor).

That makes perfect sense being the batteries potential is primarily a chemical reaction. So the flux of power is absorbed by the chemical nature of the battery.

But wouldn't it be more cost effective to increase the number of bridge rectifiers in an alternator thus preventing the battery from having to buffer in such a way?

Being as sensitive our fuel injection engines have become and the demand for more efficiency, one would think producing a smoother voltage signal would be the first place to start. My HO 220A Alt has 12 diodes in it, instead of 6...if that is the same thing or not?


btw, I've read about new technologies in capacitance that are beginning to surpass battery function. Cheaper, more light weight and can hold far more a charge.

[pocket-rocket]

You would have to have a multiphase alternator to work that way. Our alternators are no where near that complicated in design, and I doubt there are any multiphase wired ones on the market. You wouldn't need one when you have a good positive and negative wire set going to your battery

Your HO alternator has to have twice the diodes in it because it puts out twice the power. Each diode is rated for a certain amperage. Uprating the alternator and modifying it to put out twice the power requires twice the power capability by the diodes, and the easiest way to get that is put twice the amount of diodes in it.