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Discussion Starter · #1 ·
I just changed my fuel pump and my hot start problem went away, so I though everything was great. Well yesterday another issue started again. The idle would start to swing up and down, until it finally got so extreme that it caused the engine to stall. Recently I noticed that when the AC clutch engaged, the engine had a dip in RPMs, so I thought I would test a theory. The next time it started to swing up and down, I shut off the AC. The idle returned to normal instantly. I appears that the cycling on and off of the AC compressor corresponded with a harmonic of the engine's ability to adjust for changes, that caused the idle swings to amplify instead of cancelling them. I put some refrigerant in the AC (it was low anyway) and now it even idles better at all times, AC on or not.

I have heard of numerous issues with LO3/5s having unstable idles. This may solve some of them. Especially those that continue to go up and down in a rhythmic manner.
 

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Your issue is because of E10 fuel. A bump in fuel pressure and a chip adjustment, and it will go away.

If you havent already, unplug your EST and set timing to 4* advanced. Reconnect ect, shut down engine and unplug negative acable to reset est code. Key on engine off, go under hood and open throttle all the way. After a few seconds, unplug the IAC. Release throttle and start engine. Set idle to lowest stable idle in gear. Shut down and reconnect IAC. Undo cable again. Reconnect and see how it feels.

Also look into a 3 wire o2 sensor upgrade from a 95 s10. Will help quite a bit for drivability and economy with this fuel too.

Chris
 

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Discussion Starter · #3 ·
I am currently running 13 psi, and have a custom chip in the computer. I am not sure if I want to set the timing to 4 degrees, because I do not know the basis for the change, unless the LT1 cam requires it. Aren't all of the sensor inputs fluid, and are learned over time by the computer?

When I reengaged the AC, it would start the idle cycle again, but other than that, the engine idles at about 550 in gear, and is smooth. After running for about 2 minutes the issue seems to go away except that with the AC on, the idle tends to have to settle out for a few seconds.

On a separate but maybe related issue, occasionally I get a rich code (45) at idle from a hot start, so I was going to reduce the fuel pressure a little.
 

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Check for a vacuum leak with a smoke machine first.
The IAC could be getting weak.
Fuel pressure won't effect the rich code. Pressure is well within spec unless the chip actually needs more fuel & it is running it too lean.
 

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Discussion Starter · #5 ·
It does the same thing with two different IACs.

I may check for vacuum leaks, but the engine runs very nicely with the AC off, and now is not acting up to any noticeable degree.

The injectors are higher flow than stock, so it is possible that the rich code is caused by the higher fuel pressure. If I turn the engine off, and restart it the code does not reoccur.

I recently replaced the TBI base with a rebuilt one, and it helped with the stability of the idle.
 

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My 92 had that problem forever. I tried replacing the IAC, vacuum leaks, MAP, o2, various other sensors. Never did get it to stop. It would do it whether the AC was on or not.

It would do it when the car was first started after sitting all night, once it warmed up it rarely did it for the rest of the day. Although on rare occasions while sitting at a stop it would start doing it briefly then go away on its own.

It only stalled maybe once or twice form it. Very weird. One day I just noticed that it had not done it in a long time and it did not do it ever again?!?! Maybe it was E10 related because I believe that I started buying fuel at a station that at the time sold regular gas.
 

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My 93 stumbled and occasionally stalled at low rpm's. It was the EGR on mine. Easy check is to pull the vacuum line off the EGR and stick a golf tee in the vacuum line. If the stumble goes away, replace the EGR.
AC will drop your RPM, which could make this problem seem worse, or you could have a completely separate issue related to some of the advice already posted.
Good luck.
 

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Discussion Starter · #9 ·
Your issue is because of E10 fuel. A bump in fuel pressure and a chip adjustment, and it will go away.

If you havent already, unplug your EST and set timing to 4* advanced. Reconnect ect, shut down engine and unplug negative acable to reset est code. Key on engine off, go under hood and open throttle all the way. After a few seconds, unplug the IAC. Release throttle and start engine. Set idle to lowest stable idle in gear. Shut down and reconnect IAC. Undo cable again. Reconnect and see how it feels.

Also look into a 3 wire o2 sensor upgrade from a 95 s10. Will help quite a bit for drivability and economy with this fuel too.

Chris
I may look into the three wire O2 sensor. I was contemplating what could cause the rapid fluctuation in idle, and a change in fuel would do that, because it also affects the slightly open throttle as well. It acts like it is not getting enough fuel, as well as the fluctuation. I believe the O2 sensor is the only thing that could make a quick change in output if it was not up to temp, or the mixture was off. That would affect the engine running most dramatically. I also checked my fuel pressure again, and it was about 14 psi with the new pump. I adjusted it down to about 13.5, but that does not seem to make a difference.

First, I may just try another O2 sensor first just to see what happens. I do not remember if I used the same one from before the engine swap, and the symptom was present before the swap.
 

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Discussion Starter · #11 ·
No scanner available.
 

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Discussion Starter · #12 · (Edited)
Your issue is because of E10 fuel. A bump in fuel pressure and a chip adjustment, and it will go away.

If you havent already, unplug your EST and set timing to 4* advanced. Reconnect ect, shut down engine and unplug negative acable to reset est code. Key on engine off, go under hood and open throttle all the way. After a few seconds, unplug the IAC. Release throttle and start engine. Set idle to lowest stable idle in gear. Shut down and reconnect IAC. Undo cable again. Reconnect and see how it feels.

Also look into a 3 wire o2 sensor upgrade from a 95 s10. Will help quite a bit for drivability and economy with this fuel too.

Chris
I put a new O2 sensor in the car yesterday, and it has not acted up since then. I stopped for an hour a couple of times today, and started and ran without issue. I guess the O2 sensor was the issue. I believe I used the same one that was on the old engine when I did the swap last winter, because the problem followed the car, even with the replacement engine. While I was under there, I found that my oil pressure sender was leaking profusely, so I replaced it as well. Now my oil pressure reads steady again. BTW, the fuel pressure is about 13.5-14 psi, with no ill effects.
 
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