This is NOT a rough idle problem .....Engine has a new IAC valve, ICM and coil and PCV valve and hoses. Opti was replaced about 30K miles ago, and that was 10 yrs ago.... Idle is fine up until about 20 minutes after starting and engine has warmed up... Then the idle starts to step between 500 and about 1000 rpm, but runs smooth, doesn't misfire.... If in Drive, it will try to surge to 800 or 1000 rpm and you have to keep good pressure on the braked to stay put !
Finally, when these conditions are present it will sometimes stall when taking off from a stop, not always ...... but starts back up. When these low rpm problems are present, if you get on the highway and cruise at 60 to 70 mph,, runs fine ......
Any ideas ? Could a problem with the EGR system cause something like this...
Finally, I did take a short trip in it the other day, long enough to get it nice and warmed up.... Amazingly, it ran fine ... no idle speed up and slow down,,, even after I let it sit and idle for about 10 minutes. I did have the check engine light come on .... I checked for codes .. I now have a 43 in Module 4 ....... Knock sensor circuit ?? But the idling probelm has temporarily gone away ..... Could a bad Knock sensor cause idle speed to cycling between 500 and 1200 rpm or thereabouts ?? Where are these mounted ?? And how difficult is the replacement procedure ??
Thanks ............

 

I highly doubt the knock sensor issue is causing the crazy idle. If the car is running better you may want to try resetting the ECM by disconnecting the battery for a few minutes, then reconnect. See if the check engine light stays away and the car runs well.

There are two knock sensors on the LT1, accessed from under the car on both side of the engine close to center. They have shields that need to be removed to access them. They are sensors used to correct detonation during acceleration. Basically detonation occurs when the air/fuel ratio in a cylinder does not occur correctly during ignition spark and a second explosion is taking place when it shouldn't. It's called knocking, engine ping or spark knock.

Have the symptoms on your car gone away for good or is it still having idle problems?

 

Never heard of code 93 in module 4. I believe they only go up to 72 for the basic LT1 car and engine.

Start with the basics. Check fuel pressure to see if it is correct. May have to drive with the gauge connected and visible on windshield to verify if there is a problem or not.

Look for a vacuum leak and cracked hose. Also check the 3 way check valve under the right side plastic cover on engine.

Inspect and clean throttle body and speed density wires.

I would not completely take the opti off the list and consider the ECM but only go to these places when exhausted all other primary troubleshooting procedures.

 

I assumed that the 93 was a typo and the OP intended 43. Once the title thread is established and "submit" button clicked the title becomes unable to be edited by the original poster. A mod can fix it if asked.

 

Makes sense, 43 is knock sensor and what he was talking about.

 

Code is 43,,, not 93,, my bad.....
These intermittent problems are the tough ones to diagnose, as we all know... What is throwing me here is that it has always been fine until it warms up, then the idling will go high, low, high, etc.... sometimes.. That's why I was thinking EGR, since I believe it does not get vacuum until after the system goes closed loop .... Right ??
Or maybe I have one or more O2 sensors that are on the edge ???
Comments ???

 

Yes, the LT1's do have 2 knock sensors. One is located on the side of the block just forward of the oil filter and is behind a small heat shield. The passenger side is under the exhaust manifold and also behind a heat shield. Be careful removing them because they are in the water jacket and they'll give you a bath if you're not real quick or the coolant is drained. As a matter of fact the factory procedure for draining the block says to remove these.

Remove the wire from each and with the key on you should have about 5 volts on each wire. If you do then the ECM and wiring are probably OK. Turn the key off. Next check the resistance of each sensor. Check from the terminal that the wire plugs onto to ground. I've always found these to be somewhere around 3000-5000 ohms. Your meter may say 3K or 5K indicating thousand ohms. If either one is way out of spec you'll need a new one and the part number for your vehicle will be AC Delco part# 213-96. Most parts stores carry this and they're about 30 bucks or so. Clear your code and you should be good to go.

The knock sensor can change your timing over 20 degrees so it can definitely cause the problems you're experiencing. The reason it doesn't show up for 20 minutes is that the knock sensors and some other sensors aren't "read" during open loop mode. At least I think so. Open loop is when the engine's cold. OH, BTW, if everything checks out OK make sure to listen for an actual knocking condition. I chase a knock that was caused by a loose starter for a few hours a while back. :laughing::laughing:

If that's not your problem feel free to holler back at us. We've only just begun.

 

1963SS,, Thanks for that information.. It is very helpful. I will try to under there and check the voltage / resistance... but I think I am first going to replace the EGR valve ... It looks original and may have a flaky diaphragm..... plus it's not too hard to get at ......

 

Cool. Just letting you know that the EGR does nothing when the engine is cold and the pintle on the sealing end of it is held closed by a spring. Cabon build up on that pintle is sometimes a problem and can usually be cleaned off with some carb cleaner and a green scotch-brite. Find a steady vacuum source and hook it up to the EGR valve. You should be able to see the shaft inside moving back and forth as the vacuum is applied. If it's moving then the diaphragm is probably good.

Your original post was very good and you said after 20 minutes ot so all is good. That sure sounds like an open loop problem and the EGR isn't involved. But..........stranger things have happened.

 

1963SS,, Actually..... maybe I wasn't clear in my write-up.... The idle is fine when the engine is cold and warming up.... it starts to cycle up and down in rpm after it is warmed up.... That's why one of the possible causes on my list was the EGR valve..

 

That is the code I am getting..... based on the Service Manual there is an issue with the knock sensor circuit. Have not had time to get under there yet,, but will soon.... Has anyone ever had to change these out ? Are they hard to remove..... anything else have to come off to get at them ??