Help! Idle issue

[MARCUS VETTE]

Hi,

I have a small problem with my idle. When I start my Corvette, the idle is OK, running at 7 RPM and very well but after 2 minutes (2:06:00) the idle starts fluctuating; it speeds up and slows down repeatedly. I can’t identify the cause? So far, I suspect a bad sensor or a bad signal. I know it has something to do with the mixture of air and fuel. For this reason, I eliminated a vacuum leak as a possible cause. When I drive, the engine runs well, the RPM is regular and works great. The problem is only at idle. What else can I check? Which sensor could affect the ratio of the mixture (air and fuel)? I have 2 theories, first it has something to do with the cold start. Second it could be related to the intake tempera sensor in the plenum. I suspect the issue is related with temperature because 2 minutes after the engine is started, it starts running badly. If I wait a few minutes and I restart the engine, the idle will start fluctuating immediately. If I wait a few hours to restart the engine, then the idle is ok for 2 minutes and the issue reappears. :ugh

Thank in advance.

Marc

 

[Hib Halverson]

I'm not sure how you eliminated a vacuum leak as a possible fault but one of the most common causes of idle instability of a stock engine is a vacuum leak but there are a variety of other causes. Finding the fix will be much easier if you have a Factory Service Manual and a scan tester.

The reason the engine idles smoothly after a cold or cool start is right after you start the engine, the system runs in open loop. Once the ECT comes up a bit, it goes closed loop, it can't control idle speed hence the surging.

 

[MARCUS VETTE]

I'm not sure how you eliminated a vacuum leak as a possible fault but one of the most common causes of idle instability of a stock engine is a vacuum leak but there are a variety of other causes. Finding the fix will be much easier if you have a Factory Service Manual and a scan tester.

The reason the engine idles smoothly after a cold or cool start is right after you start the engine, the system runs in open loop. Once the ECT comes up a bit, it goes closed loop, it can't control idle speed hence the surging.

Thanks,

For the vacuum leak, I checked with a propane torch, I checked all vacuum lines, I checked aroundthe plenum and I saw nothing. It couldbe a vacuum leak, but I can’t find it!!!! I tried really hard without results and that’s why I’m looking for somethingelse.

 

[Kevo]

How does car run other than at idle? SES light ? Access to scanner? Anything repairs prior to this happenning?

 

[gmjunkie]

Thanks,

For the vacuum leak, I checked with a propane torch, I checked all vacuum lines, I checked aroundthe plenum and I saw nothing. It couldbe a vacuum leak, but I can’t find it!!!! I tried really hard without results and that’s why I’m looking for somethingelse.

Oh,There are allot of other vacuum lines than that! AC has them inside.Cruse Control has them inside and out,Purge canister has them,Vacuum Brake Booster Diaphragm could have a small leak, And it don't take a very big leak!~~!!!

 

[MARCUS VETTE]

How does car run other than at idle? SES light ? Access to scanner? Anything repairs prior to this happenning?

Hi Kevo,

The only thing I have, SES it a code 36! For the, the engine run well, expect the idle, it pretty good. On the Highway, the engine keeps the RPM perfectly, no up and down.

Thanks Marc

 

[MARCUS VETTE]

Oh,There are allot of other vacuum lines than that! AC has them inside.Cruse Control has them inside and out,Purge canister has them,Vacuum Brake Booster Diaphragm could have a small leak, And it don't take a very big leak!~~!!!

Hi Gmjunkie,

Do you have a trick to find a vacuum leak? I try with propane, but can we check with something else? Can we isolate or separate a vacuum line. Example, can we isolate the line from the plenum to the A/C and make sure this line is OK. I think for the plenum, I have to go with propane?

Thanks Marc

 

[Kevo]

Dtc 36 relates to MAF sensor burnoff. Sensor is supposed to clean itself when engine is turned off. Relays are a common cause of MAF sensor troubles. Also look at condition of "foil" in sensor sometimes spray contact cleaner can take the soot off. MAF can cause you troubles, check wiring and relays first however.

 

[KOPBET]

Hi Gmjunkie,

Do you have a trick to find a vacuum leak?
Thanks Marc

IMO, get a hand vacuum pump (gmjunkie always keep a couple on hand I hear...otfl) so you don't waste your propane or your gas. Start at the source and work your way back while segmenting backwards as they come. Keep looking for a line that won't hold vacuum and follow it back until you find the problem. I've had issues including the vacuum controller for the HVAC, plastic cruise control line that goes thru the firewall, and the little plastic check valve gizmo under the passenger side fuel rail. Of course, it could be anywhere. So just start looking.

 

[MARCUS VETTE]

IMO, get a hand vacuum pump (gmjunkie always keep a couple on hand I hear...otfl) so you don't waste your propane or your gas. Start at the source and work your way back while segmenting backwards as they come. Keep looking for a line that won't hold vacuum and follow it back until you find the problem. I've had issues including the vacuum controller for the HVAC, plastic cruise control line that goes thru the firewall, and the little plastic check valve gizmo under the passenger side fuel rail. Of course, it could be anywhere. So just start looking.

Hi,
I found a leak and now the engine runs better. There’s still some up and down idle, but now stable idle stretches are longer. I probably have to reset or adjust my minimum idle. The idle runs around 5 and 6. It’s a little bit low and sometime goes up to 7 for short periods. I suspect it’s due to a very small vacuum leak.

 

[92VetteNY]

Idle Issue

Hey can I jump in. My 92 also has an Idle Issue. When in neutral it idles at about 580. When I put it in drive drops to about 500 feels like it want's to stall at a stop. Gets the wife nervous. Runs great on the road and runs great when I nail it. I was told you can't adjust the idle because the computer takes over and will reset any adjustment you make. I was also told it may be the gas, not enough octane. I am running 93 octane. I have gone through a couple of tank fills and still have same problem.
Any suggestions?

Thanks

 

[hcbph]

Vacuum leak

I found I had a vacuum leak on mine that was tracked back to the cruise control. There's a line that goes between the battery and a brace that was cracked. Fixed the line and it's run much better since.

 

[Kevo]

Low idle make sure throttle body is clean of carbon build up. Check for vacuum leak. Base idle is adjustable in a sense. You could check min air rate and tps if they're out of spec could cause your problem

 

[Bwil64]

Hey Marcus, I had that problem and I changed the front throttle body gasket and it runs great. I also change fuel and went with Shell gas and sea foam in the tank.


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[WhalePirot]

Unmetered air drives the ECM nuts, most noticeable at warmed-up idle (see Hib's comment).

An electronics troubleshooting principle, called the half-split rule, can apply here. It means splitting the circuit in half by individually disconnecting each vacuum tap, plugging that tap and looking for improvement. Once the leaky vacuum circuit is found, it alone can be further scrutinized for leak(s).

With so many electrically controlled, vacuum activated devices on modern cars, good connections and good hoses/sensors/diaphrams, etc. are crucial to proper, smooth and safe operation. Each of those is subject to age deterioration, especially with the high underhood temperatures of these cars and the cleanliness most of us maintain (read: corrosion from water and other chemicals for that nice engine look).

ECM codes are nice, but the basics of engine tuning still apply; the things the ECM cannot 'see'.

 

[Robertwav1]

I didn't see the mention of the IAC motor or cleaning of the valve?