Fixes for fluctuating idle

[Red94Chev]

Seems like this is a common problem. Thought I'd share a few tidbits on correcting this in the .bin (chip) after engine modifications.

If you have a stock engine, things to check would include varying battery voltage at idle, low fuel pressure, dirty/faulty IAC valve, EGR malfunction, and vacuum leaks. Make sure to check for ECM/PCM codes. This may help to narrow down the problem. Faulty EGR valves seem to be a common problem in a stock truck.

Okay, assuming your engine is modified, in good shape, and you've ruled out the above conditions, here are some other suggestions. If at all possible, try to datalog when the condition is occurring. You can then see if stall saver is active, MAP values, battery voltages, spark advance, etc.

1. Stall saver settings: You may also need to lower the stall saver thresholds. Often missed. The target idle is controlled by the PCM, not mechanically at the TB. The idle stop screw is not adjustable, but can be made adjustable by removing the plug. At temp and in "Park", set the screw so your IAC steps with your scantool are between 10-15. You may need to adjust the TPS after this so that voltage output at idle is 0.53-0.58V. TPS% is self-adjusting, just cycle the ignition key on and off waiting a few second in between. Your scantool will report TPS% now at 0% again. Reducing IAC steps allows for maximum adjustability and lessens controlled "air leak" that can cause idle problems in all driving conditions (cold/hot weather, warming up, etc.). Make sure to remove the throttle cable slack from the throttle linkage adjustment screw after doing this procedure being careful not to put tension on the linkage.

2. VE/Main Spark tables: Find out the MAP value at idle at normal operating temp. If you've replaced your camshaft, your engine may have a higher MAP value (less engine vacuum). For a stock TBI engine, idle MAP is usually between 30-40 kPa. With a larger camshaft, you may now be idling at 50-60 kPa. You will need to smooth/flatten out the cells in the VE table AND main spark table around where your engine idles.

3. Coolant spark compension table: If your idle varies only when the engine is cold or after a few minutes after engine start-up, you may need to smooth/flatten the cool/coolant spark compensation vs load/vac table. The table adds timing during cold engine operation only. If your modified engine has less vacuum at idle, this table is often out-of-spec and contributes to the out-of-spec main spark table for a double-whammy.

4. Varying battery voltage: Usually not a problem but if your alternator is subpar or if you've installed underdrive pulleys, you may have low voltage at idle that can cause a whole host of problems. Think about re-installing your stock pulleys if this is happening to you. Underdrive pulleys don't contribute that much to power anyway. Install an electric fan instead and you'll remove the biggest resistance factor on the accessory drive system.

5. Check commanded idle RPM: This is an area I don't completely understand yet as I'm still interpreting the $0D hack. There are a number of idle sub-routines. Some you have access to using the TunerPro .xdf. Some not. Try and keep the idle settings as stock as possible as this may mess up the idle routine causing the fluctuating idle. It is also the reason why when you lightly tap the gas pedal, the problem disappears for a few moments and then returns. The light tapping of the gas pedal allows exiting of the idle sub-routines and then it is initiated a few seconds later after TPS% is at 0%.

Hope this helps. I had this problem and it has now disappeared using the techniques I outlined above. Been one of the most frustrating issues to solve for me. I'm sure others share my frustration. It was very annoying!!

My only other advice is make one change at a time and re-test.

[92SierraTBI]

and dont forget a simple thing as checking the fuel filter,as I found it to be my culprit!

[FastChev]

thanks for posting this information, i am going to see if i can't get rid of my flucating idle with these methods!

[Tbi-MAX]

great thread... so the 02 thresholds has nothing to do with this? mine idles find in OL.. but someday, i can here it jump maybe 50 rpm.. but all that seems to be history runing @ 25psi of fuel now. you hit the nail on the head with the idle timing.