Changing out the IAC valve did not solve my surging idle problem.
My car is in the shop. Today I stopped by the shop and spoke with the tech who had a scanner connected to the car. While we talked about what was happening with the idling and cooling problems, the idle starting surging when the temp reached approx. 240 degrees. The scanner showed that there was a 3400 rpm command being inititated when in fact the idle wasn't close to that. But the command caused the idle to surge and then go down and back up with no regularity. The tech figured something was outputting an erroneous signal causing the engine to try to compensate for a false increase in RPM.

Some other factoids that came out of our conversations included a high reading for the MAF. The tech mentioned that the MAF should operate between 4 and 7 (pulses?) but it was pretty consistently hanging at 7. He didn't say anything about needing to replace the MAF, but I have the feeling he was hinting strongly to that task.

Also he said that the engine temp was running too hot. I was there when the scanner indicated 240 degrees without the secondary fan coming on to cool down the engine. Earlier before the engine had started its surging idle episode, the fans would come on and off normally at the prescribed temps. The old radiator has lots of crap in it and will be replaced.

Does anyone out there have any thoughts or comments about this?

I'm all ears.

 

I have a bigger question - why the hell was your engine reading 240?

Fix that, I think everything else will fall into shape.

 

You will need to get the temps down a little. I don't know why the engine revved when the temp reached 240*. The MAF is measured in g/sec. At about 1000 rpm you MAF reading should be around 10 g/sec. Don't let the mechanic replace things that are already good. You can turn the A/C on to acutate the fans (if they are working properly). Is the SES light on?

 

I am leery of this mechanic anyway since he asked me what engine was in my car to which I replied the LT-1.
The SES light has never come on.
And I don't know why the engine is running at 240. I never got any indication of the engine overheating. This reading is coming from his scanner. A new water pump has been installed because of the work done to the opti-spark and a new radiator to replace the crusty old one was supposed to be installed yesterday afternoon.
Today I hope to liberate my car from the shop and finish chasing down the idling gremlin if there is one left.

 

...did you peek inside the car to look at your analog temp or digital gauge? When you get your car back, you can retrieve your error codes by shorting A & B terminal on your ALDL connector. But like Bogus said, need to get your temp. down. Hopefully new WP and rad. will fix this, also check your aux. fan fuse...Check your coolant level to make sure they "burped" your system properly.

 

The whole time I drove the car (before I put in the shop), the analog gage never read hot. It usually read around 185. A new water pump has been installed and a new radiator is to be installed this morning before I get the car back. The temp reading was coming from the tech's OBD II scanner, not the dash gages, analog or digital. In the future I will from time to time monitor my temps digitally. After I get my car back in my hands, I'll then get with my fellow club members with the Corvette Club of Texas and seek some of their sage advice and assistance--along with info I get from this forum. Thanks loads!!

 

Just let them install the radiator, for now. Get the car and drive it a little and watch the digital guage (more accurate than analog). If the temp ever starts to climb over 240*, it's time to shut it down. You will find after driving ,if it was just some freak thing or what. If you notice anything strange after you get it back, just let us know.

 

This morning I drove my Tiburon (the DD) to the dealer/shop and told them to stop work on the car, but go ahead and install the radiator.
This 240 reading came from the scanner, not the gages. I did not see the actual reading, the tech told me while he watched the readout. Plus he was not 100% confident in the scanner readout he was getting. I assume he was using an OBD II scanner on a OBD II non compliant system. Once I get my car back then I can do some of my own diagnostics based upon this advice.

 

I think you allready know you need a new Tech. Complete scan data would be nice. He's telling you that the Engine Coolant Temp Sensor is indicating 240 degrees - ok - now he needs to verify the accuracy of that signal (which usually starts with a cold engine); whether or not both fans are on and coming on within specs; IAC counts, BLM's, a/c signal and whole bunch of other stuff to help figure it out (or just aiming a hairdryer at the ECM with the engine cold can tell you alot about a heat related problem). You can use the troublecode charts (even without a troublecode) for this stuff, depending on what the diagnostics tell him or you - doesn't seem that the tech knows how to use them. As planned, put in the new radiator and go somewhere else or get your own scan and post the data. Good shops use capture and graphing data and know how to work with the info.

 

I'm part way there now. Since I own two cars, both of which use OBD II, I purchased an Actron super scanner today. First thing I'll do is do a scan of the Vette's codes and see what I have, then share them on this forum.
I doubt that the car will be ready today, but I will have it on Monday.
And you know I need another tech, one that knows what he's doing. That may end up being me with assistance. And that's cool with me.

 

Ok - when you get it hooked up to your Scanner, here's some things to think about. All the ECM can do is add or subtract fuel. It can't make air or spark, but it can tell it when to spark and it can let in more air through the IAC valve.

Since your problem is heat related, I would aim a hairdryer at the ECM with the engine cold. If there's a weak solder joint or cracked board, this will often duplicate the problem on a cold engine. I somewhat doubt it's an Engine Coolant Temp Sensor Problem. If there was high resistance in the wiring, the Temp would be lower than what's reported and the fans may not come on at all. More likely, if there's a problem in that area, the fans are always on - no resistance - or the ECM isn't reacting to it's signal - overheating, but no idle problems per se.

Once the engine is hot or in closed loop, the ECM commands a couple of things: 1. EGR, but the throttle (TPS) should be above idle, so with it idling, make sure it isn't suddenly commanding EGR. If so, that will kill the mixture, and the MAP sensor voltage will drop, which tells the ECM to add some fuel so that it can maintain the idle speed, so take a look too, at what that MAP voltage is. 2. Air Pump should turn off. If it stays on, the ECM reads the extra air flowing pass the Headers (O2's) as a lean condition and then dumps a bunch of fuel to make up for the perceived shortfall. Make sure it's commanded off or just clamp off the hoses to the headers and if the problem goes away, you'll need to pull the check valves to make sure they're not broken. 3. A/c signal - when a/c is requested, the ECM let's some more air in through the IAC and the MAP sensor pumps up the fuel to match the load of the compressor. Make sure it doesn't suddenly have an a/c signal with the a/c off when the idle starts to surge. Make sure that when you turn the a/c on it reads yes and that the idle counts go up. 4. Fuel Evaporative Controls - should suck the fumes out of the tank, but again the TPS voltage should be above idle voltage - look at both numbers when it's surging. 5. MAF - usually reads too much or too little air when bad and that will show up as a lean or rich condition which usually makes it stall. You don't have that, but I'm pretty sure you can simply disconnect it and if the problem goes away, it's in the MAF or it's circuitry or the ECM - You'll need the troubletree chart to troubleshoot the circuit or someone can point you to what wire does what (I don't have the schematic for your Year).

Someone else needs to jump in and point out some tips for troubleshooting the Opti on this Year. A bum module or pickup coil will give you these symptoms but I think an OB2 would spit out some type of misfire code if there was an ignition problem. I'm somewhat surprised too, that there isn't an IAC overrange code given the high idle. Maybe there is?

 

FYI. The IAC valve has been replaced. I provided it to the tech thinking it may be the problem. It wasn't. Also, the ignition module and other parts were replaced in the opti spark as part of the tune up I asked for.
Here's something else that happens. When I'm at a light, tranny in drive and the idle starts surging and carryin on, I can put the tranny in neutral and the idle will go up and then settle down to a more normal RPM. I told this to the tech, but I don't think it sunk in.
Thanks for the info. I'll be using it ASAP.

 

Drive part is interesting - Look at the Park/Neutral and Gear Selection on your scan. If it loses that signal, you can get the surge. I don't know where the postion switch is on your Year - internal/external - but something is there to tell the ECM what gear it's in. There should also be trouble codes for the tranny.

 

I sure hope you get an answer because I have the same idle issue on my 94.

When you say your idle is "surging" do you mean like a "blip", as in, I'll be at a light and the idle will go from 700 to 1200-1300 just for a spilt-second and makes the car creep forward a little This only happens when the temp has reached in the 220 range. Actually it only happens on a return trip from going somewhere, like the heat soak has finally set it. I've never had it happen when I first take the car out to go somewhere, even when it get hot. Let it sit for an hour though, or even a couple hours, and the return trip is guaranteed to have a "blip" at a stoplight. I thought it only happened while in drive on mine, but once I put it in neutral at a light and it still did it. Only that one time though.

I'll be watching for any answers you get, good luck.

 

You pretty well described the same problem I'm having. Putting the tranny in neutral doesn't really solve the idle spiking/surging, but it sure does keep the car from wanting to launch like John Force's funny car. LOL...Monday I'll get my car back from the perplexed Ford tech (he's in better condition than the Chevy baboons in town) and then tackle the problem myself with a little help from my Corvette friends.

 

With some of the recent talk about ICM's going bad, I thought I'd give mine a good cleaning a few weekends ago. Took me awhile to find it because I was looking for the heat sink fins...man mine were so covered with grime they were almost filled in flat. I thought, yes, this has to be the problem. Cleaned it up, put some new heat sink compound on and put a few washers in between the block and the module. First few times I drove it, no problem. But, as always, on one of my "return" trips, after heat soak, at about 220, got the idle "blip".

Is it possible the actual module is a little fried and needs to be replaced? I'm running out of ideas.

 

The ignition control module in my car was replaced by the shop as part of the "major' tune up and diagnostic enema.
The radiator is being replaced. The water pump has already been replaced. Earlier in this thread I was told that replacing the radiator and water pump should resolve the "excessive" heating situation and the idle problem too. I don't think that the tech has made that connection yet although he believes that a new radiator should fix the heating situation. Keep in mind that I have not had a SES light or other indication that the engine coolant was too high, i.e., no idiot lights have come on.
As soon as I can I'll hook up my new super autoscanner to my car's DCL connector and see what the gremlins are saying. But first I'll take my car out for a 30 mile run and see if I get the idle problem we are talking about.
The plot thickens.......

 

Maybe it's just TOO damn hot in North Texas

 

BlueMonster, you just need to drive it and see how things work out. Be aware of the engine temp and don't let it get to hot. See if the idle does anything strange while your out. You can always turn the key off or put it in neutral, if it gets an uncontrollable revv. Also use the scanner that you have just to see. I would disconnect the battery and reconnect then scan during the first few minutes and then again after you go for the ride (battery still connected).

 

....just hit 101 F in Austin....and being that I grew up in NY.....Faggetta'bout'it!!