Hi Guys/Gals:

I just bought a beautiful blue 89 IROC coupe with the 350 vette motor and 4 spd auto tranny. It runs great but I am having a problem when the car idles. When the car is warm and has been running for an hour or so, the car idles at 500 rpm and many times, it will die if I don't give it a little gas. The car sat for about a month in SE PA before I bought it. I put a fresh tank of 93 octane gas, STP injector cleaner, and STP water remover in when I filled up the tank. What could be wrong? I was thinking that it had something to do with the timing or the low speed idle screw or something like that? Could any of you please help me with my problem because winter is approaching fast and I would love to drive my new "beast." Thanks.

Steve

 

First things first. If that is the stock engine, it is NOT a Vette engine.
Second thing. Your low-idle speed might very well just be an adjustment on the throttle body. (I am not too sure)

 

Everything is stock on the car. I was told by the previous owner that this was the same motor that was found in the Vette. Where on the throttle body would I find the adjustment?

 

There is a place to set minimum idle setting. Check out www.fl-thirdgen.org and scroll down to their tech article section. How to adjust idle and TPS.

Your car is a L98, but is not a corvette motor if stock. You car has cast iron heads. The corvette had aluminum heads.

 

Take the TB and clean it out. Clean out the IAC motor and the IAC housing. Reassemble and try get the TPS about right. Run the motor till it is in closed loop and use a scanner to see what the IAC counts are. Adjust the set screw till the counts are about 10-20 counts. If it is more than 20, turn set screw clockwise. If it is less than 10, turn it counterclockwise. When you are done, set the TPS to about 0.54V

 

Some of the instructions on the web do not mention disconnecting the set timing connector. Timing does affect idle speed.

Aklim,
Correct me if I'm wrong, doesn't "minimum air" refer only to the air getting past the throttle blades? The instructions say to disconnect the IAC after the pintle has completely blocked the air bypass so that the only path the air can take is through the Throttle body.
You seem to be setting idle speed with some amount of bypass air, please explain why this method works, and if you're not disconnecting the set timing connector, or putting the ECM in diagnostic mode, again why?
If this works better why don't we make it a sticky, or a tech article?

 

Idle speed is set by the EPROM. You cannot change that without reburning. Timing affects idle so assuming everything is right on, you want to have a certain amount of adjustment for the ECM. Hence the 10-20 counts. If it has 0 counts, it means that it is either right on or it is not opening at all (vacuum leak possible). Say 100 counts means it is opening a lot. Something else you probably don't want since when it is cold, it will try open even more. This is why I said to do it when it is hot and in closed loop. At that point, the ECM will try keep the programmed warm idle speed which is the lowest speed. So, if it is at say 10 counts, it can close up a little more to lower the speed if for whatever reason it needs to. Also, it can speed it up for the coldest of days if it needs to by opening it up more.

I have used the other method too. Just not so fond of it. Also, I remember reading in a GM tech manual about how if everything was right, the IAC should be about 20 counts. That and my conversations with other tuners who also use the method i described has led me to believe it is the easier method.

 

It's not the EXACT same engine (slightly different heads), but its more than close enough. Remember, the early 85-86.5 Vettes had the same heads. I know the difference, and still usually refer to the L98, LT1, and LS1 found in F-bodies as a "Vette motor" to those who aren't as familiar with these cars as I am.

 

The previous owner was a retard I'd suggest you don't repeat anything he says, to save face... ya know?

As for the idle, You should probably check the IAC or just replace the damn thing. Its the module with the plug that hangs off to the passenger side under the throttle body, 4 prongs set up in a square configuration. While you're at it, I'd say do the TPS too. Thats the one on the passenger side of the throttle body with the plug pointing forward. 3 prongs, in a straight line. If that fails you'll need to adjust the idle manually.

get back with us

 

Even if you do replace the IAC, you need a IAC housing gasket so you can take the housing off and clean it and the passages.

 

I have a 90 corvette motor and 4spd that came in my car, and i got a deal on it so i didn't really look into much on it...it was running a bit rough when i got it, but it was sittin for a good year, so i figured the injectors are just a bit dirty...no big deal...so i got some money, took it too a mechanic, he said there was still the 5.0 computer in there that came stock, and there was NOT, get this, any type of incoming air sensor....no maf/speed density, no nothing...there was some wires, but they were just snipped and tucked away....so i got a new vette computer, and the mechanic wired up a MAF sensor...the car still runs rough, an idles between 500-1100....just goes up, then dips back down...its just like yours, its fine when its cold, but as soon as it gets hot, thats when the idle goes spratic....i'm gonna try some fuel injector cleaner, and if that doesn't work, that TPI is going on ebay and i'm getting a carb...

 

my freinds jeep was having a bad idle problem and when he brought it into the shop we found out it had a bad TPS, make sure its getting .45V at closed and close to 5V at WOT. if you can look at it on a scope and make sure it doesn't have any hash on the waveform, if not then a DMM is better than nothing. i'm not sure if our cars can monitor systems like IAC through a scanner but if you can look at one on it it'll tell you how much (in percent) its open. if the solenoid isn't opening it up enough or its sending the wrong signal then you won't get enough air into the cylinder to burn all the fuel. this will make your ECM read rich and lean out the cylinder till it dies. i'd check the TPS first and then look at the IAC second.

 

I doubt that will work well. You need a SD computer and the correct harness. The 91 Vette (therefore I suspect the 90 Vette) has 4 plugs to it. How did you get it mounted? I have a 91 Vette and I see that the computer is totally different from my 91 Firebird unit which I will gladly sell you. Are you talking of the EPROM? they are different.

 

You can monitor the IAC counts thru a scanner. For the TPS, you will need a scanner or a voltmeter. You need to see what the voltage is at startup and then slowly depress the pedeal with the engine off and see if there are any dead spots in the voltage.

 

I got the idle speed problem solved, sort of. I had it the second day and took it out one night to the grocery store with my dad. Well anyway, we pulled out of the shopping center and as I was driving down the road at like 30-35mph, a stupid 16yr old kid driving a POS Dodge Dynasty (well it DIED nasty) made a left in front of me just after another car did and I gave him a good wack, screwing up his right front axle and gave him and a passenger a good bump. My dad and I were fine and if it hadn't been for my quick response and the Camaro's excellent handling and braking, it would have been a lot worse. The frame is intact and I didn't even puncture the radiator (methinks). Well anyway, the front of the car is alright. I am going to need new front quarter panels, new hood, new windshield, new top dash panel and some alignment work. This is all after having the car 2 days and not even 75 miles. If you want any more details, please feel free to ask. Thanks for all your help and hopefully, I will be able to have my car back shortly.

Steve