Nathan Plemons

Ok, so it's not really a problem, just an annoyance. Most of the time when I start my car it's cold. I drive it to work in the morning, park it all day and then drive it home. I park it and don't get back into it until the next morning. On the weekends though, or on the off chance that I should have to fire it up hot, it acts a little funny. I've got a thoery, let me know what you think.

When cold it starts and runs perfectly, other than the noise of the exhaust and the obvious cam lope, you wouldn't know that it's not stock. When it's warm though it's a different story. First and foremost it takes longer to start. Not a real long time, just a couple more revolutions of the engine. It's one of those things that probably nobody but me would ever notice. When it does start instead of roaring to life, it kinda stumbles and sputters up to idle. As soon as it does figure out it's idle the car runs perfectly.

I know it's not a case of leaking fuel injectors. I have a fuel pressure gauge in the cockpit, it'll hold fuel pressure for hours as it should.

Now time for my theory, see if it sounds reasonable. With all my mods the computer program has been tweaked just a little bit on multiple occasions. I also don't think this is something that has just started happening. It's probably done it ever since I put the cam in it, I just never really noticed it. In the computer there is a setting for "Idle Steps vs Coolant Temperature" or something to that effect. The values in the computer are MUCH lower for higher coolant temps. I can only assume that this value in the computer is related to the IAC steps required to maintain an idle.

I'm thinking that the hot values in the computer are simply too low. When I turn the key on it's not opening the IAC enough, thus causing the longer start and the stumble. Once running it tweaks the IAC to maintain the proper idle speed. If this is the case all I have to do is data log the car when it's warm and see what the actual IAC counts are vs what's in the computer, and change it accordingly.

Sound feasable?

VR'92

Interesting. ttt for you. :lurk:

JCAIRE2

Try giving it a little throttle when your trying to start it warm. If that helps the problem, you're probably right about the IAC.

Nathan Plemons

I may give that a shot. You've got to be careful with that on an LT1 though because of the self zeroing TPS.

CFI-EFI

Here is an observation from a computer illiterate. Why would the computer be able to maintain a proper idle speed shortly after it's started, if the hot values in the computer were too low? I could see a sticky IAC not positioning is self properly, before and during start up, but it seems to me, if you tweak the hot idle for starting purposes, you will change the desirable hot idle. Am I making any sense? Or do I just not understand how all this works?

RACE ON!!!

Nathan Plemons

A good question, let me see if I can explain.

The computer has a target idle setting. Assuming the IAC can adjust far enough to do so it will maintain that idle.

What I think is happening is that the values in the computer for IAC position are just "guidelines" to get it close. IE, when you first turn on the key this is where it sets the IAC so it will be close enough to start. Then after it's running it will adjust it as needed to find the desired idle.

So ultimately by tweaking those values I wouldn't be changing the desired idle speed, that's in another table "Target Idle vs RPM"

Both tables have higher values when cold, and then lower values as the temps go up. From the factory the car will idle as low as 700 or so when it's hot. With my cam and such I've bumped that up some. If these values are out of sync that could be my problem.

Clear as mud? It makes sense to me, I just thought I would run it by everybody else. BTW, I don't think the IAC is sticky. I replaced it not so long ago when I was having some other problems and a supposedly trusted mechanic said that had to be the problem. However much money for a new one later I still had the problem I was having. :smash:

CFI-EFI

I misunderstood the first post. Apparently the "target idle speed" is an additional table. I was thinking you were going to be changing the hot idle speed to improve starting. Carry on.

RACE ON!!!

VetteBandit

A different stab at this might be what I have encountered in the past. The starters on the cars are subjected to a LOT of heat from the engine (hence why a lot of aftermarket places sell shields for them). On my 85 if I run it and then turn it off and if the starter hasnt cooled down enough that can sometimes delay or prevent re-starting first try. Just another outside the box suggestion, I think the computer ideas you have are very valid aswell.

Nathan Plemons

I don't believe it's starter heat soak because it still spins the motor over plenty fast enough. I am very familiar with what you're talking about though. My dad's 46 pickup does it very bad. When it's warm the engine cranks really slow and it sucks the battery down hard to start it. When it's cold it does just fine. It's gotta be heat soak on the starter.