Slowly Rising Idle With FAST

[ZBass28]

It's almost getting near time to bring the car out of storage. I ran it the other day for the first time in over a month and realized I had forgotten about a very minor problem that has been bothering me since last summer.

As the car warms up and gets to operating temperature, the idle has a tendency to rise anywhere from 100-150rpms on its own.

On cold starts the engine will idle exactly as it is told to do, and as it warms up idles perfectly at about 950rpm, which is where I have it set. It will stay dead set at 950 for a good 15 minutes, but sure enough as time elapses it begins to creep up higher and higher and usually tops off right around 1050-1100 once everything is fully warmed up. I have tried a few things to compensate but it always insists on doing this. I am probably missing something incredibly obvious.

 

[Craig Smith]

What does the target IAC position do under this condition? Does it drop down to 10?

Sounds like you may be developing a vacuum leak when it gets warm.

 

[ZBass28]

Target IAC stays relatively steady at about 38

 

[HighPSI]

You sure the Throttle Follower isn't holding it at 38?

 

[ZBass28]

Hmmm...I'll have to check that. I haven't really looked at any of the programming since October so I can't remember specifics.

How would you go about checking that in itself? Just keep dunking the throttle follower and see where it ends up at?

 

[HighPSI]

No. Go to the Throttle Follower screen and verify that the Throttle Follower is at it's lowest setting (10, I think) at the idle TPS percentage.

 

[ZBass28]

I will go verify this today. I am almost 100% positive that the throttle follower is nowhere near 38 at idle, but I will check again just to make sure.

 

[ZBass28]

Throttle follower is right in the 34-38 range it would seem at idle, so I would assume then it is keeping it there.

I have checked for possible vacuum leaks before with a 'benz-o-matic' portable torch with the car running and came up with nothing....unless something new has developed.

I assume if I move the throttle follower down that IAC target (and idle) will go down with it. I know I used to have it at or near 10 before but bumped it up because I was getting poor drivability down low off-idle, but that may have been due to poor programming at the time as well.

Crap, any suggestions for a course of action to follow?

 

[HighPSI]

Originally posted by HighPSI
No. Go to the Throttle Follower screen and verify that the Throttle Follower is at it's lowest setting (10, I think) at the idle TPS percentage.

 

[ZBass28]

Out of curiosity, why is '10' the goal I am looking for here? Other than the
slowly rising idle the car runs perfect and I would rather not mess with it.

Is there any significance to having the throttle follower at '10' at idle as
opposed to say '20' or another value in that range?

 

[HighPSI]

10 does not have to be the "goal" The goal should be below the IAC opening needed to maintain the idle. Otherwise the idle will not be allowed to come down once the car warms up. If the idle can't be attained then the FAST will try reaching the traget idle by playing with the timing. I would be willing to bet once your car gets warmed up and starts idling high, the timing drops real low.

Playing with this screen does not affect the way the car runs other than if you have it too high and the timing is doing crazy stuff. I consider it more of a "stall-saver" screen when decelerating.

 

[ZBass28]

I guess I will have to revert back to one of the older ways I used to have it set and see what happens. I know its pretty difficult foy anyone here to make a spot-on diagnosis for a car they have never seen or know nothing about. I'm just thinking though if it was the throttle follower why does it take that long for it to start acting like that. I would expect for it to happen sooner. The car can sit at operating temp for sometime before it bumps up 100rpms on it's own and prior to that idles perfectly.

The timing never jumps around at all. Even when the idle rises slightly after warming up the engine runs smooth. At first I thought that maybe I had some extra fueling going in on another part of the program.

I think my main reason for having my throttle follower where it is at now is that when I had it low at idle (like around 10-15) I would get a whole bunch of cam surge when under 1500rpms around town. I found that it helped to alleviate it by messing with the follower. I used to try and just compensate with opening the blades a little more with the idle screw but that didn't always work and more times than not I would have to open them so much that the idle was worse off than it is now with just messing with the follower.

Next time I run the car in the garage I'll mess with a couple things and I will try and be more aware of exactly when the idle starts to rise and if I can see any noticeable changes in any readings. Thanks for the advice! -Tom

 

[bobc455]

Zbass28,

If the IAC target isn't changing, then I don't think you'll find an answer in the ECM.

Is there more vacuum as the engine speed changes?

-Bob Cunningham
bobc@gnttype.org

 

[ZBass28]

Bob-

Not completely sure how to answer the question. Do you mean as the idle slowly rises does engine vacuum as well? I would guess fairly certain it might be. I am pretty sure as the engine warms from being dead cold that it does. I think at start up I am usually seeing about 12-14" of vac and as the engine gets to operating temp it usually ends up around 18" of vac at idle. I honestly don't know what it does when the idle starts to creep but seeing as that I usually look at the vac/boost gauge a lot, I don't recall seeing anything out of the ordinary during those times.

What might you be thinking is going on?

 

[bobc455]

Kinda just what Craig alluded to earlier, a vacuum leak that gets worse as the engine gets hotter...

Somehow, more air is getting into the motor. If the IAC isn't letting more air in, then it's leaking in somewhere else.

I guess another possibility is that the engine is running very inefficiently when cold (due to incorrect warm-up, afterstart, etc.) and as the engine warms up it becomes more efficient and the idle speed increases. But I don't really think that is the problem since the engine would probably run very roughly too, you'd know that was the problem.

-Bob Cunningham
bobc@gnttype.org

 

[ZBass28]

What about a faulty bypass valve? No idea whether mine is or not, but it is old. Could it be allowing a little more air to be entering at idle than should be, and thus causing this?

I'm beginning to wonder if I am reading into this little problem more than is needed since the engine runs well. All my vacuum leak tests to this point have been negative. Crap, I guess I will just wait and see what happens when the car comes out for springtime. Kinda hard now with it being in storage and whatnot to really test and diagnose.

 

[bobc455]

Leaking PCV, leaking brake booster (assuming you have power brakes), leaking tranny modulator, sometimes a vacuum leak can be in a strange place.

I assume you've already checked the operation of the IAC, so that 10 = closed and 180 = open...? ( I suspect you would know if this were the problem, but just double checking)

-Bob Cunningham
bobc@gnttype.org

 

[ZBass28]

Interesting thought with the PCV...perhaps a suspect in the crime

Reason I say that is because I am running an electric vacuum pump which is run like this: (keep in mind this is an LT4 engine)

The pump is directly connected (pulls vac) to the port on the driver's side of intake manifold where the PCV used to be. The front PCV port on the intake is blocked off with a brass plug. The hole in the pass. side valve cover is just plugged as well. I'm not sure how familiar you are with the config. of LT1/LT4 engines, but now I am wondering if perhaps this system could be involved. It's probably a little confusing the way I have it explained.