Vacuum issues and idle questions

modular46

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Joined
Jan 19, 2022
I have a Grand National that's pulling 16-17 inches of vacuum at idle. The car doesn't want to run under 900 rpms for idle. The car is modified with 120 pound injectors, bigger fuel system, 62-62 turbo, Precision dog house and Accufab throttle body. I have the ECUGN and I have it tuned pretty good. It's running on flex fuel that has about a 80% alcohol content. I have the stock cam with upgraded valve springs and I replaced the stock cam gear set to a heavy duty set. Air to fuel is nice and clean at idle and I have adjusted the timing a bit and it doesn't seem to matter. It idles fine at 940 RPM's but I can't really get it to want to idle under that. Can anyone tell me what they seeing for idle RPM's and how much vacuum? I suspect I have a vacuum leak or two I definitely don't have a boost leak as I am seeing 28 pounds of boost without adjusting the HD actuator too far and it runs really strong under WOT. I was going to go ahead and replace the vacuum block and eliminate the EGR and hoses but before I did that I wanted to see what ever one else is seeing. And the last question is in regards to the IAC motor and where it's set, is there a specific way to set it? Do you set it for a certain voltage with a meter?

Thanks
 
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Find your vac leak and then adjust the IAC and TPS again.. A vac gauge works good as well.
I have a vacuum gauge (read above) I am trying to get some feedback as to what is normal for a stock cam car at idle. Do you have any feedback?
 
He’s trying to tell you that you can look for vacuum leaks with a gauge. You say you suspect one. Look & test your vacuum hoses for leaks. Can you hear one?

My stock ECM idles my car just over 800 RPM’s.

As for the IAC it’s not set at a certain point. It’s what your car likes. I go between 10-20 with the car at operating temp. You can reset it, then adjust it by the screw on the DS of the throttle body linkage. You may have to do the dance between the TPS & the IAC. Change one the other changes.
 
He’s trying to tell you that you can look for vacuum leaks with a gauge. You say you suspect one. Look & test your vacuum hoses for leaks. Can you hear one?

My stock ECM idles my car just over 800 RPM’s.

As for the IAC it’s not set at a certain point. It’s what your car likes. I go between 10-20 with the car at operating temp. You can reset it, then adjust it by the screw on the DS of the throttle body linkage. You may have to do the dance between the TPS & the IAC. Change one the other changes.
thanks, how much vacuum do you see at idle?
 
I see about 13-14 but I live at 5,400’.
My GN is near stock. Modifications shouldn't effect idle too much as it is mostly at the big end. I have 60lb injectors and an LT-1 MAF sensor with translator. Engine is rebuilt less than 10,000 miles ago so everything is tight. Idles at around 875-900 at 18-19 vacuum reading. IAC was set at 10 as it is supposed to be between 0 and 20. It occasionally goes off on its own but normally stays in the 0-20 range. I agree that poor idling is often due to a vacuum leak but stuck egr valve (if you still have one) can also be a culprit.
 
How old are all the vacuum lines? I changed all mine as they were all weather cracking. Looked ok til u bent them and the small cracks showed up. My idle was wonky and the vacuum gauge would flutter at idle
 
17 in is good vacuum @ 900 rpm. I would hope you have the EGR blocked. My car likes closure to 20 IAC, all cars are different. My car idles at 800 smooth a kitten and has 210 cam. I don't think ECUGN has a EGR command in it. What is fuel pressure at Idle? what is injector pulse width ? I would not know what it should be on flex fuel with a injector that big. But with a stock cam and you did not say if the port size is stock but I have a filling it is. You have enough induction to make 800 hp I would say your drowning the poor car. and it will never idle with that combo.
 
That is normal idle rpm with 120's, amazing the things you miss when you have been awake 48hrs...:sleep:
 
yes you are correct , and usually vacuum will go down and IAC will go down. we don't have a IAC # but 16-17 in is a very good number. For vacuum.
I personally think its too good but its a good sized turbo 6262 on stock heads and cam and probably can't spin the turbo at idle enough to build some boost, it won't go + but should be heading that way. Example my wife's Subaru forester ideas in gear at about -2 in and my 5857 BB is at -6 in. with a number like that you would think I have a hell of a leak , but its tight as it can be.
His injectors are twice as big as mine so I can assume at 43 lb fuel pressure it should be about half the pulse width as mine but it flex fuel so that blows that theory.
I have not read that he has set his IAC or even knows how. 87 TurboT gave him the short version on how to adj.
TTA308 clams that normal for 120 and I can believe he is correct.
 
I have the same inj. and E85, chased my tail for a few days thinking somethings not right with the high idle then i got the bright idea to ask Eric... I guess that is why he won't burn a chip for anything bigger than 120's.
 
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