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This high idle problem continues. This time I plugged 5 ports on the Manifold and the the brake booster(#6).
My Vacuum gauge shows 25psi. 1200/1500 RPM idle and approx 1000 RPM with spark advance disconnected.
TPS/IAC /all good. Did the procedure to set the idle @ 450....perfect. plug everything back....NG
she's a little rough at 450 not terrible. generally runs excellent, so no mechanical problems.
QUES: Did I miss a vacuum port?
can a partially open EGR create a vacuum problem?
possible bad Spark controller?
Sprayed carb cleaner everywhere except I dont believe I got the EGR good enough, will try tomorrow.
any ideas?
If you can get it to idle at 450 rpm with the IAC fully closed and its idling under normal conditions at 1500 rpm, then the IAC is fully open (145 counts) causing the 1500 rpm idle.
The IAC has enough range from closed to open to add about 1,000 rpm. My settings are exaggerated a bit here, but I can go from 800 rpm to 1800 rpm on demand via the IAC as can be seen in the log below.
If you can get it to idle at 450 rpm with the IAC fully closed and its idling under normal conditions at 1500 rpm, then the IAC is fully open (145 counts) causing the 1500 rpm idle.
The IAC has enough range from closed to open to add about 1,000 rpm. My settings are exaggerated a bit here, but I can go from 800 rpm to 1800 rpm on demand via the IAC as can be seen in the log below.
Why is your IAC fully open? Low coolant temperature and high tps voltage are possible causes along with modified programming. Scan it.
hmmm, with the IAC and timing spout disconnected, yes I can Idle at 450(as per the book). when you say IAC "fully open" i'm assuming that means
fully retracted? I dont believe the ECM has been messed with but I will be checking the timing today. I dont have a scanner, so I dont know
if its open/closed. if there is a mechanical way to manipulate the IAC please tell. I am now throwing a "22" (TPS tested good) and a "43" ESC.?
thats todays fun. starting to suspect the ECU? the EGR is OLD, will spray that today suspecting it might be partially open, but that makes no sense
due to the vacuum gauge showing 25PSI. Over the years god knows how many folks messed with this. The IAC (which is new) misbehaving sounds very plausible!
I will try to find the procedure for it.
PS, LOVE your explanation, it makes sense. Also, Ive heard about this scanner. Where do I get it?
Looks like I will make much use of it
I have a 1988 C4 as well and when I was working on it I had problems with the idle caused by a "slow moving" IAC. I needed to buy a new IAC so I went to a NAPA store and they got me a new IAC but when I compared the new one with my old IAC they had differently shaped Pintles. The pintle is the tip of the IAC that moves in and out to control the idle.
Be very sure that you find the same pintle as your Corvette came with, the parts are not interchangeable. The wrong IAC would do weird things to your idle and might even make the problems you are experiencing. I would check it out by looking at an early 1988 and the 1989 model Pintle. The new shaped Pintle was introduced sometime in 1988 during the production run. The new Pintle shape is used on the 1989 model C4's.
I hope it is something this simple that is causing issues on your 1988 Corvette!
Just an update: I used a system check chart for the IAC today. As it turns out
two of the circuits have no power. Going to or coming from The ECM In this
case it CKT,s 441/643 I traced back to the firewall and no power......ECM?
Anyone who can help me with this would be appreciated!
Thanks to all
I did check the pintle and its good. the idle somewhat goes to normal when i disconnect the plug for timing.
I did check the timing and it was set at 8 degrees, I put it back to 6. it seems no matter how many times I set
the TPS @.54 it always changes. Reference signal for the TPS eventually goes to .5 volts. I pulled the ECU and sending it to
an outfit called G7. for $225 they will test and repair if needed, if nothing is wrong they send it back with a full refund.
I found a Reman ECU in the car, so???
and on it goes! I WILL solve this
I did check the pintle and its good. the idle somewhat goes to normal when i disconnect the plug for timing.
I did check the timing and it was set at 8 degrees, I put it back to 6. it seems no matter how many times I set
the TPS @.54 it always changes. Reference signal for the TPS eventually goes to .5 volts. I pulled the ECU and sending it to
an outfit called G7. for $225 they will test and repair if needed, if nothing is wrong they send it back with a full refund.
I found a Reman ECU in the car, so???
and on it goes! I WILL solve this
Thanks
Bobb
As it turns out it seems the ECU was bad. The folks at G7 told me the ECU had some hardware problems. As I stated earlier I was getting differing values on the TPS. I would set it at the prescribed .054 and when checked again it wold show .029,.071 etc. Next I checked the signal wire. it is rated at .5 volts, it would change from .010 to .6, sometimes .5volts. Anyway, put in the rebuilt ECU........done and perfect. Thanks to all for the help!
cant stop high idle!
