Looking for Help/Advice
Thread Starter
Cruising
Joined: Aug 2011
Posts: 10
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From: Kailua Hawaii
Looking for Help/Advice
New here and new to Corvette...I have a '88 with a few mods...most history is unknown except that it has not been running good, and sat for a few years.
It had a SES light and loss of power, since then the MAF sensor, IAC and AIT sensors have been changed, and it's running well with no SES light...BUT there is a high idle problem....When first started cold it idles around 700rpm, then about 1-2 minutes into warm up, in jumps to 1300rpm like a swith was turned on (close loop?)
I did some testing and know that base idle is correct at throttle valve, Base timing with wire diconnected is 6 degrees, and TPS sensor is at 0.54 volts at idle.
What I did find is that it idles perfect with wire disconnected...the wire that takes timing away from ECM.... As soon as I hook this wire up, the timing jumps and stays at 38-40 degrees...and idle rises.
I'm thinking the timing jump is why the idle increase.
Can anyone tell me why this happens as soon as wire is connected? Or where to look or test? Why would computer put timing up at idle like that?
Appreciate any and all help...
Andy
It had a SES light and loss of power, since then the MAF sensor, IAC and AIT sensors have been changed, and it's running well with no SES light...BUT there is a high idle problem....When first started cold it idles around 700rpm, then about 1-2 minutes into warm up, in jumps to 1300rpm like a swith was turned on (close loop?)
I did some testing and know that base idle is correct at throttle valve, Base timing with wire diconnected is 6 degrees, and TPS sensor is at 0.54 volts at idle.
What I did find is that it idles perfect with wire disconnected...the wire that takes timing away from ECM.... As soon as I hook this wire up, the timing jumps and stays at 38-40 degrees...and idle rises.
I'm thinking the timing jump is why the idle increase.
Can anyone tell me why this happens as soon as wire is connected? Or where to look or test? Why would computer put timing up at idle like that?
Appreciate any and all help...
Andy
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
What history DO you know? What mods? Closed loop normally doesn't kick in until 5min after a cold start (though it drops in half for a warm start and 1min for a hot start).
O2 sensor good?
Check the harmonic balancer to make sure it hasn't slipped.
How does it run otherwise?
O2 sensor good?
Check the harmonic balancer to make sure it hasn't slipped.
How does it run otherwise?
Thread Starter
Cruising
Joined: Aug 2011
Posts: 10
Likes: 0
From: Kailua Hawaii
Here's what I do know about car...
It ran for over a year after mods without any problems, it was modded by a pro corvette shop. Looks like a crate motor, with a roller cam and roller rockers, I can hear a small lope with the cam. It has long tube headers, and the TPIs "Mini Ram" intake manifold. The rest of the TPI and distributor, ECM all looks stock or original.
There are no codes coming up, and the crankshaft damper did not slip.
The timing is going to max advance when the EST wire is connected and computer has control of the timing. When this wire is disconnected and computer does not have control, the idle is perfect and timing is at 6 degrees BTDC. Why would the computer want max advance at idle sitting in park? Hoping someone can tell me why computer is commanding max advance, or what is telling computer to go max advance....
Thanks again...
It ran for over a year after mods without any problems, it was modded by a pro corvette shop. Looks like a crate motor, with a roller cam and roller rockers, I can hear a small lope with the cam. It has long tube headers, and the TPIs "Mini Ram" intake manifold. The rest of the TPI and distributor, ECM all looks stock or original.
There are no codes coming up, and the crankshaft damper did not slip.
The timing is going to max advance when the EST wire is connected and computer has control of the timing. When this wire is disconnected and computer does not have control, the idle is perfect and timing is at 6 degrees BTDC. Why would the computer want max advance at idle sitting in park? Hoping someone can tell me why computer is commanding max advance, or what is telling computer to go max advance....
Thanks again...
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
I'm not certain, but an aftermarket cam could do the same -- if not yet tuned.
Another POSSIBLE reason is an incorrectly entered cell or chip for tuning.
I'd have to look up in an FSM how the EST wire works. The EST module might be bad...or maybe the reference wire is feeding the ECM bad info about the current amout of advance?
Do you have a local tuner? Have you tried a mechanic?
Last edited by GREGGPENN; Aug 21, 2011 at 02:14 PM.
Thread Starter
Cruising
Joined: Aug 2011
Posts: 10
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From: Kailua Hawaii
Thanks Gregg,
I think I eliminated a vacuum leak and O2 sensor...since without the advance the engine runs normally...you're suggestion of a cell or chip is very possible...I don't know what a stock chip looks like, but this one plugs in, is real wide with a lock tab on each end...it also looks like a replacement ECM...I don't know how the EST fits other than retarding timing from a knock sensor...
We have a local expert here, BSM Performance (Bodie Medeiros), looks like I will be calling him on monday for some help on this...
Thanks again...
I think I eliminated a vacuum leak and O2 sensor...since without the advance the engine runs normally...you're suggestion of a cell or chip is very possible...I don't know what a stock chip looks like, but this one plugs in, is real wide with a lock tab on each end...it also looks like a replacement ECM...I don't know how the EST fits other than retarding timing from a knock sensor...
We have a local expert here, BSM Performance (Bodie Medeiros), looks like I will be calling him on monday for some help on this...
Thanks again...
