Vetracr

Who's got best prices on rebuilt ECM's. I think I need one for my 93 LT-1 6 spd.

Background;
New Opti
New ICM
New IAC
New TPS
Cleaned throttle body
Checked, no vacuum leaks
Checked throttle blade shaft, no leaks, no hang up
Throttle closes against stop screw
No trouble codes set

Symptoms;
8 months (1000 miles) ago when shopping for the car I started it and it idled at 1800-2000 rpm. Seller had car fixed by shop before I bought it. He did not know what they did to repair it. When I got car it ran fine but developed an erratic idle problems which I thought I had chased down, see background. Car was idling and running great. Last night when I start it, it idles at a steady 1800-2000 rpm. Nothing will change idle even when engine warms up. kick the throttle, no change. Problem repeats after half a dozen starts. No check engine light.

With throttle closed (visual check) only way engine could idle that high is from 3 possible sources. 1) stuck open IAC but IAC is new. 2) Gross intake manifold leak but how could that happen over night?? Tach is reading o.k.so ECM is getting idle speed info from Opti. Only thing I can think of is new IAC went bad overnight (not likely) or 3) ECM has intermittant bug. I"ve had intermittant ECM problems before and this sure looks like one. Any suggestions would be appreciated.

Want to get spare ECM anyway just in case. Any ideas???? THNX

Larry

FYI: duplicate post to C4 test and tune

Tom Piper

Not all scan-tools are bi-directional.
A bi-directional scan-tool can send commands to the ECM to allow you to check the results -- most other scan-tools can only read data items.

With a bi-directional scan-tool (like the Vetronix Tech-1A, Tech-2, or MasterTech), I would command the idle to a selected value and watch to see if the engine responds (I would do this several times - up and down). If the engine responds correctly and the IAC counts change proportionally, the ECM and IAC are functioning. With all scan-tools, you should be able to read the IAC counts.

With that said, for some trouble-codes (not all of them though), the ECM will go into limp-home-mode where the fans will run all the time and the idle speed will increase quite a bit.


Tom Piper

Jet-Jock

Hmmm you have one of the best persons to help onboard this post. Tom is unprecedented with his knowledge.

If you need to replace your ECM, I got mine from Chris at Superior Chevy. I'd have to pull the invoice but I believe it was either 124./164. plus exchg.

The symptom I had with a leaking intake manifold was miss in the idle. Actually two aspects to the intake manifold; RTV failure on the fore/aft rails, or port gasket leak. The former can be seen by a nasty oil leak, the latter a miss in the idle.

93 ragtop

Well here is a good price. http://www.madvet.com/shop?frame=1.875
I got one about 2-3 years ago. I believe the core comes off of that price which made it about $100.00

bogus

hm... I agree with Tom... if you can get a better scan tool on there, you might learn something.

Qx: where did the IAC come from? I had an AutoZone part, and it died after about a year... Then again, for 30.00, you get what you pay for. I just installed a NAPA part.

You say a new opti was installed... this is a real stretch... a guy came onto the forum with an LT1/Fbody that was running poorly. The final cause of his idle/miss problems was an oil leak into the opti.

Vetracr

I just priced a Tech 2 and MasterTech. Lets just say I can't afford to buy tools that cost half the price of the car. The IAC is a Delco (If I remember correctly) I got from GM Parts Direct. I sealed the new Opti with silicon sealer before the install. I also vented it. With less than 500 miles on it, I doubt its an oil leak into the opti.

This morning when I started the car the idle went to 1800 but after a few miuntes of running, it dropped to 1000 rpm, still high but lower than yesterday. One thing I have to do again is check the alternator for diode leakage. I tested it awhile ago and it was o.k. Its possible the alternator has a flakey diode. Hmmm. Still working on the problem.


Larry

89vettefromhell

dont know if this is what youre looking for but PCMFORLESS

Tom Piper

Many shops (including dealers) have these scan-tools, it might be worth having them check it out.
Myself, I don't like to guess at what is wrong -- it usually leads to replacing multuple parts and could cost more than paying someone to check it.

Tom Piper

corvette1989bham

Have rats been nibbling at your harness by chance?

Vetracr

Tom,

Can't argue with you one iota. Does the Tech 1 tool have the two way communication cabability?. I may have a line on one of those.


Larry

Tom Piper

Yes, the Tech-1 (most of them are really Tech-1A, which is updated) has that capability.
The Tech-1A and MasterTech use the same software cartridges.
The Tech-1A has a smaller screen than the MasterTech, so you can't see as many data items on one screen.

Tom Piper

bogus

I would pay someone a $100.00 to have them run a diag.

the oil was getting into the opitical side, not the high voltage.