So I think the computer on my 86 is in it's death throws.

I am having issues with idle - staying up at 14 - 1700 rpm - and some other stuff. IAC is replaced - throttle body is clean as a whistle, extra return spring on the throttle - nothing helps. Sometimes it comes down by itself to normal - other times not - tapping on the gas petal doesn't have any effect - (in terms of releasing the autochoke).

I'm guessing computer - come on - it's only 20 years old - sitting there in a very hostile environment - why would it go bad so soon?

anyway - I have a ZZ4 running on the stock TPI (very well thank you - it is not starved for fuel - contrary to the nay sayers!)

Do I just replace the computer with a regular "stock" replacement? If so - pep boys? - autozone? Napa?

I know many don't like autozone - but I have had great luck with them - got lucky- my local AZ has a car guy working there - very knowlegable - & helpfull - and the lifetime warantee helps to - as well as the prices!

Bogus thinks NAPA - what do the rest of you guys think?

Carl Johansson

 

I think you should do some more testing before buying a new computer. I know it is 20 years old but this does not sould like a typical computer problem. Check with propane for vac leaks first and put a scanner on that new IAC to see what it is doing and post the results.
Jerris

 

I've always bought replacement ECM's from the Dealer.

If you haven't done so, you might want to try some other tests first. Your Vette may have air getting into the plenum from somewhere other than the IAC opening (a vacuum leak, worn throttle shaft, etc) or the IAC won't extend into it's bore. A scanner simplifies everything, but you can make do without one.

A basic test would be to ground the diagnostic link with the ignition on. That should fully extend the pintle and you usually can hear the motor running as the pintle moves. Disconnecting the harness and starting the engine should then produce a base idle of 425 to 450 rpms. If it won't do that much, check wiring. Each wire should flash a test light with diagnostic link grounded. No light could be the ECM or an open in the wire. You could do the same check at the ECM to narrow it down. The IAC pins are C3, C4, C5 and C6. A steady light could mean that the wire is shorted to voltage. Confirm that by disconnecting the ECM and if the test light remains lit, you'll have to unwrap the harness to find the short.

Anytime you replace or disconnect the IAC harness, it will have to be reset. Do this by depressing the accelerator slightly and starting it. Let it run for 10 seconds, then off for 5 seconds.

You might also verify that the TPS signal isn't above .62 volts. If it is, the ECM will think that the engine is accelerating and won't pulse the IAC motor.

A couple of other items to look at would be the a/c signal and air pump plumbing. The a/c signal is at Pin B8 and should be 0 volts with the a/c off. If it was shorted to voltage, the ECM would retract the IAC pintle increasing idle speed for the perceived load of the compressor. If one the header check valves is broken or if the air pump is pumping air to the headers after startup, it would have the same effect as a vaccum leak. You can pinch off the hoses to see if that's a problem.

 

I agree. Sounds like something OTHER than a computer problem to me too. I'd first suspect a vacuum leak.

Jake

 

Because this IS the C4 section, and you don't say otherwise, I take it this car has EFI, and likely is a TPI. Carburetors don't have IACs and throttle bodies don't have an "autochoke". Nothing you have mentioned points the blame at the ECM. Your problem is un-metered air. As SunCr said:When you have disconnected the IAC you have disabled the only means that the ECM has of regulating the idle speed. Did the idle speed change when you disconnected the IAC? I'll bet not. If not, the ECM isn't the problem.


There are a lot of other items in this car 20 years old. With THAT philosophy, you have your work cut out for you.


Fuel delivery isn't the problem with TPI performance. One can always buy larger injectors, up the fuel pressure, increase the pulse width, etc. Getting fuel in, is nothing. The problem is getting AIR in. Just like your high idle is an AIR problem.

First you need a FSM so you can begin to understand how your car works. It will have trouble-shooting charts to help track down faulty components and help keep you from buying unneeded parts, like an ECM (and an IAC?).

RACE ON!!!

 

first of all - thanks very much for the input. now the first part of your answer is completely unessisary - I did identify the car as having TPI as well as being an 86 - i guess I need to put it in the first sentence (BTW the info is also in the profile)

Now this is good info - thanks!.
Nothing you have mentioned points the blame at the ECM. Your problem is un-metered air. As SunCr said:When you have disconnected the IAC you have disabled the only means that the ECM has of regulating the idle speed. Did the idle speed change when you disconnected the IAC? I'll bet not. If not, the ECM isn't the problem.


You have given me many items to check before I go through with the computer. I appreciate that -

Your condescending attitude however, left me a little cold to you. You obviously are very knowlegeable about your car (although I do see some of the old wives tale - mechanics passed down stories - that don't hold up under close inspection!) regardless, Can I assume your doctor or your dentist take that same attitude with you when you go to them for help? Course - there is a difference - they are professionals!

i have already ordered the helms book - But sometimes - being one less learned than you about cars - I need someone to give it to me in plain straight forward english - I thought thats what this forum was for. Apparently you think it is a forum for you to belittle those who know less than you about corvettes.

13 second club? pretty impressive! unless your talking with the 9 second club member in this forum - (get my point?)

Carl "dumber than dirt" Johansson

 

yea, as soon as I read this, I thought IAC, too.

Or a vacuum leak.

I did recommend NAPA, but based on the simple question "my ECU is on the fritz!"

Another point with the earlier C4s, the wiring harness is, well, junk. They don't last. Check the connector ends and the wire. The insulation will crack and pull back, leaving bare wire to short and such.

 

Thanks Andy - the more I thought about it - computer dosn't make alot of sense - I think that was a diagnosis from a frustrated mechanic - looking for answers!

I'm going back into diagnosis mode - unfortunately because of my time restraints I'm communicating this info to a guy whos working on the car - so i sometimes loose information in the passing!

 

Before you start trouble shooting set the idle & IAC according to the repair manuel. I have Chiltons. With ignition off, short the pins in the ALDL as mentioned above and turn on the key. The service engine light will flash once, then twice. It will do this 3 times. Then, if there are codes set, it will flash each code three times. record them. Next , un plug the IAC connector. Remove the jumper wire & start the car. Let it warm up if not at operating temp. Adjust the idll with the throttle valve adjuster 450 in gear with an auto, and 600 in neutral wit a manuel. Turn off the motor. Adjust the Throttle position sensor to 5.4 volts ( I think?) Reconect the IAC & TPS. Start the car. I have also read, and it does seem to work, that you can dreive the car at 40 mph, about 1000 RPM in high gear, and the computer will adjust the IAC. If the propper adjustment does not solve the problem, then stert trouble shooting .

 

I don't see TPI mentioned anywhere except in reference to your ZZ4 . You do say it is a 1986. There are more and more of these earlier cars being converted to carburetors. Even though you mentioned the TB, you also spoke of the non-existent "autochoke". It seems every time I ASSUME on here it turns around and bites me.

Your post doesn't exactly leave me feeling all warm and fuzzy, either. Please pardon the time and effort I spent on trying to assist you. I will try not to make that mistake again.


The avatar isn't there to "impress". It is merely a statement of fact. And that isn't half bad for a stock, high mileage, Crossfire, with minimal investment in performance.

RACE ON!!!

 

My dentist always gives me Valium, so I consider him an expert. Try all the other tests first, then maybe it is time to replace the ECM. Ignore the "condesceding" attitude and listen to the advice given. And never ever ask about air-foils or 160* t-stats.

 

Or header tube size.

RACE ON!!!