Hi all,
well.... this is a weird one. The 90 c4 will run like a scalded ape one time, the next it runs like it is starving for fuel. I had to have it towed home yesterday; it left me dead in the store parking lot. Today it started right up and runs fine, smooth as silk. Turn it off for 1/2 hour or so and it will start up, but runs like 4 of the plug wires are pulled. You can't touch the gas at all or it will die. It will idle but very badly. Turn it off and wait a day and it runs fine again.

I have replaced the plugs, wires, cap, rotor, fuel filter, oil, oil filter, air cleaner (k&n), idle teensier, serp. Belt, and filled it with 91 gas with a bottle of 7 point oct. booster. Fuel inj. cleaner, short of removing the radiator cap and driving a new Vette under it, I’m at a loss to know what's going on here. Anybody ever have this happen to them before, or is my Vette gone mad?

Could the computer be going **** up? It was scanned and didn't throw any codes. Someone told me the ign. module might be going bad. Would it run one time and not the other with that going out? When it is acting up it will start up and die off. After a while it will start up and idle poorly, but die off if you even think of touching the gas. This car has always ran very good, it is in great shape and well taken care of.

Any help would be great,
Gregg

 

I have a 90 coupe that did the same thing. I ended up taking it to the dealership and come to find out it was the injectors. It was very weird how it happened because there was no codes that appeared. If you need any help with this I live in laguna niguel CA so I can drive down to help you figure this problem out. Let me know

 

I would check the fuel pressure constantly, especially when it is giving your problems. Possible fuel pump going bad or injectors. Could be some sort of an electrical problem causing one of them not to work properly. Fuel pressure though would be a good start.

 

Ive got my money on the ECM

 

what did the dealer charge to find and fix?

 

I don’t trust anyone to work on my car. If you’re going to maintain it yourself there are a few things you’re going to need.
Factory Service Manuals, Craig Moates ECM Software, and Interface cable for your laptop.

The software will allow you to monitor the vehicle’s systems in all driving conditions real time. I tested every sensor with a multi meter before I broke down and purchased the cable.
I was able to see exactly what the ECM was seeing and found it misinterpreting the TPS as full throttle all the time, but my multi meter indicated the TPS as working correctly.
Replaced the ECM and problem solved.
Its things like that you’ll never see unless you have it.

 

what did the software and interface cost? sounds like a good investment. is it easy to find?
thanks for the info / reply
gregg

 

Check out the C4 Scan and Tune section. Should find all the info you need there

 

well,
i have an ecm on the way. found a guarenteed ecm on ebay for $45.
should be here in about a week.

i ran the car today as well. perfect running. shut it off, wait 1/2 hour, total crap. wait another 1/2 hour, it's perfect again. it will run all day if you don't shut it down.

wierd!

i'll keep you posted on what happens when the ecm gets installed.

thanks to everyone who posted ideas,
gregg

 

Sounds like a fuel vapor lock

 

I would not order a ECM before you looked at the fuel pump or relay. Your problem is intermittent, so you really won't know for sure unless you catch it acting-up

 

There could also be a possibility of water in your gas or a fuel pump problem.

 

well..... it wasn't the ecm either. unless the chip is the problem? i had to put my old chip in it.(can chips go bad???) i put the new one in today and it fired right up and drove fine. drove it around for nearly another hour. seemed fixed to me. turn it off and on after i got back and all was good. i went in and cleaned up. came back out about an hour later...... same thing again.
i just don't understand how it can run for over an hour at a time just fine? just don't turn it off for over a few minutes. if it were a fuel pump going bad, wouldn't it get worse the longer it ran? and as for water in the fuel, it has a fresh tank of 91 octain and fuel booster. and injector cleaner. all running in it as long as you want.....just don't turn it off. no codes, no other symtoms. when it is acting up it has emissions from hell. it's like the mixture is all out of wack.
i've now lost what little mind i had left.......

 

if it was a vapor lock problem, how would you know? or fix it?

 

My guess is an injector problem or fuel filter or fuel pump issue. If you are going to fix this without taking it in to dealer or mechanic you have alot more leg work to do starting with a few tools if you dont have them. You need a fuel pressure guage to hook to schrader valve and see if your fuel pressure is OK with motor cool and warm to check fuel pump and also to see if it holds with no signifigant drop with motor off for say 30 mins. If it drops drastically you have one or more leaking injectors.
Then you need a good volt/ohm multi meter to check things like injectors resistance. Pull the clips on injectors and put meter on ohms and check resistance of injectors, they all should be around 15.5 to 17 ohms with no greater flucuation between all 8. Check this cold and hot as sometimes heat causes a difference hence the trouble when warm.
Also need to put a low power test light across wires to injectors and check for steady pulse from ECM when running.
Finnally, you need a mechanics stethoscope to listen to injectors for a steady ticing pulse with no pauses. A homemade stethoscope can be made of some rubber tubing. Listening lets you know if one or more are sticking even though they have electrical pulse and good resistance.

Your symptoms can be caused by many things but this is where I would start. My 90's injectors failed at 76k miles and its a pretty common issue with the 85 thru 91's and I had symptoms like yours. Eliminate these possibilities and let us know what you find we'll go from there.

 

Dave has some real good advice there. Having the right tools is a must. I know it’s a large initial investment but you gotta have em to work on these FI cars.

It is a possibility that your ECM chip is bad. Mine went bad the first year I owned the car, but at least the ECM threw a code (52 or 53 I think it was)
I just upgraded to an after market chip and it worked fine after that.

Either way a spare ECM is good to have. I keep mine in the back of the vette at all times.
The replacement I purchased from Napa burned out within two weeks. They replaced it under warranty, but at least I had my spare to get by on till the replacement arrived.

You can get a fuel pressure gauge from Sears. It’s equipped with about a foot of hose so you can lay it on the windshield face down and monitor the pressure as you drive.

Keep us posted and get a Factory Service Manual if you don’t already have one

 

Mine did and it was a bad fuel pump. Once I could get it started, it would run fine. Turn it off, come back and it may or may not start. Finally stranded me in a parking lot and I had to have it trucked back to the house.

 

I vote for the injector problem

 

Experience with several L98s with similar symptoms indicates the injectors. A TSB published in '91 directed GM to replace them when problems were indicated. Problem is that they are wired in parrellel and when one fails, they all fail. (Should be more like Christmas lights, huh?) They can be well within spec (ohm) when cold, but as the "bad" one warms up, it fails and causes all the injectors on that bank to fail. It's a straightforward repair, but plan on 3 hours time. GM stopped making the orginal design injector in '91, so if you have NCRS or Bloomington plans for your car, you're going to have to find a set of NOS injectors (saw a set once for $1200 -- eke). The GM/Delco replacement injectors (similar but the gray color is "shorter") run around $90 each. I'd recommend checking out fiveomotorsport.com -- he's got various "take outs" which will works. I got a set for my daughter's Firebird GTA for $278!

 

well,
it wasn't the fuel pump either. drove it for ten miles after the $60 install, turned it off in the garage, came back half hour later......won't stay running.

let's see.....

new cap
new rotor
new wires,
new plugs,
new fuel filter,
new fuel pump,
new "ecm" box,
new intake manifold gasket (had an ugly oil leak),

i'm looking at an ignition module and then injectors next i guess.

i'm getting a new vette ..... one part at a time!!!!