Well hello everyone!
So I've had this 1988 corvette for about a year and a half, everything was fine for the first year aside from some simple things;

• Throttle body gasket (Changed)
• original plugs (changed ac delco)
• 2 year old gas when i got it (Ran 93 and Techron for 4 tanks)

Ran tip top for about the first year, got a flat tire and work/life happened and the car sat for a couple of months, went out to start it and it would fire and die, could keep it running with throttle noticed the MPG on dash was 99 constant
after a couple miles and up to temp everything would be perfect, engine/idle/MPG reading. so I dealt with it for a while and didn't think much of it.

*Car sat as there was a brake issue, Caliper tried to slide off but pinned against the wheel, the pin slipped out, car was dead before the next part and had to jump start it*

Fast forward a few more months I Installed new Idle air control, checked the TPS for proper voltages (good) and changed the coolant temp sensor, cleaned MAF... and then it was harder to keep running, backfiring and misfiring.
Cleaned up all connections, reset ECM (Battery disconnect) re-seated the IAC and changed O2 sensor, cleaned the throttle body completely. and changed out the 2 relays for the MAF.

I can now tap the throttle, start it up and it'll idle around 1k, dropped to 400rpm randomly and then corrected, If i don't tap throttle before cranking it'll die off immediately, Once it's idling if i put it in gear it drops to 300-400 rpm almost dies and runs real choppy, put it back in park and tap the throttle and she'll level out around 1k again, no difference whether MAF is plugged in or unplugged.

*During this Choppy idle my eyes water from the exhaust. and when i get the RPM up with the throttle to around 1500/1800 (holding the runners open with pocket screw driver in linkage) the headers start to glow.*

*I can now get it to idle and warm up, SES light will stay on and sometimes flicker out and on, throttle responds well and will come back down without issue once warmed up. Have not driven it to see if she still dies coming to a stop.*



Waiting on adapter for OBD1 so i can scan the car with Snap on Solus edge as jumping the port does nothing, the SES light stays solid.
*SES light now will either stay on while trying to read codes or It'll go out for 2 seconds and then turn back on and stay on*

*I know this thread is a tad bit old, but as the OP I figured I should give an update.
This part will be added to the OP so here it is, I've hunted down the appropriate diagrams and followed a few avenues given here and around the forum.
long story short I pulled the ECM, Found it to have been a re manufactured unit with no branding available anywhere. at this point there was nothing to lose further.
I opened the ECM to check for any visible corrosion or damage. I found the Cork stand off to have moved from this original location and lodged the board upward.
I assume by relieving the pressure on the board I closed a cracked solder joint on the board that I could not see. put the casing back together made sure the connection was clean on harness.
The car fired up and idled just as it had a year ago, not only that but I could communicate with the ECM with my scanner and no codes were available as all data shown was accurate.
I Appreciate everyone's time and support with this and am happy that I've finally pin pointed the issue to be within the harnessat the ECM / ECM itself.*

 

Fuel pressure regulator...injectors maybe ? Somebody with way more knowledge than myself will chime in I'm sure, but those are the first two things that popped into my head. Good luck, and I hope it gets resolved quickly and not expensively...

David

 

I had Ohm checked the Injectors when I was dealing with the old fuel when first assessing the car. I'll give them another check here shortly. Haven't checked the fuel pressure lately so I'll do that as well.

 

I hate to throw ideas at a problem without being directly involved so take this with a grain of salt. But one symptom you called out I have personally experienced; when the mileage display goes to 99 mpg in my case it was a bad ECM. Now the phrase "bad ECM" can mean a lot of things; corroded connections, bad ground, connectors needing a remove and re-seat, etc..... in my case the ECM was actually bad. I got a rebuilt ECM unit and transferred my factory calpak (PROM chip) over, and the car was perfect. This was at least 23 years ago. The same ECM is still in the car.

Have you tried getting to the ECM with the battery disconnected and removing / reseating all the connectors? Look for corrosion on the pins, etc.? May be worth a shot.

 

When you say you jumped the ALDL pins and the SES light stays solid, this sort of points to ECM also unfortunately....

 

I have not gone that far yet, Speaking with a Master Tech of 25 years (my dad.. lol) he mentioned the same thing but advised me check the other things that I had mentioned before hand,
he also had me cut the positive lead rubber housing and clean the corrosion that would be under it, and he was correct there was but it didn't pertain to the issues.
did you have any other symptoms come along with the ECM being bad?

 

Alright, I can work with this.
Do you know of any other tell tale signs? I know the symptoms i have listed can be about 20 different things with these cars.
Would a scanner even be able to read a bad ECM?

*EDIT*
Additionally the MPG is no longer 99 all the time, it's 12.8 all the time, no matter reset or sitting "idling" instant is 0 and range varies

 

No, in my case it was only the MPG reading going to hell. That was the only symptom. The car was only 8 years old at the time so it was somewhat of a surprise since it's never been in a heavy corrosion environment (I live in Northern CA) but that's how it goes.

For what it's worth, all that other stuff you did (IAC, TPS check, cleaning, relays, MAF, etc.) is needed on an '88 so that work is not wasted. If I'm right, and it's the ECM, then when you correct that all the other work you did will make the car run better than it has in a long time probably.

DEFINITELY replace/rebuild the fuel pressure regulator as puufgone mentioned. The diaphragm in there is complete garbage by now if it's the original.

 

Alright, I will check connections and go ahead and address the pressure regulator as it is original, car had 96,000 on it when I got it, but those miles were put on early in it's life.

 

Do yourself a favor and don't put the dash back together fully once you check and reseat the connections. It's likely that you'll end up replacing the ECM and since you'll have easy access to it once the right side dash is apart you'll want to keep it that way temporarily. Good luck.

 

Hello TylerJ!

Welcome to the Corvette Forum!

I too have a 1988 C4 coupe and have had my fair share of problems over the 25+ years of ownership. I have had a ton of different issues and have never had to replaced my ECM.

Did you replace the Idle Air Control valve with a replacement valve? When I did mine the NAPA store had one under the right part number that had the wrong shaped Pintle on it. It seems that mid 1988 they switched to a newer style pintle for the IAC.

I kept opening boxes until I found one that matched the original. The Pintle is the tip that moves in and out controlling the air leaking into the system to control the idle speed. It is critical to have the correct Pintle.

I am not sure when they changed or why but that is a big potential problem if you have the wrong pintle. Do you have the old one still? I would compare the parts at the auto stores before buying one. You might try looking at the IAC for a 1989 when they were different and then pull an 1988 or even a 1987 and compare them.

The one item that might have something to do with your problems is the Cold Start Injector. It will squirt fuel into the intake to help the car start when cold or warm. It will activate up to about 30* (C) and then stops. It is not controlled by the ECM. The one on my 1988 C4 was leaking fuel when it was activated and this made warm starts and hot starts more difficult. The CSI is a Fuel Injector mounted on the drivers side of the plenum. The control system is explained in the Factory Service Manuals and how to troubleshoot it as well. It is what does the Choke did in a carburetor and your Corvette sounds like it has a sticky choke giving it too much fuel at first until the CSI turns off.

From your post above I assume that you have the Factory Service Manuals, if you don't then you can buy a hard copy from Helm Publishers or a DVD copy of the manual from RockAuto for a lot less. The 1988 set is two books, one Mechanical and one Electrical and they have more information in them than any other aftermarket manual like Haynes or Clymer's.

Good Luck and be sure to tell us "how you fixed it" when the Corvette is back on the road and happy again!

Get your FPR Diaphragm replaced First and then go on IF you have to.

 

@ctmccloskey wow, well put together response! I appreciate that a lot actually!
Thank you for the welcome! I haven't even introduced myself to the forum yet sadly. long time lurker just a new member though.

I will be doing the FPR Diaphragm next weekend for sure and I'm trying to hold out on tinkering with other things too much so that I can pin point the issue once it's resolved.
That being said the IAC that I replaced did match the one that was in the unit previously, pintle shape wise that is, the new one has a paper gasket that the previous did not.
I'm going to edit the main post with some additional information that I forgot / have recently stumbled upon.
Which includes around 1800 rpm the headers glow and around the 300/400 rpm and rough idle an eye watering exhaust
not sure if this still indicates an issue with the CSI or not. but I believe the FPR may be at fault for these type symptoms?

 

Hello Again TylerJ!

Thank you for your nice comments, You are too kind!

I really like doing this as when I first got my C4 there wasn't even an internet yet and the only way was to read and learn. The amount of experience available on this Forum at any time is just mind blowing! I learn new stuff all the time just reading and learning from the posts people put up.
When I noticed your "new" membership I felt "Someone" should step forward and make your welcome a Formal event. I speak for myself but I am glad that you have decided to join us in the Corvette Forum! I too "lurked" as you call it for a while but when I started learning I joined and I still get the best of the deal as I get to learn from a lot of really amazing guys and gals.

Your Fuel Pressure Regulator's Diaphragm is clearly the prime suspect as it would have an affect on the idle and they don't like sitting especially with the Ethanol they add to Gasoline. Do you have an External Fuel pressure gauge that you can use on the Schrader Valve on the fuel line? You will need one to verify that your fuel pressure is where it needs to be at some point. Most auto parts stores have them and they come with a nice long hose which allows the gauge to be taped to your hood while you test drive it. Just be sure and have a Fire Extinguisher Handy and Available. Once you push on the center pin of the Schrader Valve you can have pressurized gasoline vapors sprayed into your face or the surrounding air, vaporized gasoline burns very easily. Don't push it unless you are expecting to find pressure behind it...

The Headers glowing I am not familiar with but likely has something to do with the rich condition your engine is experiencing when Cold. I have seen the Exhaust System on a aircraft Radial Engines "Glow" when climbing in Altitude.

I have been reading up on the Cold Start Injector system. This is what I found:

1985-1988 L98: The MFI (Multi-Port Fuel Injection) includes a Cold Start Valve otherwise known as the Cold Start Injector mounted on the engine. When you turn the ignition "ON" the CSV Injects vaporized fuel into the intake manifold, in addition to the fuel injected by the port injectors. The CSV operates with a temperature/Time switch, Independent of the Computer control.
The Cold-start Valve's Bi-metal switch opens when heated, whether by coolant or by the heating element. The switch grounds at or below 95* (F). The heating element then opens the switch, to stop CSI delivery. The delivery is: up to 8 seconds at -20*F or 1 second at 95* (F).

That is from the Book By Charles O. Probst called "How to Understand, Service and Modify Corvette Fuel Injection & Electronic Engine Management", Bentley Publishers

The Cold Starting system has plenty of potential for causing an issue like yours and that is why I wanted you to understand "How and What it does". It sounds like this could easily be A problem but the diaphragm is critical for getting your Corvette back "One the Road Again".

If that heater is bad or the thermal time switch has failed it might cause your CSI to operate excessively and "possibly" flood the engine while starting. It is a simple system that is supposed to make our cold starting easier, when it works properly that is.

I am pretty sure that you will find the Fuel Pressure Regulator's diaphragm is full of hole or crumbling. Once that is replaced and the new one working properly the Corvette should run better for sure. I suggest you just do a stock FPR Diaphragm unless you are tricking out the engine. I don't use the adjustable type as I have not needed to. Check your Vacuum hoses and be sure that they are all nice and tight and sealed. My 1988 had vacuum lines that literally crumbled in your fingers, replace every piece you find, I ended up with buying a roll and doing it myself.

Let us know how it does after the FPR diaphragm swap, that will make the biggest difference!

 

My 1989 idles all over the place. New TB, new IAC, TPS set perfectly. Runs great above idle, just can't figure out this idle problem. No codes, starts right up. Runs hard all the way through the RPM range.
Back down to idle, and hunting everywhere. Even stalls once in a while. No vaccume leaks anywhere.

Getting annoying...

Only thing not OEM are the injectors, accel 21 lb'ers were the closest available back then when the muiltec's failed.

 

Just because,
have u thought about installing a heated wide band oxygen sensor in it?
i think you will find that your mileage will go up and idle will be better.
But it’s not much more than a bandaid if you have other problems .
1. Good injectors.
2. No vacuum leaks
3. intake tube from mass air to throttle body is air tight
4. Plug wires are good.
5. Plugs are properly gapped.
6. Engine is timed properly.
7. The Y vacuum connector at the back of the distributor is leak free.
8. The cruise diaphragm is leak free.
9. The brake booster holds vacuum
10. the fuel filter is not partially plugged.
11.
don’t go out and buy all this stuff.
every component can be checked to determine if it’s the problem.
people lose patience halfway through the troubleshooting process and start just buying parts, hoping they will stumble upon the offending part.
this us a very reliable fuel/ignition system.
But occasionally, parts do fail.
Hang in there and troubleshoot.
your expanding knowledge will give you the confidence to tackle more difficult tasks.

 

You really think any of the above can cause a "hunting" idle?
The car runs amazing above idle. I already have a heated O2, just not wide band.
Next step I'll just cap off all vacuum lines.
#3 I know is good.
Plug wires I'll check in the dark tonight.
I used to keep up with the fuel filter annually, but I haven't in a while. But if it were fuel related, it would starve at top end.

 

Another thing
have u been playing with the idle screw on the throttle body?
if so, you can do a mini reset.
crank the idle screw in about one turn.
reset the throttle position sender to .5v,
and try that.
the iac cannot correct the idle if the idle is too far open or closed.

 

I had the same problem..harder to keep running, backfiring and misfiring. found a vacuum hose off.. on one of the throttle body this might not fix your problem but check your vacuum hose's around the carbs... good luck

 

I try to start with the simple parts and work your way up from there. It still sounds like a bad Diaphragm in the Fuel Pressure Regulator based on your description. I know I mentioned it before but you might want to check the diaphragm in the FPR. It is a fairly inexpensive part but the labor will hurt if you have to pay someone. Get some long torx bits and go to it...

The fuel pressure regulator makes the horrible fumes you described . Pull the vacuum hose and smell inside of it. If there is gas then you have your problem...

Let us know when you test it!

Best Regards,
Chris

 

I know this thread is a tad bit old, but as the OP I figured I should give an update.
This part will be added to the OP so here it is, I've hunted down the appropriate diagrams and followed a few avenues given here and around the forum.
long story short I pulled the ECM, Found it to have been a re manufactured unit with no branding available anywhere. at this point there was nothing to lose further.
I opened the ECM to check for any visible corrosion or damage. I found the Cork stand off to have moved from this original location and lodged the board upward.
I assume by relieving the pressure on the board I closed a cracked solder joint on the board that I could not see. put the casing back together made sure the connection was clean on harness.
The car fired up and idled just as it had a year ago, not only that but I could communicate with the ECM with my scanner and no codes were available as all data shown was accurate.
I Appreciate everyone's time and support with this and am happy that I've finally pin pointed the issue to be within the harnessat the ECM / ECM itself.