KevinRR

So a couple years ago my dad gave me his old 85 vette (auto) that he's had since the early 90s. He bought a c5 and let the c4 sit. It has 117k but pretty much sat the last 7 or 8 years. It still had 02 tags when I got it from him in 07. The car still ran but ran roughly and barely idled.

Last summer (08) I tore the top of the engine apart to do maintenance and tune up. Here is what I replaced:

-lower intake to block gaskets
-thermostat (160)
-fuel injectors (new/updated stock replacements from fuelinjectorconnection)
-runner and plenum gaskets
-cleaned throttle body
-ISC (idle speed control) valve
-blocked off EGR
-plugs
-wires
-cap
-rotor
-belts
-hoses
-oxygen sensor
-valve cover gaskets
-pcv valve
-waterpump gaskets
-fuel filter
-fuel pump strainer
-shopvac'd old fuel out of tank and put in fresh 93
-new a/c orings and vacuum/charge with freeze-12
-set timing to stock per manual
-cleaned AFM with brake cleaner
-new K&N air filter (stock panel dropin)
-eliminate stock air pump

So after all this, the car still runs the same. I sent the car off to the body shop and just got it back recently, still running the same if not worse. The body man is a racecar guy and he said he tried bumping the timing up a little to get better throttle response but it didnt help.

The engine takes 3-4 seconds to fire on cold startup. Then it immediately dies. This is repeated 2-3 times along with pumping the throttle once it does start to try and keep it running. You have to keep it around 1500 for 15-30 seconds before it will stay running. Then it drops to the 450-750 range and goes up and down, almost dying and then surging, repeat every 3-5 seconds.

If you try to give it throttle too quick it sputters for about 1 second, then picks up and revs pretty normally. The throttle response SUCKS. Once it picks up and clears out, it runs normally. Once you take it out and drive it, it runs normally, except for a rough idle at stoplights that makes you wonder if it will die or not.

Bumping the idle up to 800-850 smooths it out in neutral once warmed up, but when you pull it into gear it goes back down to 500-600 and acts like it wants to die, and starts surging again. It never actually dies, but it goes as low as it possibly can without dying...sometimes the stock tach indicates 440rpm for a second.

IF you shut it off warm and then restart it a few minutes later, it still takes 3-4 seconds to fire up, and it likes a little throttle to get it going and clear it out or it will want to sputter and die. Once you clear it out then it idles as described above but does not die.

It acts to me like a car with timing off, maybe retarded 10 degrees or something, with the poor idle and poor throttle response, and slow starting. But the timing seems to be dead on.

IF you take it out and floor it, it runs what I would consider 95-100% of what it should, for a 230hp car (I have several faster cars). Just driving normally it also runs well enough, except when at or just off idle.

I have done everything I know to do. Most everything on the top end is new or inspected. The coolant temp sensor reads normally and that is the only other part I can think of, other than the airflow meter, that can effect how a fuel injected car runs.

Am I better off to just do a carb swap on this car? I know nothing about carbs...I am an import fuel injection guy (I build rotaries) but I can't see it running much worse than it already does.

I am also pondering an LS swap of some sort, however I'd like to drive the car as-is for at least a couple years before I undertake that task.

Demonic85

I see you blocked off the EGR, did you have the computer tuned for this? If not its very likely going to run like crap especially at idle. I would go buy a new EGR valve and put it in to see if that helps, cheaper than tuning the computer.

As far as doing carb swaps, thats entirely up to you and you should weigh the pros and cons of doing so. Early EFI systems can be a pain when they're not working properly, but that doesnt mean you have to resort to a carb. There's always a LS1 swap

KevinRR

What would the EGR have to do with idle? Is the EGR valve supposed to be used at idle on this car?

I dont know specifically about the corvette, but in other cars I've worked on, the EGR wasn't used until the engine was cruising, and if used at idle would cause the engine to stall. And this is why they are controlled by a seperate solenoid, so the computer can trigger it while driving but not while idling.

In fact that is the reason I blocked it off...the valve didnt look all that healthy and I actually thought it may be a source of a vacuum leak or perhaps could be stuck partially open, causing the exact problem I was trying to resolve (****ty idle).

Muffin

Since you know your way around EFI can I assume your adjustment of the IAC, TPS and timing were done correctly?

KevinRR

I was not aware there was an adjustment for the ISC/IAC. I have never heard of them being adjustable on any of the more modern cars.

The TPS is adjusted but it did not seem to make much difference to how the car idled or ran one way or another. As the issue is mostly only present at idle and throttle tip-in/throttle response, I figured the TPS was not to blame...in my experience TPS problems are usually present at a certain throttle position that is repeatable every time, say 2500rpm for instance.

Perhaps someone knows something specific to this car that I do not, which is why I have posted here.

fbelz

My 85 was running really rough the other day. It would start and pick up but power was lacking. Took it to the shop as I was baffled. Found out it was a blown fuse on the left bank fuel injectors and they were not working properly. After replacement the engine runs nice and smooth. Can almost balance a nickel on it.

coupeguy2001

assuming you checked fuses, etc.
1. while it is running or idling badly, try disconnecting the Mass air flow sensor. If that does not improve things, try disconnecting other sensors.
The computer has a limp home mode that will revert to a look up table for fuel computations.
2. Are you sure the MAF module is working? it is a separate unit that is plugged into the haness under the dash. '85 only.
3. It is possible that the oxygen sensor is causing this. sometime they get damaged if it is dropped in the store before you get your mitts on it.
4. Sometimes you do your darndest, and you still have a vacuum leak. check all areas that you visited and double check.

samsonb

Where are these fuses located at? The fuse box, or in the engine bay?

Demonic85

fuse box by the passenger door.

KevinRR

How would one test it? The MAF does respond to airflow changes...with the engine running if I blow into it from the front the engine dies (presumably from rich correction to perceived airflow increase). I did clean the screens and fins with brake cleaner a couple of times just to be sure it was clean.

I guess anything is possible, but the o2 sensor looks fine and the box was not beat up that I recall. I can hear no vacuum leaks anywhere. All of the intake and vc's are powdercoated, so I dont really want to go spraying starting fluid on everything trying to find a leak as it will discolor the powdercoat.

KevinRR

Also, I found a nice writeup about the IAC/ISC relearn procedure along with setting the TPS (which I had already done anyway). I did the procedure for both as described there and it did seem to help significantly with the hunting, I suppose the IAC was never zero'd out (which I was unaware had to be done in a specific procedure).

I would still not say it runs like new but it is a lot closer to being acceptable now. The car is up on stands being lowered right now so I dont know how it will run in the long-term once I get to driving it, but hopefully it will stay as stable as it seems right now.