Photomania

My 1990 Corvette six speed won't start half the time now. When it does start, it quits after two or three minutes. It runs smooth and then suddenly cuts off.

I just put a new fuel pump in it two days ago. But I can't even drive it now.

The problem started off as nothing more than an occasional stumble once a month. Then it got worse and worse until I couldn't drive the car more than 25 miles without it stumbling and sometimes cutting off. I would let it sit for 10 or 15 minutes and it would crank up and run fine.

Since April, I've replaced the distributor cap, the rotor, the ignition module and the ignition coil. I replaced the spark plugs and fuel filter back in the fall of 2004.

I thought that surely a new fuel pump would solve the problem. But it didn't.

Now I'm learning that a fuel pump relay can cause problems. Where exactly is the fuel pump relay located on a 1990 Corvette? I looked on Eckler's and didn't see one listed.

I did see a fuel pressure regulator on Eckler's, and I think I see that on my car. Do those ever need need to be replaced?

Someone please help me. I haven't driven this car beyond my neighborhood since May. Last week, I got halfway down my driveway and it cut off and never did start back that day. But it cranked right up the next morning and allowed me to back it up the driveway and back to the house.

I can't even drive the car to the shop. I hate to have it towed. Any thoughts on this would be greatly appreciated.

Thanks.

-- Eric

65Z01

If you are sure it dies due to no fuel pressure, replace the oil pressure switch.

It is located on the "tree" in the rear of the engine block just to the left of the dist and it the vertical sensor with two wires.

skateparkdave

65Z01 may be on to something here if you're sure it's fuel starvation. If oil pressure drops to a dangerous level the "working" oil pressure switch cuts the fuel pump off therefore saving the motor. A "faulty" oil pressure switch will do the same thing, but with good oil pressure. Definetly something to check.

You may also want to ohm out your injectors, or listen to them with a mechanics stethoscope for a steady ticking of injector pulse at idle with no pauses. I had one sticking intermittently and causing simular conditions that would clear after cooling.

It also displays simular conditions to that of a clogged cat.

Has your SES light came on? Have you pulled codes if any??

Just a few things to think about.

Photomania

I guess I need to buy a fuel pressure gauge. But it sounds like the engine just isn't getting the fuel. I've had one other vehicle in which the fuel pump died and I remember what that sounded like.

Can you tell me where the fuel pump relay is located because I read in another part of the forum that the wires go bad sometimes on the relay.

If it is the catalyic converter, is there some way to check for that? A mechanic at a Corvette repair shop told me that my problem sounded like the cat converter.

The SES light comes on, and I've checked for codes but gotten only the 12, 12, 12.

I've watched the oil gauge when I'm trying to crank up the car, and the oil pressure seems to be fine. But like you mentioned, a faulty oil pressure switch could cause problems.

Thanks guys for your information. -- Eric

brookman

The function of the oil pressure switch is described incorrectly.

The Oil pressure switch is a fail safe - but not to shut off the pump. The oil pressure switch works in tandem with the fuel pump relay. If the fuel pump relay is faulty the oil pressure switch will keep the fuel pump running. They are in parallel - not in series - check the wiring diagram.

On the troubleshooting note.... It sounds like you might have a large vacuum leak? or faulty spark module in the Distributor.

have you checked to see if you have spark - when it quits?

rick lambert

Don't know where the relay is on yours, on my 87 it's located by the wiper motor-and mine had the wire insulation shrunk back so far bare wires were touching each other-easy fix-but-I don't think the relay is your problem-simply because if the relay goes out the car will start and run fine after oil pressure reaches 4 lbs.Still, the relay is only $12. I've had a plugged cat before, the car would start fine but had no power.
With alot of bad gas going around I'd probably check out a fuel filter first.

skateparkdave

I havent checked the diagrams but was going on what I had read in here or somewhere that I believed to be a reliable source. I cant remember exactly where. Just something I picked up along the way.

So you are saying it's only function is to power the pump when the relay fails and wont cut off the pump if oil pressure is low?

I'm not disputing you. It just seems crazy that they would create a fail safe anticipating relay failure to keep fuel pump running but not one to protect the engine in the event of oil loss.

Good ol' General.

brookman

I actually have that from an unfortunate incident that occurred to a friend of mine. on the racetrack the oil pump shaft sheared off and he ran for about a mile at full throttle with no oil pressure.

Not to be argurmentative - but you can pull the connector while it is running and see the results for yourself.

I think it is beside the oil pressure sender for the ecm which is under the distribuitor on a tee fitting. (the sender for the guages is near the oil filter)

CFI-EFI

It may seem crazy, but you can confirm the fuel pump relay and oil pressure switch relationship by looking in your Helms.

Crazy? How many of the cars do you, or have you, owned, that have a provision to kill the engine when the oil pressure drops? Your "crazy" Corvette, and the "Good ol' General" is no different than anything else, in that respect.

RACE ON!!!

skateparkdave

Although it would be nice if it did come with that provision as a safety. I didn't mean I EXPECT it, I just thought I remembered reading that thats the way it was.

What I thought was crazy is if they made a provision in expectation that the fuel pump relay would die. Why not something to protect the engine if oil pressure is lost?

I said "I'm not disputing it". I believe. I just read somewhere that it was different. Now I'm considering a low oil pressure cutoff switch for my nitrous kit to go along with my low fuel pressure switch as I thought the car already had that covered.

CFI-EFI

You more than likely DID read that the oil pressure switch was a safety shut down. I have read it many times, and I try to correct the misconception, every time I see it. If you're memory is long enough, you may recall ME posting that the oil pressure switch would shut off the engine. I was told no, and I hauled out the Helms to prove my nay sayer, wrong. It wasn't long before my Helms proved him right. Ever since, I have tried not to tell anyone how something worked, unless I was very sure of myself.

RACE ON!!!

Joshman3382

Try back probing at the fuel sender assembly the gray and black wires. Gray is power black is ground.

When the key is in the on position (engine off) the fuel pump relay supplies the fuel pump with power for approx. 2 seconds then power is shut off. When the engine starts the oil pressure switch closes and supplies the fuel pump with power untill the switch is opened or when you loose oil pressure (I.E. when the engine is shut off.) The PCM or in your case ECM controls the fuel pump relay. I belive, and i haven't checked yet, but i think the only function of the relay is for starting the engine. The only way to check is to start the engine, remove the relay with engine running and messure voltage at terminal B2 on the relay socket. If there is voltage present then it proves my theory wrong.

Drive around untill the problem occurs. You have a 6 speed, so when the engine dies leave the car in gear and the clutch engauged. This will make sure you still have adiquit oil pressure to leave the switch closed. Check voltage to the fuel sender assembly if you still have voltage the the problem is not electrical. Make sure the car is still moving and has oil preesure when you check the voltage.
If you have voltage, check for a kink in the fuel line or any type of restiction in the supply line.

biggestjohn

You can remove the Oxygen sensor from the exhaust system to check to see if the cat is plugged. If you have a vacume gage, you can attach it to the engine, if the cat is plugged you will usually have low vacume that will steadily decline. Remember, if the cat is plugged, something caused it,and if you don't fix that, you will have the same problem. Another indicator of a cat going bad is if the car was running like a pooch before the problems started. A fuel pressure gage is very expensive, I made one out of an old Freon refill kit. The schrader valve on the end is the same size as the one on my TPI fuel rails. I just put a fuel pressure gage on the other end of the hose.Should read about 30 psi as I recall.(Make at your own risk)

CFI-EFI

"The only way to check is to start the engine, remove the relay with engine running and messure voltage at terminal B2 on the relay socket.". Or read your service manual. Once again, the fuel pump relay and the oil pressure switch are wired in parallel. When the key is turned to the "ON" position, the ECM energizes the fuel pump relay for approx 2 seconds. When you crank the engine over, the distributor sends pulses to the ECM. On receipt of those pulses, indicating that the engine is turning, the ECM again energizes the fuel pump relay. The fuel pump relay remains energized as long as the engine is turning, either by cranking or by running. Again, whether by cranking or by running, when the oil pressure reaches approx 4 psi, the oil pressure switch closes and supplies current to the fuel pump directly, in a circuit parallel to the relay. The fuel pump relay is a source of power to the pump whenever the engine is turning. The oil pressure switch backs up the relay.

RACE ON!!!

CentralCoaster

And I believe Gordon Killebrew said the same thing. You could easily rewire them into series...

Although, that wouldn't work, since it would never prime the pump.

CFI-EFI

At least I was in good company, even if we both were wrong.

RACE ON!!!

Photomania

Thanks guys. I thought this thread had slipped into oblivion. But I'm glad to see advice still flowing in. I'll study these tips and try some of these.

I'm almost at the point of having the car towed to a shop. I've replaced the ignition module, ignition coil, distributor cap, rotor, fuel filter and fuel pump. Now, the car is running worse than ever. I don't think I got to the root of the problem.

I have a question: Does the 1990 six speed Corvette have two pre-cats near the engine and one larger cat near the transmission?

I temporarily removed the main cat converter and the car ran well for 20 minutes. In fact, it didn't cut off. I had to turn it off and go somewhere. I didn't drive it though. I put the cat back on and the car wouldn't start. So, I took it off again, and it wouldn't start. This car is such a mystery.

-- Eric

CentralCoaster

I wonder what kind of garbage you pumped through the fuel system.

Right after it craps out, do you have fuel pressure, if you do, then don't mess with that stuff. Do you have spark? Bring your timing light with you.

I had a similar problem on my Ford, the ignition coil would create an open circuit when it warmed up. I hooked up the timing light and cranked it, it was pretty obvious then.

CFI-EFI

You have wasted enough time and money throwing parts (and money) at the problem. It is time to do some carefully coordinated trouble shooting.

The 1990 came with the three cats you describe. Connected or disconnected would have no bearing in whether the engine will start or not. Connected, if they were super clogged up the engine might die, but after the pressure in the exhaust system dissipated, the car would at least start again.

RACE ON!!!

GreyHairedTopGun

Go to www.misterfixit.com/fuelrlay.htm. (note fuel relay is missing the e in relay).

There you will see an explanation of how the fuel pump relay is used for starting until the engine builds up oil pressure. If the car is in a bad accident with engine stoppage, obviously there is no oil pressure, so the oil pressure sending unit shuts off the fuel pump to prevent a fire if there was a ruptured line.

I bought a relay for like 14 bucks at Corvette Central for my 89 and just put it on yesterday. On the 89, it is on the engine side of the firewall, driver's side. It is held into position with 2 small metric nuts/bolts. Mine had an electrical schematic printed right on the top of it, which made it stand out. The replacement part had the same schematic.

This was about a 5 minute job, and I wish I could tell you if it worked, but I got interrupted and haven't turned the key over. My problem was with hard starts when it was cold. I will know the next time I take it out of the garage if it worked or not, but.....you spent a lot of money on parts already, so what's wrong with throwing another 14 bucks and 5 minutes at it for a potential cure?