My 1991 corvette won't start when the engine is warm. It just cranks and won't fire. When the car is cold it starts immediately, no problems. I just put rebuilt injectors in it, although they are multecs... I ohm tested them (when warm) and on a few of them the needle jumped around a lot before finally settling at about 13 ohms. It has a brand new distributor and spark plugs. Ignition wires are not new but they are high quality MSD wires. When the car warms up it drives just fine, but idles rough, however it's never stalled while idling (it has got close though). Any ideas?

 

What is the FP reading when you shut the car down, and does it hold steady after shutdown.

 

Fuel pressure is about 45 PSI and holds after the key is turned off. I also found out the car will start with starting fluid when it's hot, so it looks like it is a fuel issue. I would guess injectors?

 

How long is the FP holding?

What is the TPS voltage and IAC counts from a scanner readout?

 

I haven’t left it pressurized for more than a few minutes but it holds throughout that whole time. It doesn’t drop off at all for quite a while.

unfortunately I don’t have a scanner at the moment. I’m taking the car to a muffler shop today to check for a clogged cat, so I won’t be able to do much on the car for a little bit.

 

Ok I got my car back from the muffler shop. The engine still runs rough at idle after its warm. I unplugged the MAP sensor and started the car and it ran fine, as soon as I put the sensor back it continued being rough. Also there is a clicking sound that comes from the engine after it shuts down. Any ideas?

 

I would put the module from the old distributor into the new distributor and see what happens when warm.
don’t forget the heat transfer paste

 

I'm sorry for the space between replies. I don't have a lot of time to work on the car. Unfortunately I don't have the old distributor. The shop that changed it offered to let me keep it but my dad picked up the car for me since I was at work and didn't take it.
However, I did take the cap off and the whole inside of the distributor is still cold when the issue occurs. The ICM and everything else in there was cold to the touch, although the engine was hot enough to cause a no start.

 

Also I found out that the longer I let the car sit while its still warm, the harder it is to start, to the point where it won't start even with starting fluid. If I leave it long enough to cool down it'll start right up though, and if I try to start it with starting fluid right after shutdown it starts right up too.

AND when the car is running I can hear and feel the injectors ticking, even when the problem occurs.

 

usually with tpi’s, a hard start when warm is the coolant temp sensor. the cts reads low, thinks its cold, instructs ecm to enrichen fuelling (like a choke) and it wont start.

if you could find a member close to u wath a scanner or a local shop, they could tell you what temps the ecm is seeing from the cts.

note its not the same coolant temp sensor that feeds the dash!

the ecm coolant sensor is under the throttle body.

the orher thing is yes, icm or ignition related. you could try and put a timing light on cyl1 and observe the spark timing to see if spark is going crazy.

also didnt u say you unplugged the maf and it ran fine?

 

I replaced the coolant temp sensor a while ago while chasing down a different issue. Also wouldn't it give a code? I'll check it anyway, if I can.
I have a map sensor, but yes when that's unplugged it runs fine, and as soon as its plugged back in it starts idling rough again. That leads me to believe it's not a mechanical issue but rather a sensor somewhere but I'm not sure.

 

I mentioned earlier that fuel pressure does not drop
this is INCORRECT
the pressure tester I had was broken, and I realized that and got a new one today. When I turn the key, pressure goes to 40psi, and drops to 30 in less than 30 seconds after the pump shuts off. It continues dropping and seems to hold at about 20psi. I'm gonna do some research and figure out how to isolate where the leak is coming from. I'll update this when/if I do.
I have a new FPR diaphragm so I doubt this is the cause but I'll check anyway.


Ok I've done some work and it seems like the problem is caused my low fuel pressure. At idle the fuel pressure drops from about 30 down to 20 or so. The drop is slow, and I suspect the problem isn't caused by heat but is caused by time, and the car just needs to be given time to build pressure again. Once the pressure dips below 20 the idle gets rough and it starts having issues. Any idea how to test for what part is causing this?

 

If that is true, then leaking injectors are likely the culprit, and that would explain the hard start when warm, its basically flooding.

 

OK I pulled the rail and pressurized it, but didn't see any leaking injectors. Also fuel pressure while the car is idling is only 10-20 PSI, and drops slowly but steadily (while the car is running) When the car is off it drops quickly.
I pinched the return line and pressure still dropped. I tried to clamp the sending line after the pump shut off, but for some reason it only pressurized to about 20psi (it did hold though). It did that out of nowhere and I haven't tried clamping the sending line again yet, although I have pressurized the system again about 15 minutes later and it went up to 40 like before.

It seems like the more I run the fuel pump the less pressure it wants to give. Both pressurizing the system and starting the car repeatedly caused drops in pressure between tries, but it went back when I waited a while.

 

if the injectors when warm have a materially lower resistance measured in ohms, then yes, when warm, injectors are likely the culprit.

if you have 10-20 psi at idle....i think u know what you need to do. Though before removal, i would first apply 12v directly to pump to ensure its not some weird heat related electrical issue that is dropping voltage.

the fact that pressure held after clamping the sending line it tells me the check valve is leaking.

while ive never personally dealt with that issue ive read about it many times here.

 

All injectors ohm about the same hot and cold, at about 15 ohms. The car started leaking coolant out of nowhere.
Looks like its from the water pump.
Now I've got 2 problems I'm gonna put a drip pan under the car and try to solve the fuel issue first, the leak is small, but I'll need to get that fixed before I start driving the car.

 

believe me there are alot more than two problems with your car you just havnt discovered them yet.

its quite possible to take a c4 corvette and make it 100% reliable, it just means you have to completely go through it !

most people, including me, just drive them and as things come up they get addressed.

The water pump is not a hard job but if you are new to diy wrenching it will take a weekend.

Ive not done a 1990, for any others who read this, is there a “hidden torx bolt” on the 90’s like there is in 85-87? I think the ac bracketry system changed in 88 so hopefully gm removed that pita.

As you go theough stuff on your car, it behooves you to tidy and fix other botched or neglected areas too. It takes longer and uses more head bandwidth but its worth.

let us know about the fuel and waterpump

 

Trust me I am aware of several more problems than just 2 lol. There's just 2 that need my immediate attention. The coolant leak seems to have been a loose coolant line that was dripping coolant down onto the water pump and making it look like it was leaking. This has been resolved and the leak is gone. I put a new fuel filter and pump in it as well.
Now the car seems to idle fine, even after getting hot, and drives great, however it still does not start when it is hot (except with starting fluid).
I took it on a very short drive, after it had got hot, and the idle hesitated once backing out of the driveway (while clutch was in, so the engine was completely disengaged from the wheels), but it didn't stall and it didn't do that again.
After I got back I let it idle for a few minutes, and tried to turn on the A/C to see what would happen and it immediately stalled, and won't start again as it's still very hot.
Any ideas?

 

what is fuel pressure doing when no hot start now that fuel pump and filter are new

 

I apologize for the late response I've been at work all week and haven't had time for the car.
When I turn on the ignition, the fuel pump runs and pressurizes the fuel system to 5-10psi. When I crank the car, the fuel pressure rises steadily until about 36psi, then it holds there. When I stop cranking fuel pressure holds and doesn't drop.

I started the car with starting fluid. At idle, the pressure is between 19-22psi. Revving the car does not change the fuel pressure, it holds steady, although the idle speed does change the fuel pressure. (At startup, the car idled at around 1100 for a few seconds, before dropping to about 900 RPMs, which resulted in a drop from about 22 to about 19 psi at the same time). Removing the FPR hose does increase fuel pressure a little bit.

While messing with the FPR while the car was running, I got shocked by the distributor. Would this indicate that the wires are going bad? I got shocked through the distributor wire on the cap. Maybe the spark is arcing at the plugs too?

This is all for when the car is hot and not starting without fluid.


It seems like the fuel pump isn't creating enough pressure, until after cranking, which makes me think of the fuel pump relay. However, even when the system is pressurized to 37psi or so, it still doesn't start, but does start immediately with starting fluid.
Also, turning the heater on immediately stalled the car. I forgot if I mentioned that or not.