My 71 would vapor lock when the fuel would get to hot but that is a carb and mechanical pump occurrence. Does this happen with FI 91 C4's? What switches or sensors would go bad that would cause fuel smell, excessive idle, irradic idle when coming to a stop?

Out on the road it runs great but the above mentioned symptoms are occurring. I just replaced cap,rotor,coil and ICM. This was due to it not starting up for at least 15 minutes after being driven. I have not had this occur again sense replacing those parts but have not driven that far yet. The car shows no codes.

Thanks
Wade

 

Are there any visible fuel leaks on the ground if you park it and let it idle for lets say 20 minutes?

 

This is a VERY interesting subject !

the EFI vapor lock question.....

in theory, this CANNOT happen since the fuel is always under pressure so there is no 'void' in the line. It can't go thru an injector unless its running, and IF the pump is in good shape it cannot get back to the tank, back thru the pump.

I use "IF" a couple times...

Vapor locking is fuel boiling and turning liquid into vapor or a semi-gas instead of fluid. If the system has pressure, this cannot happen. Again.....IF.

So, as long as the system has pressure the fuel cannot boil. That's the big "IF".
I've seen it happen though...a fuel system that's got a bad check valve in the pump will boil the fuel at the #6 & 8 exhaust manifold where the steel fuel line comes very close to the hot exhaust. "IF" there is a leaky injector that allows the pressure to bleed off, remaining fuel can vaporize.
In EFI though, all this means is that it'll be hard to start until its cooled off a bit. OR, you can manually bleed off the "vapor" at the fuel rail Schrader valve and you're good to go again. I would not advise this because if you think about it, a hot engine, spraying GAS liquid as well as heavy fumes/vapor....sounds like a car-b-que to me.
The only other means of starting it would be to crank on it so long that the vapor was pushed thru the injectors as they cycled during cranking...thats assuming that the rails are under so much pressure from the vapor and now the pump is sending more fuel. That "void" of gas vapor has to go somewhere before gasoline can get to the injectors again...that could take a while and its hard on the starter....long cranking is hard on everything.

IMO, yes, the EFI can potentially vapor lock BUT its also able to solve the problem since its not depending on a running engine to supply more fuel. The electric fuel pump runs when the engine does not.
if you stick a small electric pump inline on a carbed engine that will end most vapor locking issues. Just be sure the float valve needle & seat are ok !

Vapor locking is not your problem. A bad fuel regulator probably IS.

Take the vac line off the reg after some run time. See if its wet and smells of gas. If so, the reg diaphragm is faulty and must be replaced.

next would be stuck fuel injectors. These inj are batch fired, meaning the whole bank fires together off of a single circuit that's split 4 ways. If one inj shorts out , that means they all short out. If its a short to ground then all 4 inj are stuck wide open dumping fuel.
This is unlikely as the engine will not run with 4 inj dumping fuel.
What does happen is 1 inj may be stuck and dumping gas. Maybe 2. You might just have the whole set leaking and bleeding down. A system that keeps fuel pressure on the rails will have an endless supply of gas to get dumped into the cylinders...as long as there is pressure the gas is gonna go where it can. Leaking inj are proven by the leak down time with the pressure gauge connected to the rails.

Pull the dip-stick and again, smell for gas. Stuck injectors will put so much gas in a cylinder that it gets squeezed past the rings and down into the crankcase in the oil. I've seen 2 qts TOO much in the pan ! All gas.

The reason that I am going straight to the fuel system is because that's about the ONLY thing that cannot set a code and give the symptoms that you describe. Rich smell at idle, rough running etc...

I can also tell you that your fuel injectors, if original, are TRASH and need to be replaced asap. Its not about how many miles, its about the age and the fact that they are well known for the problem they have.
They are the cause of most of the rough run, hard to start problems. These stock injectors were not designed for alcohol blended fuel so they get damaged internally by the internal seals failing and this is where/how they get stuck open and dump fuel, running or not.

Do this to PROVE the problem.....

get the fuel pressure test gauge on the engine. Turn key to ON but DO NOT start.
The pressure should go to about 40 psi and stay.

Wait. Watch. This test is about 1 beers worth of wait time.

It should hold every bit of that 40 psi for 10 minutes and then it should hold about 35-38 psi for the next couple hours. It should take all day to drop to 20 psi and it should hold the 20 for more than a day. THATS a good, tight fuel system.

if the pressure drops immediately after the pump stops, that's a problem.

If it takes more than turning the key 2 times to get the pressure up to 40 psi, THAT is a problem. Remember its only running the pump for 2 seconds, so give it a chance !

When you start up the pressure should be stable at 38-40 psi. The gauge needle should be steady and NOT bouncing around.

Once you have this information then we can figure out what part of the fuel system is at fault. But, regardless of what else we find, you DO NEED injectors if these are stock ! The usual "fix" is to call Jon @ FIC and he will fix you up. Good service, good parts. Its DIY so have a rag and your tools handy. The replacement injectors will cost you about $300. They are Bosch-III, the latest generation for this design. Never seen a complaint about these injectors.
The 1st time you pull the plenum on a L98 will be challenging....after that its cake.

My money is on the regulator for the current problem. It may be only the injectors, but you have to go thru it to find where and why there is that much fuel going thru the engine. That's real hard on these engines....that extra fuel is killing the cat and its destroying the engine by washing the oil off of the rings and cylinder walls.

Do the fuel pressure test and let us know...regulators are about $75 IIRC and there are some bad ones in circulation, so do let us know whats going on so someone can guide you thru the reg "inspection" process if that's what you need.

Every other sensor, air/fuel metering system will set a code if there is fault somewhere. Even relays...so no code, not a sensor issue. Go to the basics of spark and fuel delivery.

Good Luck !

 

fuel line leaking is the most common, Injector orings would probley be next. in thory a bad oring will also cause a vacume leak and this could also cause your other problems. i would look hard for a vacum leak. egr sticking, brake booster, intake leaking any place you could get a vacume leak would be where I would start