leesvet

That says it all...
I don;t believe in it, because in theory its not supposed to happen.

BUT.....


Vapor lock,
to anyone thats been around since the quadrajet is simply a condition caused by heat that allows the fuel to boil in the lines so that its not getting liquid into the float bowl of a carbonator.

This creates a higher pressure going the wrong way in the lines so the weak fuel pump (8-15 psi) on a carb motor can't push enough to over come the boiling...so the carb starves for fuel and the motor dies. Fuel lines are full of air and gasses, not liquid fuel.

Now in theory, this simply isn't possible in EFI since we're talking about 40+ psi fuel pressure instead of 10 psi in a carb. The EFI is also pretensioned, meaning that it always has tension on the injectors so the pressure is relatively stable and consistant. So, fuel should not be able to boil if its under pressure and has absolutely no place to go.

BUT,
Its happened a couple times now...My fuel pressure is 42 @ key ON and stable @ near 50 idling. Pump is good, filter is good, injectors are less than 1 yr old. B-III's.

Symptoms.... while cruising or mild accelleration the engine stumbles as if running out of fuel. (only when temps are EXTREME) If I back off and give it gas gently it might pull but it eventually starts to starve again. It will die if allowed to drop to an idle, like it did today on a Texas black-top road in 105 degree heat....

Its happened 2 or 3 times in the last few months. This time I was just getting to a point where I could cruise again.; stupid assed people actually stopped traffic on the 4 lane freeway to look at a p/u overheated on the side of the road. Of course with 105 ambients...mines next. I wasn't too concerned,just annoyed but it did creep up to 245 with the a/c...thats when I chickened out and cut the a/c and starter to sweat, literally.

It was HOT,. but after a few minutes we started moving again. Thats when the engine started to stumble.

FF to pushing a Corvette across 2 lanes of traffic with morons that refuse to even slow down as they pass less than 18" away.......

Its hot...and the starter DOES suffer from heat-soak in extremes, so its barely able to crank. It tries to fire but no go. I have to let the starter cool off or it'll melt down.

After sitting for 15 or 20 minutes, hood up to get some air in there, and venting the gas tank by pulling the cap, it cranks and fires right up. Runs fine even though its still sitting at 220.

I do not believe that vapor lock is possible in fuel injected engines...But this sure smells like vapor lock.

I can;t find anything else wrong with it. Besides the tired starter (thats getting replaced by a mini-torque soon) there isn't much else wrong with this engine.

Any thoughts? Has anyone really experienced the vapor lock symptoms to the point of it stalling the engine and NOT allowing restart until it cools down somewhat?

one more thing...

On the L98 the location of the steel fuel lines is damn close to the exhaust toward the bottom of the evaporator box...maybe 3" away. Is there or WAS there some form of heat shield there at one time? There sure seems like there should be, but I do not have one and wonder if someone removed it before I got the car many many yrs ago?

Any thoughts and ideas would be appreciated.

383vett

The C4 fuel injection is a recirculating system with 40+ lbs of pressure. Vapor lock is not possible. Fuel cannot vaporize with that amount of pressure and underhood heat. I would look elsewhere. The ignition module comes to mind. They can act strange in hot environments.

James93LT1

Cats going bad.

gunnerjuju

why this theory? just curious

leesvet

I'm wondering about VL after having ruled out just about every other possible fuel system problem...

So, I'm down to a process of elimination... and there are people that swear that VL is still possible even in EFI. Vapor locking was a common problem with street rods in the 50's and 60's...with simple fuel systems that were not under pressure, it was pretty easy to boil the liquid and build gas pressure that prevented liquid from flowing to the carb until it cooled off. Mechanical fuel pumps only push 10 lbs give or take...It takes a 16lb cap on your radiator to prevent that from boiling over ...and even then it still happens. It happens more when the heat source is more like 800 degrees just a few inches from the lines.

As previously stated, I DO NOT believe that vapor lock is possible with 40 psi fuel pressure. Thats plenty of pressure to prevent boiling and gassing inside the closed fuel lines.

BUT

thats what it acts like. Stumbles, like there is no fuel, sputters, dies. I can throw the gauge on there and there will be the usual 40+ psi at key ON. It usually takes a few minutes to find the fuel test gauge and et it on,. by then its cooled somewhat and I get pressure. I dunno... My regulator is set high, so it hits near 50 at idle.

It may be hell to restart untill I let it sit and cool for 20 minutes,. then it lites off and runs fine. This won;t happen again for weeks...months.

I am also wondering IF the ICM could be behind this bizzare behavior...so I think I'll swap in a new one tomorrow. Seems to only happen when its close to melt-down when ambients and a/c drive the coolant temp past 240. Engine runs like a top even at those temps...as it does after these strange shut downs..

I hate pushing my Corvette to the edge of the road and having a couple women in a mustang ask if I need some help....



Still got the question of the heat shield by the evaporator...was there or is there supposed to be a shield to cover the steel fuel lines since they are acually less than 2" from the exhaust? That is the factory mount too....there is a small stud in the bottom of the evap box that the fuel line clamp sits on. Secured by a nut.

93Rubie

Run a much higher octane fuel and see what happens under the exact same conditions. I had a lawn tractor so what your describing and I used 93 instead of 87 and it ran just fine. I eventually rebuilt the carb and re-routed the fuel line. Problem solved.

Might be worth a shot just to confirm a possible VL issue. I do not believe you have a VL issue however.

Cliff Harris

The reason carbureted engines vapor lock is because the fuel pump is mounted on the engine and SUCKS fuel out of the tank. When it vapor locks the pump loses prime and can't suck.

I don't see how an EFI system could vapor lock. The fuel is circulating constantly so even if it did boil the pump would just push more liquid fuel into the fuel lines and push the vapor into the tank through the return line.

C'mon Lee, you never mentioned a fuel pressure gauge. After seeing so many posts from you here and on C4 Corvettes, I'm sure you HAVE one. Hint, hint... ;-)

I have heard of ICMs failing from the heat. From what I've heard, they usually recover after they cool down, without permanent damage. Usually they get flakey because somebody used DIELECTRIC grease (or no grease) instead of THERMAL grease (though you should get the right stuff in the box with the ICM).

Tom400CFI

NO! That is not at all what happens when a carb'ed engine vapor locks. Think about what you're saying and how a carb works.

Here is (partly) why you're wrong:The other thing is same as in a modern EFI; if fuel boils in the line from the pump to the carb, the "pressure" built from the boiling will push fuel into the bowl then the vapor will enter the bowl and simply exit out the float bowl vent. The fuel pump will continue to push fuel into the float bowl. The other way boiling can affect a Carb is when there is rampant boiling in the bottom of the bowl, in the vicinity of the main pick-up point. A situation that doesn't exist in an EFI system.



You're right that EFI won't boil, but your reasoning for WHY is way off. It won't boil for two reasons; 1. Increased pressure raises the boiling point (dramatically -like into the realm of 400*+, depending on the fuel) and 2. the (relatively cool ~110* on a HOT day after a lot of driving?) fuel from the tank is constantly circulating through the system. Put a temperature reading device on the fuel rail while it's running. I bet you won't see temps higher than 150*F. Explain to me how fuel can boil at 150*F at 4.4 BAR? It can't and it has nothing to do with in having "absolutely no place to go" In fact, it DOES "have a place to go"...IF you did infact boil your fuel, which I guarantee you, you're not, but if you did, the expansion would simply push fuel and possibly vapor, out your FPR and return line.

No it hasn't. Something else is happening. Diagnose it using logic, not "lore".

So why don't you hook up a gauge and leave it there...or better yet, somewhere where you can see it from inside the car?

Rather than throwing parts at it, why not diagnose the problem? You said that is needs ~20 minutes to sit cooling before it restarts again. In the 20 minutes that you can't get it to start, you should be able to EASILY determine what is missing; spark or fuel. Get a gauge on there and leave it, (if you can't hook one up in less than 20 minutes), and carry a spark tester with you (bought or home made). Next time the symptom occurs, figure it out. You need spark and fuel. Which is missing?





Octane rating has no affect on a fuels boiling point. That is not to say that your experience is false. The boiling point of gasoline is affected by the make up, additive package etc. In fact, I went through this with my vendor at work b/c my mechanics thought the same thing. I got a report from the vendor and in OUR CASE, the 85 octane had a slightly higher (6*F) boiling point than the 91 octane. But that was incidental; lower octane doesn't universally have a higher boiling point than higher octane. The gasoline you bought may have been opposite of the fuel that we buy here, but not because of the octane rating.