'96 LT4

I have had the car for 12 years with no problem until today. I have always had the TB bypass. The only change was made several months ago; Ported intake and heads, 32# injectors. Car has run strong with no problems. Today in Tn. it is humid and high 90's on the temperature. After a short stop of 5 minutes, the engine would not hit. Turned the engine over a half dozen times, a few seconds at a time with no firing. I let sit 20 minutes and it fired right up. Ran fine on interstate until driving through several lights in town. It stumbled after leaving a traffic light and then died on the road. After a short rest of 10 minutes it fired right up and drove 30 miles on the interstate home. No codes set, no stumbling once running again. Did the engine heat cure the vapor lock?

 

Well, since there's no place to put the clothespin or the tin foil I guess the vapor lock talk comes up. I have seen vapor lock on fuel injected engines but it usually is in the Spring when the temperature gets warm too fast for the refineries. In the winter the refineries add more highly volatile (evaporate faster) compounds to the gas to make it evaporate faster and allow for easier starting. In the summer they reduce the highly volatile compounds so that it doesn't boil off too quickly and cause vapor lock. Sometimes when it goes from 50 degrees to 80 degrees in a week or so, the winter formulation is still in the pipeline. That's trouble.

If you were in the Dakota's where it's usually 80 and now it's 100 that could be a problem for the gas in that area but you're in Tennessee. You're used to winning football games (Go Orange) and high temperatures. Too bad Tennessee doesn't have a basketball team. (UK all the way)

It's highly doubtful that you have vapor lock this late into the season. Since it doesn't seem to be a problem with the Vette's maybe you should check for any codes that may help in determining the cause of the car stopping. Nothing is more frustrating than not knowing if your car will get you home or not. Although your problem sounds exactly like vapor lock I think the problem lies elsewhere.

Good luck and check those codes.

 

IMO, vapor lock is almost impossible on fuel injection....especially multiport injection. Perhaps under some extreme conditions with some old single/duel inj like a cross fire, but not a TPI. The system is always under pretension as long as there is any heat...think about that. If the fuel system is in decent shape, it will hold pressure for 6-10 hrs at 20 psi. The first 4 hrs after shut down at near normal running pressure. It should hold some pressure all night...Turn the key, 40+ lbs instantly. If its less, the problem is the pump or screens, not necessarily vaopr lock.
There are no voids, no place for cooling contraction to make voids, and no place for fuel to go in any hurry. It bleeds down naturally and very slowly back thru the return line. The pump has a check valve to prevent rapid back flow.
I've seen TPI fuel try to boil in the steel line next to the passenger side exhaust manifold, but it did not present a real problem because as long as the fuel system was in acceptable condition the pressure prevented it from being a problem...that fuel line does need to be looked at to be sure that its clamped down and NOT riding on the header or collector. The mounting bolt nut at the bottom of the a/c evap falls apart and the fuel line flops around...

Whats probably happening in your case is a dirty fuel filter. After a run the pressure and flow force the crud in the filter. After it sits for a while the pressure bleeds off and the crud backs out and floats out. It starts again and this cycle repeats until it is so plugged up that it cannot flow anymore. A simple fuel pressure check will indicate the need to investigate further.

You may also want to look at the coolant temp sensor. Faulty temps make for hard starting under some conditions. (hot)

 

Fuel Injection will vapor lock. The reason that I know a little about it is that my company car was a Mopar 2.2 turbo and my personal car at the time was a Porsche 928. One year both were vapor locking badly. Chrysler acknowledged this and said they couldn't do anything about because of the proximity on the turbo housing to the metal fuel line.

My 928 was vapor locking badly and I am lucky enough to live in a refinery town. A good friend of mine worked at the product application lab. After one irritating episode and getting a ride home in a VW Bug, I drove the Porsche down to the lab just to see what was going on.

They pulled fuel out of my car and after some serious chemical testing their answer was, "Well, you caught us". There was still the winter blend of about 15% butane in the gas and it can and did boil in the lines causing a higher pressure than the pump could overcome. It was in the Late winter and the Reid vapor pressure was still set for cold weather but we had temps in the 80's.

I didn't see how it could happen either but when you drive a 928 and get some hillbilly in a Bug offering you a ride home well............you know. Kinda like driving a new Z06 and it keeps stalling and some guy giving you a ride home in an Aveo. Irritated me, it did and I was going to find out why or whoop somebody's butt.

 

I appreciate the replies. I adjusted the fuel pressure to 43.5 just last week. The scanner says 'no trouble codes', 'no pending codes' and no 'freeze frame data' (of course, since no codes set). Although the two instances of trouble are indicative of vapor lock, perhaps pressure is compromised by possible filter contamination. The two in concert could be the culprit, no? But the engine runs fine on open highway at speed......aarrrghhhh.

 

Actually, Bama fan here with a Tn. registered Alabama Alumni tag that reads "ROLNG". I just can't see pulling off on the shoulder with my hood up while sporting that tag.

 

If you have had a marginal condition, heat will bring it to a head. You may not be retaining fuel pressure after turning your engine off that will cause a vapor lock and a hard start condition, especially in hot weather. You should acquire a TPI fuel pressure guage and monitor your fuel pressure after turning off your engine when hot. It should retain 30-40 psi for more then 20 minutes. I have seen this test pass when cold but not when hot.

Where did you fill last? Some fuel stations, especially in rural areas do not sell much high octane fuel and still have a winter blend in their storage tanks. If so, you will have such problems in hot weather.

 

Actually, while adjusting my fuel pressure last week, I could turn on the key (not start) and watch the pressure hit 43.5 for about a second and a half and then drop to zero even though the key was on. So, what gives?? I could then cycle the key and the same thing again.

 

Ahhhh. Check valve in fuel pump not holding pressure? No, otherwise the pump would keep running, true?

 

There is your problem, you either have a defective fuel pressure regulator or fuel pump. There are diagnostic proceedures to pinpoint.

 

Would the procedures to check be regulator specific or can you point me in the right direction? The regulator is an aeromotive (several years old now).

 

Yes, there is a procedure. You have to remove your fuel fill lid and rubber boot, find the fuel return line hose at the top of the tank and pinch it closed with something like a vise grip. Now turn on your ign switch to the on position, if you now hold fuel pressure, it's your regulator, if you do not, it's your pump. To know if you pinched the right line, your fuel pr will build to way over 40 psi when you turn on the ign switch. (If it's an old style Aeromotive, I'm betting it's the regulator.

 

Looks as though I have a chore or two to perform under the clamshell in the morning. and @Leesvet, I may just put a thin sheet of SS between the line risers and that exhaust.

Thanks guys.

 

..... LTx engines seem to have a love / hate relationship with their Optispark ignition systems ... If I were you I would start doing all the diagnostics for an ignition failure ... This sounds like a classic ... use the search function for details .....................

 

... With the key on - engine NOT running , the fuel pump will run for only 2-3 seconds ..............

 

Correct. At which point, the pressure then drops to zero. That is why I believe we all think it is bad regulator, pulse modulator, or check valve. Pump thinks it still has pressure since it quits after pegging the pressure at 43.5.

 

New dynaspark opti. a few months ago during reassembly of ported intake and heads. Bad opti will usually fire, miss, stutter, hit, miss, cough, choke, run, die. My symptoms are vapor lock due to lack of maintaining good pressure at shutdown.

 

fuel pump check valve, injector or pulse modulator leaking. GM Pinch test should narrow it down to which one.

If pulse modulator, you can replace it with a couple inches of fuel injector hose (submersible grade) and a couple small hose clamps to connect the sending unit feed directly to the pump.

 

..... The fuel pump does not stop because it thinks it has pressure ... The pump quits because the ECM tells it to stop after priming the system for 2 -3 seconds for start-up ... it does this to remove air ( vapor ) from the fuel rails ... it will only continue to run if the engine is started ... the fuel pressure regulator determines where the pressure pegs ... not the pump .......... I would have the ICM checked or replaced ... at least its easy to get to ...

 

Good info. I did not know that. Problem has been determined to be bad aeromotive regulator. I tried a cheapy from autozone that held pressure but, unfortunately, the advertised pressure of 45psi only got to 40. New holley adjustable 507 on order at this time.

Thanks to all. There was a great deal of quality input on my problem from all of you and I greatly appreciate it.