What keeps fuel pressure in the fuel rail when the engine isn't running? Is it the fuel pump, fuel pressure regulator, or what? Whatever it is I need to replace it. I've got a 7.4L and it takes some cranking for fuel to get to the engine, then it fires right up and runs great.

The 7.4's have a schrader valve on the fuel rail in front of the engine and after the truck has sat for a while there is no fuel pressure in the fuel rail. Once its running, its got pressure. Also, does anyone has a fuel system schematic they could share? Thanks.

How long is "a while" it wont maintain pressure in the rail for long periods of time.

When you crank the engine pressure should build quickly.

A while is within an hour or two.

It'll take about 4-5 seconds of cranking to fire up.

I would check for injector leakdown, also a faulty fuel pressure regulator.

Is pressure within normal range when the truck is running?

Check the fuel filter. When you turn the key on the pump should come right on and pressuize the rails. A dirty filter will fuck things up. Hey its happened to me! Check the simple things!

There is a checkvalve in the fuel pump. I wouldn't say it's any less or more reliable than any other brand, but you don't see problems with it often on GM fuel pumps because they die long before it becomes an issue. :laughing: That very easily could be the problem, as could a faulty regulator or leaky injector. However, if your LTFT's are normal it's most likely not an injector or regulator issue as they would be dumping fuel (via the diaphragm or the pintle) while it was running and you'd see it trying to lean out the mixture a lot. If it is a fuel pump I'd probably just get used to cycling the key once before cranking it up. It just isn't worth the cost and trouble to me and has never been for my customers when the check valve fails. I'm not aware of any generic check valve that can be put in the supply line.

Did some testing last night and I'm thinking bad injectors. Heres why: Within a split second of turning the key to the on position, I've got 54psi, when running 46psi, both within specs according to the Chilton manual. When I shut the engine down fuel pressure builds back up to 54psi (the fuel pump runs for a couple seconds). Within 5 minutes I'm back down to 30psi.

Dr. Chilton says its either the injectors leaking into the cylinder or the fuel pump. I'm thinking its the injectors because after the truck sits overnight it fires right up in the morning. I think the fuel leaks out of the injectors, into the cylinder and past the rings into the pan. Now, if I park the truck and try to restart in an hour or two, their is too much fuel in the combustion chamber and the spark plug isn't igniting.

Thoughts? Comments?

What are the fuel trims at? You should see them running negative as it tries to pull fuel since the injectors are leaking. I haven't seen a ton of problems with the newer GM injectors (multec 3?), but that doesn't mean you don't have an issue. A little BG 44K can fix a lot of injector problems. I'm not one for mechanic in a can, but the stuff works.

Its the fuel pressure regulator. Its vacuum referenced ionto the lower rear portion of the intake.father in laws truck had same trouble. I busted my ass trying to diagnose it and ended up discovering an inch deep puddle of raw fuel in the lower intake.If you leave it sit for 8-10 hours it always fires up on the first rev right? thats cause that fuel puddle is evaperating sitting in a hot manifold. His would spray fuel into the manifold upon the key on fuel pump cycle.Jim

experience - I've never heard of these injectors failing either.

86k30 - yep, if you let it sit overnight or just for a long time, it fires right up.

I guess I'll start with the fuel pressure regulator, its the cheapest. I know exactly what part you are talking about. I thought Napa had the wrong part but I realize now how it works.

I have seen the isolater between the pump and metal tube on the sending unit assem fail which causes the the system have weak pressure or bleed down problems after turning off engine. The factory replaced the coupler with regular house and clamps some time ago, bit who knows what you got in the tank. Only buy a factory pump or assem if at all possible. Is the engine oil smelly, dirty and/or do you have any CEL/SES?

Injectors are know to have problmes at varying age and mileage points as does the FPR. Both require the removal of the upper intake to repl. Depending on mileage, consider repl injectors, FPR and plastic dist at one time. Easy job.

You can get up speed by reading start to finish the following:

http://www.fullsizechevy.com/forums/general-discussion/technical-maintenance/213662-96-99-454-vortec-owners.html

Andy

I have this same problem with two different trucks, one personal truck and one work truck.

On my personal truck I diagnosed it as the fuel pressure regulator. Pressure would drop dramatically after I shut the truck off with a guage on it. When the truck was warm it would have to crank for a while to start then it would spit out black smoke.

My work truck had the same exact symptoms but a local shop diagnosed it as the injectors. The truck is a fleet truck which we are not to work on (long story). The truck was fixed after that but they also put in a regulator at the same time as the injectors since it is only one $40 part versus eight $80 parts.

hope this helps.

Don't get me wribgm tghese trucks have injector problems. Thye just aren't like the old 4.3's. Here's my issue with blaming injectors. The injectors are located centrally with electronically operated pintles like any other injector. However, the fuel must travel through the spider tubes to the nozzle ends and push the poppets open. Being that these trucks will fail to start with fuel pressure that is even 2 psi too low because the poppets won't move I have a hard time believing that the issue here is a bum injector pintle that's sticking open and allowing fuel out to a poppet valve that is so weak that it can bleed the fuel pressure down to nearly nothing when they don't typically open below about 45 psi.

The fuel pressure is held in the "rail" (these trucks don't really have a rail in the traditional sense) by two things. You have a check valve in the pump on one side so that when the pump isn't on the fuel (and pressure) can't return to the tank that way. On the return side you have the regulator which contains a calibrated spring that requires a certain PSI to be forced open and allow fuel past it. A vacuum line on it works on the diaphragm making it easier to open at high vacuum dropping fuel pressure and harder at low vacuum. With the engine off there is no vacuum so it would require full fuel pressure to open and bleed off...unless it had mechanically failed or the diaphragm was leaking into the vacuum line.

If the regulator was bad then engine would have less fuel pressure to work with and therefore would need to greatly increase the injector pulsewidths to get enough fuel to make up for the lower pressure. Typically these trucks will cut out or not run with a bad regulator. If you have a fuel leak in a poppet tube or injector then the pulsewidths would be much shorter since there is unregulated fuel being dumped that the O2 is going to pick up on. If the check valve is bad they just require extended cranking time, but there is no ill effect once it starts.

I could easily be barking up the wrong tree here. I'm just basing this on my experience. The regulator is going to be easier and cheaper to change than the whole fuel pump. Pop the upper plenum off and replace it. Check for liquid fuel in the vacuum line to it. Also, look for liquid fuel pooled in the intake. Maybe you have a bad injector and one of the spider tubes rubbed through.

What are the fuel trims at? You should see them running negative as it tries to pull fuel since the injectors are leaking. I haven't seen a ton of problems with the newer GM injectors (multec 3?), but that doesn't mean you don't have an issue. A little BG 44K can fix a lot of injector problems. I'm not one for mechanic in a can, but the stuff works.

The problem is with his fuel pressure regulator, if it's his injectors leaking i'd also recommend the same solution for him.

Buy the upgraded Multec 2 spider assembly (you were close :D) it cost a couple hundred bucks online, but it replaces the stock poppet valve injectors with a much better design.

It's still a 19# injector, but isn't so susceptible to minute injector clogging. Since you're gonna have to remove the upper intake to get at it anyway, you might as well get the whole thing replaced (FPR, spider,injectors). I'd also include the redesigned intake manifold gaskets as well. The stock ones are prone to leaking and can cause a myriad of problems.

This will prevent some of the common 1st gen vortec issues from biting you in the ass later, leaving you with a very nice engine at the end.

The problem is with his fuel pressure regulator, if it's his injectors leaking i'd also recommend the same solution for him.

Buy the upgraded Multec 2 spider assembly (you were close :D) it cost a couple hundred bucks online, but it replaces the stock poppet valve injectors with a much better design.

It's still a 19# injector, but isn't so susceptible to minute injector clogging. Since you're gonna have to remove the upper intake to get at it anyway, you might as well get the whole thing replaced (FPR, spider,injectors). I'd also include the redesigned intake manifold gaskets as well. The stock ones are prone to leaking and can cause a myriad of problems.

This will prevent some of the common 1st gen vortec issues from biting you in the ass later, leaving you with a very nice engine at the end.

96 up big blocks are MPI unit injectors, not like the 5.7 vortecs as you've described.

I drifted in and out of small block land too. :laughing: I'm still waiting to hear what his fuel trims are at. Or...if replacing the regulator as a shot in the dark worked.

Did some testing last night and I'm thinking bad injectors. Heres why: Within a split second of turning the key to the on position, I've got 54psi, when running 46psi, both within specs according to the Chilton manual. When I shut the engine down fuel pressure builds back up to 54psi (the fuel pump runs for a couple seconds). Within 5 minutes I'm back down to 30psi.

Dr. Chilton says its either the injectors leaking into the cylinder or the fuel pump. I'm thinking its the injectors because after the truck sits overnight it fires right up in the morning. I think the fuel leaks out of the injectors, into the cylinder and past the rings into the pan. Now, if I park the truck and try to restart in an hour or two, their is too much fuel in the combustion chamber and the spark plug isn't igniting.

Thoughts? Comments?


I don't think the problem is your injector, most likely the seal's on the injectors. I had to change all the seals which was a little bit of a pain but now I have no problems.

So you were leaking fuel past the rail, down the injector body, and having it pool up on the intake? That should be evident enough. The lower injector o-rings just seal vacuum, not fuel, so there's no way those seals would allow leakdown.

I haven't had a chance to work on the truck this week, hopefully this weekend.

I'm now thinking its the FPR because when I tore the engine apart the first time - about 2 months ago to replace the lifters and a broken valve spring - I didn't see any fuel pooled up in the intake.

96 up big blocks are MPI unit injectors, not like the 5.7 vortecs as you've described.

Frig i was not paying enough attention, i apologize. I needed alot of sleep that day :shaking::homer:

Upper intake manifold is off right now.

I replaced the Fuel Pressure Regulator and I don't think that was the problem. With everything apart I turned the key to run and let the fuel pressure build up to 52psi. After 5 minutes it was down to 35.

A couple of the runners in the lower intake manifold (right under the injectors) look more damp than others. I'm thinking those might be the leaky injectors. Or they just didn't dry as fast as the others and my theory is Ka-put.

IDEAS? Tests I can run while this thing is apart?

Allright, my Dad is thinking its not even fuel related. If it fires right up after sitting for 12 hours, but has problems and stumbles if it sits 1-6 hrs, hes thinking something like a crank position sensor that goes bad when it warms up. I have plenty of fuel when cranking over and it still doesn't start for a while.

Here are some pics:

http://i7.photobucket.com/albums/y281/MrZ71man/Dans%20Truck/IMG_2141.jpg

http://i7.photobucket.com/albums/y281/MrZ71man/Dans%20Truck/IMG_2142.jpg

And here you can see injector 4 is damp compared to injector 2.

http://i7.photobucket.com/albums/y281/MrZ71man/Dans%20Truck/IMG_2143.jpg


EDIT: The pics make it look like there is rust in the manifold, but there isn't. The whole thing is cleaner in real life.

The big blocks (and small blocks for that matter) can do some goofy things when ignition parts are on their way out the door. I've seen a module do similar things to what you're describing. Sadly, the GM strategy for diagnosing it is to just replace it and see if that fixes it at $90 or whatever they are. :shaking: We managed to catch it with a Vantage Pro...it was the damnedest thing. It would produce a perfect square wave, but not with the same frequency as the new module meaning misfires that you could hardly feel other than the lack of power (like retarded timing almost).

So I put the truck back together and the fuel pressure regulator was part of the problem. The truck never started right. Now when you fire it up it either starts right up or you have to crank it for 4-5 seconds. No in between like there used to be.

I sold the truck over the weekend so we will never know what the real problem was. Talking to a couple GM techs, they were banking on the fuel pump being the culprit.

Thanks for all the help guys. Hopefully this thread may help someone in the future.