Fuel Pressure for 7.4L vs 5.7L

[Supercharged111]

So what's the cause of the "damp cranks but won't fire?"

I've got a 4.3 that has been doing this for about 2 years, maybe 3. I'm about to yank it and do a V8 swap because of low oil pressure, but would like to figure this out. Fuel pressure is good, about 58 psi on mine. I thought it was electrical?

Does the pressure hold by turning the key on and leaving it on but not starting the engine? 2 things cause the damp intake and hard starts: leaking FPR or leaking poppet(s).

 

[Supercharged111]

Absolutely, no vacuum line to the regulator in the 5.7 Vortec. But OP has a 7.4, as far as I can tell..

Richard

I'm making the assumption that the swap is a 350 and he has a 454 in his truck given the title of the thread. We shall see when he replies.

 

[stutaeng]

Does the pressure hold by turning the key on and leaving it on but not starting the engine? 2 things cause the damp intake and hard starts: leaking FPR or leaking poppet(s).

Yes, fuel pressure holds.

I was actually thinking about this last night and all of the electrical components have been changed by now.

I did diagnose a "stuck" injector on my other truck with 5.7 and replaced the injectors with MPFI, and it solved the lean code issue. I did an injector balance test to diagnose it.

Maybe I'll do the same on the 4.3...

Thanks!

 

[Crazydavez28]

I know you're talking 454, but just to be clear the 350 doesn't have a vacuum line. Also, the 454 line is easy to forget to hook up for some reason. OP how old is the regulator in the 350? They're known to fail like the 454 and cause starting/running/idling issues.

The 350 had 275k on the clock when I yanked it out of the Tahoe 7 yrs ago...and I have touched nothing in\on the motor. I used the in tank pump from the same Tahoe. IOt used to start first time every time now is sounds sluggish and the check valve only works if you give it a quick bump. The pressure actually goes up once you get it started.

On the 454 your saying my fuel pressure would be high if the vacuum line fell off and there would be a vacuum leak? This might explain the crappy mileage and poor starting. It did however sounds like there was a lot of return flow with the engine off and the relay jumpered

 

[Crazydavez28]

So what's the cause of the "damp cranks but won't fire?"

I've got a 4.3 that has been doing this for about 2 years, maybe 3. I'm about to yank it and do a V8 swap because of low oil pressure, but would like to figure this out. Fuel pressure is good, about 58 psi on mine. I thought it was electrical?

GM says it has to do with the crank position sensor but I ended up changing the pickup in the distributor. The one time I replaced the crank sensor it did not help at all. My best estimation is the extra condensation in a high mileage motor gets thru the seals in the distributor and I had little corrosion crystals all around the pickup. I also assume the water droplets mess with the crank trigger.

I tried using ether once when this was the issue and it blew up the little VORTEC box. Very loud...I was impressed.

 

[Supercharged111]

With no vacuum reference, you will have higher than desired pressure at an idle. But the injectors and regulator suck and cause issues on their own.

The thing with the distributors is that the vents get clogged and it doesn't breathe.

 

[Crazydavez28]

Im gonna hijack my own thread. Since this pertains to my fuel pressure issue... Does anybody know the center-2-center distance on fuel tank straps for a 2-door Tahoe and/or suburban?? (I sold my Tahoe or I would measure it myself)

My Elco measures 21" and it doesn't have any good options for in tank pumps. The Tahoe tank should be near perfect and can be purchased.

 

[Crazydavez28]

Conclusion #1 - 5.7L Swapped into the El Camino. I used a new Airtex sender unit from the RockAuto and after suffering thru the required modifications to make it fit in the wagon style fuel tank (see pic) she starts and runs like it used to. Using the same crap gauge engine off fuel pressure measures 58PSI. I can actually rev over 3000rpm in gear and make power again.

One other thing I did was pull the schrader valve and run the old pump with a clear hose running into a gas can. At first I got a decent stream and then I got lots of bubbles and a "slug" of fuel every now and again. Since this was the OEM plastic 4-door version sender I think the screen going to the can was plugged and the pump was also weak.


GMT400 parts on a G-body

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[Crazydavez28]

My issue with the 7.4 continues. I did some driving yesterday with an empty gooseneck and it kept up but I know it could do better. Checked fuel pressure today outside at around 95deg and it was 40psi key on and 35psi running.

I have the new FPR so I will install that first then a new filter. If that doesn't pep things up the fuel pump will be next.

 

[Crazydavez28]

Just a follow-up. Finally got around to changing the FPR on the 7.4 and now I get 45psi Key on, 40 psi idle, and 50 when quickly burping the throttle. Not the numbers I was hoping for so I may do a fuel pump as preventative maintenance.

Replaced the reference line as the old one was cracking on the OD.