MAF Sensor/Fuel Trim/ Vacuum Leaks

[Schurkey]

I have a 96 gmc sierra with 4.3 V6. At first truck idled normally but bogged down when accelerating, sometimes would shut off. Borrowed a maf sensor from a buddy and is driveable now and made a big difference but if i get on it from a stop it's sluggish at first. If i gradually get into it, it drives good. Now im getting some long term fuel trims that are off big time. On LONGFT1 and LONGFT2 im getting between 16 and 25 which points to a vacuum leak. I made a homemade smoke machine and im not getting any leaks. Ive searched up and down using a flashlight and all... ...What else could cause the LONGFT's to be off like that? Fuel pump is new and has the right pressure. Any ideas anyone?

Aint the brake booster, just checked and made no difference. Any ideas anyone?

Goofy fuel trims don't "point to a vacuum leak". At least, they don't point ONLY to a vacuum leak.

Low fuel pressure, failed "spider" injectors, defective O2 sensor(s), could all cause this problem.

Connect a scan tool, look for issues in the data stream. Verify that there's no misfire.

What do you consider "right pressure" for the fuel system? Will it maintain that pressure under heavy load?

 

[Marcos Beltran]

Goofy fuel trims don't "point to a vacuum leak". At least, they don't point ONLY to a vacuum leak.

Low fuel pressure, failed "spider" injectors, defective O2 sensor(s), could all cause this problem.

Connect a scan tool, look for issues in the data stream. Verify that there's no misfire.

What do you consider "right pressure" for the fuel system? Will it maintain that pressure under heavy load?

Ok so hooked up a fuel pressure gauge and turned key, pressure shot up to 61 psi and then dropped after the fuel pump turned off to 20 psi and slowly lowered. Did this a few times with the same results. Symptoms of a leaking injector or fuel regulator right? I cranked on the truck and fuel pressure ran at a constant 50 to 52 psi. Tried accelerating by hand but car truck would bog down which is the original issue i was having. I replaced fuel pump probably around 2 months ago. So i ended up taking plenum off and checking the injectors and fuel regulator. Turned key on without cranking and nothing, no leaks from injectors or regulator. Dumb question but if these are bad arent you supposed to see some time of leakage? I heard that fuel pumps can cause the same problwm and im starting to think that it is. Is there some type of check valve in the pump that keeps the fuel line pressurized?

 

[thegawd]

I'm not an expert in this area... I have not opened the intake... but they could also not be receiving the signal to fire.

theres a way to test each injector with a 9V battery to see if they spray. in not familiar with this though....

 

[Marcos Beltran]

I'm not an expert in this area... I have not opened the intake... but they could also not be receiving the signal to fire.

theres a way to test each injector with a 9V battery to see if they spray. in not familiar with this Ayv

I'm not an expert in this area... I have not opened the intake... but they could also not be receiving the signal to fire.

theres a way to test each injector with a 9V battery to see if they spray. in not familiar with this though....

What else could cause the pressure loss when the keys turned on?

 

[Marcos Beltran]

Ive been doing some research and from what ive been reading is that a failing pump could be causing the issue too with the pressure dropping. These fuel pumps have a check valve that fail apparently. Anyone run into this issue before. Like mentioned pump was replaced 2 months ago but it was a cheap one from Autozone.

 

[99bluetahoe]

What else could cause the pressure loss when the keys turned on?

I believe there is a check valve at the pump in the tank but this is not going to cause your fuel trim issues.

 

[stutaeng]

To check for a leaky check ball, you have to somehow plug the fuel return line. If the pressure holds when the line is temporarily plugged, then that would be your problem. On flexible hoses I've seen guys use hose pliers, but all the lines on the Vortec engines are hardlines.

Have you done an injector balance test? Could also be FPR. I can't remember the diagnostic for it off the top of my head.

 

[99bluetahoe]

Ive been doing some research and from what ive been reading is that a failing pump could be causing the issue too with the pressure dropping. These fuel pumps have a check valve that fail apparently. Anyone run into this issue before. Like mentioned pump was replaced 2 months ago but it was a cheap one from Autozone.

I have always had good luck with oem delco pumps vs aftermarket.

 

[stutaeng]

Actually, now that I think about it, more like a leaky FPR than check ball failure. I would just go ahead and replace the spider injector assembly while you are in there. They come with a new FPR.

 

[kennythewelder]

To check for a leaky check ball, you have to somehow plug the fuel return line. If the pressure holds when the line is temporarily plugged, then that would be your problem. On flexible hoses I've seen guys use hose pliers, but all the lines on the Vortec engines are hardlines.

Have you done an injector balance test? Could also be FPR. I can't remember the diagnostic for it off the top of my head.

I thought the same things about all the lines being hard lines untill recently. If you crawl under your truck, and look on the drivers side, you will see a small section where both of the fuel lines have a rubber hose on them.it can be crimped there. Its rite about where your feet are when driving, but under the floor board.