'96 7.4 issue...

[TerryH]

My new to me '96 7.4 dually tripped the check engine light about 200 miles into the 400 mile trip home. It had lean condition codes for both banks. The previous owner had the o2 sensors and intake gaskets replaced a couple of months ago. Truck idles fine and runs fine at part throttle and cruising speeds but bogs at full throttle. Suggestions on where I should start?

 

[someotherguy]

#1 suspect fuel pressure. Should be roughly 58~62 psi on your Vortec 7.4. If it's a little low I'd try the filter first. The Vortec big block will start and run on seriously low pressure, unlike a poppet-valve injector equipped Vortec small block. I've had one run even at 30 psi but would fall on its face under any throttle.

Richard

 

[TerryH]

#1 suspect fuel pressure. Should be roughly 58~62 psi on your Vortec 7.4. If it's a little low I'd try the filter first. The Vortec big block will start and run on seriously low pressure, unlike a poppet-valve injector equipped Vortec small block. I've had one run even at 30 psi but would fall on its face under any throttle.

Richard

Thanks. I noticed last night that the fuel filter looks like it's been on it for a long time. I'll start there.

 

[TerryH]

Cleared the codes and changed the fuel filter. No change. Starts fine cold or warm but is very cold natured on cold starts. Runs good at idle and part throttle after it warms up for a few minutes but still bogs at full throttle and the check engine light is back on. I'll scan it in the morning but I'd guess it's the lean condition on both banks again. Going to check the fuel pressure and go from there. Also going to change both the temp sensors as the gauge doesn't read over 140 degrees. I've also noticed that the oil pressure gauge is ocassionally erratic. Engine is quiet so I don't think these actually is an oil pressure issue. Guess I'll change that sender as well and hope for the best.

 

[TXVortec454]

Check the fuel pressure. Period.

 

[TerryH]

Check the fuel pressure. Period.

Fuel pressure is 40 lbs. and falls to zero immediately when the key is off. I suppose that means I need to do the FPR and the injectors even though it does not have any sign of hard starts when warm?

 

[someotherguy]

My first suspect would be the pump, not the FPR. You can try (carefully) pinching off the return line just long enough to turn the key on for pump prime and look at the pressure - if it spikes up over 60 or so then it's definitely the FPR, but my bet is it will be the pump instead.

Lucky you that '96 pickups still had the separate pump/sender setup, so you can just buy the pump (about $75 for a Delphi) vs. having to buy the whole "plastic trash can" pump assembly the later models used.

Richard

 

[TerryH]

My first suspect would be the pump, not the FPR. You can try (carefully) pinching off the return line just long enough to turn the key on for pump prime and look at the pressure - if it spikes up over 60 or so then it's definitely the FPR, but my bet is it will be the pump instead.

Lucky you that '96 pickups still had the separate pump/sender setup, so you can just buy the pump (about $75 for a Delphi) vs. having to buy the whole "plastic trash can" pump assembly the later models used.

Richard

Interesting. I thought for sure it would be the FPR. I had read about pinching the return line but not exactly sure of the exact spot to do it. At or near the tank or?

 

[someotherguy]

If you look under the truck between the engine and fuel filter you'll see where the lines are rubber. Not the one from the filter to engine but the one coming back from the engine. It's not really recommended to pinch the lines but if you do it carefully you should be OK. I have a very thick piece of plastic I fold over the line and then clamp with vise grips, just enough to deform the line and restrict it.

Again though my bet will be the pump. The injectors and FPR are known issues on these trucks but the symptoms don't usually manifest as significantly low fuel pressure. The bad FPR will just about always give you the hard warm starts.

Richard

 

[TerryH]

If you look under the truck between the engine and fuel filter you'll see where the lines are rubber. Not the one from the filter to engine but the one coming back from the engine. It's not really recommended to pinch the lines but if you do it carefully you should be OK. I have a very thick piece of plastic I fold over the line and then clamp with vise grips, just enough to deform the line and restrict it.

Again though my bet will be the pump. The injectors and FPR are known issues on these trucks but the symptoms don't usually manifest as significantly low fuel pressure. The bad FPR will just about always give you the hard warm starts.

Richard

Thank you, Richard.