Low fuel pressure in first gear.

[Doughnuts]

Hey everybody!

Due to the terrible nature of things going on I have been labeled non-essential and have been laid off.
But never the one to sit on my butt, I decided to work on the old girl dealing with one of the most annoying features of the truck.

It has a tendency to, when warm, bog down heavily in first gear at wide open throttle.
I know it is a fuel related issue due to hooking up a fuel testing gauge and seeing it plunge to nothing then bouncing back up to 10 p.s.i.
It only happens around the 2-3 thousand rpm mark and only lasts a second. after that it'll run through all the other gears just fine without stuttering or bogging down.

I replaced the fuel pump last year hoping to fix the issue but it still has the same problem.
and I also doubt it is the fuel pump due to like I said both pumps had the same issue along with the fact that its a good name brand Delphi pump.

One of the things I believe could be related but am skeptical of is a new ignition control module, due to the fact this started happening around the time the bogging issue started occurring. but to me that wouldn't make sense because that system only controls the timing of the engine.

I am open to any suggestions, and hoping I can get this fixed on a slim budget.

Here's hoping that everyone is healthy and staying safe in this troublesome time!

 

[Doughnuts]

Edit
Another note is it only happens from a dead start, as in you floor it from a dead stop. I don't like doing that to the old girl but there are times you have to get out of the way and stalling in the middle of the road doesn't help.

 

[Supercharged111]

How's the filter? I'd look at that and the regulator, they're pretty notorious for sucking.

 

[PlayingWithTBI]

I know it is a fuel related issue due to hooking up a fuel testing gauge and seeing it plunge to nothing then bouncing back up to 10 p.s.i.

Going back on your previous posts, I gather you're talking about an 89 TBI truck? I haven't looked to see if you've done anything to the motor so, let's start there - 10PSI is weak at best (pressure regulator?), the OE fuel pump for a TBI system can't support the higher flows you need to support much more than 190HP. The better solution is to go to a pump from 96 - 98 Vortec IE. ACDelco EP381 IIRC. Or your fuel filter mat be partially plugged, as @Supercharged111 just mentioned, HTH.

 

[Doughnuts]

Its a 91 350. Basically bone stock however, I have rebuilt the TBI along with some of the ultimate TBI mods it a few years ago. It is running between 10-12 psi while running, edging towards the lower end of the spectrum while driving. I did replace the fuel filter, but that was probably two years ago so I might switch that out tomorrow. Also I am testing the fuel pressure either right before or right after the filter so that might have something to do with the overall pressure, even though i would assume the pressure would remain the same throughout the entire system.

 

[PlayingWithTBI]

Also I am testing the fuel pressure either right before or right after the filter so that might have something to do with the overall pressure, even though i would assume the pressure would remain the same throughout the entire system.

Theoretically the system should show the same pressure from pump to regulator IF, your filter isn't plugged or your supply line isn't pinched somewhere in between. Your test should give you an idea.

 

[PlayingWithTBI]

Thinking about it, did you replace the suction sock in the tank?

 

[Doughnuts]

I did not, however I did hose it down with brake cleaner and gave it a good blast of air. the tank is also spotless so I doubt it any contaminants in the system.
I still don't think it has anything to do directly with the fuel pump its self do to the fact that it runs fine and only does it in first gear and a certain rpm range. I feel like
if it did have something to do with the pump or filter I would be having drivability issues at higher rpms and in higher gears due more strain on the engine thus requiring more fuel.

 

[L31MaxExpress]

One though I have...The fuel pump wiring has always been sub standard in these trucks. Add in 30 years of corrosion, wear and tear and it is likely that the pump is not getting adequate voltage.

On my Vortec Express van I added a Racetronix Hotwire kit. It supplies the pump with power directly off the alternator output stud through a dedicated 10 gauge circuit, fuse and relay. My Delphi replacement pump stayed at 70 psi at WOT after which is what I had the regulator set at. I now run the Corvette filter. It runs at a steady 58 psi now.

The Vortec pump works well in a TBI application. Ran one in my old G20 van. A TBI generally runs best at 13-14 psi with a stock engine.

 

[Doughnuts]

Replaced the fuel filter and got about an extra pound in pressure. It now sits around the 12 psi while driving which should be fine for the entire throttle range correct?
It's still bogging down in first gear though so I'm thinking it has to be some sort of sensor or controller shutting off the fuel pump for a second before it comes back to life.
I'll look into the wiring here in a bit. I'm guessing the signal for the fuel pump come directly from the ecm?
At any rate I'm wishing I would have put in a vortec pump in while had I had the chance, getting that fuel tank down is a real PITA.