91 12v fuel pump

[opofus]

How do I know if my fuel pump is getting weak. My truck will only do about 85mph WOT. Is this normal or is it starving for fuel.

How do I get more acceleration and speed. Also, the tranny does not shift to passing gear when stomped on...

Any suggestions are welcome.

[Begle1]

What year truck?

Depending on axle gear ratio and year, 85 is probably about right.

That condition is easily fixable for a few dollars.

The way to diagnose the lift pump is with a fuel pressure gauge, but if you can get to 85 stock it isn't holding you back too far.

Passing gears are lame anyways.

Regarding acceleration and speed, buy a pyrometer and boost gauge and you can go from there.

[cummins king]

when my truck was stock thats as fast as it would go, in OD with 4:11s

[dpuckett]

A diesel wont hit "passing gear" like the old big blocks will. They just dont have the RPM capability to drop into 2nd (or direct if you have OD) at 60-70mph.

First, I'd get some gauges- pyrometer, boost, and fuel pressure at the very least. Tach and tranny temp are optional at this point. Boost on a 91 should be in the low teens if intercooled; upper teens if not.

Change your filters- air and fuel. I've seen fuel filters clog up in 1/2 tank, so a spare in the glove box is a good idea. The turbo will pull in more air at idle than a 750 Holley will at WOT. The turbo diesel needs clean air and lots of it. In actuality, the stock-size filter is marginal if you go to tinkering with things.

If you have a nonIC truck with a 3spd auto, or 4.10s with OD, you are going to top out at 85 in stock trim. If you have 3.54s with OD, you should be able to close in on 100 if all is well. My 93 W250 will only do 88, but I think it has issues- 2400rpm is it in OD

Daniel