Fuel issue

[5.4slowstang]

Took the L to get a tune the other day an the high speed fuel pump I think is what it's called would not kick on instantly. It would go lean for a second then the pump would come on. Tuner thought it was the relay so we changed that an it didn't fix the problem so he couldn't tune the truck he said it was either a resister that is located under the front bumper or weak fuel pumps. Has any one ever had this problem or know how to fix it? Just curious because a friend said he read some where that a guy had the same problem replaced relay file pumps an resistor just to find out the tuner was able to change when the pump would kick on?

 

[lvgolfer962]

if you go over to Lrodder and search for fuel pump resistor mod it will pull up a bunch of threads on it.

never heard of a high speed fuel pump, but we have 2 in the tank and sometimes the hoses will rot or split and maybe that could be your issue:shrug:
just guessing fwiw

 

[Robert Francis]

If you have the stock fuel system with no alterations, you do not have a "high speed" pump. The standard system has two fuel pumps which are wired together in parallel and operate at the same time. It's the "speed" of the pumps that varies. There is a resistor located underneath the front of your truck, and under light load and throttle conditions current flow goes through the pumps and that resistor which limits voltage to the pumps to about 8.7 - 9 volts. When you get on it hard and there is more demand for more fuel, the CPU, using several different parameters, commands what is often called the "high speed relay" to drop out, which in effect allows full voltage to flow to the fuel pumps and they now operate at maximum speed which increases your fuel flow for maximium power. Note - the CPU, using several different parameters, determines "when" the high speed relay is and is not activated and this can be altered in the CPU, as it's not likely to change on it's own.

Sometimes the high speed relay does fail and needs replacing. Sometimes the resistor fails but your tuner can find this out by jumping this out of the circuit at the high speed relay. I'm assuming your tuner checked for any DTC fuel pump circuit failures and had a fuel pressure guage hooked up. He can jump out the both the fuel pump relay and high speed relays, check voltages at the relays, can measure fuel pressures with one or both relays jumped out - also he can jump the relays and close off the return line and check maximum fuel pump pressure output. When he checks fuel pressure readings under these differing conditions, he can tell if you've got weak fuel pumps, a leaking fuel system, etc. and what to do next.

Now if he hasn't checked fuel pressures under those varying conditions - he's just guessing. I'm not saying he isn't right in his analysis of your problem - I'm just saying he doesn't know for sure.

 

[5.4slowstang]

Thanks Robert that's what I ment high speed relay he didn't check anything but the relays