OK, I've had this problem for a while, hope someone can help:

Basically at idle, my fuel pressure is 38psi or so. Occationally, the engine will stumble and the fuel pressure will go to 60psi (and stay there).

I'll occationally get a SES light for running to rich.

Anyone have any idea what is going on? The only thing I've done to modify the fuel system is 255 pumps and the pressure gauge.

I've tried: cleaning the IAC, new fuel filter, disconnected the battery etc..

Coldie

 

I got two ideas

I would check relays for pumps sounds like second pump is going on for some reason ti should only be like 32-34 psi @ idle

I would also check fuel psi regulator and all vacuum hoses it one has a leak or is off I think it will send more fuel psi if disconnected not positive on that one but pretty sure it does this

Slick

 

maybe check the fuel pump relays?

 

Both pumps run all the time. There is a resistor circuit that drops the voltage to the pumps at idle, cruise, and below a certain throttle position or load to lower the pressure. Then there is a high/low relay that kicks in for full voltage which jumps the pressure to 55-60 psi. It sounds like for some reason your relay is switching at idle when it should not. It is controlled by the computer in the tune, but possibly the relay is bad and would be the first thing I would try.

The high/low relay is under the hood in it's own separate little black box next to the main fuse panel under the hood, drivers side. It's between the main box and the master cylinder and will have black tape holding the cover on most likely unless someone removed it. There are two relay positions but likely only one relay in there; either way the high/low relay is the one closest to the front of the truck.

Jody

 

So should I just replace the relay? or should I use a voltmeter and see if the computer is triggering the relay?

Any chance the engine is hitting a lean spot and the computer is kicking the fuel pressure up on purpose?

Coldie

 

The computer won't compensate like that if it's lean. The relay is definitely completing the circuit, but whether the computer is signalling it or it's defective is the issue. I would grab another relay, they're pretty inexpensive.

Jody

 

OK, fairly certain it's not the relay failing.

Sat in the driveway a bit and tried to figure it out.

truck idles at 500rpm (is this to low?)
voltage at idle starts at 14 and drops to about 12.2 over time
electric fan kicks in and voltage sags to 11.9
as the voltage sags, fuel pressure jumps

Is my voltage a problem? I'm running an electric fan and electric waterpump and a 200A alternator. Optima battery that is 3 years old (or so)

I'm going to try and disconnect the electric fan and see if the problem still happens. Truck shouldn't overheat if I leave the hood up...

How is the idle adjusted on our trucks?

Coldie

 

in the computer

 

So just to summarize where I ended up with this:

I have an electric fan and electric water pump. at 500rpm, the alternator (a Mr. Alternator 200A) isn't strong enough to charge, so the battery discharges slowly.

If the voltage drops below 12V, the computer sees the pumps are starved and bumps them to full voltage.

The solution is to simply modify my tune for a 700rpm idle, at which point the alternator is making enough juice for all my electrics.

Coldie

 

how did your trucks idle get set at 500rpms?

 

Both chips I have set the idle in Drive to 550. Either it's a wierd fluke on two vendors' part, or that's normal...

My neutral idle is around 700 RPM.

 

im no expert,,,but 550 is pretty low

 

I agree 550 is pretty low. Not sure why they are set there...