okay in my quest to go through this whole truck I did the fuel stuff the other day. I installed a grainger pressure valve and it was like 8psi at idle and up to about 18 at 2000RPM. I then replaced the heater/strailer the fuel filter the feed and return lines and stretched the pressure relief valve spring back to .500" started it up and it was sitting at about 20-22 idle. Figured problem solved. Went to take it for a short drive and in less than 1 mile it was back to sitting at 8 at idle and a bit less than 20 at 2000. Next day I checked and redid all the fittings on the gauge and they seam to be okay. weird thing I tried using just compressed air to check the gauge regulator was set to 120psi and the gauge showed like 35psi, I checked the boost gauge and another gauge I had laying around and they all read 35psi, I can't explain that one bit.

I know this sounds like it is a lift pump problem but I am not convinced, since it did show the correct readings if only for a brief amount of time. FWIW the truck does still run like crap, I know the timing is in need of adjusting and the egt's I see are 670 MAX POST TURBO.

So any other ideas or convince me it is the lift pump, anyone know where to get one for less than $200 or have a napa part number (I heard they are a bit cheaper and without a part number they are hopeless to try and find anything)


Thanks

 

My experience with stretching the spring is that is usually doesn't last long, better to buy a $25 OF valve.
On all the gauges reading 35, sure your regulator isn't set to 35?
Every mechanical gauge I've ever used either worked, didn't return to zero when it didn't or had a sticky spot. Pretty easy to tell when one is bad.

 

I plan to get a replacement valve but I would expect it to last a little longer than 2 minutes after stretching it. In fact I am quite sure cause I took it out again a day later and it was still .500 inch.

AS far as the regulator on the air tank I am sure it works and the gauge is accurate, it matches the tank pressure gauge when the valve is full open and there is definitly a difference in flow between 40psi and 120 psi. Furthermore it drives all the other air tools just fine which it cetrainly wouldn't do at less than 90psi.

 

okay I was doing a little more reasearch last night and came across a test procedure that says pinch off the return line and see what happens to the pressure.
Here is my question I assume they are talking about the return line that I just replaced that is also the one that likes to leak behind the fuel filter?

 

You are correct. If the pressure goes up with line pinched the problem is the OF valve. If the pressure remains the same it's the lift pump.

 

So I did the pinch the return line test at idle it went up to about 30psi and went up from there though I didn't push it past 37psi since that is as much as it should run If what I read is corect.

The weird thing is and please try to explain this to me. at idle it sits at 5psi at 200 no load it is 18ish at 50 mph 2300 RPM it sits around 15-16 BUT if I get on it build 15 psi of boost or so the pressure drops to around 12. This sounds to me like a lift pump issue but it passed the pinch test. I should have a new overflow valve by this weekend or whenever the UPS man gets here but I am still leaning toward the lift pump. any ideas?

 

It sounds to me like a restriction problem, it's starving for fuel.
Plugged filter including the ones in the tank and fuel heater or a pinched/kinked feed line somewhere.

 

yeah I though about that too, I just replaced the fuel filter and the prefilter/heater screen. also replaced the feed and return line, no way to screw that up too bad. The metal feed line looked just fine.

 

If you have an air compressor try removing a feed line near the filter then pressurize the tank though the fill cap, 5 psi max and see what kind of flow you have. Pinch off the return line beforehand or fuel will feed back though it and make a mess.