So here's the story:

First mod to this truck was gages, and based on Infidels great advice I installed a hardware store needle valve and ran the little line right into the cab. My fuel pressure was around 18 PSI, and moved up and down 2 or 3 psi at most no matter what I did. Added an 11 plate, same deal. Then the nylon line got a crack in it from rubbing (my fault) so I replaced it with braided stainless, and added the isolator while I was at it. All said and done 17-19 PSI.

Now it gets weird. Hit 10K and changed the fuel filter, overflow valve (because I thought my pressure was a little on the low side from other posts I've read here, and becasue I'm now over the sometimes quoted lifespan of 80K) and had it timed, put in the 10 plate and 3k gsk (since the clutch was done) For the first week I had around 25 PSI and was happy. Then it started falling to 20, and now it hangs around 16, with only the slightest fluctuation. I'm worried it's my lift pump, but I don't really know how weather affects diesels (its getting cold here too) and I haven't had enough seat time with the new plate/kit to be sure that I'm down on power, although I seem to be, but still have decent smoke, and fairly low egt's (they are actually lower with the timing and 10 than before with the 11)

Background is that I replaced the fuel sender when I bought it, along with the fuel lines in back, and screen in tank, so I don't think they are an issue. Starts like a champ, still pulls my 6k lbs well and really has no issues, but I can't seem to get the fuel pressure out of my head.

So, What is the correct fuel pressure I should see, and if mine is low, what do you think the issue might be? If its the lift pump is there a special part number I should get (updated for ULSD, or specific to the 215 pump?)

Also is there a chance I got a bad overflow valve? (from TST) and just need a higher pressure one? I've read that I you can pinch the return line and watch the pressure, but I'm not sure I can do that on a 98 with the quick lock lines (the Haynes manual I have says not to on 98's)

 

your fuel pressure sounds fine. it will drop a little in the cold because the viscosity of the fuel is higher when it is cold.

 

Factory specs are 17 - 22 psi at idle and 25 - 35 psi at 2500 rpm (no load).

With the engine running pinch your return hose at the rear of the engine with pliers. If the pressure shoots up your lift pump is fine, overflow valve is the problem.

 

My concern is that my fuel pressure is still 14-17 at 2500 RPM no load...i.e. it doesn't move up with RPM, and never gone above 22 since I've owned the truck.

I'll try pinching the line tonight and see what I get