'98 won't hold fuel pressure
 

First of all I'm sure this is on here somewhere already but I must be searching wrong because I cannot find it.
Story goes,
I just finished swapping a good used engine into this '98. Fired it up, sounded good, no CEL. Took it for a cruise and the CEL came on. Then it started flashing. I could tell it was now missing, kind sluggish. Code reader says random misfire.
So I check fuel pressure first of all. KOEO it hits almost sixty, which by my research is low. The worrisome part though, is how quickly pressure drops. In less than twenty seconds it is already down to thirty. So I swap regulators. Same issue. Then I check for leaks with the upper plenum off but fuel lines connected. No leaks under plenum.
So my question is, why else could I lose fuel pressure?
Thanks peeps..

Test the pressure at the fuel filter, and check for pressure leak down there.

For that test, you're running straight from the pump to the gauge, the fuel line to the engine is disconnected. You can connect the gauge at the outlet of the fuel filter, or to the pipe coming into the fuel filter.

Pressure should be >70psi when the pump runs for prime, and should hold over 60psi after 10 minutes. If it's dropping there, the pump, or some of the other parts in the pump assembly inside the tank are leaking.

If there's no leak down at the filter, but it's leaking down when the injector assembly is hooked up, then the regulator could be leaking down the return pipe.

Awesome. I'll check it out now.

Or tomorrow I guess. I'll have to find an adapter or make a hose. Any suggestions?

Most diagnostic "fuel pressure gauge sets" include a set of adapters. The "loaner" ones at Autozone or O'Reilly have a bunch of adapters with them. There's usually one that fits one end of the fuel filter or the other.

Is your filter threaded at both ends? Or does it have the "quick connects" you need the fuel line tool for?

On the tank side it's threaded and on the engine side it's a quick disconnect. Thanks for the tip!

Okay, my buddy at the parts house hooked it up with some fittings. After I tightened everything up enough it quit leaking all over the place, pressure would hit 80, and hold pretty well, maybe drop a pound a minute. So after 20 minutes it was still above 60 pounds. Where to look now?

I guess since everything is already pulled apart I might try to swap entire spiders on the engines. Unless you guys tell me not to.. :D

So you've already swapped the regulators.

Now, it sounds more like one or more poppets are sticking open. You've swapped regulators, am I right? I think I missed that before.

You could swap the spider assembly. Or you could get a new, upgraded spider assembly and install that.

Yessir I swapped regulators. I would put a new one in if it were my vehicle. I will ask the owner if he wants to but I'm gonna guess he'll ask me to see if the old one would work.


So really the problem has to lie in the spider assembly somewhere, right? Either the regulator or poppets or something else. It can't be anywhere else in the fuel system?


By the way, thanks for your help so far.