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TBI Fuel Pressure Problem

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kennedy6949

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This is my first post, although I have been lurking a while. This is the first question out of about a thousand that I couldn't find with the search.

I have a 1987 Sub 3/4t 4x4 with the TBI 350. When I purchased it, it took a little to long to crank, so I replaced the fuel pump, strainer, and for good measure the relay, with a new Airtex unit. This fixed the problem and it ran pretty well.

The last owner broke the hard line to the TB and replaced it with hose. I went ahead and put in a fuel pressure gauge with new hose and fittings. I also went ahead and rebuilt the TB, including making the regulator adjustable.

Now for the problem. After starting it cold, the pressure comes up to about 13.5 lbs, which is good, but drops slowly and steadily, until it warms up and hovers around 8.5-9lbs. When I adjust the regulator, the cold pressure moves, but always ends up at the same 8.5-9lbs. If I let it cool down, it will come up to 11lbs or so then drops back down. I can't figure it out and really don't want to drop the tank and change pumps, unless I have to.

If anybody has any ideas, please let me know, it's driving me crazy. I don't know if the pump is weak, its electrical, or heat related.

Thanks in advance.
 
The pressure gauge is on the inlet side isn't it? I would suspect either a bad regulator diaphram, or a bad pressure spring.
 
Yeah its on the supply side. Regulator diaphragm is new and spring is solid.
 
kennedy6949 said:
Yeah its on the supply side. Regulator diaphragm is new and spring is solid.
Click to expand...

If you're not careful when installing the diaphram the seat may not be seated correctly against the injecotr pod casuing it to leak and bleed off pressure.
 
I'll reassemble it and try again. It acts the same though before the new diaphragm and after. No leaks either.
 
Hate to say it but your symptoms sound to me like a weak fuel pump. A bad FP will usually drop pressure the longer they run. Did you replace all the rubber lines in the tank when you did the fuel pump? Check the voltage and grounds to the pump?
 
I used an Airtex pump. I replaced the lines in the tank, strainer and filter. I'm gonna double check the lines for a possible leak and see what happens.

It's looking more and more like the pump.
 
Well replaced the pump, and voila....same problem.

Holds 13lbs of pressure, declines as it warms up, until it stabilizes at about 9lbs.

I'm stumped, anyone?
 
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