Hi all, following suggestions here for a lack of power issue on a 350 TPI I replaced the FPR diaphragm with an AC Delco original, and installed Southbay Delphi 22lb injectors. Checked for vacuum leaks and smoke tested the intake manifold. Also replaced O2 sensor, fuel filter and checked timing.

Symptom: car has no power throughout the majority of the RPM range. It will start and idle just fine, but say I am at 30mph and put my foot down even a quarter of the way, the car will actually go slower. If I give it less throttle (just a touch) then I can gradually increase speed.

Did a drive around with a fuel pressure gauge attached to the windshield. When I do the 30mph acceleration test, the fuel pressure starts at 40psi but drops slowly to 10psi and all the time I feel the car getting slower. Fuel pressure at idle is around 35 psi and steady. At prime with engine off it looks to be right at 43.5.

Does this confirm that there's not enough pressure at the fuel rail when there is demand for a lot of fuel? I'm unclear on exactly what I should see there when putting my foot down. Given that I replaced the FPR diaphragm and did a vacuum leak test there, does this indicate that the original fuel pump is likely to blame, or should I check anything else first?

 

Seems like a fuel delivery issue. Could be failing pump, or a clog somewhere in the line.

When's the last time you replaced the fuel filter?

 

Thanks for the input. Fuel filter was replaced last month.

 

The static and idle fuel pressure appear to be nearly textbook perfect. The issue may be insufficient flow under higher demands, as Komet advised. You can test the ultimate pump flow by removing the pressure line at the fitting feeding the rails, or somewhere else convenient. Route the disconnected line to a container suitable for fuel. Jumper the pump power and measure the fuel captured after 30 seconds of pump operation. It should be in excess of a quart.

 

Thanks, I will try that! In any case, I'll be fitting a new fuel pump shortly and a Racetronix hotwire kit.

 

Sounds like a weak fuelpump as others have said.

 

Don't instantly blame the pump. 30+ year old fuel lines - both rubber and steel - could be restricting flow. Ask anyone who has had a brake caliper "stick" only to replace everything before finding the internally degraded rubber brake hose.

 

Yeah, I plan to do the 30 second test before and after replacing the pump. I don't mind replacing the pump (well, my wallet cares since I decided to buy new springs/shocks/struts while I have things in pieces) but I do want to make sure the problem gets solved.

 

I did a fuel flow test at the flexible feed line in the engine bay. It does indeed pump in excess of a quart in 30 seconds (it managed it in 20 seconds). I did clean and rebuild the entire fuel rail so I am convinced there is no blockage there. So I am puzzled about this issue.

The 9th injector is not leaking (I removed and tested it) and the Delphi injectors are all brand new.

I will pull the plugs and check everything there.

Since I cannot log anything while driving, what are my options here? I was looking at dynamicefi.com as a possible solution for that.

 

Actually it pumped 1 pint in 22 seconds which appears to be OK for a TPI engine. Imperial units are confusing…

 

1 pint should take less than 15 seconds, especially free-flowing (no head pressure). That translates to 113 LPH, which is the minimum for a stock pump.

1 pint in 22 seconds is less than 85 LPH, and again, that is with NO head pressure. I suspect there is some flow problem or impediment.

 

Good info, once again Haynes fails! Fuel pump replacement happening as soon as my Quickjack shows up, and as before I will post an update here. Much appreciated, great help on this forum. 👍

 

If you are confident that the inline filter is intact, none of the presure lines and rails are kinked or restricted, and that the pump is getting full voltage while operating, there isn't much remaining but the items in the fuel tank. The pump, pickup (sock) screen, and connectons between the pump and its pedestal should be checked.

I suspect you have already researched the site for hints and tips for dropping the fuel tank, since that involves several other steps before the tank itself is actually out and accessible. There is a lot of good informaton available.

 

Yep, Quickjack arriving today (with crossmember accessory due to the offset lift points), and while I am doing the job I will be replacing the original springs and shocks, and also upgrading the power supply for the pump.

 

Obviously, you are planning ahead. If it hasn't been done, it may be of some benefit to get under it and apply penetrating oil to the fasteners, joints, and parts that you will be removing. That includes the fuel filler neck and quarterpanel trim, as well as the exhaust and suspension parts. Even if the vehicle has not been subjected to salt and corrosive elements, 30+ years of oxidizing can create some struggles.

 

It sounds like you have decided to replace the fuel pump and have accepted the fact that it is a lot of work.

But, before you do, I would disconnect the fuel line under the hood and hook up your air compressor to blow about 10psi of air backwards through the fuel line. I had a car that was stumbling on tip in when you needed more fuel and by blowing light pressure air backwards through the fuel line it must a have displaced a clog since the car started running fine.

If nothing else, I would blow out the fuel line when you have the whole system apart.

Just a thought.

 

I will definitely do that since I will have everything in pieces! I am getting good at adapting random hose A to random line B these days.

 

Just got everything back together - easy but time consuming job! I found the root cause - drumroll...

The car did not have the original fuel pump. It was a Delphi, and the rubber hose connecting the pump to the solid fuel line inside the tank was loose. It was secured with shitty plastic clips. Pressure was being lost there, otherwise the pump appeared to be fine. There is a Walbro 255lph in there now anyway with a Racetronix harness.

I drained the tank by running the fuel pump with a fuel line draining directly into a gas container. The flow was low, and when I got down to the last gallon and a half, air was coming through the fuel line. I suspect at this point the rubber hose inside the tank (top of the fuel pump) was being exposed to air, and air was being drawn in.

 

I'd bet those clamps and the wrong hose used in the tank (non submersible hose) have caused more issues than pump failures.