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What it is: 1989 GMC with 1987 Formula 350 TPI engine, stock.
Okay, so a couple of weeks ago it was starving for fuel and it turned out my new fuel pump was bad. Replaced pump again and truck ran great until today. Again, it seems starved for fuel.
I hooked up my fuel pressure gauge and the pressure was 30-34 running (always fluctuated a little) and did not go up when revved up. I unhooked the FPR vacuum line and fuel pressure did not go up. I smelled gas in the line so I figured maybe the NEW FPR diaphragm was bad as I have heard horror stories so I installed the old one. The truck runs better but now the fuel pressure fluctuates wildly between 25 and 35 PSI. Vacuum line off and it jumps back and forth wildly between 20 and 50. It seems to drop pressure a little when revved up which it shouldn't.
Ever since I built the truck, the fuel pressure will go to 30 with the key on, not running, and drop to 10 or 0 immediately at the filter and at the fuel rail. I've replaced the fuel pump, the submersible line from the pump to intake line, fuel filter, FPR diaphragm, ohm'ed the injectors (16.4 on all 8), checked the hoses for kinks and internal damage and nothing.
Honestly, I am at a loss here, I can't have a truck that will run fine one week and not the next or I'll have to get rid of it.
Anyone have any ideas?
Last edited by TPI Sierra; Sep 12, 2016 at 08:30 PM.
So it runs good currently or no? If it is, maybe that gauge is unreliable, try a different one?
It's running but still feels as though it's starved for fuel. I haven't road tested it yet.
Originally Posted by thtanner
When you swapped the pump, did you replace the pulser with a rubber hose (usually included in the pump kit)?
I never used a pulsator, I was told the Firebird never came with them (guy could have been a usual parts store fool though) so I used a short length of fuel injection hose.
Have you had a look at the fuel filter? I changed mine and gained a significant amount of flow. Perhaps your tank work jostled some crap loose, who knows. It's a cheap and easy check.
Have you had a look at the fuel filter? I changed mine and gained a significant amount of flow. Perhaps your tank work jostled some crap loose, who knows. It's a cheap and easy check.
I put a fuel pressure port between the filter and the tank and no change but thanks.
I took it on a road test, it runs good down the road now but the fuel pressure still fluctuates a bit and when the vacuum line is unplugged, it still wildly fluctuates. I tried another gauge to see if that was the problem and same thing.
I pinched off the return line and the fluctuations went to normal with vacuum, without vacuum it went wild.
Now though, it's taking 3 times as long to start. It was starting instantly, now it cranks and cranks and finally starts after about 20 seconds of cranking. I want it starting instantly again and running good!
I put a fuel pressure port between the filter and the tank and no change but thanks.
I took it on a road test, it runs good down the road now but the fuel pressure still fluctuates a bit and when the vacuum line is unplugged, it still wildly fluctuates. I tried another gauge to see if that was the problem and same thing.
I pinched off the return line and the fluctuations went to normal with vacuum, without vacuum it went wild.
Now though, it's taking 3 times as long to start. It was starting instantly, now it cranks and cranks and finally starts after about 20 seconds of cranking. I want it starting instantly again and running good!
Pressure and flow are two different things. The demand from the engine is a lot higher than reading pressure from the gauge, regardless of where you tap into it. Your pump may be able to produce the pressure but not the volume. It could be due to a restriction.
Figured out the fluctuation problem which was a kinked fuel line, right now though I have the fuel line running around Robin Hood's barn to the front of the engine. Going to have to figure out something else. Not sure what yet though unless I can find a factory 3/8s line with about a 100 degree bend and not completely flattened at the bend.
Also, when I unhook the vacuum line to the FPR, the pressure doesn't go up. Stays around 35. Need to figure that out as well.
Used the wrong type of hose in the fuel tank, it is likely split. Need to use fuel line that is designed for submerged use: SAE 30R10
RBob.
I did us 30R10 hose, RBob. I guess I will take the bed back off and see what is going on in the tank because I replaced the diaphragm again and put on the AFPR, still have 34-36PSI at idle, no change when vacuum line is disconnected. All hoses are clear and not kinked. I really can't afford a third fuel pump for this thing and may end up having to sell it or part it out and put the old TBI back in.
So I pulled the bed and pump, the small line from the pump to feed line looked okay but I replaced it anyway and put fuel injection hose clamps on it since I had some extras. Blew out the feed line and put it all back together. Seems to run okay and fuel pressure is at 45PSI at idle now and with the return line pinched off, it goes up to 75PSI, plus the pressure goes to 55PSI with the vacuum line unhooked.
However now the only problem is the pressure drops back immediately to 8-10PSI after the truck is shut off. It goes to 35 with the key on, but drops back to 8-10 once the pump shuts off. With the return pinched off, it does the same.
I know this isn't right but is it going to be a problem?
So I pulled the bed and pump, the small line from the pump to feed line looked okay but I replaced it anyway and put fuel injection hose clamps on it since I had some extras. Blew out the feed line and put it all back together. Seems to run okay and fuel pressure is at 45PSI at idle now and with the return line pinched off, it goes up to 75PSI, plus the pressure goes to 55PSI with the vacuum line unhooked.
However now the only problem is the pressure drops back immediately to 8-10PSI after the truck is shut off. It goes to 35 with the key on, but drops back to 8-10 once the pump shuts off. With the return pinched off, it does the same.
I know this isn't right but is it going to be a problem?
I sure appreciate all y'alls help!
I don't think it's a big deal that the pressure drops..maybe a sticky injector? As many pumps as you've had would suggest it's not the check valve there unless they're all that way. Does it hold that 8psi for a while? Does it take too long to build pressure after sitting?
Also get that fuel pressure down to 43ish without vacuum. Stock motor on stock 22lb injectors, 55psi is alot of extra fuel.
Fuel pressure is at 42 now. Been driving it for a week with no problem except a high idle on occasion but I think that's a small vacuum leak as I don't have a clamp on one hose, keep forgetting to put it on. Took it to a car show on Sunday. Only one third gen when I was there, a blue 1991 V6 Firebird that was really nice. A brown 1982 Z28 pulled up as I was leaving.
Last edited by TPI Sierra; Sep 12, 2016 at 08:31 PM.
I believe a combination of things, a bad FPR diaphragm, a kink in the line that was hidden in the braided hose and the submersible hose from the fuel pump to the feed line being weak and collapsing under pressure.
