Fuel Pressure ?????

[wrenchaholic]

I'm hoping someone out there can help me with this.

I have a 69 Suburban with a close to stock 396, Holley 670 Truck Avenger carb and Holley red electric fuel pump plumbed in 3/8 line. When I start it cold I have 5-6 lbs of pressure at the carb but as it heats up it drops all the way down to 0, but maintains idle and even seems to drive OK. It just won't tow without bogging. I put a different gauge in just after the pump, which is mounted about halfway up the framerail, and it stays at 5-6 lbs. I thought the fuel line in the engine compartment was getting too hot but I can still feel them with my bare hand and they don't seem that hot. I put a return regulator and line in to get more circulation but that didn't fix it. Iv'e tried running just the pump without the engine running and noticed no drop in the same time frame. I replaced the 3' of braided line feeding the carb in the engine compartment with steel line and made sure it was not in direct contact with anything and no change.

I thought that the fuel pressure problem could be causing a lean condition and that is what is causing it to run hot and eventually overheat

I'm almost certain that fuel pressure at the carb is not suppose to drop to nothing even if the bowls are full, so what gives.

I've been working on this thing for years and any help would be greatly appriciated

[nitrowarrior]

Point of curiosity, Where is pump mounted? And give us some detail on fuel line routing from front to rear please.

[wrenchaholic]

The pump is mounted about 5' from the tank on the inside of the passenger frame rail. The tank is in the stock location at the rear of the truck and the lines run straight down the inside of the frame rail to the front, even with the front of the engine where the return reg. is installed and the feed is routed up through the accessory brckets from there.

[nitrowarrior]

That doesn't sound bad at all. Have you double checked pick-up inside the tank for restriction or crimping? Is the pump fairly new? I have run into a couple of the red pumps losing their vane clearances inside and bypass the fuel when they run long. Armature and winding amperage getting too high from heat or wear causing a slower action. Without being hands on with this project, it's hard to give a good diagnosis. I'm only throwing ideas out there for now. Be patient for a little bit, I know somebody here has also experienced this and figured it out.

[wrenchaholic]

I thought the same thing as far as a restriction in the tank, so I temped in a different guage near the fuel pump outlet and had no drop in pressure. I assumed everyting from the pump back was OK.

[robot]

What is the brand/model of the return regulator? Have you
recently tried the system with the return blocked?

Also, if the fuel pump is wired with too small a wire, the voltage
drop will cause the pump to run hotter.

[nitrowarrior]

Okay, let's assume (I hate assuming things because of the bite in the butt which follows) the line(s) are clear to the tank from the pump back. Are the lines from pump to engine new or original? Are there any AR type fittings to make a turn or bend in the line? I fishing around to see your setup in my head.

[wrenchaholic]

Yes to both. I temped in a long peice of 3/8 hose from the pump, left it laying on the driveway, ran it in front of the grill, hung it from the top of the hood and dropped it down to the carb. Although it took longer and didnt quite drop to zero, the same basic problem was there.

The wiring thing I havent checked , but would'nt pressure at the rear guage drop if that were the case.

[wrenchaholic]

Okay, let's assume (I hate assuming things because of the bite in the butt which follows) the line(s) are clear to the tank from the pump back. Are the lines from pump to engine new or original? Are there any AR type fittings to make a turn or bend in the line? I fishing around to see your setup in my head.

I don't have any unnecessary fittings and the lines are only as old as the project, about 5 years. The bend in the return line back to the tank is a little tight, but not kinked.

[wrenchaholic]

Yes, I took the gas cap off.

[nitrowarrior]

Have you talked to your local Priest to have the demon excersized? You really have a interesting fuel drop problem. One more silly question, What line size are you running?

[NTFDAY]

I agree with Denny, 5 foot is a looong way from the tank and I wouldn't wire with anything less than 16 gauge wire and preferably 14 gauge.

[wrenchaholic]

I'll try moving the pump back, but I'm pretty sure I used 14 AWG.

Oh Oh boss just got here! I'll get back to you later!

[R Pope]

Dare I suggest hooking up the stock pump???

[kitz]

The best suction pumps are the Gerotor gear types and not the vane pumps.

However seems to me if you had a suction problem it would occur from the very start and might also be sensitive to tank level.

As Denny said it sounds more like a voltage drop problem. Is the pump on a relay and/or not running through a toggle switch directly? Wires not too small, properly grounded with no looseness etc. The pump motor could also be sick; this would in fact tend to show up as it gets to operating temperature.

Kitz