Fuel Pressure ?????

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

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

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.

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.

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.

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.

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.

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.

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Originally Posted by 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.

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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.

Yes, I took the gas cap off.

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?

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.

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!

Dare I suggest hooking up the stock pump???

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

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Originally Posted by R Pope

Dare I suggest hooking up the stock pump???

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It's always the simplest things that get overlooked! That does sound like a good idea, I don't think I ever tried the stock mechanical on it.

Thank you all for all of your input. I'm sure you know how frustrating it can get to be so close yet so far.

Still got problems. :( I tried moving the electric pump back as close to the tank as possible, tried the stock mechanical, tried the stock mechanical and electrical at the same time:confused: and even talked to the preacher, but he wouldn't do the exorcism. Always get the same result- works fine with 6-7 lb until it heats up then slowly drops down to nothing. I've felt all the lines by hand and figured if I can touch them, they can't be too hot. Is it still possible the lines in the engine compartment are still too hot? Will a bigger line work? I'm getting a little ticked off about this.:mad:

One simple test would be to temporarily hook a full peice of rubber line from pump to carb. Doesn't have to be pretty for now, it's only temporary. This will answer the question of existing lines under the hood or any other hard lines running down the frame. Have you had a chance to run a volt meter at the pump connections while this thing acts up?

I think you're gonna have to drop the tank. There must be a chunk of crud floating around in there, or the sock filter is plugging.

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Originally Posted by R Pope

I think you're gonna have to drop the tank. There must be a chunk of crud floating around in there, or the sock filter is plugging.

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I thought about waiting till it acts up then using a 5 gallon canister to draw out of instead of the tank, but haven't tried it yet. I'll ckeck the voltage, but I already did the temp hose thing. :mad:

My bad....I overlooked your post on the 24th. My thought at this time would have to follow along the lines of bad pump or junk in the tank. Proceed with voltage check first please.

One time I had a guy come in that had a problem where the car would die unexpectantly. After it died, the thing would restart and run OK until some random time when it died. It would idle forever. Turns out he had a shop rag in the tank that wandered around and eventually covered the sock. When the pump stopped, the rag wandered away and the car restarted....I dont think that yours is that problem since it appears to be predictable.

Do you still have a regulator in the return line? What is it...is it specifically made for this application in the return line? Something is fishy here...something that appears to heat up and affect the performance of a component....regulators can heat up, pumps can heat up, fuel lines can heat up.....by the way, has the tank internal lines been changed....as in the return line modified or is it GM stock?

mike in tucson

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Originally Posted by robot

One time I had a guy come in that had a problem where the car would die unexpectantly. After it died, the thing would restart and run OK until some random time when it died. It would idle forever. Turns out he had a shop rag in the tank that wandered around and eventually covered the sock. When the pump stopped, the rag wandered away and the car restarted....I dont think that yours is that problem since it appears to be predictable.

Do you still have a regulator in the return line? What is it...is it specifically made for this application in the return line? Something is fishy here...something that appears to heat up and affect the performance of a component....regulators can heat up, pumps can heat up, fuel lines can heat up.....by the way, has the tank internal lines been changed....as in the return line modified or is it GM stock?

mike in tucson

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I've tried it with and without the return line. Matter of fact I put the return line in after having these problems hoping for a fix. When I plumbed in the mechanical pump a few days ago I 86'ed the regulator and return line all together. It originally did not have a return. I had the tank custom made when I started this project by a reputable tank maker, but who knows, something may be in there. Something is definitely fishy.:confused: I've talked to the tech guys at HOLLEY, who usually do a real good job, and it's got them miffed too.

By all means use the process of elimination. If you can run it off a 5 gal. jug,do so,then you know it's tank related. I had one give me problems years ago,somebody switched the sender,and used rtv and the cork gasket and then ran in the 10/32 screws.the little strings of rtv swell in gas. When I finally got them out they were3/4" dia and 3" long. Hank

OK, Ive tried everything I can think of and still haven't figured out what the deal is. I'm back to thinking my origanal though which is that it is just getting too hot in the engine compartment. Is that possible? I have a 396 that is .060 over and have never had any kuck keeping it to run cool. Is it because the cylinders are too thin and creating more heat then I can get rid of. I thought the fuel pressure was leading to the engine running hot but now I'm leaning toward the engine running hot effecting the fuel pressure. Any other ideas?

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Originally Posted by wrenchaholic

OK, Ive tried everything I can think of and still haven't figured out what the deal is. I'm back to thinking my origanal though which is that it is just getting too hot in the engine compartment. Is that possible? I have a 396 that is .060 over and have never had any kuck keeping it to run cool. Is it because the cylinders are too thin and creating more heat then I can get rid of. I thought the fuel pressure was leading to the engine running hot but now I'm leaning toward the engine running hot effecting the fuel pressure. Any other ideas?

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Is the fuel tank properly vented?

Don In Austin

To my way of thinking your problem has to be in the pump. They have two check valves,inlet and outlet,with the outlet maintaining backpressure all the way up to the carb. The only other thing that I can think of would be a fuel hose collapsing internally blocking the line. If you keep the fuel line away from engine and exhaust heat then vapor lock shouldn't be a problem. Hank

I'll take a few pics tonight when I get home from work and post them.

The tank is ventilated and I even ran the fuel out of a 5 gal. container after I started it and the pressure dropped. When I did this I could feel hardly any suction from the end of the hose through the mechanical pump.

Except for a few small peices of rubber hose at the filter, tank and where I temped in the mechanical pump its all 3/8 hard line.

I've used 2 different electric pumps and 1 mechanical so I didn't think it was the pump. However, I ran out of fuel once on the way to the river in this vehicle and pumped fuel from the boat to use. Since then I had to replace the fiberglass tanks in the boat because the resin was breaking down and doing all kinds of bad things. 2 of the pumps I've been trying have been subjected to this bad gas so could that create the same problem. The one in the boat still works fine. :CRY: :CRY: :CRY:

SUCCESS!!!!!!!! Well, kind of. I bought another new mechanical fuel pump, 5' of 3/8 hose and a new hard tube. I went from the carb to the fuel pump in 3/8 hard steel line and from the fuel pump straight to a 5 gallon container. I started it up and in about 20 minutes the pressure dropped to 0. Then I took the pressure gauge and carb fuel feed line off and capped it. Finally I started the truck with the remaining fuel in the bowls to read the pressure and it was at 7lbs. For good measure I reinstalled the fuel line to the carb and started it and it was still at 0. SO. This tells me it's the carb. Through the site glass I can see the floats are adjusted right, but is there some way the needle/ seat are staying open? The first thing I'm gonna do is rebuild it. then if I still have a problem, HOLLEY is getting a call. I've had nothing but problems with this thing since I bought it. They might get a call anyway for the rebuild kit.

Bypass the pump with a foot or so of rubber line and then use an air hose to to blow out the line from the carb back through into the tank. Use common sense with amount and duration of air pressure back into tank so yuo do not cause a problem here )short blasts of air). This is to blow off a sock filter in the pick up side if present and prove no obstructions. Cap on tank must be off and be aware of gas fumes and dangers.
Hook system back up and rig a volt meter up to wiring at the pump. Watch voltage as it starts up and has pressure to point of losing pressure. If voltage remains constant it is a bad pump that may be failing from a internal heat related fault but faulty regardless. A new pump is cheaper than all your time and old pump is 5 yrs old anyway. Also easy to check flow rate or gallons per min of old pump, time how long it takes to fill a 1 quart can at carb inlet and do the math on output from time to fill 1 qt. good luck .... be safe