Hello!

I've been working on a 1986 F250 with the 460 V8 trying to get it to the point of becoming a daily driver. I have some fuel-related issue that I'm having trouble figuring out, the best way I can describe it is it has an intermittent loss of fuel pressure, which happens under hard acceleration in the upper rpm. The truck starts and idles fine, driving in town with light acceleration is normal. However, under hard acceleration, after about 3000 rpm the truck starts to buck and jerk and doesn't want to accelerate at all. If I let off the gas pedal and slow down a bit and then step on the gas (even just a little bit) again it will buck and jerk, sometimes stall. After it acts up like this it's very hard to get it to run right. I'll pull to the side of the road, let it idle for a few moments, then drive some more and it will continue having problems accelerating, I usually can't get above 40 mph. I have a clear plastic fuel filter installed on a rubber gas hose going from the hot-fuel orifice to the carburetor. I noticed that after this problem happens, when I check that clear plastic filter, it appears completely empty, like there is no gas, only maybe vapor seems like. Does this sound like vapor lock?

This is a little bit of background on this vehicle, will try to keep it short. I bought the truck as a non-running vehicle and the previous owner did quite a bit of modification, long tube headers, dual exhaust, msd ignition, mild cam. The truck sat for ages, and I have no idea if it ever ran right after he did the work to the truck.

I put on 2 new fuel pumps, both old pumps were trash, full of corrosion, quite a bit of rust inside the tank. I did my best to clean the inside of the tanks as much as possible before reinstalling the new pumps. I checked my fuel pressure with the engine cold and I have 5.5 psi. Right now, all I have for a carb is a 500 cfm summit 4 bbl that I have from a different project. I know that is a small cfm, but I bought the truck without a carb and I didn't have a chance to get something bigger, bit I will. This is just a short term thing. I don't believe the carb has an issue, I actually took the carb off this engine and put it on a different 460 engine and it ran perfect throughout the rpm range.

In my spare used parts collection, I have a complete stock duraspark setup and I installed that in place of the msd ignition but this didn't help me fix the problem.
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Anyways, not sure if this is an issue with the fuel pump, maybe doesn't put out enough volume? Maybe vapor lock because of the heat from the headers? What do you think?
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Did you replace all the rubber fuel lines?
They may look good on the outside but be bad on the inside.
Dave ----

 

As clean as you think you got the tanks if the pumps were that bad the tanks should have been replaced too.
I say this after pulling my carb apart a few weeks back.
​​back.My tanks looked good and think were being used when I bought my truck & parts truck. The carb was full of rust in the bowl. Truck ran great and has 2 filters and still had rust in bowl.

Oh have you tried to run the tanks with out the caps?
Vents could be plugged.
Dave ----

 

The truck has nylon lines going from the tanks to two steel lines up to the engine into the hot fuel orifice valve. The nylon feed and return lines all look in good shape, no kinks anywhere. I replaced the short rubber line going from the hot fuel orifice to the carb. I didn't notice any other rubber lines. Also I replaced the fuel filter twice, the first time around I did notice some rusty sediment in the bottom of the filter. I recently took the top of my carb off and the fuel bowls were very clean, no rust inside. I did pour out what was in there and blew out the jets before reinstalling. Didn't seem to help the problem though.

I did not try to run the truck with the gas cap off though, will have to give that a try. I don't have the original evap system hooked up, parts of it are missing off the truck. On the fuel tank, there is a rubber hose attached to the vent on one side, not connected to anything on the other side, just laying on the gas tank. Not sure if this is a problem.

The truck will start and idle all day long in park. Seems normal right up until you give it about 3/4 throttle while driving the problem begins. It seems really odd to me that after this happens the clear filter is completely empty, no liquid gas at all.

Also forgot to mention, while trying to figure out the problem, I bypassed the fuel pump cutoff relay so there is power going to the fuel pump at all time while I am driving. I disconnect my jumper wire from the battery after I'm done driving, of course. I was thinking maybe the fuel pump relay may have been cutting off power intermittently but that doesn't seem to be the case. Still have the same problem with the relay plugged in, or if I use my jumper wire and bypass the relay.

 

Can you tee the gauge into the fuel line at the carb inlet? Tape the gauge to the windshield and carefully route the line so you can observe the fuel pressure while the engine is acting up.

 

It's very difficult to vapor lock a vehicle that has the fuel pump in the tank, and has the return line system also. The return system keeps the fuel circulating back to the tank, keeping the cooler fuel from the tank circulating through the lines. I suspect your fuel pump socks may be clogged again. Whatever is causing it, sounds like a fuel delivery problem.

If you want to try an experiment, after it starts acting up, pull over to the side of the road and turn the engine off. But leave your pumps hot wired. Let it sit for 5 minutes to fill the carb back up, and then take off and see if it runs good for a little bit and then starts stalling again. If this experiment works, it's just another verification that it is starving for fuel, and when it runs good it is running on the fuel stored inside the carburetor. Once that runs out, the fuel pump can't keep up with the demand. I would pick one tank and pull the pump assembly back out and see what it looks like. If it's filthy, I would go ahead and buy a new fuel tank, they are not that expensive.

 

Another experiment to help rule out heat-related issues:

Let the truck sit overnight so everything is cooled off. When you first start up the engine, get onto the freeway as quickly as possible. If the truck behaves before everything warms up, you've probably got a heat issue. If the truck still misbehaves, it's likely a fuel delivery issue.

 

While anything is possible, (well, ya can't dribble a football and walk on the ceiling)

Vapor Lock in Michigan in January would not be my first choice of "What Could Be The Problem?".

 

FWIW, would this info help ?

460 Fuel Systems - Gary's Garagemahal (the Bullnose bible)

Does the 460 fuel system have any filter after the tanks, before the engine as the EFI trucks do ?

My 86 was doing the same thing, after replacing the fuel reservoir filter the truck runs great. I got that idea from my 10 year old 25HP Husqvarna lawn tractor. It started to "like run out of fuel" after about 25 minutes of mowing and stall. After a few minutes it would start and run fine, took me a couple times before, DUH, I figured out the fuel filter was dirty. Since replacing it, all is fine.


I see on this page BULLETIN NUMBER 84-24 of the TSB 84-24-14 shows a in-line filter. Seems it's page 26 which is the second page of the TSB link.
Hope that makes sense and you can find that page. I tried to copy and paste, but can't.

FUEL VALVE SELECTOR ASSEMBLY

Looking further down that TSB I see more pictures of the inline fuel filter/reservoir like the my truck has.

 

I appreciate all the replies. This just came to my mind after looking back at everything from the beginning. I think back to when I replaced the fuel pumps and I remember just how nasty and corroded the old pump assemblies were. I can't help but think maybe somehow through the gas cap area that water was able to get into the tank causing it to corrode so badly.

Franklin and FuzzFace you are right, I should have replaced the fuel tank then and I will end up doing so. I was able to clean the inside really well with a few bottles of evaporust. Probably the new gas tank would have been cheaper than the bottles of evaporust that I bought. LOL

When I did reinstall the tank, I remember it was completely dry inside. Yet when looking at the gas in the clear plastic filter near the carb it does look rather hazy, maybe like water mixed with fuel? I guess in that case water contamination would cause bucking and jerking and problems accelerating.

I know that vapor lock in this kind of setup sounds crazy with the electric in-tank pumps and fuel return system. I find it hard to believe as well, but I've never actually had a vapor lock problem which is why I wanted to ask you guys. I have an 84 F-250 with a 351W single tank with a mechanical fuel pump and I've never, ever had vapor lock issues, no difficulty starting or restarting even during the hot summers. As you guys have said, it must be that sock on the pump is plugged with debris causing it to intermittently stop the flow. I'll try to blow it out with some air on an air compressor and see if it helps it any just in the short term before I get the new tank and fuel pump.

 

Looking further down that TSB, there is a sintered bronze filter in the carburetor housing. have you replaced that ?

 

That's a good point. I think he said he had a clear fliter in the line before the orifice fitting, so I think I would pull that bronze filter out and leave it out and just use the clear inline filter.

 

I got that he added a clear filter before the carb. So I'm thinking he may not have replaced the sinter filter. It may have all the crap in it from 35 years...


I'll assume that sinter filter is in a fitting attached to the fuel bowl. He did talk about removing he fuel bowl but not about the sintered filter.

 

The filter you guys are talking about is attached to the inlet of the carb right? I don't have the stock carb, I'm running a summit racing 4bbl 500 cfm, just for the short term til I get a bigger carb. I bought this truck without a carb and this was the only carb I had.

The clear filter I have is after the orifice and before the carb. Is there a filter inside the orifice?

 

Yes, the sintered filter is in that fitting. Did you see it in the link I put above ?

Not sure about a filter at the orifice. Could the orifice itself have a issue ?
I think you talked about blowing the line out...might work if you could blow reverse flow through it... ?

The other smarted guys may know...


So you have a 500cfm 4bbl, IMO that's a small carb for that big engine. I read some of the "related threads" below, one found the jets in the carb had crap in them.