I'm having a minor problem with my "Frankenstein" truck. ('89 body/'84 drive-line) 351W with C-6 trans., DS2 ignition and 2150 carb.

It has a new fuel tank, new fuel line, new mechanical fuel pump and new fuel filter. The carb. has been tuned with a vacuum gauge to 19 "/Hg reading. It starts well, idles smoothly and runs smoothly throughout most of the RPM range

The problem is that on hard acceleration, especially uphill, it will accelerate and then start "bucking/stumbling". It still continues to accelerate but with the erratic running. When you back off the gas, after a few seconds it smooths out again. Sure seems like a fuel problem and not ignition.

To eliminate concern that it may be water or junk in the fuel bowl, I removed the carb. and cleaned it out. This didn't help. I also suspected water in the fuel tank but with an alcohol content of 10% in the gas in this area, I thought this shouldn't be too much problem. Also filling the tank again hasn't helped.

Does anyone have any ideas on what could be causing this? It really is just a minor irritation but knowing that something isn't right just bugs me.

 

What did you do with the fuel line? What did you replace and did you run it the same way with a steel line?

 

I ran new steel line from the new tank to the new mech. pump.; the line from the pump to the carb. was the original from the '84 (my F150, frame rusted out). Did my best to duplicate the route that was used on the '84. The tank is for the '89 with a new sending unit (no pump) with the pickup extended. No sock at the bottom of the pickup was used, however.

 

What does the end of your pickup look like? If it is a hose did you cut a vee in it? I hope it's not sucking on the bottom of the tank, or curled around and sucking up high in the tank. Sounds like you have a fuel problem somewhere, and a lot of non-stock parts. So go over your install and see if you can find any weird problems.

 

The pickup is a metal tube soldered to the sending unit down-tube, where the pump would normally attach. We essentially replaced the pump that would be there for an '89 with a metal tube. Yes it was cut on an angle. We did connect the sending unit output tube to the steel line with rubber fuel line, but the install is only 6 months old so the line shouldn't be deteriorated. I'll see if I can check that it hasn't pulled loose in some way. If I remember correctly, we hose clamped it down on both ends.
Thanks for the reply. I will go over the install. Any other thoughts?

 

The problem could be a leak in the new line between the tank and pump. This line is under a light vacuum, so you may not see fuel leaking out because it is not under pressure. Some parts are higher than the tank, so nothing leaks out at rest. But when the pump is working hard, it's much easier for it to draw in light, easygoing air, instead of that heavy, troublesome fuel. I'm not sure of the best way to test for this.

Another possibility is the fuel tank not venting properly. This is done through the cap, I think. You could try loosening the cap to make sure excessive vacuum can't build up in the tank and interfere with fuel flow to the pump.

Also, this is a new problem, right? Or did it exist before all that recent work?

 

Problem did not exist before the switch-over. I'm sure the '89 tanks don't use a vented cap.
We did re-use the old rollover vent valve and old hose to the charcoal canister. I guess it is possible that the valve could be defective or the old hose is somehow collapsing under hard acceleration and not allowing the tank to vent. I will investigate this possibility.

 

Uff da, no doubt you are correct. Still, temporarily loosening the cap would be a quick way to rule out a vacuum buildup in the tank, no matter how it is vented.

Have you tried the volume/pressure tests for fuel delivery?

I've added a fuel pressure gauge on my truck, with the sender at the carb inlet. For your situation, I'm trying to think of some simple means to confirm adequate fuel delivery under the conditions that cause the apparent fuel starvation. Any thoughts out there, short of adding a fuel pressure gauge?

 

Considering it is a high load situation when you are getting a stumbling issue don't be to quick to rule out ignition issues as that is when they will initially show up. Also change the fuel filter once again. Rule out the simple stuff first.

 

After rereading your original post, I wonder if this could be heat-related vapor lock. If not the entire issue, it could be part of the equation. Before I converted to an electric pump, my truck would vapor lock sometimes, almost always on acceleration or climbing a hill.

Where do you live? (If in your signature, it doesn't show on the mobile version I'm using) How hot is the weather?

How about trying a different brand of fuel? I've noticed some brands are very sensitive to heat issues while others are very tolerant. Cheap troubleshooting...

 

Fuel filter was changed very recently in an early attempt to rule it out.
The weather here has cooled off recently but has been hot. Didn't think it was heat related because of how fast it clears up.
I'll recheck my fuel lines for issues, then try loosening the gas cap and making a run. If is is venting related it is probably the rollover/vent valve or hose. Another brand of gas is easy enough to try also.
If neither of those things resolve it I'll move on to ignition testing.

Thanks to all! I'll report back whatever I find when I get a chance to roll under the truck and check things out.

 

In my experience vapor lock problems happen after you stop and are idling, not when going down the road and there is a lot of air movement. That is also why I asked about the fuel line routing, if it's routed the same path as the factory line, there should not be a problem with vapor lock.

I know you said you soldered a steel line on the sending unit. I just want to make sure there is no rubber line anywhere submerged in fuel. The reason I ask, "fuel" line you buy in the store usually has different layers to it. They have a cheaper rubber on the outside, and the good fuel resistant rubber on the inside. This type of hose will fall apart if it's submerged in fuel inside the tank. They do make hose rated for submersion, but it has to say that on the hose and it's more expensive.

 

I had intended to use rubber fuel hose to extend the tube in the tank but my buddy that was working with me suggested he just break out the torch and solder something in so as to eliminate any problems with "hose".

The engine has shorty headers on it but they were installed at the same time as all the other work was done and I'm sure we were careful about routing the fuel line. I will check that out again just to be sure.

 

Ah, the plot thickens. I'll readily admit I'm biased, as a card-carrying member of the "Header Hater Club". Had you asked, "How can I cause a rash of heat-related underhood issues on my truck?", I'd have replied, "It's very simple, just add headers."

Considering this problem started when you added headers (in addition to the fuel tank work), this could be your culprit. In my self-inflated opinion, headers just pump out too much heat into the engine compartment. The stock fuel pump and lines may have previously been okay against vapor lock, but not so with the extra heat radiating from the headers.

Years ago I had another vehicle with headers installed by a previous owner. It would buck and chug at high power settings on warm days. I tried all sorts of things, such as carb rebuild, new fuel pump, header wrap, fuel line insulation, etc. The only thing that fixed it was adding an electric boost pump near the tank to keep the fuel line pressurized feeding the stock engine-driven pump. If I switched off the electric pump, the chugging would return immediately. It didn't even have to be a hot day, maybe 70 plus. The line between the tank and engine-driven pump is under a vacuum with the engine running, so it's a prime candidate for vapor lock.

I'm sure others will say they've run headers for years with no ill effects, but my experience has been just the opposite.

I'd still suggest trying a different brand of fuel. One of my old Jeeps (no headers) is very finicky about fuel due to underhood heat. It's very happy on Chevron, for example. Conoco or 76 can mean a long wait by the side of the road. I could be in left field about your headers, but trying a different brand of fuel won't cost anything.

 

I checked the fuel line routing in the engine compartment and it is well away from the headers.

I found out that it is not as simple as a venting issue. I removed the gas cap and made a full power run up a steep hill by my house and it almost choked out. Just after I let off the gas, I could take off normally again. I have climbed this hill many times under normal acceleration so I don't think it is a pickup problem induced by the angle of incline.

I will check the fuel line connections at the tank and engine when it is not raining (which it is right now).

I'm beginning to suspect that my "new" fuel pump may not be up to the job. It is a Carter pump #M60318 (made in USA) which is the same brand and number that was previously on the engine. I only replaced the pump as a precaution since the engine was out and it was easy. I kept the old pump since it was performing well. If nothing else shows up to fix this I may swap pumps and test further. All it will cost is time and few scraped knuckles.