Fuel Starvation? (1 Viewer)

[BTG88]

1975 2F with pretronix and Downey headers - Engine runs really well, starts with no choke, smooth idle, excellent compression in all 6 cylinders.

Has new plugs, air filter, fuel filter, coil, voltage regulator, recent oil/filter change. When going 55+ for a couple of miles it will hesitate like it's not getting enough fuel. If I back off a bit it's fine. Around town at slower speeds - never any hesitation.

Could it be time to replace the OEM mechanical fuel pump with a new one?

Thanks for any suggestions.

 

[Splangy]

Is it a hesitation or an all out loss of power? I've had bad fuel pumps that resulted in an all out loss of power above certain throttle positions, but never a hesitation issue.

 

[sggoat]

What does the fuel filter look like?--If the filter is one you cannot see, I would recommend getting one that has a glass case, so you can actually see if there is any gunk plugging it. Before going the fuel pump route, pull off the fuel connection from the tank and see what kind of flow you have--should be a good strong pencil-sized stream--if not, there's prob gunk in the tank. If the flow is good, reconnect and pull off the hose going to the carb. Crank the engine over and see if you get good flow. If not, then you might have a pump issue(if this is the case, the pp. diaphragm may be cracked(check the engine oil to make sure fuel is not getting in there--elevated level or smells like gas).
But--given the symptoms you describe, seems like the pump may be ok--the highest discharge pressure the pump will ever see is at idle--when the float bowl is full and the inlet valve is marginally open. At speed, the flow will be much higher-the float inlet will be open much more and the pressure will be quite a bit less-so flow that would normally go through the internal recirc valve will now go to the carb(it could be that the internal valve is sticking, thus bypassing some flow that should be going to the carb)

 

[BTG88]

Is it a hesitation or an all out loss of power? I've had bad fuel pumps that resulted in an all out loss of power above certain throttle positions, but never a hesitation issue.

It's a hesitation. As soon as it happens, I back off, so not sure what will happen if I keep the speed up. I'm going to go out tonight on the highway to see what happens.

 

[BTG88]

What does the fuel filter look like?--If the filter is one you cannot see, I would recommend getting one that has a glass case, so you can actually see if there is any gunk plugging it. Before going the fuel pump route, pull off the fuel connection from the tank and see what kind of flow you have--should be a good strong pencil-sized stream--if not, there's prob gunk in the tank. If the flow is good, reconnect and pull off the hose going to the carb. Crank the engine over and see if you get good flow. If not, then you might have a pump issue(if this is the case, the pp. diaphragm may be cracked(check the engine oil to make sure fuel is not getting in there--elevated level or smells like gas).
But--given the symptoms you describe, seems like the pump may be ok--the highest discharge pressure the pump will ever see is at idle--when the float bowl is full and the inlet valve is marginally open. At speed, the flow will be much higher-the float inlet will be open much more and the pressure will be quite a bit less-so flow that would normally go through the internal recirc valve will now go to the carb(it could be that the internal valve is sticking, thus bypassing some flow that should be going to the carb)

It's a clear filter. I replaced it about 100 miles ago. The old filter looked fine, but I replaced it anyway. No fuel smell in the oil. Fuel flow is good, the tank is an OEM replacement that the PO installed along with all new fuel lines. For what it's worth, the carb is a Mark's Offroad one.

 

[Splangy]

It's a hesitation. As soon as it happens, I back off, so not sure what will happen if I keep the speed up. I'm going to go out tonight on the highway to see what happens.

A hesistation is more of a hicup where the truck hesitates before accelerating.

 

[Flatlanderfj40]

I had similar symptoms and put a vented gas cap on and problem solved. Try taking the gas cap off and see, just a thought.

 

[Pin_Head]

When it is not getting enough fuel, it usually bucks and spits back through the carb, which is not what I would call hesitation.

 

[ducktapeguy]

Does it happen even if your'e standing still and gas it or only when driving? If you suspect fuel starvation, take it up to speed and when it starts to cause problems, shut off the engine and pull off the road and look at the sight glass on the carb. When I first got my 40 I had a similar problem, the PO had the inlet and return lines to the carb switched so it wouldn't get enough fuel at high RPM's.

 

[Splangy]

Does it happen even if your'e standing still and gas it or only when driving? If you suspect fuel starvation, take it up to speed and when it starts to cause problems, shut off the engine and pull off the road and look at the sight glass on the carb. When I first got my 40 I had a similar problem, the PO had the inlet and return lines to the carb switched so it wouldn't get enough fuel at high RPM's.

This doesn't always work. The cam can still turn enough to work the pump and fill the bowl.

 

[ridefastflyfar]

well, since your description is close to what I experienced, you might consider checking the supply line in the tank. You'll have to use a scope or blow back into the tank or some such thing to see if that tube is partially blocked. Here's a link to my post earlier today: Strangest fuel filter ever

 

[brian]

the oe fuel pumps can fail in different ways, from all out quitting, to pumping enough to get it refired after the engine shuts down and anything in the middle.

 

[BTG88]

Update: Coming back from my morning rowing, cruising along the highway, get off the exit and it dies as soon as I stop at the light. Tried re-starting - cranks, but does not catch. Tow truck takes it home a few miles away. I assume that the pump gave up the ghost. I was going to replace it myself yesterday, but it was not as plug and play as I had hoped. Even though it is the right part, Toyota changed the angles of the the inlet and outlets, so now the hard fuel lines would need to be adjusted accordingly. Going to have a local shop do that and the requisite oil change for any fuel that may have got in the crankcase.

 

[brian]

Swap the nipples around on the pumps.
Assuming they are the threaded variety.

 

[BTG88]

Swap the nipples around on the pumps.
Assuming they are the threaded variety.

The outflow is, the inflow is not. It's about 20 degrees off the original pump alignment.

 

[BTG88]

Update - new pump is in, but status remains the same, will crank but not catch. New plugs, wires, cap, rotor, fuel filter, battery cables and now pump. Trying to think what would have caused it to go from running fine one minute to stopping the next.

 

[brian]

is it getting fuel to the bowl?
and is the line from the tank to the pump clear?

 

[BTG88]

is it getting fuel to the bowl?
and is the line from the tank to the pump clear?

That's the next step to see if blowing the lines makes a difference. The strange thing was how it went from running fine to completely dying. Stumbling, spitting, coughing would have been expected, but it was just like the key had been turned off.

 

[Pin_Head]

Why not describe the problem in greater detail. I don't know what you mean by "hesitate like fuel starvation" because fuel starvation doesn't cause hesitation in my experience.

 

[BTG88]

Why not describe the problem in greater detail. I don't know what you mean by "hesitate like fuel starvation" because fuel starvation doesn't cause hesitation in my experience.

I doesn't hesitate any longer. Now it just cranks and does not catch. As it's getting spark, fuel delivery was the next port of call. With the replacement of the pump, and the clean filter, a clogged line would be the next logical call. I was commenting that it was strange that it went from running fine to dead stop. I would have thought that a blockage would have some symptoms first, but not in this case.