Hello,

I have a 1975 F750 dump truck with a FT 391 engine. When I got it, it had an inline electric fuel pump on it. I was told they thought the mechanical one was bad. I rebuilt the carb and got it up and running with the electric pump. Not great, but running. I had an issue with the carb leaking fuel. I couldn't tell where it was coming from, but I wanted to rule out having too much fuel pressure. I hooked the mechanical pump up and kept the electric inline, but unplugged it. It ran for a while, but eventually quit and wouldn't start back. I plugged the electric back in and got it running again.

Somewhere along the line, it got to where it didn't want to crank. I figured the starter was old and tired, so I replaced it. While replacing, I noticed the bottom of the oil pan covered in liquid. It turned out to be gas. I popped the drain plug and got about 4 gallons total of fuel and oil out of it.

Is it possible the bad fuel pump allowed gas to pass thru it into the pan?

Drained, new oil and filter in it. Starter installed. Still doesn't want to crank. Almost seemed hydro locked. I removed all of the spark plugs and cranked it over. It spun fast enough to lift the bed enough to drain water. I expected gas to have shot out of the spark plugs holes. Not a drop. It rained here today, so I didn't get a chance to work on it.

My next action plan is to put the plugs in one at a time and see where it gets to the point of not wanting to crank again. Hopefully this will show me what cylinder is the problem. Maybe I won't have to pull both valve covers to see what I am dealing with.

Does it sound like I am on the right track? Possible stuck exhaust valve, bent pushrod, and or broken rocker arm?

 

Welcome to FTE Wayne

Yeah it is definitely possible for a defective fuel pump to leak into the crankcase. I always recommend using a troubleshooting or repair manual and go step by step, because unless someone is a full time mechanic it is almost impossible to remember all that stuff.

 

Thanks! I have a new mechanical fuel pump to put on it, but I am not certain it will fit. They sent me one that has a screw on filter on the bottom. I think it will interfere with the oil filter. I am going to get the old one off and see what I need to do there. Looks like the power steering pump will have to come off first to get to it though. I agree on the repair manual for the truck, but it looks like a lot of stuff there is not original to the truck or the engine.

 

Right, but the basic troubleshooting or diagnostics are always going to be the same. Compression. Ignition. Fuel in that general order.

Usually there is also some kind of troubleshooting or diagnostic table or chart. "Engine fails to crank" and then steps to take. "Engine cranks normally, but fails to start." and then the steps to take. "Engine starts, but idles rough" and then the specific steps to take. A big part of diagnostics is often just finding out what the problem isn't.

The repair manuals are often very clever, with simple quick tests that will isolate or divide a particular system in half, thus proving that component A over there, is doing just fine, and then getting more specific and narrow in the isolation tests in area B. It's important to be methodical in testing because it saves time, and money.

The problem with forums sometimes is fifty different people chiming in with some variation of "Well it could be this, or it could be that".

 

Any updates?

Make sure the engine turns freely by hand. With the plugs removed check for a good spark. A nice fat white hot bluish spark, not a yellow thin weak spark for example.

 

Sorry, no updates yet. With the time change, it is mostly dark by the time I get home from work. I hope the weather cooperates this weekend for me to get some work done on the engine.

 

"Hello Darkness my old friend, now it's dark at 5 PM"

Yeah, and in the summer that extra hour burns the lawn. Wish they could make up their mind.

 

Weekend update.

Saturday the weather didn't cooperate.

Today I put the spark plugs back in 1 at a time. I cranked the engine after each one to make sure I didn't have a certain cylinder stop it from cranking. Everything went well. After all were installed, I put the plug wires on and then hooked just the electric fuel pump to it. It started and ran, but the carb leaked fuel on top of the manifold. I stopped and removed the fuel lines until I can get my mechanical pump replaced. I think the electric one is supplying too much pressure to the carb.

I think I did have some kind of hydro lock going on somewhere, because it cranked really good this time.

 

Measure the fuel pump pressure output, if it is within spec, it may simply be a matter of adjusting the carburetor float down a bit. A mechanic's vacuum gauge measures fuel pump pressure. Static pressure of 4 to 5 psi is plenty for most engines. More is not better.

The float height spec seen in the rebuild instructions is just a bench setting to get close, it isn't the final adjustment, the "wet height" should be checked after carb installation on the engine itself.