If a mech. fuel pump is starting to go could it make a intermittent clacking noise? I am trying to fined an clacking noise that doesn't really sound like metal to metal. Doesn't do it at idle. I have an 83 LG4 with v-belts. This is what i have already tried:
- removed the v-belt off the AIR pump,
- put all new check valves on the AIR/exhaust.
- Checked the exhaust for leaks at the joints.

Only thing left that I can think of is the Mech. fuel pump.

Any help would be greatly appreciated.
phil

 

I had one a few years back with a bad spring that made the engine sound like a diesel. But, it made that noise at idle as well.

How's your oil pressure? I also had an engine with a wiped out bearing that was quiet at idle but made a dull thudding noise above that.

 

thanks for your response. oil pressure is good. Engine runs great. The thing is that it is intermittent. I only noticed it now when it is colder outside. Doesn't do it under load all the time. I drive out of the driveway and I go about a quarter mile, crusing at 35mph and it will start. It is not very loud. Then if i slow to a stop, about half way to stopping it will stop. I have put it in to neutral and reved the engine a little and it definitly is connected with the engine, not the drive train. I can stop and get under the hood and rev and nothing. After it is fully warmed up it seems to be gone. I thought sure that it was in the AIR system but that is now none functioning until i find this problem. I have also disconnected the vaccum from the EFE. I thought that if it were an exhaust leak I would hear it all the time. Is it possible that the EGR is pulsing and clacking. It is a new EGR though.

thanks for any more help i can get.
phil

 

An exhaust leak can come and go. Some manifold leaks will be apparent when cold but as the engine warms may go away (steel gets hot and expands...approx. 6.6 x 10^-6 in/in per degree F which is real close to the thermal coefficient of expansion for concrete by the way, but I digress). Torque transmitted to the engine through the drivetrain will place different stress on exhaust parts. Drive to Park will even cause the engine to shift some (especially with worn motor mounts). Have you dismantled the AIR system and plugged all lines or just removed the belt? With the belt removed, the valves will still be opening to the manifolds during warm up. You can temporarily route one of your rubber AIR tubes (the long one) between the two manifold connections to rule the AIR system out completely (well nearly completely).

Fuel pump is unlikely, but possible.

 

yeah the belt is off the air pump and i have put new check valves on all 3 air tubes. It is weird because once it starts make the noise it will continue for a minute or so even with varying throttle. Then if you slow to a stop it seems to go away and if you speed back up it may or may not start right away. It almost seems like it is some valve that is controled by the computer that is wearing out. I wonder if the EFE is rotted out somehow. But i would think that would be more constant. There also is the EGR. Does it pulse?

thanks for all the feedback.

 

It was the Mechanical Fuel pump! It actually surprised me. It was a Carter (6626) from napa that didn't have very many miles on it. Maybe 8000. I am ordering an ACDelco from gmpartsdirect.com to replace. Thanks Damon for heading me in the right direction.

phil

 

Good to hear. How did you narrow it down to the fuel pump? Only thing I could think of was the metal rod/stethoscope trick, other than pulling it and letting the engine idle with the bowl full.

As as side note, my fuel pump died on me last Thursday morning, bout 100 feet from my driveway. If your pump has to go out, make sure it's within walking distance of your house....and I had a new pump in the garage still in the box. Sometimes it works out right.

 

well here is the deal. I don't drive my car that often and therefore my carb dries out. So I got the idea that to save the starter i would install an electric pusher fuel pump back by the gas tank, with a switch under the drivers seat, to prime the carb on startup after a long no-run time. After the car starts I shut the pusher pump off and go about my drive. One problem I had with this setup was on a full throttle accel the mech. pump could not draw enough gas through the electric pump (which is off), so I put a bypass line around the electric pump with a check valve to solve this problem. It works GREAT!!!

Now to your question. When I am driving and the intermittent noise started to happen I just flipped the electric fuel pump on and the noise stopped.

That's how i figured it out.

phil