Engine/Transmission (1994 - 1998) Lift pump tapping.........

[BigPapa]

Why would my lift pump be tapping? I replaced the original lift pump and overflow valve @70K miles ago trying to diagnose a fuel knock. It has started tapping pretty hard at idle. It may be tapping at higher RPM's, I just can't hear it over the engine and road noise. Fuel pressure was a little off at idle and higher speeds. Clamped off the return line, not totally closed, and raised it a little but I figure that's normal. The spring in the return valve was . 5" but I put 2 shims out of an old valve under the spring anyway. No change in the tapping. Truck runs great and I'm getting my usual great MPG's so I'm just wondering why it's tapping and will it hurt anything? Is there a diaphram in the lift pump or is it piston action?



Thanks,

Scott

 

[DieselDoghouse]

I THINK (emphasis think) its because the return spring above the pump piston is weakened or has broken turns. The piston lags behind the pushrod and gets 'tapped' as the rod rises for the next cycle. I noticed defueling at the top end which resolved when I replaced the original pump. I have heard that these Carter pumps are rebuildable from parts from Case tractor dealers - same pump.

 

[Lightman]

Why would my lift pump be tapping? I replaced the original lift pump and overflow valve @70K miles ago trying to diagnose a fuel knock. It has started tapping pretty hard at idle. It may be tapping at higher RPM's, I just can't hear it over the engine and road noise. Fuel pressure was a little off at idle and higher speeds. Clamped off the return line, not totally closed, and raised it a little but I figure that's normal. The spring in the return valve was . 5" but I put 2 shims out of an old valve under the spring anyway. No change in the tapping. Truck runs great and I'm getting my usual great MPG's so I'm just wondering why it's tapping and will it hurt anything? Is there a diaphram in the lift pump or is it piston action?



Thanks,

Scott

You might want to check your fuel pressure before your fuel filter you might find that it is Plugged on my 94 when the filter got Plugged real bad it would knock and it will destroy your pump if left very long

 

[BigPapa]

You might want to check your fuel pressure before your fuel filter you might find that it is Plugged on my 94 when the filter got Plugged real bad it would knock and it will destroy your pump if left very long

I changed my fuel filter and the tapping is gone. It was nasty. I must have gotten hold of some really dirty fuel. The filter didn't have but 8K miles on it. I generally change them at 10K and I've never had one get that clogged up.



Thanks,

Scott

 

[Timebomb]

Its funny I came accross this thread. I just noticed my truck making a tapping noise it seems to be comming from that area. when I crawled under the truck wile it was running it seemed to be comming from the bell housing area. My attention was drawn to the clutch but it very well could be the lift pump. Fuel filter has about 10k miles on it, Ill check it.

 

[BigPapa]

It's tapping again... ... ... ... ... ...



Scott

 

[guns101]

Not to be a smart *****,but did you get rid of the fuel in your tank after changing the filter? If it was that dirty, you may want to sample what is in the tank. JMHO

 

[Vaughn MacKenzie]

It seems a high percentage of 12-valves I've heard running have tapping lift pumps. Both of my trucks do it even though the LP are quite new (30k on one LP and 8k on the other) and they do it just as loud as the old.



Vaughn

 

[BigPapa]

Guns, anything's possible.



I'm with you Vaughn but it gradually got to the point where it was pretty loud, changed the fuel filter and it quit totally. Now it's back to being as loud as it was. It's only been @700 miles since I changed the filter. At one point, since I changed the filter, it was tapping in an irregular way. In other words, it would tap a couple of times and miss three times then tap one time and miss three times then tap a couple of times and so on... ... no regular tempo.



If you think it's normal, I'll let her go. I just don't want it to leave me walking but I also can't afford $140 for a new one right now.



Scott

 

[Joe G.]

One thing to note is how the lift pump works. The cam pushes a rod and piston that compresses a spring. The spring pushes the piston back to move the fuel. If the spring can't push the fuel fast enough there will be some slack in the cam, rod, piston, and spring sequence. Thus you will get a tap. That's why an almost plugged filter will cause a tap. As long as your fuel pressure is ok, I would not worry about tapping.

 

[JFaughn]

I just had to replace mine and took it apart [ I do that with every part I take off anything ]
I was looking for a rebuild kit becouse it was just 3 check valves , 2 pistons and 2 springs ,
the only thing that was wrong was that the check valves were coming apart ,
I've a mechanic long enough to have rebuilt the gaser fuel pumps so I figured it should be easy , everybody I talked to said that there were no parts available , even the places that said there pumps were rebiult .
I would take the pump back out look at all the parts to see if you can find something , broken springs , check valves and see if there is some hammering onthe push rod to cam & piston to make sure your not gainning space for the knocking to happen .

 

[Lightman]

Guns, anything's possible.



I'm with you Vaughn but it gradually got to the point where it was pretty loud, changed the fuel filter and it quit totally. Now it's back to being as loud as it was. It's only been @700 miles since I changed the filter. At one point, since I changed the filter, it was tapping in an irregular way. In other words, it would tap a couple of times and miss three times then tap one time and miss three times then tap a couple of times and so on... ... no regular tempo.



If you think it's normal, I'll let her go. I just don't want it to leave me walking but I also can't afford $140 for a new one right now.



Scott

As quoted above you may have a load of fuel with fugus in it and it will plug your filters very fast. When it happened to me it went about 200 miles and pledged my new filter and it was nasty I had to drain all fuel out and start fresh with new fuel.

 

[JoeBioDiesel]

I've had 2 12V trucks. A '96 and now a '97. Both had noisy lift pumps... and noisy replacement lift pumps. At 249,000 miles, my '97 had been tapping since at least 150,000 miles. The tapping comes and goes, and is louder at some times than others. If you've got good fuel pressure I wouldn't lose any sleep over it.



Joe