I replaced my dying carter this morning with a Walbro 392 and got 28# of pressure for a short time.
The pressure dropped to 20 then 16# and when I drove less than half a mile it dropped to 0# !
The pump is running and pumps up pressure slowly then begins dropping. I removed the relief valve and ran direct but that made no difference.

What could cause this ?? could the bypass valve in the IP have gone bad??
Harold

 

Wow, what a slow weekend. I'm surprised nobody has tried to help you with this yet.

I have not heard of any issues with the Walbro and your's would be the first. (I'm not an expert) If you have already bypassed the new pressure relief valve (PRV) and are still having issues, then the next test I would do would be to see if the pump is pumping anything at all.

To do this, I would find the return line and direct it to a clean container and have someone turn on the key and bump (not start) the engine. I would do this with new PRV installed.

I would expect (again, not an expert here) your fuel pressure guage to show good pressure and maybe a little flow to the container. If you get little to no pressure and lots of flow to the container, I would guess your Walbro is good and look to your PRV. Since you have already tested after bypassing your new PRV, I would suspect your IP's PRV.

If you get little or no pressure, and little or no flow, I would suspect your Walbro. Hmmmm..... changed your fuel filter, right?

Last test would be to put a guage directly on the discharge of the Walbro.

 

Fuel filter reasonably new? You said you removed the pressure relief valve? The pump will just pump back to the tank, no pressure, try taking off the fuel filler cap and bump the starter and see if you hear fuel running into the tank. I would be suspicious of your pressure relief valve, you should not have seen pressure over 20 psi and then dropping like it has, mine has never been over 19 and never dropped under 16. One way to test that I use (read use at your own risk) is to use a plastic spring clamp to clamp the striped end of a piece of wire (use #14 or larger) to the positive battery terminal, then I can touch the other end of the wire to the pump positive terminal and run the pump, if you hear fuel running into the tank you know the pump is pumping

 

Before you start pulling everything apart, start slow. Take the fuel cap off the tank then check the pressure. If the tank vent or cap has failed, symptoms would be similar. Check you connections on the suction side. Check the fuel filter, make sure it's clean.

 

Thanks for the replies guys.
I straight lined the walbro( removed the prv/cross over from the system and got the same result.
I replaced the filter a couple days ago because I thought that was the problem with the Carter.

After your suggestions I ran the walbro and opened the fuel cap then the water drain on the filter. I got a good flow but no pressure.

I have removed the PRV from the VP44 and using an allen wrench it takes very little presure to open the bypass valve.

Going to the Cummins dealer in Hilliard,Oh in the morning, Hope that solves the problem.

Thanks again for your help,
Harold

 

It is not possible to have good flow but have no pressure. If there is flow and you stop it, pressure will build up. I would guess the electric sending unit is not working properly. I would be careful running the 392 with no bypass valve as it can and will develop 160 psi, I tested it to that personally. If the bypass you have is backed off you would see no pressure, correct, but if you took it out and plugged the return line, you will have the full pressure from Walbro to the VP44, which will do damage. I would double check the pressure gauge if you are running non-bypassed Walbro and seeing no pressure.
Well I suppose it could be a bad 392 but unlikely.
-P

 

Thanks
PaulDaisy, I was real careful with the full pressure test, but got the same result, 28# to Zero.
I replaced the VP44 by-pass-regulator with no change in result.
I finally located a mechanical gauge and put it in place on the input to the fuel filter, no pressure there. I removed the output from the tee at the pump,installed the guage, and still no pressure.
That only leaves the pump, it runs but no pressure develops.
The flow I thought I had must have been the filter draining?
I found the original Carter pump and re-installed it and everything is working at 22# idle and 12#WOT with the comp on 5x5.
Harold

 

Sounds like your diagnostics walked it right back to the new pump. I was thinking since you were replacing your "dying" carter that there must be something shared between the two pumps that might have been bad like maybe a bad pickup. But, since you put the old carter back on and everything is fine, go get your $$$ back for the bad pump.

 

There are a lot of people on here that have Walbro pumps with no problems, I think I'll just ask for a new one.

Harold

 

First question is do you have a prefilter? If not you may have debri in the inlet side of the pump causing it to fail. There is no way to get that debri out. Second, have you checked all connects to the tank? A leak from the inlet side of the pump to the tank would not really show that much diesel, but show a definate drop in pressure. It really sounds like you have a pump issue. If you don't have a prefilter, get one. The best prefilter for these systems are the Billet filters that Vulcan performance and Wide Open Diesel sell, they are 35 micron and reusable.

 

Yes I have a pre-pump filter, one for each tank, and I replace them every 5K along with the oil change.
Good point though.

 

What micron prefilter do you use before the walbro? If you are running a 140 Micron strainer and the new 15 PPM low sulfer diesel, chances are that some trash got by them and clogged up the pump. Seen this happen two different times.

 

I use Fram G3 filters on both lines, the mesh filter on the Carter pump (6401HP)that died was clean.

Should be 4601HP pump

 

Yeah, that pretty much leaves the pump as the cause. Send it back, have it replaced. Looks like you can get a lemon 392 too
-P

 

It sounds like something restricting flow to the inlet of the Walbro to me as well. Is the tank that you are using nearly empty? I would sure double check to make sure that you are not getting a load of crud from the tank.
Anyway please post when you have it figured it out and let us know what you find. Good Luck.