2nd Gen Non-Engine/Transmission Where is the best place for a fuel pressure tap?

[JGheen]

I would think the best place to tap into the fuel system would be on the banjo fitting on top of the fuel filter? Can you remove that 10mm bolt where one bleeds the filter housing and install a boss fitting to run the gauge off of there? I am getting an Autometer fuel pressure gauge soon and wanted to get some info on the proper -3 AN fittings to purchase for the line to the gauge.



Thanks, Joe

 

[Joe G.]

You need a special fitting if you mount it in place of the bleed screw.



I drilled and tapped the banjo fitting where the fuel line from the filter connects to the injection pump. That works very well and the fitting is not in the way every time the fuel filter is changed. The way the fitting is made if you drill from the inside the fitting insures the hole is centered and straight. I installed a needle valve in the fitting and used a grease gun hose from there to the fender well when I mounted the sender. Have to be carefull and make sure everything is clean before reinstalling the tapped fitting.

 

[JGheen]

So there is no 1/8" NPT plug anywhere, for example, that I could install a boss fitting and run it? What size fitting did you use for the drilled and tapped hole you machined into the banjo fitting? What do you mean drill from the inside? Why would you use a needle valve? Is it really necessary?

 

[icarus33]

I would use the 1/8" NPT port on top of the filter housing to install the pressure transducer. I beleive the inner plug is the filtered side of the housing, same pressure that is being delivered to the VP-44. My '98 has them like yours, there were only a couple years I think that had those plugs on top of the fuel housing. Makes bleeding and pressure taps easy.



ic

 

[Steve St.Laurent]

The ports on top of your fuel filter head will work fine. As said above they are just 1/8" NPT ports. Here's pics of how my fuel pressure senders are hooked up on my 98:



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[Joe G.]

Joe,



You need a needle valve because the mechanical lift pump used with the P7100 pump will beat the pressure sensor to death with pressure spikes. I use a Westach 40 PSI gauge. The first fuel pressure gauge I used was mechanical. I couldn't stand the racket from the needle bouncing. The needle valve stopped that. The Westach pressure sensor on this gauge is supposed to be ok with those spikes, but there have been quite a few posts about failing pressure sensors so I am using the needle valve with this gauge also.



I tapped the fitting for 1/8 NPT.

 

[JGheen]

Originally posted by Steve St. Laurent

The ports on top of your fuel filter head will work fine. As said above they are just 1/8" NPT ports. Here's pics of how my fuel pressure senders are hooked up on my 98:



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Steve,



So are you running a pressure sender for both supply and return? Is that why you have two sending units? I think I have a different filter housing that you being mine is an early '98 w/ the 12v. Do you get the pressure spikes as Joe G. was refering to or are you using a needle valve as well?



Joe G. ,



where is the best place to find a needle valve for use in this application? Do you have a pic of the one you used or one tat it similar?

 

[Joe G.]

I think Steve's truck is a 24 valve so it doesn't have the pressure spike problem.



You can get a needle valve any place that sells brass fittings. Adjust by closing it until the gauge does not work. Then open it until the gauge barely works. The minimum gauge that will work in a 12 valve is 30 PSI. The problem with it that it will be pegged a lot. A 60 is better, but then the readings will mostly be in the first half of the dial. I got a 40 from Westach. It still pegs if I lift at highways speeds with the TCC locked. But it doesn't stay that way very long.

 

[JGheen]

Is the needle valve a 1/8" NPT male thread on both ends of it? Just having a hard time picturing what it looks like. I got all kinds of places around here that sell brass fittings so they should have a good selection for my application.

BTW, Joe G. , do you happen to have any pics of your fittings and lines on the filter housing?

 

[Steve St.Laurent]

Correct, my truck is a 24v so I haven't had a need for the needle valves. Our fuel filter heads are the same - the two ports are both for the supply side, one is before the filter and the other is after the filter. Cummins specifies a max pressure different pre and post filter of 5 psi. I change my fuel filter when I see a 2 psi difference across the filter - that's part of the reason I have two senders. Also, I once had a bad load of fuel which I was able to troubleshoot by comparing my fuel pressures before and after the filter. I would change the filter and within minutes I would have a 2 psi differential across the filter again - without measuring both I wouldn't have known it was a fuel quality problem.

 

[DanielH]

Sounds like a very good idea using 2 gages. Where did you mount them? I ran out of room in my pillar mount and was going to mount a small fuel pressure gage in the speaker hole.

 

[JGheen]

Steve, I wonder if you could tap into the pre-filter side and just take note of the pressure with a new filter and if it drops any, then that would be a sign of the filter becoming clogged? Do you think that would work or is it easier to have one on the post-filter side for a comparison?

I guess the reasoning behind running a needle valve for the 12v application is due to the lift pumps being different from the 24v to the 12v, right?

 

[Joe G.]

The lift pump on a 12 valve uses a piston to push the fuel. Everytime the piston works there is a pressure spike. The 24 valve pump is some sort of electric gadget that provides steady pressure and fails often. A loss of fuel pressure on a 12 valve does not necessarily mean a clogged filter. Lots of things can cause that. If you monitor fuel pressure with one gauge you want it in the system somewhere on the output side of the filter. If you have a pressure drop shortly after fueling the first assumption is bad fuel. If it's an on going low pressure problem then you have to look at the overflow valve, fuel hoses, fuel heater, pre-filter, fittings, and the lift pump.

 

[Shovelhead]

I have a '99 24 valve and I used a mechanical gauge plumbed with stainless steel line to the dash mounted gauge.

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I use a needle valve (closed almost all the way) to keep the gauge from buzzing due to the pulses in the fuel supply system.



Ed

 

[Steve St.Laurent]

With the reliability of the 24v lift pumps you definately cannot assume a drop in pressure is due to the fuel filter. It looks like Joe listed a number of 12v issues that could cause low pressure as well. In my case I used a single gauge and have a switch on it to switch between the two senders - so if I want to check the other one I just throw a switch. You can see my entire gauge setup here - https://www.turbodieselregister.com/forums/showthread.php?s=&threadid=6113 . Keep in mind that's for a 24v so the pressures you'd be looking at would be different.