Why Don't We Care About Vacuum?

[JayTheCPA]

Punch-line up-front: My vote is that among the list of necessary auxiliary gauges, a vacuum gauge measuring just upstream of the lift pump is among the most critical.


Scenario:
Just came off of three filter clogs over the past ~6 months which either cut-off the motor or got close enough to severely cripple it. Two were tows where at first I thought I ran out of fuel. The third was close to home and at a convenient enough location that I was able to park and change the filter just prior to loss of ignition.

The Burb has a filter just upstream of the lift pump where for the 2'nd and 3'rd filter clogs, all I did was replace that one to get back on the road. Both of those filters had a very fine sand / crystal type sediment pour out along with the fuel. Also, they clogged 600-ish miles apart from each other.

Roll the story forward a bit, and a drop of the tank confirmed that I had some sort of debris (fine grit to small rock-type objects) in the tank. Also learned that the tank still had a sock on the pick-up tube which is a different topic that I am fixing. While somewhat a side-topic, the stuff in the tank was definitely new to the system as I replaced the tank at the same time as the motor.

To bring the story full circle . . . While the Burb does read pressure just upstream of the IP and it did drop to under 1 psi just prior to each filter clog, the filter was discounted as a dead lift pump (even if making noise) will reflect the same behavior and allow the truck to keep running. Had I measured vacuum just upstream of the lift pump in similar manner to marine counterparts, I expect that warning would have come a lot earlier.


Feel free to poke at my theory . . .

 

[WarWagon]

As the IP has a pump in it the entire suction side of the system from the IP to the tank could be in a vacuum and vapor lock the system as diesel boils off. A vacuum gauge before the lift pump would tell you what? (It's always in a slight vacuum from the lift pump to the tank. Only the IP pump will really "suck" as the lift pump won't suck enough to move the gauge: unless it's an engine driven lift pump.) IMO it's better to have a vacuum pressure gauge on the IP inlet. This way if anything clogs or fails you get loss of PSI and a vacuum reading. The vacuum reading itself is important because 0 PSI could be ok with high RPM/demand on a lacking lift pump, but, you don't know if it's 0 PSI or a vacuum if you don't have a vacuum gauge. Some random hose gets soft from biodiesel and kinks shut hidden under a wire harness on the back of the engine... Fuel filter clogs still reads trouble in the same place on the IP inlet where it wouldn't before the lift pump.

I have used a vac pressure gauge on my truck forever on the water drain, but, I only measure it during an oil change. A pressure switch with a light or gauge would have helped me in the past.

Just because the pressure is low don't assume it's anything specific. It's not always the failed lift pump.

 

[Twisted Steel Performance]

Me thinks ya need a locking fuel cap... about the vac part... don't have any input their...

 

[Will L.]

A fuel pressure gauge just before the ip is crucial for knowing whats going on. I have always said a pressure gauge that can read vacuum is best.

Having more gauges doesn’t hurt anything if you want them- Put one before and after LP.

If you have a gauge before LP reading 25” of vacuum, what does it tell you. What counts is pressure goin into IP.

Want to do something to make positive change, get the LP at the tank. Inside the tank would really be best. Remember it is a “pump”, not a “suck”. Cost of assembly is why the lp is not at the tank.

 

[schiker]

Your are right 1 guage really only tells you the IP is being fed it won't pin point any particular reason its not being fed. I think it was Kennedy who had a double guage system one before and one after the fuel filter(s). An appreciable difference in pressure meant the filter was clogging and you could change at a certain delta pressure as a little better than X miles. If the prefilter guage was low then the LP was weak or fuel problem. But that would still not alert you in the pre filter clog scenario.

Since you have pre-filters a vacuum guage could warn you the pre-lift pump filters are dirty, plugged or gelling. It should read some vacuum all the time and as filters get dirty it'll pull more. It would work similar to air filter vacuum guage on the intake. That would be a pretty good guage set up just more $$$. Just depends on how much you filter before LP if its warranted. Most can live with some pressure before the IP is OK and if it can't maintain it then start with changing filter(s) and checking again if still low look at LP or towards tank the 1 guage is just a place to start. You would have to have guages between every element if wanted to know which element to start with first on diagnosis for the quickest find.

Sort of like splitting systems in half then half again each time to narrow down the unknown the most efficiently.

 

[JayTheCPA]

Your are right 1 guage really only tells you the IP is being fed it won't pin point any particular reason its not being fed. I think it was Kennedy who had a double guage system one before and one after the fuel filter(s). An appreciable difference in pressure meant the filter was clogging and you could change at a certain delta pressure as a little better than X miles. If the prefilter guage was low then the LP was weak or fuel problem. But that would still not alert you in the pre filter clog scenario.

Since you have pre-filters a vacuum guage could warn you the pre-lift pump filters are dirty, plugged or gelling. It should read some vacuum all the time and as filters get dirty it'll pull more. It would work similar to air filter vacuum guage on the intake. That would be a pretty good guage set up just more $$$. Just depends on how much you filter before LP if its warranted. Most can live with some pressure before the IP is OK and if it can't maintain it then start with changing filter(s) and checking again if still low look at LP or towards tank the 1 guage is just a place to start. You would have to have guages between every element if wanted to know which element to start with first on diagnosis for the quickest find.

Sort of like splitting systems in half then half again each time to narrow down the unknown the most efficiently.

This ^ pretty much nails the logic.

WW / Will, Completely agree that the vacuum gauge will show a vacuum all the time simply due to the restrictions from the filter and 3/8" line as the lift pump sucks fuel through them. Sure, clogging will lower psi at the IP where this is usually interpreted as either a LP going bad or OE filter clogging and ignores whether there is a restriction upstream at the tank sock or any aux filter. Never was convinced that simply monitoring psi at the IP gave a complete picture as the LP would (for the most part) overcome resistance from the upstream filter / tank sock until it got severe.

Expectation is to have this measurement as one of the items to monitor for when it starts to increase above a normal baseline. Seeing as nobody is using a vacuum gauge (that I am aware of), will have to learn what is normal. schiker's idea of dual vacuum makes most sense in terms of getting a complete picture.


Then again, perhaps I am simply reacting to the more rare occurrence of debris in the tank and the pain in the wallet of going through the entire fuel system

Me thinks ya need a locking fuel cap...

Already ordered a locking fuel cap!

 

[Will L.]

Yeah, the pre ip gauge is the go/no go stuff that counts. All the rest is diagnostics for once there is a problem, unless like mentioned knowing the filter restriction counts as $ savings and better preventive maintenance.
I have been rethinking an electric fuel pressure gauge for convenience. If you did electric, just use 3 sensors and selector switch to 1 gauge maybe to avoid 747 cockpit scenario? Then the rare scenario thing is more easily justified.