fuel pressure question

[smokin150]

OK, now I know many questions about fuel pressure and where to test from have been asked... I've got one that idk if it has been asked so here we go...

When checking pressure I know we take it from the schraider valve on the filter head and for me I show 5psi stable since my recent electric pump swap... better than the 2psi I was seeing. Now just on the other side of the filter I get 3psi stable. My main question is where should I be seeing the estimated 7psi... before the filter or after but in front of the IP. And are there any numbers related to allowable pressure drop across the the filter.

 

[gandalf]

If you have the filter head shown below, and you're taking your reading at the schraeder valve, that's not the best spot. As you pointed out, you're taking your reading before the filter, and then wondering why you have a pressure drop after the filter. The safest answer to that would be that the filter is restricting the flow. Change the filter.

You must be registered for see images attach

The question is, "What pressure is the IP getting?" The procedure, then, is to measure the pressure at the last available spot before the IP. See the picture. If you have the previous style filter head, then you've done about all you can do.

 

[smokin150]

Ok , well that answers that but begs more questions. I understand that I'm going to get some loss in pressure thru filter media, I guess I'm just wondering what psi the IP needs to see (min and Max values) and what would be a nominal drop in psi thru the filter. As is now the filter has less than 500 miles on it and the truck still runs strong.

 

[idiabuse]

great pic that perfectly explains how air is introduced into the pressure side of your injection pump.
I took the clear plastic line in the pic and rerouted it to the return line back to the tank.
All air intrusion issues disappeared for me since that move.
I have a cowl mounted pressure gauge on my truck that psi reading is basically taken from
the same location.

Again great picture!


Javier

 

[PackRat239]

2 things come to mind. First, Gandalf, bad idea using that brass nipple as it is very soft, and with the vibration,and the weight of the gauge, it will break sooner or later. Engine room fires are not a pretty sight! Use a short close nipple if black iron at leasts. Second. Smoking150, The IP only needs fuel volume. No pressure required as any fuel that is not needed is returned to the tank anyway, so building any appreciable pressure is not possible unless you have a restriction of some sort.

 

[icanfixall]

Lots of good information posted above. As Kens filter head pic clearly shows the fittings on the head. The schrader is the first line. The second is the filter air bleeder to the fuel return line system. Some members feel this is the return flow from the injecters back to the filter.. Nope.. Thats not the direction of flow for that valve.. BTW that brass valve somethimes fails and flow will constantly happen thru it.. It has a check valve in it and when the fuel reaches it the valve shuts.. Only air is supposed to flow thru that. Now the last line with the vacuum sender and pressure gauge on it is the last place pressure will be felt. This is after the filter media and a good place to check pressure. ?Actually it would be great if we could see the in and out pressure. Then we can judge the restriction seen in a dirty filter and save a problem. In common language its called delts t and delta p readings on a special gauge.. Nice to have too but expensive. Now what pressure must the injection pump see... Not much if any.. All it needs is flow and volume but pressure is nice to have up to about 7 or 8 lbs at any throttle position. If your seeing the dash filter lite coming on under hard acleration you have a bad lift pump or a bad dirty filter. The lite comes on only when the fuel system from the filter to thew injection pump sees a vacuum. We don't like to starve the pumps for fuel either. The flow lubes them and cools them... More flows thru the injection pumps than the engine can burn too. That was just proven by Al when he flushed the pump and injecters with a cleaner.. Nice article too..

 

[racer30]

I found two of the four filter heads I have don't have a check valve in the head, only a small hole for return of fuel. seems like there might have been some changes in the filter head over the years.

 

[icanfixall]

I found two of the four filter heads I have don't have a check valve in the head, only a small hole for return of fuel. seems like there might have been some changes in the filter head over the years.

So are you telling us there is no drilled and tapped hole for the fitting or there is a plug in the drilled and tapped hole... Just wondering what you have compared to what Ken posted a great pic of..

 

[79jasper]

I think maybe the check valve flapper is gone, so it looked like a small hole.
I believe the newer trucks used a ball and spring?

Also a bad restriction sensor can suck air/leak fuel, and also not show a clogged filter when it should.
Which is why I believe in having a pressure gauge. (Soon to come)

 

[eastsideauto]

Also a bad restriction sensor can suck air/leak fuel, and also not show a clogged filter when it should.
Which is why I believe in having a pressure gauge. (Soon to come)

My fuel filter light stays on all the time even after a filter change less than 100 miles ago. Truck still runs great throughout the rpm range. Can/will this restriction sensor/fuel filter light sensor go bad? Is it a replaceable part?

 

[icanfixall]

Simpley replace the filter pressure switch on the filter head. In Gandalfs pic the clear line is connected to the air bleeder and allows air to escape the filter and not run thru the injection pump. That fitting is a check valve. It has to work. Oregon Fuel Injection sells both sizes of them. One is 3/16 and the other is 1/4 inch line size. Return lines are 2 differnt sizes so be sure what you have before ordering a kit or a fitting.

 

[79jasper]

My fuel filter light stays on all the time even after a filter change less than 100 miles ago. Truck still runs great throughout the rpm range. Can/will this restriction sensor/fuel filter light sensor go bad? Is it a replaceable part?

Cheaper to just add a gauge.
The restriction sensors are pricey and known to eventually leak. Rather it be a few months or 100k miles, I would rather see actual numbers than a dummy light.

The 7.3 Powerstroke's use the exact same restriction sensor, I pipe plugged mine, as I change my filters pretty regularly, and carry a spare. The Powerstroke running low on fuel make a pretty distinct sound. (Though I never get to that point)

 

[TWeatherford]

When I had my IDI, I had my fuel pressure gauge plumbed into the line that goes from the fuel filter to the IP. I never saw more than 4-5 psi (only saw that when I was off the throttle, using the engine to slow down), usually it was right about 3. Sometimes, it would run at 0, or somewhere between 0 and 3, but would always run great. This was with four different mechanical fuel pumps, including a delphi and carter. New fuel filters didn't change anything, so I must have always changed them before they plugged. By the way, my gauge was in the cab, so I was always looking at it.

 

[jim x 3]

Another possible cause of filter warning lite

.All it needs is flow and volume but pressure is nice to have up to about 7 or 8 lbs at any throttle position. If your seeing the dash filter lite coming on under hard acleration you have a bad lift pump or a bad dirty filter. The lite comes on only when the fuel system from the filter to thew injection pump sees a vacuum.D

To all: Good info above. To add, I find only 2-3 psi at schrader valve with no running problems at any speed. Another possible cause of a lighted filter warning lite is low fuel pressure/flow due to blockage in the fuel supply line(s) from the tank(s). My warning light would go on during acceleration because my front tank had a disintegrated "shower head". Bits of black plastic were sucked into the steel pickup line and were blocked at the line restriction at the fuel line connector at the top to the tank. No warning lite at idle, but insufficient fuel flow under acceleration. Blowing air back thru fuel supply line at mechanical pump temporarily cleared the problem until I dropped the tank, cleaned out debris, and fixed the pickup.

Regards,