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Fuel pressure test point- Filter Cap vs. Test Port
While chasing an issue on the truck for the last few weeks I have come to the conclusion that I need to add fuel pressure monitoring to my truck, so I'm looking at various ways to plumb into the fuel system for this purpose. I'm seeing that several companies offer a replacement upper filter cap that has a threaded test port, which seems like a nice way to have easier access versus the factory plug in the filter housing.
First, is there any variance in pressure readings from the factory test port versus these filter caps? Second, I noticed in the description of at least one of these caps that it was mentioned to reinstall the factory cap after you are done testing. Can you not leave one of these modified filter caps installed permanently? I wouldn't see why you couldn't but thought I'd ask.
Finally, if anyone has a suggestion of the best cap to use I would appreciate that as well. Eventually I will use whatever option I go with to add in a fuel pressure sender for use with my Edge Insight, but until I do that the test port will be used with a temporary shop gauge to aid in diagnosis.
I’ve never done a comparison of the two sources. I bought the set of amuminum caps maybe 10 years ago as I wanted to monitor fuel pressure but also use the oil cap as a source for an oil bypass filter, so it was a good choice. At least from the caps I bought they have been in use since without issue.
But I’m in the semi-unique position of having a disassembled 6.0 next to my family room in the garage.
With taking the fuel pressure from the cap it’s pressure is shown on the outside of the filter, on the pump and regulator side. It’s an OK position, informing you of both the HFCM and regulator operation. And it’s an easy install.
The test port is indicating the pressure on the clean side of the secondary filter, indicating the pressure of one of the two head feeding circuits. Considering your and many other members suffering from fuel paraffin drop out, this side would indicate if the secondary filter was blocked, paraffin or dirt. The other side of the filter would only indicate an issue with the HFCM filter.
For those who may have installed 6.4L fuel banjo bolts, the test port has the possibility of an effected pressure reading if an injector has excessive spit-back, the reason for the dampening check valves in the design.
So at the test port you would definitely know if your injectors were about to be hurt, better then at the cap.
I’ve never done a comparison of the two sources. I bought the set of amuminum caps maybe 10 years ago as I wanted to monitor fuel pressure but also use the oil cap as a source for an oil bypass filter, so it was a good choice. At least from the caps I bought they have been in use since without issue.
But I’m in the semi-unique position of having a disassembled 6.0 next to my family room in the garage.
With taking the fuel pressure from the cap it’s pressure is shown on the outside of the filter, on the pump and regulator side. It’s an OK position, informing you of both the HFCM and regulator operation. And it’s an easy install.
The test port is indicating the pressure on the clean side of the secondary filter, indicating the pressure of one of the two head feeding circuits. Considering your and many other members suffering from fuel paraffin drop out, this side would indicate if the secondary filter was blocked, paraffin or dirt. The other side of the filter would only indicate an issue with the HFCM filter.
For those who may have installed 6.4L fuel banjo bolts, the test port has the possibility of an effected pressure reading if an injector has excessive spit-back, the reason for the dampening check valves in the design.
So at the test port you would definitely know if your injectors were about to be hurt, better then at the cap.
Thank you for the detailed response and pictures. I guess I've got some thinking to do. The easy access of the cap is very appealing and would be useful especially in the interim as I'm diagnosing this truck with a temporary shop gauge, but wouldn't be much of an issue once I get a permanent setup on there.
If memory serves, most of the caps are 1/8" NPT. To remove the cap you'll either have to disconnect the hose/sender at the cap or somewhere else and snake the hose around the bay while removing the cap. You could also adapt the NPT to something like an AN fitting but that's just more joints to spray diesel on your exhaust manifolds. Couple that with a reading upstream of the filter and I went the test port route with a relocation hose. YMMV.
Not to hijack the thread but I am taking the fuel pressure reading off my regulated return port. The ADII is cranked up and fuel pressure regulated by the return regulator.
01-30-2018, 08:54 AM
