Anomalous Fuel Pressure Reading 4.0L 1989 Comanche.

[Dal3]

The engine would start as usual, but run for only 20-30 seconds then die.

 

It seemed like a fuel supply problem and and the Troubleshooting section in the Haynes manual indicated this problem would be covered in the fuel supply section.  It wasn't, specifically as far a I could tell, but I went through the test procedure, connecting a fuel gauge to a fitting on front end of the fuel return manifold and starting.

 

The meter didn't deflect from the peg, either initially or when I removed the vacuum hose or when I pinched the return hose as instructed in the manual.

I didn't feel confident enough in the gauge reading at that point to start disassembling the fuel supply lines.

A search directed me to a discussion in this forum about a similar problem, where several possible electrical issues were discussed, including a defective "ballast" resistor.

 

I found a resistor mounted on the driver's side front fender.

When I pulled back one of the wires to bypass the resistor a (broken) connector lug pulled away from the resistor.

I connected the leads together, started the engine and ran it for over a minute before I shut it off.

 

At this point, my concern is the incorrect gauge reading.

 

I'm not an expert and just learning the engine systems as I'm studying this problem.  The pressure test equipment is a new-looking unit borrowed from an auto parts store. I doubt the engine would start at all if the 0 psi. reading were correct.

So I'm wondering if there might be an error in the way I connected the gauge.

 

I used the GM coupling that came with the kit.

I didn't remove the check valve from the fitting on the truck, but I believe I was not instructed to.

I have run the engine with the test assembly connected and there is no fuel leaking in the vicinity.

 

Thanks,

Dale

[Keyav8r]

hg in (inches of mercury) is a vacuum measurement. Did the gauge also have a PSI range on the face? If it has both ranges, at 0 PSI you would be reading no pressure and no vacuum.

The ballast resistor is in the fuel pump power circuit and, being broken, may have prevented fuel pump operation.

I'm assuming you rechecked the fuel pressure after you jumped out the broken resistor. If you still had no pressure reading you may not be getting the fuel pressure through the test rig (or the gauge could be bad - those loaner tools get treated pretty roughly sometimes). Your truck would not have run for a minute without pressure to the injectors.

The fitting on the fuel rail is a Schraeder (sic?) valve (as used on tire valve stems). The center pin has to be depressed to open the valve and allow the pressure to be read. It may be that the kit's GM coupling did not open the valve. A refrigeration gauge hose fitting will open the valve as I have one with a gauge adapter on one end that works well.

As a test, find a tool that you can use to depress the center pin in the fuel rail valve. Put on some goggles or safety glasses and gloves, have someone start and run the fruck for a few minutes and then shut it down. Immediately open the valve briefly and see if fuel squirts out. Be very careful getting ready to dothis as your hands will be in close proximity to the fan and the belt drive. If you have fuel pressure at that point get a better test kit to read the actual pressure. And be sure to do this outdoors, wear safety glasses or goggles and gloves. You can wrap a shop towel around the tool and valve to keep fuel from spraying everywhere.

[HOrnbrod]

Obviously the open ballast resistor was the run problem. The ballast resistor has to be in the circuit after the FP relay energizes or the engine dies shortly after starting. As to why your pressure gauge wasn't indicating I have no idea. You do not have to remove the Schrader valve from the fuel rail for the pressure gauge to indicate. And I use the GM adapter on my personal gauge set. But if the engine ran for a full minute you do have some fuel pressure. I'd bring the gauge back to wherever you got it, exchange it for another and try again. Pressure should be 30 psi (w. vacuum on the regulator) and 39 psi w/o vacuum on your Renix.

[HOrnbrod]

Hg/in (inches of mercury) is a unit of measurement for pressure. Used for barometric pressure normally. 0 hg/in = 0 psi and 1 hg/in = .491 psi. As said above, read the psi scale when you find a good pressure gauge.

 

Conversion calculator:  http://www.convertunits.com/from/in+Hg/to/psi

[Keyav8r]

True, in hg can also be a pressure measurement. But, since I sell pumps (liquids), blowers and compressors (gas pumps), and vacuum pumps (gases) I'm accustomed to using PSIG or feet of water for positive (above atmospheric) pressure and in hg for negative pressures.

[Dal3]

I've corrected my error in the original post.

I knew it should have read psi, but I'd just been considering checking the vacuum and  goofed the post.

I'll check the gauge and fitting and try a different gauge if something's amiss.

I do believe I found the running problem at the resistor connection, but I'd like to look at the supply pressure while the procedure is fresh in my mind.

 

Thanks,

Dale