A fuel pressure flummox!

[danny boy]

Hey all,

My darn Comanche's at it again. It started as normal on thurs but just wouldn't stay running and now its resting on the side of the road.

Symptoms: Fuel pump primes as usual but upon depressing the schraeder valve on the fuel rail nothing comes out.

So I was thinking (after a debacle of replacing the pump and filter a while ago and finding major debris) a clog in the fuel rail. Well I pulled that apart and it was as clean as can be. So then I try cranking the engine with the fuel supply line off and my thumb over the hole to check for pressure and it definitely gave a healthy squirt.

So now I'm thinking maybe the pressure regulator is just stuck and just letting all the fuel just go back to the tank. Unfortunately I don't have the tool to test for pressure accurately and the truck won't start anyway so I'm kinda thinkin of getting a couple of junkyard regulators and trying that road.

By the way: 89 comanche, 4.0, fuel pump and filter from o'reilly (Kragen).

Any thoughts?

thanks,
Dan

 

[old_man]

Take a pair of vice grips and clamp off the return line and see if it starts.

 

[lawsoncl]

Are you getting fuel flow with it in run as well? Just wondering if the ballast resistor died, in which case it can be bypassed and removed.

 

[danny boy]

Thanks guys. I pulled the ballast resistor a while back, so that's not it. Good idea OldMan. Why didn't I think of that?

Is there any way to test the regulator with the vehicle not running? Im wondering because I might pick up another at the junkyard tomorrow.

Thanks and keep 'em coming.

Dan

 

[joe_peters]

Take a pair of vice grips and clamp off the return line and see if it starts.

Yes, but do it BRIEFLY and wear eye protection--the system can quickly exceed 95 psi when the FPR return line is clamped off and something could turn loose on you.

 

[danny boy]

Ok,

The regulator seems to work. I had a friend hold the schrader valve open while I held the key to prime it several times. After 2 or 3 primes, a good squirt would come out. So now I'm thinking that what ever tells the pump to keep running once it's on isn't doing it's job. I had notes about what's what somewhere but I don't recall which of the many sensors or relays are responsible for idle and above. It does start and idles smoothly for several seconds then just kinda dies off. Also if you hit the throttle it is very reluctant to climb the rpm.

I had a spare CPS so I figured I'd throw that in but the yahoo PO rounded off one of the already difficult bolts. Great!

I guess this is what a $700 DD gets you...


Dan

 

[joe_peters]

Yeah, Renix fuel pump circuit--lots of fun.

Ok, key ON, power flows to the fuel pump relay and B Latch relay from the ignition switch. The ECU provides the energizing ground to both of those relays, then cuts that ground when it doesn't receive a signal from the crank sensor. Key to START, 12 volts is sent to the fuel pump by the STARTER RELAY, bypassing the fuel pump relay and the ballast resistor on the driver's side fenderwell. Key ON, engine running from idle to just short of WOT, 12 volts is provided from the fuel pump relay to the ballast resistor where it is reduced to about 6 volts and that runs on to the fuel pump. Key ON, engine running at WOT, 12 volts is sent to the fuel pump from the o2 heater relay, bypassing the fuel pump relay and ballast resistor.

Have you bypassed the ballast resistor? Just patch around it with a piece of wire of equal gauge, see if that helps.

 

[danny boy]

Thanks Mr. Peters, those were the notes.

I removed the ballast resistor a few months ago, so thats not it. Makes me think the START portion of the system may be flawed. Can you refresh me on which relay is which inside the engine bay? Also I can't help but consider the CPS, however, I usually don't like changing things (especially with used pieces like my spare) that might not solve the problem but could possibly introduce another variable.

I'm almost tempted to just wire the pump to a toggle on the dash and go full Hillbilly! The fuel system on this truck has been a nightmare so far. Oh well, there can't be that many gremlins left...can there?

Dan

 

[joe_peters]

Quick and dirty pull the fuel relay and hook up a ground wire to the negative contact in the connector then push the relay back in--i.e., if you have power from the ignition switch providing a constant ground to the fuel pump relay will make the pump run--if it works--and you can at least confirm the pump operation.

As it starts and then dies with the key in ON, engine idling the fuel pump relay may be the problem, the engine is simply using up the fuel provided by the starter relay when the key was in the START position.

The following image is posted under the "fair use" section of the copyright laws:

Note that some people have found the above image to be in error, so I don't know if there were production changes or just a bunch of clowns on the assembly line rearranging things for a goof.

 

[87manche]

Quick and dirty pull the fuel relay and hook up a ground wire to the negative contact in the connector then push the relay back in--i.e., if you have power from the ignition switch providing a constant ground to the fuel pump relay will make the pump run--if it works--and you can at least confirm the pump operation.

As it starts and then dies with the key in ON, engine idling the fuel pump relay may be the problem, the engine is simply using up the fuel provided by the starter relay when the key was in the START position.

The following image is posted under the "fair use" section of the copyright laws:

Note that some people have found the above image to be in error, so I don't know if there were production changes or just a bunch of clowns on the assembly line rearranging things for a goof.

that's pretty much it for testing the fuel pump. I like to run a straight 12 GA jumper wire across the positive terminals though, it eliminates any goofiness from the ignition and the possibility that the relay is bad.

You can also access the fuel pump 12V wire in the DRB-II diagnostic connector. I don't have my FSM here at work, but the pinout is in there. You can also jump 12V to that pin in the diag connector and feed the pump 12V straight from the battery if your ignition wiring/fuseblock is not providing voltage. At least that way you can get it somewhere more convenient to work on. Obviously you should use a suitable sized jumper wire that has a fuse inline.

Edit: Don't forget to check the Fusible links! an ohm meter is best, but you can do it with a quick tug as well.
87 "don't go anywhere without a multimeter in a Renix rig" manche.

 

[danny boy]

Thanks guys.

I doubt I'll get a chance to work on it until after thanksgiving, but the more info, the better. One thing I noticed is that my relay bank only has 3 spots. Could it be that since it doesn't have power anything the power latch relay could've been omitted? Not saying that caused the problem, just trying to soak it all in.

I still wonder about the CPS, too. As I continue going through this truck my desire to strangle the PO grows...