Ok this is the oddest thing, as per previous thread my suburban was stalling at the same distance from my house@ 2miles.I changed the pmd and relocated it out of the engine compartment with no result. I did the lift pump test on my 3 month old lift pump and no fuel out of the t valve. Changed lift pump and ops again. While doing this I blew out the fuel lines into the tank just to make sure no junk in sock. Tvalve gave off a little fuel but not much same with filter vent. Car began to stall again. I then disconnected the ops and the car ran fine. Continues good although slight bog w/o ops connected.So I reconnect ops car stalls within 30 seconds. Could this be clogged fuel lines between the lift pump and the filter or injector. I know of brake lines that collapse and form a sort of one way valve and maintain pressure on calipers. Maybe lift pump pressure is doing same thing on fuel line and when lift pump off the vaccuum of injector opens line. Or maybe I have no clue and someone can please give me an idea. Any help would be great. Thanks

if engine runs with ops bypassed, but stalls when connected, you have 2 possibilities 1. low engine oil pressure 2. bad ops

a fuel pressure gauge is an excellent diagnostic tool that will save time and frustration when troubleshooting fueling issues

X2 You should check fuel pressure to the IP w/ an inline gauge right on the line and see what happens under the conditions your talking about.

I had a problem with my truck that it would blow the fuse when the ops would power my new lift pump. I just installed a relay and ran it that way, no more blown fuses. But it wouldn't start when the fuse was blown, might want to look into that.

I probably wasnt clear, with the ops connected and the lift pump operating the engine stalls with the ops disconnected and the pump not working the engine ran well. The t handle when open and the lift pump running very little fuel comes out the tube. Also with the pump off and the t valve open the engine didnt stall. Thats why I think its weird. Thanks for all your help and I hope this is clearer.

Huh?

Any chance the lift pump was installed backwards??

The second new lift pump was labled OUT which I put toward the engine but that was a great idea. Thanks Pete

Bad OPS?

Two new ops and I jumped lift pump wth direct current and it stalled no current to lift pump the damn truck ran

I probably wasnt clear, with the ops connected and the lift pump operating the engine stalls with the ops disconnected and the pump not working the engine ran well. The t handle when open and the lift pump running very little fuel comes out the tube. Also with the pump off and the t valve open the engine didnt stall. Thats why I think its weird. Thanks for all your help and I hope this is clearer.

I want some of what you have been smoking.:crazy:

Seriously, that makes no sense. How long have you had the truck? Did the previous owner bypass the ops?

When you had the LP off did you have fuel running out of the line coming from the tank? Disconnect the outlet side of the LP and connect a short piece of hose to it, stick it in a container and apply power to the LP. If you don't get any flow, you can work from LP back to tank to find problem. If you get a good flow from LP, work from LP toward engine to find problem.

Another thought -- I haven't had to replace my lift pump [yet] and wonder if the pump will run backwards if wired in with the wrong wire going to ground? Most DC motors will run in either direction depending on polarity, but I don't know the internal configuration of this device... Possible??

Pull outlet off supply pump and energize the supply pump. You should get a good stream of fuel out of the pump. Right?

I like the reversed polarity idea. I installed a universal electric fuel pump on one of the cars ive got and it says boldly to make sure its wired correctly. It says if its wired backwards that it will run, but wont pump anything.

I don't smoke but wish I did with this truck. Removed output side of lift pump and got good flow. Thanks for the great ideas about reversing wires. Bought truck in 9/07 with lift pump not working. Installed new lift and first ops then. I think my next move is to take apart thehoses from the lp to filter and filter to injector pump and tvalve. As there is no flow to tvalve with lift pump on and with lp diconnected the engine doesnt stall with t valve open. Does anyone know if the fuel lines are steel or rubber in these areas. Thanks for all the great ideas. I may be drinking before this is over.

Have you changed your fuel filter?

Changed fuel filter three times in 4 months never too dirty .Thanks By the way autozone standyn filters boxed in autozone box 13.00 in ny

I had a similar issue I kept fighting. I had replaced my fuel filter housing from a 93 parts truck(just because it was cleaner) that I had when I replaced the heads. I kept having a stumbling issue and low pressure at the t-valve. The inlet on the 93 filter is at a different angle and I believe the rubber line was getting kinked or collapsing at times. I ended up putting the original housing back on and trimming the ruuber line and making sure it wasn't kinked. now excellent pressure at t-valve.

I would pull your housing out and check the pressure at the feed line to the filter housing with the lp jumpered and start from there.

I replaced my ops yesterday and have an issue with low oil pressure at the gauge. Napa's part number is the same for a light or gauge unit. I wonder if there is suppose to be a difference and if that difference will relate to o/p reading.

Today I will take off the ops and put in a gauge to manually check the pressure then replace the unit as needed possibly locate a OE unit.

My point is there seems to be new ops's out there that are faulty

My lift pump is not running so must do more testing!

Thanks again for some great ideas. The kinking may be the issue. My ops is good because the pump pumps with engine on. As far as kinking, i assume this is from the filter to the injection and the t valve. Do you think i could us braided fuel hose if so what size? Iam starting to feel better thanks to all your help. Pete

Pete, my problem was the feed hose into the back side of the filter housing, the one that comes from the lift pump, there is a short rubber piece from the top of the bell housing to the back of the filter. I think it was 3/8. I wouldn't bother with the braided, just make sure it is a quality hose.

Don't forget to check the screen inside the filter housing....pull it out and make sure it's clean !!!

Steve

This is just an idea on why it runs without the lift pump. I had a combine with an air leak in the supply line. A real joy to find in 15 feet of line that is hidden. It would run but lacked power. The mechanics said it was probably drawing fuel from the return line. I don't know if thats possible on these trucks. Diesel Pro could probably answer that. Leo