Fuel filter, OPS, Pump Too Many Questions...

[B Lake]

OK, double check my work guys. New to me 95 Suburban with 97000 miles. Truck was running fine until yesterday. It stumbled bad when I really got on it merging into traffic. I let off the pedal, it ran fine, got into it a little more and it stumbled again. Classic "not enough fuel problem".

Today, I changed the fuel filter. It was about as dirty as the pictures in the Member's Area. Is that about normal or is that a really dirty filter in that picture?

Now the good part. With the engine running, I opened the drain and expected to see fuel drain, got nothing but a stalled engine. That tells me there is no pressure on the filter side, correct?

I shut the engine off and proceeded with changing the filter using the procedures in the R&R guide. Open the fuel cap, open the water drain valve (nothing came out), open the air bleed valve, pop the filter out. As soon as I popped the filter, fuel drained out of the drain line, curious. Put the new filter in and purged it by putting the tranny in Drive and hitting the key until it filled up. Started the engine after a few coughs and quits.

When I open the drain valve with the engine running fuel should drain out without the engine dying, correct? If the pump works when the starter is engaged but doesn't work when the engine is running means a bad OPS, correct. At this point I'm thinking I had a really clogged filter but I want to be sure.

Thanks for your help. I searched the posts and did my best to piece together the info but no one post covered everything going on here.

[schamp]

Sounds like the lift pump. Located underneath between the frame rails, actually on the inside of the divers side frame rail. Pump is tied into the ops but if ops was bad it wouldn't run at all. I am kind of suprised it started back after the fuel filter change. You might want to get some one to turn the key on while you are under the suburban. Should here it pump. If system is presurized pump will only operate a very short time. Might have to turn the key on and off a few times. Do a search, I am sure there is all kinds of info on the life pump and ops. Good luck.

[B Lake]

From reading the old posts on this, it sounds like the lift pump can be powered from two directions, when the starter is engaged and through the OPS when the starter is off but engine has oil pressure. Is this correct?

I can hear the pump when I engage the starter with the tranny in Drive. I can't hear the pump when the engine is running, too much other noise.

[jspringator]

Hot wire the pump and see if your problem goes away.

[fastcat800]

I just went throught the exact same thing and it was the OPS. It sends the power to the lift pump when the engine is running and has oil pressure. I opened the valve on mine and no fuel came out and it quit running after a few turns (after replacing the filter and lift pump). The lift pump gets power when cranking from another source, that's why you can get it started and the injector pump has enough suction to keep it running. Buy a 1 1/16" deep oil sender unit socket at the parts store (about $9)and remove the two bolts that hold the fuel filter and connector bracket. You then can get the the connectors out of the way to get at the sender. Took me about ten minutes to change it. Runs like a new truck now.

[ 02-15-2005, 10:31 AM: Message edited by: Dave Schumacher ]

[ucdavis]

What Dave said. OPS R&R req'd. On the 95, there should be a spade connector w/red wire IIRC, next to the relays on the firewall. Originally it had a paper tag that said Fuel Pump, and it is a hot-wire point to test the lift pump. Run 12V to that to be sure, but if the water bleed valve open shuts off engine, it is because the IP is sucking fuel then air back from the valve.
IP can develop enough suction to pull fuel thru the LP in most cases, so that's how you stayed on the road this long w/the LP only running while on start cycle.

[B Lake]

The new switch is in and works correctly. Now I know what to feel for to test for a working pump, can't hear it but I can feel it. Fuel dumps out when I open the drain, engine keeps running. Guess I need to keep a spare in the glove box at all time.

Say I'm stuck on the road on a dark and stormy night, what's to stop me from jumpering the two terminals and hot wiring the pump?

I took the old switch apart. Very interesting. It looks like it has some sort of coil to help hold the contacts closed. One of the contacts had a black spot on it, probably why it wasn't completing the circuit. The contacts look more than large enough to carry the load of the pump but I don't think they hit square so it burns the tip, gets crudded up and stops completing the circuit. The coil might also be burned out. I'll test it with an Ohm meter and see what's up.

[MTTwister]

B Lake - now's a good time to wire in a Relay between the OPS and the pump. Takes the load off those points. Search on OPS Relay - you should get a few good hits - seems I reacall writing something up about doing that.