1998 tahoe fuel pump

Yeah, the engine will shut off when there's no fuel to pump. The fuel pump will continue to run if you leave the key on.

No, once the engine quits, the fuel pump will not continue running. You can test this by simply turning the key on your car to the "on" position....you will hear the pump prime for a second or two, then shut off.

Ian

No.

Ian

Could a oil sending unit keep the fuel pump relay from not letting the fuel pump from not coming on?I'm having a similar promlem.Thanks Bobby

Depends on what vehicle, what year..etc. But in almost all GM cases...the oil pressure is designed as a parallel circuit to the relay, so that if the relay goes south, the engine will still start and run once oil pressure is achieved. So if the circuit through the oil pressure switch goes down, nothing happens as long as the relay is good.

Ian

Well, Ian, you know that I consider you to be Mr Goodwrench but when you turn the key to "on" the pump stops running because of pressure in the line. If it can't build pressure without fuel to pump, won't it just keep running? Just like connecting it across a battery? Not to argue - just to make sure!

Well today I replaced the oil sending unit along with the new fuel pump relay,and the pump still wont run unless I remove the relay,and cross it over with a wire.I will probably just run a wire from the relay fuel pump harness to the fuse panel on a circuit that supplies current only when ign. switch is turned on.Do you think this will work?Sorry for cutting in on the question conserning your 98 Tahoe,but I thought maybe you could use the info.Mine is an 88 S-10 Blazer 4.3.Any comments greatly appreciated! Bobby

As far as I know, there is nothing in the fuel system that measures the fuel pressure. Here is a quote from the service manual:

Fuel Pump Electrical Circuit When the key is turned ON with out the engine running, the control module turns a fuel pump relay ON for two seconds. This builds up fuel pressure quickly. If the engine is not started within two seconds, the control module shuts the fuel pump OFF and waits until the engine starts. As soon as the engine is cranked, the control module turns the relay ON and runs the fuel pump. When the engine is cranking or running, the control module receives the Crankshaft Position (CKP) sensor reference pulses which in turn energize the fuel injectors. As a backup system to the fuel pump relay, the fuel pump can also be turned ON by a fuel pump oil pressure switch. When the engine oil pressure reaches about 28 kPa (4 psi), and the fuel pump relay does not complete the circuit, the fuel pump oil pressure switch will close to complete the circuit to run the fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold.

Ian

On my 88 Blazer S-10 4.3.it has a regulator the fuel pump is suppose to continously run when the switch is on,One line going in TBI and one coming out,with the unused gas back to tank.I think this is the way mine works. Thanks Bobby

bobby swift wrote

I doubt that!

Ian

Ian I have a TBI with 2 injectors.The Haynes man. says An electric fuel pump located in the fuel tank with the fuel guage sending unit pumps fuel to the TBI unit through the fuel feed line and an in-line fuel filter.A pressure regulator in the TBI keeps the fuel available to the injector(s)at a constant pressure between 9 and 13 psi.Fuel in excess of injector needs is returned to the tank.Does the relay supposed to kick out the pump between these pressures.Because if thats so when I bypass the relay the pump will constantly run.Is this right,and will shorten the life of the pump?Thanks! Bobby

Look, the relay is nothing more then a switch that is used by the computer/module that controls the fuel pump. A relay's function is to allow a small current component to control a large current component. If you bypass the relay, you are just taking the computer/module out of the loop. When the computer has control of the fuel pump circuit....it will not power up the fuel pump unless the engine is running. You get a 2 second prime when you turn the key, and then the computer wants to see evidence that the engine is running.

Ian

I'm following what you're saying.I tried a new relay and oil pressure switch,neither worked,I'm just trying to figure out what could be the problem?It shows hot in(1) relay connector(with ign. off or on),which has 4 prongs.There's also another relay that looks similar right be side it.Do you suppose it could be in the ECM?Thanks for all your help,don't know were else to investigate.Bobby

Refresh my memory....are we still talking about an 88 S0-10? On pretty much any 4 wired relay, you should have two large wires....one is hot at all times usually, and then you will have two small wires...one of which will be hot when the ignition is turned on. So if you turn the key on, and probe the connector, you will usually see power at two locations. The non-powered large wire will usually go to the component that the relay powers up....the non-powered small wire usually is the ground wire that whatever is controlling the relay flips to ground when required.

So the sequence is: key on, power on one small wire, power on one large wire.....whatever controls relay grounds the other small wire, this energizes the relay and makes contact between the large hot wire and the other wire going to the component that is being turned on. So you can test the relay operation by grounding the signal wire and see if the relay clicks...and/or check to see that a connection is made between the two large wires. Or you can bypass the relay by unplugging the relay and (with a fused jumper wire) connect the two large wires together.

In most years...on GM products...there is a wire end kicking around the engine compartment that can be used to apply voltage directly to the fuel pump for diagnosis purposes. For example: on a 96-99 truck, the lead comes out of the wiring harness right below the cruise control assembly by the brake booster. You apply power to this lead, you are running the fuel pump manually and bypassing all the other items that control it.

Ian

Thanks Ian that was a lot of useful info.I can use.Sometimes it's so hard to explain things by this method.I was only getting current on one wire,with switch on.I've got it fused and bypassed for now.I must have a short somewhere.After I got the pump running,now I have a gas leak above rear driver wheel,rusted lines.The truck is running fine now,just a lot of stuff I have to fix,behind someone else.Thanks Again Ian and You have a Great Day! Bobby

I have a 98 Tahoe sport, I guess it is the Z71 package? But I have had the fuel pump replaced twice so far, then about 2 weeks ago it went out again, well I banged on the tank with a rubber hammer and it crunk back up fine. To it to the mechanic, they checked everything and said it was fine? Since it happened once, I am sure it will do it again. I gave it a full tune up couple months ago, but after that, the pump began going out. Also, when I crank it cold, sometimes it doesn't have much power, though I can hear the engine revving up, and it sounds like it should be moving alot faster than it is, rpm's go up as well. Does anyone have any ideas? I would appreciate it!