Brand new here, guys, and I am hoping for some help because this truck is driving me crazy. It's my neighbor's truck, has the 5.7 Vortec. He is laid off, and I am trying to get it going for him. He does not have a lot of money to throw at unneeded parts (or a pro mechanic), so here's what we have for symptoms and have done...

The truck will start and run briefly, but then seems to run out of fuel. Spark is present. I understand these trucks use a timed fuel pump circuit for startup, then the firing engine should produce 15 psi oil pressure, which closes the sending unit switch to restart the fuel pump for run status. (This truck has an oil gauge, not a light.)

The pump does run and shut off when key is turned, engine fires off a little but then dies, seems fuel starved. Replaced filter. Initial fuel pressure at manifold plenum port is 65 psi -- perfectly in-spec.

The truck ran great on a trip to town, then he came home and shut it off, and it would not restart and run. No prior troubles, except the fuel pump was replaced about a year ago. We have replaced the throttle positon sensor, the fuel filter and the oil pressure sensor.

With the new oil pressure switch in, engine has the same problem, so we know it was not that.

Here is the sequence: Insert key and turn to ON, fuel pump runs and shuts off. Pressure at the port is 65 psi at that point (in spec). Turn key to start, engine will fire on that pressure buildup and run for a moment. Pressure steadily drops to around 55 psi (which is below the minimum spec of 61 to fire the injectors). It runs out of fuel after initial firing and then if you keep the key at start you are just cranking it. The fuel pump never comes back on for the run phase (which is what we hoped the new oil pressure sensor would remedy).

Just for grins, I checked the operation of the pump relay. It works perfectly, and the contacts are making contact internally. If you pump up the pressure and leave the truck sit for an hour on a run to town (a 14-mile round trip for each run, mind you), it will still hold pressure when you get back, so it is not the in-tank check valve. All fuses are good, and as I said the Throttle Position Sensor has been replaced already and is brand new, as is the fuel filter.

We pretty much can be satisfied from this sequence that the ignition system is fine and that the fuel injection system is working, except the pump at run. If the fuel pump would come back on after initial start, the engine should run fine. I have confirmed by listening in the filler neck as the procedure above was carried out that the pump is not coming back on.

OK, so while we are in town we get a loaner scanner. Hook it up, just to be sure. No codes are stored, because the battery has been disconnected at some point. We cuss.

Now it widens way out as to what it could be. We know the well-defined parameters of the problem, all we need is to be pointed to the cause, and that is the sticking point. We are at the place where parts prices start at $75 quickly rise past $150 a part from there.

I have heard from a tech that the Engine Control Module (ECM) has to see an ignition pulse train from the igniton system in either crank or run to keep the fuel pump coil energized. He says it does this as a safety so that if the motor stops running with the key on, you don't continue pumping fuel. He thinks this pulse train is not getting to the ECM, so it never re-energizes the fuel pump relay. This pulse train should run thru the electrical ignition switch, he says. Says he has seen this problem on several GM's, and in these cases it was a burnt ignition switch. If I can get a wiring diagram, he says to check this signal for integrity back to the ECM (he thinks its the yellow wire...anybody know for sure??). He says I can also get an inspection mirror and look at the switch, it probably has a charred plastic body.

Does anybody have any other ideas on this?

I am completely stumped, and trying to help the owner because he is laid off so he doesn't have the cabbage to just haul it to a dealer. ALL HELP AND SUGGESTIONS APPRECIATED like you would not believe!

 

IMO you are looking too hard at the issue....lets back up for a sec and relook at the basics...............fuel pressure is 65psi on prime( in spec) how about spark ? you said it has fire but how much? do you loose spark or fuel? how about the FPR has it been checked ? you are obviously loosing one of the 4 basics needed to run an engine....how about the ignition module? has it been pulled and tested? A crank sensor will also cause this issue.... I have the procedure for testing one of these if you need to go that direction. Without any codes it is real hard to pinpoint the issue without a little guessing. I also have a wiring diagram....but you can also get that one from Autozone.com If you want to eliminate the fuel pressure and pump as an issue you can always "jump" the pump and make it run independant of the switch.

 

I would say see how much spark it has. I dont know alot about vehicles, but I know I have had the EXACT same problem in dirt bikes and atv's and it was because the spark plug was not producing enough spark.

 

Thanks, but it has spark throughout. This is not an ignition issue. It will jump 1/2" with blue spark on a tester. That was the very first thing I tried. The 98, BTW, is a distributor ignition, so it has no position sensors.

FWIW, a guy I met who works at a Chevy dealership told me the following today: "Beat on the tank with a hammer handle or a 2x4. It will start and run."

I have my doubts, but he says it is a fuel pump issue. I doubt it because the pump primes at 65 psi at the rail, which is spec. But what could it hurt? I'll try that. He says they do it to get them to run so they can pull them in the garage to swap pumps. I dunno, maybe if the pump did not come on at ALL, I would put more credence into what he says. But it can't hurt...

I definitely have thought of jumping the pump direct to battery, and will do so if necessary. That is pretty drastic and really doesn't help diagnose the problem, though, since we already know it comes on during the engine's start sequence and the einge starts with fuel present, but it does not come back on for the run sequence.

I'd appreciate any more suggestions, but I am certain from my diagnostics so far that it is the fuel pump not coming on for the run cycle. I will try to look up the wiring diagram at autozone.com, but I have tried stuff there before and not been able to find it at all.

Any more ideas?

 

The "beat on the tank" method is for when the pump will not come on. I do think you have a fuel pump problem. I think it will create enough pressure for the startup but can't keep up with the needs of the engine, thus the drop in pressure. I may be wrong but i think the pressure drop has something to do with it. And if the replacement he had put in it a year ago was an aftermarket one, the signs point even closer to that explanation.

 

The 98.............................. HAS.......................... a crank sensor my 97 has one also....humor you self (and me) and look at the passengers side of the crank right behind the ballancer and there you WILL find a crank sensor !!!!!!! I have to agree with 98white5.0 but there are several things that the pump circut relies on to run right...even if the fuel pump relay was bad the truck would start and run after it builds enough oil preassure (15 psi if i remember right) and it will start ( saftey feature and fail safe built it) I think you either have a pump or a sensor problem. Jumping the pump will tell you almost right away if the pump can keep up with the engines demand for fuel....thus eliminating or confirming the pump as the problem. this could be an ignition problem but I doubt it based on the half inch of spark you have but it is still possible....will it run on carb cleaner? this will verify the spark as being moderatly good. but it sounds like a sensor problem or a FPR (Fuel preassure regulator)

 

I would agree with Chrisbmo, I would say it is a sensor thing. I may not even throw a code and still be a sensor thing. I would change the crank sensor and try it after I have confirmed that the pump is keeping up.

 

Thanks, now I am getting a good list of stuff to run through, between here and asking around. The pump definitely doesn't keep up, since it does not run at all except on the relay starting sequence circuit. We did try it on carb cleaner as one of the first things we did, and while it won't stay running on it, it does seem to run longer on it.

It has to be something that could happen suddenly, so that points to a sensor. I have contacts inside GM engineering who say it could be the coolant temp sensor (well...maybe), or they say it could be a security system breakdown which has shut down the run signal to the pump. I pray not, since that is a highly complicated system to deal with. If it turns out to be that, I will permanently jump the pump and hotwire it through the ignition switch.

But I am some way from that, with a good list of easier stuff to check. The owner is laid off, so simply swapping out parts on hunches won't cut it. Gotta be methodical.

I am under the weather with a stomach virus now (fun), but when I get better, I will first beat on the tank (what could it hurt), then try running a voltage test to the fuel pump while the engine is cranking to determine whether it is getting juice. Then will come jumping the pump direct to battery. Then the circuit, ignition switch and sensor tests I have compiled on my list from here and talking with Chevy techs and independent techs. There is one guy who says he has replaced nearly a dozen ignition switches in GMs for this exact problem. They burn and then ignition signal confirmation does not get back to the ECM.

Could be a long slog, but we'll get there. Thanks, and I will let you know what happens.

 

All these months later, and this truck is still inoperable. Lack of time and money have kept it so, but it still has the same problem.

1. Fuel filter was replaced.

2. We replaced the oil pressure switch, same result.

3. The fuel pump kicks on at start, and pumps up 65 psi. The rail will hold 65 psi forever if the engine is not cranked, so it is not a check valve problem.

4. The engine will start and run very briefly, and pressure then immediately drops at the line gauge. If someone lays down by the fuel tank, the pump runs initially, but it does not operate for the "run" phase of the operation.

5. It is possible that the cause is the ignition switch at "run." We borrowed a tester ignition switch from a Chevy dealer shop, and plugged it in, with the same result. This would seem to eliminate the ignition switch as a cause, unless the tester was also bad.

We are going to get back on this problem now, again.

1. I want to run a jumper all the way from the battery to the pump, and try starting it that way. The trouble could be a fault in the wiring between the oil pressure switch and the pump. Jumper would eliminate the spider, pressure regulator and ignition as trouble spots if the truck runs with it.

2. My buddy has now saved up a little bit of money. He has a friend who has the spider assembly off another Vortec that the guy says works fine. Guy wants $100 for it. My buddy just called and told me this. I am completely unconvinced that replacing the spider assembly will cure the problem, that being that the fuel pump does not come on for the "run" phase.

Once several years ago, he had this problem, but it fixed itself quickly and the truck ran fine for all those years. That tends to sound like a possible circuit fault somewhere.

I am open to other suggestions. This thing is unreal.

 

Does this have a flashing security light on the dash with the key on?