Dealer can't even figure it out! What now?

[FishinGuide]

You may have read my previous posts about my truck (95 k1500 5.7 TBI) having a major hesitation during acceleration and a slight miss at idle. I took it to the dealership and had them scope/diag it. They said it was the fuel pressure reg. I replaced it and things did not change. I also replaced the fuel pump and that was not it either. I called the dealer back and told them it didn't work. They asked where I got the part and I said Autozone. He said he would talk with both of his drivability techs and get back to me.

When he called back he said both techs still say it's 100% for sure the pressure reg and that I need to buy a GM part. Apparently one thing they did was pinch off the return line and the truck supposedly ran fine. So I buy the part from them ($50 more) and install it. Nothing changes. So I call back again and he said the only other thing it could possibly be are the injectors and to bring it back down to test them. They tested fine. So now we are all stuck and I'm not sure what to do next.

So far I have replaced - Cap, rotor, plugs, fuel filter, EGR valve, pressure reg, fuel pump/strainer. It has been scoped and everything looks fine. When I changed the plugs they had at least 30k on them and they all looked identical and perfect, no oil, carbon, high heat signs, etc. Also, I personally got out the multimeter yesterday and checked the TPS and CTS just in case they missed something and they checked fine.

They say it's definately a fuel delivery problem but I don't know what to think at this point. I will say this, I have noticed some odd clues that may help track the problem down. I just don't know what they mean.

1. When my truck is cold it runs worse than when warm. A lot worse, almost dies when you press the gas. But idles better. When it warms up it starts the miss at idle but doesn't stall as bad.

2. When I sit at a stoplight for a bit I can floor it and I have all the power I want - no hesitation. But, when I'm driving if I floor it it just goes BLEH and falls on it's face. I have to put it to the floor and let it down shift to get up any hill.

So it seems like when it sits at a stop it builds pressure somehow. I can see why they thought it was the pressure reg, but it's not. Also, running worse when cold than hot means something too I'm sure, I just don't know what.

If anyone out there has any ideas I sure would appreciate hearing them. So far I'm out about $500 and I'm still at square one. Thanks a bunch.

[maxwedge]

You need to get a fuel pressure tester on this and take direct readings when the condition occurs to eliminate any issues with fuel delivery to the throttle body, were the injectors removed and the screens checked for any debris?

[FishinGuide]

I pulled the injectors myself and checked the screens, they are fine. Could a fuel line be clogged somehow? How would I test for that? They said pressure between pump and TB were fine, they did put a guage on it. But it seems to me pressure and volume are two different things. From the symptoms it displays it seems as though the TB isn't getting the volume of fuel that it needs. Is there somewhere in the TB itself that could clog? What do you think? Thanks for the response btw.

[CD Smalley]

Just out of curiousity... Have you checked the fuel pump relay?

I'm wondering if when it get's hot, underhood temps and all, if it is not restricting the voltage to the fuel pump. Thus the pump would run at a lower pressure when your truck is hot.

Get a volt meter and test the voltage at the pump connection when the truck is cold. Then go out and drive and get it good and hot and doing the symptoms you describe. Test the voltage again.

I bet it will be lower....

Test your fuel pressure at both times as well.

[FishinGuide]

You know I had that question as well. I don't understand exactly how the fuel delivery system works. Is it the relay that tells the pump when to come on? Also does the pump vary in output or is it an on and off type of thing? What exactly does the relay do? I always thought it was like a fuse, either it works fine or it doesn't. Can it "go bad" and still function but not function properly? Thanks for the response.

*edit* Another thing I want to add is a little while ago I started the truck up and got under it and put my ear on the tank so I could hear the pump run. It sounded like it was very erratic in how it was running. Not a constant "whirr" at all. Is this normal or could this be something? Again it is a brand new pump so I doubt it's bad.

[CD Smalley]

The relat is basically a switch. When power gets to it, it flips a switch and sends, hopefully 12v to the pump.

Pump is either on or off. The regulator controls the flow of the fuel through the return line. But the pump will run with less than 12v of power is the key.


If the relay is weak, as heat build up in it, resistance will increase, the amount of voltage going to the pump will decline and your symptoms will show.

[FishinGuide]

Hmmm, so the erratic sound I'm hearing from the pump shouldn't be happening? It should be on or off? When you say check the voltage to the pump where would be the best place to do that? The connector to the harness is really hard to get to without the tank dropped. Is there another place I could check voltage sent to the pump?

[CD Smalley]

You really need to test that as close to the pump as possible. But I suppose you should see nearly the same thing right at the relay.

I'd have to hear the noise to understand I guess. It should be a constant noise though. If there is a variation in the sound, then I'd suspect the relay even more.

When you replaced the pump, nothing looked melted there did it?

[Not_ Me]

Does the distributor have any thing to do with controling the injection system on this model? Tach drive controling the pump signal? Sounds like when the vacuum is higher it runs worse. When the throttle is wide open the vacuum is much less. I have no books on that model so I'm not sure if there is a vacuum advance on the distributor or not. Just a thought but check for any vacuum leaks around the intake / heads. The advance system in the distributor may have a problem as well. Good luck.

[777stickman]

I think CD has a point with the relay. Could be the contacts in the relay are not carrying the voltage to the pump. Also the fuel pump fuse. It may check good for continuity but not carrying the voltage to the pump. Sorry to say that the best place to check for power or bad ground is at the connector at the pump. It could be something at the connector or the wiring in between.

Good luck...........Steve

[FishinGuide]

No vacuum advance that I know of. No vacuum lines to the distributor anyway. I have checked all vacuum lines for cracks/leaks and found one but I replaced it. Made no diffrence.

Another thing I want to ask you guys is , and I forgot to mention this, why would it backfire through the TB? I told the guys at the dealer about this and they said that it's doing this because it's leaning way out. Does this sound right? If I hit the trottle quickly by hand from under the hood it will shoot flames out the TB! One time it even caught fire and continued to burn until I shut the truck off. I always thought backfire through the carb meant the timing was way too advanced. They say it is from leaning out and that it definately is a fuel delivery problem.

Thanks for all the help so far guys. I really appreciate it.

[CD Smalley]

The backfiring you describe, timing could cause that. Do you know what the timing is set at? Should be 0* BTDC with the EST wire disconnected.

The backfiring the dealer spoke of can be caused by running lean, usually the sign of a weak fuel fump in TBI vehicles.

Rather than guessing about all of this, we really need to know the numbers...

What is your tiing set at with the EST wire unhooked?

What is the fuel pressure with the truck cold?

What is the fuel pressure with the truck hot?

What is the voltage at the pump with the truck cold?

What is the voltage at the pump with the truck hot?

[chevrock22]

does it run cool? Your thermostat might be stuck OPEN and its telling the computer to run on choke causing major amounts of fuel to be pumped continously into the engine.

[horse482]

Pull the distrubtor cap and check for side to side slop in the shaft. Mine was doing the exact same thing, I replaced the distributor with a gm factory one and it fixed the problem.

[self-made-mechanic]

You might do what I did to solve a hesitation problem in my 1993 5.7 k1500...
PULL OFF THE VACUUM LINE TO THE EGR and run it unhooked. A big improvement in performance occured in my truck, including gas milage.