The only time I’ve had the pressure go to zero was when I had a sticking injector about 1988. I do believe there is a relief ball back there somewhere with the pump that could stick open. I don’t know what that would have to do with it though if you don’t get hard starting. As long as you have good pressure with the engine running. Still you’ve got an open in the system somewhere if it goes to zero right away.
It takes a little longer to start than it used to when the engine’s cold but not really hard to start. This drops to zero right after the pump shuts off, pressure goes right up after starting it.
I guess this pressure isn’t too high to be causing problems. I’m pretty much out of ideas now. I’m just going to change the pump and the rear lines anyway. Maybe if I just drive it, a code will pop up someday. Ugh…
I’m still reading about fuel pressure. That it should be holding pressure for 15-20 minutes after pump shuts off. I only saw one mention about bleed off being normal. Mine drops like a rock to zero when pump stops. I’ve changed the regulator twice and the injectors. Can this be something with the pump or anything else? Anybody heard of this.
I have always been advised to replace fuel pumps with the factory part, not a $15 eBay purchased one. The difference in price can be $300 but look at all the problems you are having. This might be the problem but if it wasn’t, you would be pretty confident it wasn’t the pump if you had used OEM. If this is throttle body and the injector was sticking open and leaking, you would see gas dripping down onto the throttle plate. My gut tells me the problem is within the cheap pump since you hear extra noise and the pressure drop is there. You use these to get the car running to sell it, not if you plan to keep it.
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Start the engine and while watching the pressure pinch the retrun line tight enough to completely close it and turn the ignition off. If the pressure slowly falls to ZERO your pressure relief valve is faulty. If the pressure immediately drops to ZERO there is a problem with the fuel pump.
I agree, it wasn’t my choice to buy a generic pump, that’s what the mechanic chose without asking me. I would never buy a cheap part especially on something that’s labor intensive to install.
I don’t get the zero pressure. I know there’s a check valve but mine isn’t that hard to start. And I’m not sure this would contribute to my problems anyway, but it doesn’t seem right.
Thank you, I’ll try that tomorrow. The spliced in rubber hose should make it easier.
I believe you said your engine sports an in-tank pump & engine compartment fuel rail with the fuel pressure regulator prior to the injectors. I’d guess that configuration would be designed to hold pressure when you turn the key off. But with the TBI configuration, hard to say for sure. Holding pressure may not be necessary w/TBI b/c there are fewer injectors to deal with. I’m looking at a test procedure spec here for the 4.3 Z TBI engine, from what I can tell, if you pinch off the return line some pressure should hold for at least 30 seconds. That sort of implies that the fuel pressure regulator is designed to bleed off pressure all the time, unlike most fuel pressure regulators. So that pressure drop to zero may be what it is supposed to do.
I think you should focus on the 15 psi, that’s too high. It could be your gauge is inaccurate, but I think your gauge is ok & you’ve probably discovered a problem there. Too high of fuel pressure will cause fuel trims to go out of whack. The reason is the computer assume the injectors are flow rated for a certain number of pounds of fuel per hour at 9-13 psi. If you actually have 15 psi, the injectors are injecting more gas than the computer thinks they should be for the pulse interval it is programming. And it could be a lot more gas, depending on how flow goes with psi. Might not be a straight line relationship. Your injectors might be injecting 50% more gas than the computer thinks. Not a good thing.
So what’s causing the over-pressure? Sometimes the gauge has to be air-bled for it to read correctly. Did you try that? A faulty fuel pressure regulator could cause it of course. Overly high fuel pump pressure, another possibility. But that seems unlikely. hmmm … another idea, if the return fuel line back to the tank wasn’t flowing freely, that would prevent the fuel pressure regulator from working. You might could temporarily disconnect the return line and route it into a gas can. Start the engine. If that returns the pressure to the proper 9-13 psi range, then you know the return line is the problem, probably obstructed somewhere between the fuel pressure regulator and the tank. There could be an obstruction where the regulator seats too I suppose.
I couldn’t find out much about the pressure. It’s funny it isn’t that hard to start so it may be normal to bleed off so fast. That was my thinking, if it had a restriction in the return line that might cause the problems I’m having. The restriction might cause the fuel pump noise. Maybe the line got pinched when he repaired it or a piece of rust got in.
I wasn’t sure if 2 psi would make a difference. I bought another Actron gauge, they seem decent, Can’t say if it’s accurate. I didn’t bleed it before I used it. I don’t think it’s the regulator because I’ve changed it twice now and it didn’t make any difference. Thanks a lot for the help George. I’ll keep at it.
You can usually get the gauges as a loaner tool. Do this and you can return it for another one without penalty if you are concerned about the accuracy. AutoZone, Oreilly and others offer this program. Sometimes loaner tools are pretty abused and beat up though so be aware of that. I had several compression gauges once with variable results.
The range limit is 9-13 psi. So ideally the pressure would be 11. You’ve got 15 psi. That 40% higher than ideal, and 2 psi outside the upper range limit. Little point looking for other causes until you’ve got the correct fuel pressure.
Years ago I had a intermittent problematic computer. I measured the power supply voltage at the processor, 4.73 volts, which was only slightly below the manufacturer’s 4.75-5.25 spec. Couldn’t find any other problem that would cause the symptom, but I was able to re-jiggered the power supply to 4.8 volts, problem gone immediately.
One bit of advice, if you plan to keep your truck awhile, you might want to consider installing a more modern injection system, if such a thing is available which fits your engine. Modern fuel injection systems run at about 35-40 psi. Injection systems that run on low pressure like yours are often troubled by clogging. My old VW Rabbit’s mechanical fuel injection system ran on low pressure and the resultant fuel system clogging was the main source of its problems. My Corolla runs at 38 psi and only once in 25 years has it clogged, and that was a minor one fixed with a bottle of injector cleaner in the gas tank. If a replacement injection system isn’t in the works, once you get your truck running well again, choose a good name brand of gasoline from a station that sells a big volume, and replace the fuel filter frequently.
Seriously, George . . . ?!
You would recommend that Chris somehow install a more modern fuel injection system on his 22 year old Chevy S-10?
Considering he’s got TBI, how the heck is he going to install a spider type injection system or a rail type, without going to a different style intake manifold, wiring, and a completely different fuel pump assembly
No offense to anybody, but this truck is not particularly interesting, valuable, or unique. I just don’t see how you could justify the cost of such a procedure
And if he lives in a state with emissions inspections, I imagine he would have to get the referee involved, unless by some miracle he finds a kit with the appropriate “exemption” paperwork
Your are right db, if such a thing can’t be done economically, if there’s no improved compatible fuel injection system available, then there isn’t much a person can do other than the keeping on top of the maintenance to keep the fuel system as clean as possible. hmm what else? I suppose it might be possible to remove the entire fuel injection system completely, and revert to a carburetor.
The parts stores where I am in NH don’t loan test gauges for some reason, at least Advance doesn’t. They loan lots of tools but no diagnostic stuff. It will be good to have this stuff on hand now anyway. The compression tester was $30, GM TBI fuel tester was $33. I did get the vacuum gauge from Harbor Freight for $12 and it seems to work well.
Thanks for the advice. I didn’t get to do anything with it yet. I bought new pre-made rear fuel lines and a new pump. I’m not looking forward to doing it.
What’s the situation with the fuel tank? That might be a good time to bring it up to snuff too. If it’s the original tank, the inside may be gunked up with all sorts of injector clogging debris. I’ve no experience in renewing fuel tanks, but I expect somebody here can offer up some suggestions. I’ve heard of them being steam cleaned I think, and I’ve heard of a product that you pour into the tank (removed from the car) and then you sort of rotate the tank to coat the inside surface. That apparently produced a like-new inside surface.
Only if it makes sense. I see articles in car magazines all the time about owners removing their 302’s carb & installing brand new throttle body injection systems on v8 trucks similar to mine, early 70’s. Even 60’s. Apparently this is a fairly common thing. The counter guy at the local inde-place I usually buy auto parts suggested that to me in fact, when I was having carb problems last summer. He said a lot of his customers with 60’s and 70’s era cars with carb problems were converting to some kind of Holley self-learning TBI system, and they usually upgraded to electronic ignition at the same time. The TBI unit itself just bolts where the carb would normally sit. Beyond that I think he said you had to install a new electric pump inside the tank, and plumb in a gadget into the exhaust system for an o2 sensor. Whether something similar would make sense for the more modern engine that’s already TBI, don’t know.
Tank was replaced about 12 years ago when I had the 1st fuel pump replaced. I was just going to wipe it out with carb cleaner and rinse it. I don’t want to get too involved with it. I got the mounts all loosened today, still have to lower the tank. Hardest thing might be getting the return line fitting apart in the middle.
How old is the fuel filter? if old replace it. if still not fixed Unplug the throttle position sensor and see if its fixed. let it run a few minutes before deciding its fixed. If that cures it replace the sensor. If it does not check the fuel pressure sensor and MAP sensor then check pressure on the fuel rail. If that still doesnt clue you in on the problem clean the throttle body and have a professional injector cleaning
Installed new Delphi pump, Spectra sender assembly and rear fuel lines.
No real difference in performance. Only good thing is the new pump is very quiet. The other “Precise” brand pump is junk. Glad to be rid of it. I didn’t check the pressure with new pump. That was a job I don’t ever want to do again.
Only thing left to try is a can of Seafoam… lol.
I give up, I’m just going to drive it like it is and not worry about it until it breaks or throws a code.