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97 Chevy 5.0L Misses Under Load
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mike9572
New User
Apr 10, 2008, 7:07 AM
Post #1 of 2
(4856 views)
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97 Chevy 5.0L Misses Under Load
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Help - I really don't want to just keep throwing parts at it but I need to get it fixed, It's not a daily driver but........................................
I have a 97 Chevy k1500 pickup with 5.0L & manual trans with 160,000 miles.
It runs fine if you baby it, but it is running lean, this shows on the plugs.
If you drive it at all aggressively or pull a trailer it will cutout / stumble / miss / fall on it's face (whatever you want to call it) as soon as you put it under a load.
It has not thrown the codes since last November but I haven't pushed it. Codes were PO171 I don't remember the other, they were both Bank 1 and Bank 2 O2 Sensors saying running lean.
This problem started a year ago and just kept getting worse. It's been to the dealer and two other garages so far.
Here is what has been done so far:
Complete tune-up - plugs, wires cap & rotor - I did this first and it ran great for about 5 miles when it quit running. I had it towed to the dealer thinking it was the fuel pump which is under warranty. They found the rotor with a hole burned through it, this distributor has no electrical in it other than the ignition spark. They said I had the rotor installed 180 off, I don't think so??? Pull out of the dealer and about got t-boned as I had no power, took it back, then they said it was the cat - Haven't been back to the dealer!
Here is what has been done by other mechanics:
Fuel filter - about 1,000 miles ago
New air filter
Catalytic converter
Intake manifold pulled and resealed
Throttle position sensor
Crankshaft position sensor
Timing chain checked (cover pulled)
Distributor replaced
MAP sensor replaced with known good used one
Coil & ECM replaced with known good used one
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I did a little testing and here's what I found.
Fuel Pressure
The book says 60-66psi, the way I'm reading it I think this is key on not running and then 3-10psi lower when running. What I have not running (during the 2 second pump up when you turn the key to on) is 60psi. But as soon as the pump shuts off it drops to about 15psi and it drops fast. Running it is 50-52, it's not real steady.
I just don't know where to go next, with the pressure dropping so fast when it's shut off I know it could be
- the check valve in the pump
- the pressure regulator
- injector(s) leaking
I don't see an easy way to finish the test as I don't think I should pinch the braided fuel line closed.
It doesn't start hard, if its the check valve on the pump would it make it run bad or just start hard??????
Any Idea's?????????????????????
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Double J
Veteran
/ Moderator
Apr 10, 2008, 12:04 PM
Post #2 of 2
(4850 views)
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Re: 97 Chevy 5.0L Misses Under Load
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Jogging whats left of my brain here
I remember Chevy's/GM's having problems with the fuel pump pulsators (Flexible connection between the fuel pump and the fuel sender)
Maybe something to look into..
A bad pulsator will let fuel pressure drop off quickly
Here is a pic and a descrpition of its operation.
We replaced a ton of these for fuel issues.
The pulsators with the orange seals leak.
Replacement pulsators have black seals.
Hope this helps.
Fuel Pump Pulsator 
Fuel Pump Pulsator Operation:  A small accumulator mounted between the fuel pump outlet and the fuel pressure regulator. Most are mounted in the fuel tank between the fuel pump outlet and the fuel supply line. It reduces the fuel pressure variations that occur as the fuel injectors open and close, and to reduce operating noise. Usually built-in grommets seal the unit to the tubing ends. Some are made into a section of flexible fuel line located in the engine compartment. Failure of the in-tank units would result in increased noise or loss of fuel pressure or volume, depending on the type of failure, the external units could develop fuel leaks detectable by smell or visual inspection. The pulsator isn't a routine maintenance item, although replacing the in-tank unit is recommended with a fuel pump replacement.
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