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1996 GMC Sierra P300 Random Misfire
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BadBackyardMechanic
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Oct 26, 2012, 4:17 PM
Post #1 of 30
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1996 GMC Sierra P300 Random Misfire
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OK...here's a list of what I've done just to break the ice..
Spark
Have Replaced the following: Wires, plugs, distributor, distributor cap, and distributor shaft
Coil has about 50,000 miles on it.
Fuel
Fuel Pressure Regulator
Spider probably has about 50,000 miles on it maybe less.
Extras: Replaced all 4 Oxygen Sensors
Symptoms:
The vehicle starts right up no problem. When it's driving it seems to bog down badly. Sometimes when I hold the pedal to the floor for an extended period of time the engine starts running on full power.
I initially was getting P304 (cylinder 4 missfire or possibly cylinder 7 code I forget now) but I replaced all the O2 sensors and now it's decidedly sticking to a P300 code. Even after replacing the Fuel Pressure Regulator.
(I did at one point check the cylinder that was coded, did not conduct a full compression test but the compression was a solid 150 PSI and maybe 3 years ago I did conduct a full compression test and I remember all the values were within about 10-20PSI of each other so I was very skeptical of there being a compression problem)
Fuel Pressure Test:
When I prime it before ignition, the PSI is about 58-60.
When it's idling my father and I kept testing the fuel pressure. It runs at about 50-54 PSI.
When we've pressed the gas pedal down and ran it around idk 2500 RPMS steady, the PSI did drop to 50 on occasion.
The fuel lines are metallic until you get near the tank, where they then are hoses. My father (has been a mechanic, farmer and engineer) told me to halt the flow in the return line while conducting the pressure test.
He said that...
A. If there was no change with the fuel pressure while stopping the return flow, it would be the fuel pump.
B. If there was a change with the fuel pressure, then it's the fuel pressure regulator.
Anyways when I conducted this fuel pressure test by vice gripping shut the flow on the return line, the fuel pressure spiked to 80 PSI on the shrader valve.
So I replaced the fuel pressure regulator, but unfortunately in the test drive today, there was no change and the vehicle was still bogging down.
So....
Main Question: Does this mean that the culprit is most likely a bad fuel pump?
Secondary Question: If so, I'm guessing that means there must have been something wrong with our return line test as far as determining whether it was the fuel pump or the fuel pressure regulator. If anyone knows why that test of stopping the return line while checking the fuel pressure was a faulty test, please feel free to post. Whether the logic behind the test is bad, or whether the test is just flat out wrong. I'm still just learning as much as I can while trying to get this truck fixed.
Thank you for any help,
~BBYM
(This post was edited by BadBackyardMechanic on Oct 26, 2012, 4:26 PM)
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Tom Greenleaf
Ultimate Carjunky
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Oct 26, 2012, 4:29 PM
Post #3 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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What engine but not sure it matters - fuel pressure is too low IMO or your gauge must be off. Many wont run at all below 54,55 PSI or so. I think spec is 60-66 PSI for Vortecs anyway,
T
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BadBackyardMechanic
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Oct 26, 2012, 4:34 PM
Post #4 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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I think I miswrote the AB portion and edited it.
The engine is a 305 engine. LOL yeah....all that typing and forgot that part.
I'd read on some internet sites that it needed to be running at 58 PSI and primed prior to startup around 60-66 PSI so that's good to confirm here.
Now I just have to find a place that sells a decent pump and one that lasts longer than one year. Any recommendations?
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BadBackyardMechanic
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Oct 26, 2012, 4:47 PM
Post #5 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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The primed pressure was the same as before. Around 58-60. And the vehicle had no change in performance during the test drive. P300 Code came back on after driving a couple blocks.
Haven't checked the pressure at idle but the vehicle drives exactly as if no change. I'll check the fuel pressure tomorrow but I'm pretty sure it's going to read the same as before.
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Tom Greenleaf
Ultimate Carjunky
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Oct 26, 2012, 4:54 PM
Post #6 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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When you've confirmed fuel pump which you about have go for an OE whole unit from a dealer or you'll regret it. No bargains just get the real thing,
T
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Hammer Time
Ultimate Carjunky
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Oct 26, 2012, 5:09 PM
Post #7 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Don't jump to that conclusion of a new pump so quickly.
Something is not adding up in your testing. If you truly have 80 PSI when the return is pinched off, then you do not have a bad pump and need to go back and look at that regulator again.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
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Oct 26, 2012, 5:26 PM
Post #8 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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I seriously need new glasses but can't find it pumping 80 PSI in this thread pinched return or not? Just a max of 60 so far if testing and gauge is on target,
T
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Tom Greenleaf
Ultimate Carjunky
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Oct 26, 2012, 5:33 PM
Post #10 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Found that! Shoot - that isn't a sign of totally incapable pump is it? Sorry. Now IDK - have heard of the plug right at the tank end having corrosion problems but none right in front of me YET - T
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Discretesignals
Ultimate Carjunky
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Oct 26, 2012, 6:42 PM
Post #11 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Just curious. If you unplug the MAF sensor, does the bogging go away?
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Discretesignals
Ultimate Carjunky
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Oct 26, 2012, 8:10 PM
Post #13 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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The engine computer uses the MAF sensor to calculate engine load. The load calculation is very important because that determines how much fuel the computer is going to inject into the engine.
If the MAF isn't reading the amount of air flowing through it properly, it will give the wrong information to the engine computer. For example if the MAF is sending a signal to the engine computer that there is less air than what is actually flowing through it, the engine computer is going to inject less fuel and cause the engine to run lean.
The opposite can happen too. If the MAF is sending a signal to the engine computer that there is more air flowing through it than what is actually flowing through it, the engine computer is going to inject too much fuel for the amount of air that is actually going into the engine causing it to run rich.
GM has a back up if the engine computer determines there is an electrical problem with the MAF. The engine computer will default to using the MAP sensor for load calculation.
Bogging is one symptom of a MAF problem. The easiest way to see if that is the case is to disconnect the electrical connector from the MAF. That will force the computer to default to the MAP sensor. If the bogging goes away, the MAF is the likely culprit. It is just an easy test to rule out a MAF problem. It is also important to be sure your ducting from the MAF isn't ripped or the breather pipe isn't disconnected. If unmetered air is allowed to get into the engine after the MAF sensor, the sensor isn't going to read the correct amount of air causing the engine to run lean. Usually but not always a MAF problem is followed by an O2 code.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Hammer Time
Ultimate Carjunky
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Oct 27, 2012, 3:19 AM
Post #14 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Regardless of what the MAF is doing, this engine won't run with 50 PSI so that issue needs to be resolved satisfactorily.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Discretesignals
Ultimate Carjunky
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Oct 27, 2012, 8:26 AM
Post #15 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Your right HT. The fuel pressure needs to be higher than that, especially if he is still using the old poppet valve injector assembly.
Gotta be careful when you dead head the pump the way you did. If the fuel pressure gets higher than 75 psi, that can damage the fuel pressure regulator. Don't know what the dead head pressure should be on that pump. Maybe it should be higher than the reading you got. There is no service information on what it should be. Even though it dead heads at that pressure, that doesn't mean you have the right volume of fuel. General rule is that a fuel pump should be able to pump a pint of fuel in 30 secs. Revving the engine should have caused the fuel pressure to go up when the regulator sensed high manifold pressure.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Oct 27, 2012, 8:37 AM)
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BadBackyardMechanic
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Oct 28, 2012, 7:58 AM
Post #16 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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Disconnecting the MAF sensor caused the vehicle to drive worse, if possible. A lot more bogging and sputtering going on. I'll double check the PSI at the shrader valve today while the engine is running.
I got sidetracked by the UGA game yesterday evening.
I'll probably just try replacing the fuel pump afterwards. I'll followup with the results when I complete the task. Because of my work schedule it might take until next weekend.
I'm a complete amateur so some tasks...even simple ones like a fuel pump sometimes take me awhile to plod through them.
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BadBackyardMechanic
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Oct 28, 2012, 11:57 AM
Post #18 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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I'll probably just try replacing the fuel pump afterwards. That's like throwing $500 up in the air and hoping it comes back to you. Throwing parts at the problem can get real expensive.
Yeah...just didn't know of any other viable alternative routes to go yet.
Anyways...I rechecked the fuel pressure on the shrader valve.
1. When Primed: Initially it's now at 60 PSI, then within 3-5 seconds of priming to 60, it drops to 58. I double checked this like 4 times and got the same result every time. It held steady at 58.
2. When running/idling: Dipped to roughly 50 to 52 PSI and held steady.
Suggestions?
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BadBackyardMechanic
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Oct 28, 2012, 12:21 PM
Post #20 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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When running/idling: Dipped to roughly 50 to 52 PSI and held steady. And there lies the problem. This engine will not run under 55PSI.
Right, it's running, but very poorly and the fuel pressure is the problem, I agree. So I guess I'm somewhat confused as to what to try next.
Would you recommend me taking a shot at the fuel pump or is there more testing to be done?
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Hammer Time
Ultimate Carjunky
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Oct 28, 2012, 1:04 PM
Post #21 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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You first have to confirm that there isn't any kind of voltage drop reaching the pump. If you can confirm no voltage loss, then blame the pump. Be sure to change the filter with it.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Tom Greenleaf
Ultimate Carjunky
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Oct 29, 2012, 6:39 AM
Post #22 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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BBYM: Be sure to put some PB on fuel filter flare nuts and lines now unless it looks clean as new those really stink and can break lines easily,
T
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Hammer Time
Ultimate Carjunky
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Nov 4, 2012, 11:53 AM
Post #24 of 30
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Re: 1996 GMC Sierra P300 Random Misfire
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You need to read the rules here.
You don't post any links unless they are to your own pictures or videos
http://autoforums.carjunky.com/...ORE_POSTING_P123007/
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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