2001 GMC Sierra P0300 Multiple Misfire

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#1
???
Ok getting the P0300 multilpe misfire code on my 2001 GMC Sierra with the 5. 3L engine. Got new plugs and wires. Checked spark with spark tester all coils are firing. Check resistance on injectors they checked out good. Tested fuel pressure key on 52, engine running (idle) 48, leak test for 5 mintues no drop in pressure.
Vehicle is running rough, hard to accelerate, stumble during acceleration. Dual exhaust on this truck driver side exhaust is just barely blowing out passenger side blowing out good. Was getting code before I notice exhaust pressure. Could it be Cat Converter and fuel pump?
 

Transman

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#2
Your fuel pressure is low with key on but in range idling, still a concern. GM does not like even 1 psi low and well known for causing problems. Did you do a fuel volume test?
These are also known for crank sensor problems as well as clogged exhaust. You could remove an o2 sensor or several on the side that seems to have no exhaust and see if the engine runs smoother and if there seems to be excessive pressure trying to escape through that hole, this would indicate clogged exhaust/cats. There was only a p0300 code, nothing else? Post back, Transman
 
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#3
No on the fuel volume test. Didn't know anything about it until now, I will check into it. Here are the codes that came up P0300, P0420. Exhaust pressure coming out of tail pipe was good on both sides when the codes started showing up. I would drive it down the road a couple of miles after clearing out codes and the check engine light would flash on and off showing a pending on code P0300. I have three incomplete monitors; misfire, fuel, and catalyst. They will proably show the P0420 code if I keep driving it. Ok rechecked fuel pressure today 50 with key on and 48 while running and pressure drops when I accelerate. Removed both O2 sensors from the exhaust side that didn't have the pressure. Exhaust is coming out hard on the first (pre) and weak on the second (post) so guess this means blocked cat. Thanks Transman
 
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#5
Didn't actually put the truck on the road when I removed the O2 sensors. At idle it sounded about the same.
 
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#7
New fuel pump same problems P0300 Multiple Misfire. New fuel pump showing same readings as old fuel pump. Still think that cat needs to be replaced. I'm open to suggestions.
 

olds455

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#8
when the truck is running watch fuel pressure as you connect and disconnect vacuum line from fuel pressure regulator, should go up and down (up when unplugged)
 

Transman

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#9
Take the front o2 sensors out again and drive the vehicle down the street, it will be louder but just drive it enough to see if the power has improved. If it has improved then the problem is clogged exhaust/cat/muffler. Post back, Transman
 
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#10
Hey thanks Transman. I done that with the new fuel pump (remove vaccum line from regulator and it goes up and down), pinched off the return line pressure goes past 80. I should have left the O2 sensor out when I drove it but didn't guess I'll try tomorrow.
 

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#11
Excellent, That will be the most important info you can supply. It may not recover completely but you WILL see a big difference if the cat/exhaust is clogged. Post back, Transman
 
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#12
fixed Conclusion, Hey thanks Transman should have went with my first thought on the cat/converter. Took out the O2 sensor and got my power back no check engine light. Just got the cat changed out today $120. Runs like a scalded dog, no check engine light. Can't remember what other code poped up with the PO300 when this problem started but it was for the O2 sensor and cat. :-[

FIXED, FIXED, FIXED
 

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#13
Glad you got it but just a caution here, your first instinct was correct here but it is better to take a few minutes to prove it rather then replace parts. Yes, this time you would have been correct but the odds are against you hitting it every time, by testing components first, you will be assured to replace the correct defective part.
Another concern here is why did the cat go bad in the first place, my experience is bad o2 sensors will cook a cat. Check them thoroughly before you end up replacing another cat in a month or two.
Thanks for the update, Transman