Check engine light on/rough idle

Tiny
DUBEL12
  • MEMBER
  • 1997 GMC SUBURBAN
  • 130,000 MILES
My check engine light came on and the code it spit out was p0305 which is a cylinder 5 misfire. I need to figure out where the problem is. The 2500 4x4 idles rough, has a low rpm when idling, drives with less power than before, is difficult to accelerate quickly and gets even worse gas mileage than before. Other than slightly less power when driving, it drives fine. The engine sounds a little off, but not terrible. I have replaced the spark plugs, tested the plugs and wires, replaced the fuel pump and filter, and did a compression test. Everything I have done hasn't made a difference. The only thing I noticed is very shortly after changing the spark plugs, I pulled a few out and only one was black and sooty in the 5th cylinder. I need to know what to test/clean from here. I've heard of checking and cleaning the EGR valve, but that is the only next step I can think of. I need to fix my truck! Help!
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Saturday, September 21st, 2013 AT 3:57 PM

7 Replies

Tiny
SATURNTECH9
  • EXPERT
Have you checked for actual spark to number 5 cylinder? A nice blue spark?Also. You may have a clogged number 5 cylinder fuel injector.
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Saturday, September 21st, 2013 AT 4:03 PM
Tiny
DUBEL12
  • MEMBER
My first instinct was to put concentrated fuel injector cleaner in my tank and only filled up halfway in the hopes it would push out any clog. That didn't seem to make a difference. I also checked to see if there was a spark in the cylinder and there is, so I need to know where to go from here.

I noticed also when I first start the truck and drive a while, it runs fine, but after about 15 minutes it starts to lose its power. It takes longer to accelerate and feels as if I'm running out of gas.
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Saturday, September 21st, 2013 AT 11:48 PM
Tiny
SATURNTECH9
  • EXPERT
If we have good compression and spark only thing left is fuel. Which engine do you have?Also have you done a back pressure test on the exhaust yet?If not I would do that. For the runs good for 15mins and looses power.
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Sunday, September 22nd, 2013 AT 8:33 AM
Tiny
DUBEL12
  • MEMBER
7.4L 454 small block. How do I do a back pressure test? What do I need?
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Sunday, September 22nd, 2013 AT 8:47 AM
Tiny
SATURNTECH9
  • EXPERT
You would screw in a adapter into the o2 sensor hole an use a back pressure gauge to measure the back pressure at 2000 rpms. As far as the injector can you hear hear it clicking with a long screwdriver while its running?
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Sunday, September 22nd, 2013 AT 9:24 AM
Tiny
DUBEL12
  • MEMBER
Fuel injector working fine, intake manifold gasket looked fine but changed anyway. Someone told me to check the timing, and someone else told me it could be a bad wire and to track it down or replace the whole engine bay wire harness. Any validation?
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Monday, September 30th, 2013 AT 11:59 PM
Tiny
SATURNTECH9
  • EXPERT
So can you hear the number 5 injector clicking while its running?How do the spark plug wires look?Are the pretty old? I wouldn't change the engine harness on a guess.
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Tuesday, October 1st, 2013 AT 7:13 AM

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