2000 Yukon with P0300 error code

[H8 PVMT]

My Yukon SES light is on and I had the auto parts store run the code, P0300, multiple misfire. I've read pages and links about what it could be and nothing is definitive. So, wondering if anyone has experience with this and some advice. Truck has 270k and I keep it well maintained. Some of what I've read is vacuums leak, EGR valve or bad Cat's. I could replace plugs and wires, that's inexpensive but I don't feel like chasing this problem and replacing parts like throwing darts. I know it's impossible for anyone to diagnose over the internet but if anyone has advice I'd appreciate it.

 

[fresh350]

It would be easier to diagnose if you knew what cylinders were misfiring but you can start with pulling a few plugs and checking them, check for arching on wires, and cracks or anything out of the ordinary with the coils. You can also check for vacuum leaks with a can of carb cleaner while its running and spray any vacuum lines and around the intake to check for leaks (idle will raise if there is a leak). There is a way to pinpoint what cylinders are misfiring without a scanner but I don't recommend it... Disconnect each coil connector one at a time and see if the idle changes (if it does it's a good cylinder, if not then its the dead/misfiring cylinder). Again I DO NOT recommend it and it will throw all kinds of codes but it works in a pinch

 

[1_8TTony]

There is a way to pinpoint what cylinders are misfiring without a scanner but I don't recommend it... Disconnect each coil connector one at a time and see if the idle changes (if it does it's a good cylinder, if not then its the dead/misfiring cylinder). Again I DO NOT recommend it and it will throw all kinds of codes but it works in a pinch

Autozone sells a relatively inexpensive tool made specifically for checking spark. It is a cheaply made tool, but it only costs $8 or $9....it has an adjustable mid section with millimeter hash marks .....I buy one and it lasts me 6 months or so. The adjustable portion of it is designed so you can gap the tool at the same size as the plug gap.

That being said.....

Are there any other codes present? If there are no other codes, you will have to see what the computer is seeing. I doubt you will find any vacuum leaks if there are no Evap or Lean Bank related codes.

Thinking you need to use a scanner with Real Time Display and Freeze Frame capabilities.

 

[H8 PVMT]

No other codes than the P0300 which makes it hard to diagnose. It's been a while since I've done plugs and wires, I might do that just as a tune up measure, check the old plugs and see if that helps, at 270k replacing some older parts can't hurt may not fix it but it can't hurt. I'll keep up with the thread and post what happens to maybe help someone else.

 

[fresh350]

Autozone sells a relatively inexpensive tool made specifically for checking spark. It is a cheaply made tool, but it only costs $8 or $9....it has an adjustable mid section with millimeter hash marks .....I buy one and it lasts me 6 months or so. The adjustable portion of it is designed so you can gap the tool at the same size as the plug gap.

That being said.....

Are there any other codes present? If there are no other codes, you will have to see what the computer is seeing. I doubt you will find any vacuum leaks if there are no Evap or Lean Bank related codes.

Thinking you need to use a scanner with Real Time Display and Freeze Frame capabilities.

In my experience with p300 codes, they come and go and jump cylinders... a spark tester would work but will take forever compared to having a scanner or trying to unplug each coil one at a time when the misfire starts. But as we said... a scanner is best

The plugs and wires would be a good start if it's been a while.

 

[retiredsparky]

Check the wiring harness connections to the coil packs for any corrosion. How old is your battery. Have your alternator checked for a.c. output. If even one of the diodes in the alternator has failed, strange things can happen (unlikely cause).

 

[H8 PVMT]

Plugs and wires replaced and the code is still active. Next I'll get some carb cleaner and check for leaks near the intake then go from there. Plugs did need replacing so no worries there, not a wasted purchase thank goodness.

I'll check connections as well thanks retiredsparky.

 

[Cusomano]

I just fixed a 2002 Escalade with the same code- it turned out to be the flex tube that runs from the exhaust manifold to the EGR plate had a crack/rust hole in it. It was underneath and tough to see. I had to borrow a smoke machine to find it. If all else fails from the other suggestions posted here, you could also check that.