Code PO300, cylinder one misfiring

Tiny
REDBONE
  • MEMBER
  • 1996 GMC SONOMA
  • 4.3L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
I put new spark plugs, wires and cap in and new rotor button, still has a miss. When I hook up the scan tool it shows only cylinder 1 misfiring. I changed the spark because in thought I might have gotten a bad one, still the same. I checked all vacuum lines and no leak, also the check engine light blinks off and on. I will check the fuel pressure to see if it is good. If the fuel pressure is good what else would cause this?
Tuesday, January 5th, 2021 AT 3:00 PM

8 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
This could be a number of things. The multiple cylinder misfire is just saying that there are other cylinder missing but just not consistently on every power stroke.

Here are some guides that cover all this:

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://youtu.be/UKB-ytgbnzU

However, I would suggest starting with swapping the injector from cylinder one to the front cylinder on the other bank. If it moves then you know it is the injector. If it doesn't then it is not.

Next we need to check compression because we could have some valves floating.

https://www.2carpros.com/articles/how-to-test-engine-compression

If the compression is ok, let's hook a vacuum gauge to the engine and see if the gauge is steady. This will tell us if there is an intake valve floating only while running.

Let's start with this and go from there. Thanks
Was this
answer
helpful?
Yes
No
Wednesday, January 6th, 2021 AT 4:26 PM
Tiny
REDBONE
  • MEMBER
  • 98 POSTS
I have already checked vacuum and on the gauge it points to late timing around 14 on the gauge.I will check the compression to make sure the compression on the cylinders is good. Also I have no fuel injectors, it is a CDI fuel system.
Was this
answer
helpful?
Yes
No
Wednesday, January 6th, 2021 AT 5:59 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Sorry. That does say 1996 in the heading.

Can you get a recording of the vacuum gauge hooked up so we can see what it is doing because I was expecting that to be the issue.

I attached your compression spec below for when you check it.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Wednesday, January 6th, 2021 AT 6:29 PM
Tiny
REDBONE
  • MEMBER
  • 98 POSTS
Don't know why it doesn't drop after I leave off throttle.
Video (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Friday, January 8th, 2021 AT 11:15 AM
Tiny
REDBONE
  • MEMBER
  • 98 POSTS
Sorry had gauge hooked up wrong. This is correct hook up:
Video (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Friday, January 8th, 2021 AT 11:31 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
That is most likely a valve issue. Either a sticking intake valve, loose guide, leaking intake valve or possibly a ring issue.

However, the unsteady reving is an intake valve as well. I suspect you have a leaking valve so I would do a compression check next to find what cylinder it is.

If the timing is off as you said earlier then that could be the cause of the leaking valves if they are opening too soon but I would still do a compression check just to be sure.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Saturday, January 9th, 2021 AT 4:26 PM
Tiny
REDBONE
  • MEMBER
  • 98 POSTS
I did a compression and found #1 cylinder has no compression, also did a leak down test on #1 and found out that air comes through the throttle body. So is my intake valve on cylinder 1 the problem?
Was this
answer
helpful?
Yes
No
Monday, January 18th, 2021 AT 2:06 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
You nailed it. The intake valve is not sealing so it is either the valve is burnt, rocker/spring failure, or the cam lobe/lifter is wiped out. So we need to start with removing the valve cover and check the spring and rocker arm. Then we need to dig further depending on this. I attached the process for the head if that is what needs to come off.

Let us know but this is great info so far. Thanks
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Tuesday, January 19th, 2021 AT 6:43 PM

Please login or register to post a reply.

Related Engine Misfire Content

Intermittent Misfire
VIDEO
Intermittent Misfire
In­struc­tional re­pair video
Misfire Random Code P0300
GUIDE
Misfire Random Code P0300
The world of au­to­mo­tive di­ag­nos­tics can be com­plex, with nu­mer­ous codes and po­ten­tial is­sues to keep track of. One of the more com­mon en­gine di­ag­nos­tic codes is P0300, which...
VIEW MORE