Code PO300, cylinder one misfiring

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?

8 Replies

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

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Friday, January 8th, 2021 AT 11:31 AM

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.

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Saturday, January 9th, 2021 AT 4:26 PM

REDBONE

MEMBER
92 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?

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Monday, January 18th, 2021 AT 2:06 PM

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

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