Code P305?

XEPHRE79

MEMBER
1995 GMC SONOMA
4.3L
6 CYL
2WD
AUTOMATIC
143,000 MILES

The truck listed above has a 4.3 vortex engine. Truck is giving me check engine light. Replaced spark plugs, wires, crank shaft and camshaft distributor cap and rotor, MAP sensors and fuel pressure regulators. Cleaned the injectors to check for leaks nothing. Changed out the O2 sensors too. So, after driving it for say about 5 minutes it will give me that code again. Can't figure out what the problem is at all. Oh, I also changed out the EGR valve. Checked spark plugs for spark (good). I checked the spider for any ripped or damaged gaskets. Anyone have any ideas?

5 Replies

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

It sounds like you've done a lot already. Since the misfire is consistently cylinder 5, there could be a compression issue. If it exceeds a specific allowable difference, it will cause a misfire.

I looked it up for you. No cylinders should be below 100 psi. Also, the maximum variation between the highest and lowest readings is 30%. Once you go beyond that, chances are you will have a misfire. (See pic below).

If you haven't already, I would recommend trying this. Here is a link showing how to do it. You will need a compression gauge, but most parts stores will lend one to you.

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

Let me know what you find.

Take care,

Joe

See pic below.

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Sunday, September 17th, 2023 AT 9:48 PM

XEPHRE79

MEMBER

Thank you. I did a compression test. All compression read between 130-135. Still getting code. Not sure if it's gasket now or catalytic converter. I say this because when I scan the code gives me p305 then says EGR, O2 heat sensor and mil. I'm not sure what that is if I have changed out O2 sensors and EGR valve. Anyway, thank you again for the help.

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Wednesday, September 20th, 2023 AT 9:17 PM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi,

That is odd. I say that because the EGR or an oxygen sensor would not likely affect only one of the cylinders. Have you checked to make sure the vacuum supply to the EGR is good? Also, what are the codes you are getting other than a P0305? I ask because something may indicate a rich or lean fuel mixture and may help in the diagnosis.

Also, when the misfire code comes on, does the check engine light flash? Do you feel the misfire when driving?

Let me know.

Joe

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Thursday, September 21st, 2023 AT 6:40 PM

XEPHRE79

MEMBER

Hello. Thank you for your help again. First off. On the code when reading I starts like this.
1 code- -P305, Mil, monitors,
3 INC. : Catalyst, O2, EGR
4 Ready: Misfire, fuel, comp, O2 heater.
That's what my small obd reads.
The truck does feel slightly shaky, with somewhat slight throttle hesitation. I will need to check the vacuum supply. Not sure how but. Anyway, I really do appreciate your help. I will be taking the truck for a diagnostic once I get paid next Friday (hopefully). And go from there. Thank You again for all the help.
- Juan M.

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Thursday, September 21st, 2023 AT 7:04 PM

JACOBANDNICKOLAS

CERTIFIED MECHANIC

Hi Juan, the 3 INC indicates that the PCM has not completed a full drive cycle after the battery was disconnected or the codes were cleared. The 4 Ready, I've never seen before. It appears the only real code is the cylinder 5 misfire.

As far as checking vacuum to the EGR, disconnect the hose and start the engine. Confirm there is a vacuum from the hose with the engine running. Also, if possible, check to see if there are any vacuum leaks.

Here is a link that explains how it's done:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find.

Take care,

Joe