Code P0300, misfire?

DONALDCEFAIL

MEMBER
2003 CADILLAC DEVILLE
138,000 MILES

Have been troubleshooting and replacing/repairing various components. No cylinder misfire count at this time although still showing P0300.
Ran car at high idle and it stalled. Repeated test, same result. Could this be fuel or cam/crank sensors? Live Data showing 61 (RPM) for CMP.

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

A random misfire is vague and sometimes difficult to resolve. You indicated running the engine at a high idle speed and it stalls. Does it start right back up or do you need to wait?

I ask because it could be a fuel or ignition issue. By any chance, do you have a test light for checking the ignition spark? (See pic below) If you do, connect it, and have a helper run the engine at high idle speed. Watch to see if the spark stops when the engine begins to stall.

Let me know. As far as the P0300, have you cleared it? If so, how long does it take to return?

It's odd that there are no misfires detected but you have this code. Is this the only code present?

Let me know.

Joe

See pic below.

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Sunday, November 12th, 2023 AT 9:44 PM

DONALDCEFAIL

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4 POSTS

Thanks for getting back with me. Been troubleshooting all day long.
Tightened TB bolts to intake, the think collar that joins intake to TB, repaired Vacuum lines, changed fuel filter, moved harnesses while idling, put a couple of bolts in Cat (I have heard rattling and felt the Cat was blocked so I drilled to relieve pressure). The filter was like the Mississippi River! When she died under heavy idle yesterday that pointed me towards fuel.
My shop was broken into recently and virtually all of my tools and diagnostic stuff is gone.
I bought an Innova 5160 the other day and run live data constantly. I am schooling myself on interpreting all of the values.

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Monday, November 13th, 2023 AT 9:45 PM

DONALDCEFAIL

MEMBER
4 POSTS

If I were to go fire the Caddy ip there would likely be no SES. Then it would appear after about two minutes sometimes steady other times flashing. If I were to get underway using idle speed SES would be solid. Then under load she'll blink.
The cam sensor is missing the little tit that allows the connector to hold fast to. I wrapped some thin bailing wire around both.
I replaced all 8 plugs with OEM's, clean boots and a new coil cassette in bank 2. I went from misfires in #3 and #4 to just #4. Did resistance on Injectors, cleaned, lubed O-rings and new intake gaskets. This took care of #4. Now there is no misfire count. I ran P0300 for so long that the Cat has to be messed up! Oh, new bank 2 O2 sensor. I have what appear to be good trims, O2 voltage, MAF and MAP numbers, etc. Oh, I cleaned the MAP, MAF, IAC and replaced the ECT come to think of it.

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Monday, November 13th, 2023 AT 10:03 PM

JACOBANDNICKOLAS

MECHANIC
109,747 POSTS

Hi,

The flashing CEL indicates the misfire is happening at that moment. So, it doesn't happen until there is a load on the engine.

Have you confirmed the fuel pressure is within specification? If you remove the vacuum hose from the fuel pressure regulator, is there any evidence of fuel in the hose?

You have done a lot already. If fuel pressure is good, you turned it up, and it still has the same problem, my next suspect is the noise you hear from the catalytic converter. If the internal part is loose, it could be blocking exhaust flow. Take a look through this link:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Also, keep in mind low engine compression can cause misfires as well.

Let me know.

Joe

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Monday, November 13th, 2023 AT 10:23 PM