Multiple Misfires on 2003 3.0 Automatic

[HT08]

Hello, My 2003 3.0L with 344K miles has been throwing Check Engine light recently with these codes:

P0316 = Misfire detected on startup (first 1K RPM)
P0306 = Misfire Cylinder 6
P0305 = Misfire Cylinder 5

I have swapped and tested coil pack, crank position sensor, plugs, plug wires, and injectors to no avail. I have also cleaned the MAF sensor and replaced the fuel filter and air filter. This truck gets regular service thereby why it is still going at 344K miles, so not poorly maintained.

It happens intermittently mostly after it gets up to temperature. Cold starts are right away and mostly without issues. Occasionally it will start to misfire on cold startups. I have not done a compression check yet but that is next.

It always starts right up, no issue starting. It idles fine cold most of the time but as it warms up and I start getting a miss and when stopped at lights I sometimes have to keep the RPMs at 1000 rather than 750 to get it to smooth out. When it starts misfiring the check engine light will start flashing.

Any and all recommendations are greatly appreciated. Thanks for reading!

 

[superj]

Did your compression check get done?

Since it happens warm, is the temp sensor for the ecu reading ok? Not that it would cause a misfire but something to test. I wonder if you have some leak, headgasket, intake, etc, that is causing trouble?

 

[HT08]

Not yet, as far as temperature my temperature gage reads normal when cold and once it is warm. About 10K miles ago I had to replace the front timing cover gasket and original water pump. of course a complete flush and refill. No issues with cooling system except for the leaking front timing cover.

It doesn't run well enough to drive, waiting on the weekend to rent a compression tester and see what I get. Also I know of the sunken valve issue but I thought that affected the 2004 on Vulcan engines not the 2003. I guess anything is possible. Mine is an April 2003 production.

 

[HT08]

UPDATE: Performed a compression test, here are my numbers.

1- 115
2- 120
3- 110
4- 115
5- 98
6- 100

Observation while testing compression on number 5 and 6 is once the gage started moving above 30psi as it cranked I can hear a whistle or a squeaky sound (almost like a belt slipping) all the way until the needle stopped progressing. These are the only 2 cylinders that made that noise while cranking.

Any input on what that whistle means, (guessing valves not closing)??

 

[pjtoledo]

guessing the hose was leaking in the plug hole.
did you use an adapter? grease it to improve the seal.
my 2000 has narrow plug holes, need an adapter for a good seal.

repeat the test after squirting some oil/wd40 in the holes. that will seal the rings better so the results will be about the valves.

 

[HT08]

Good point you bring up. I did have to use and adapter and yes I lubed the adapter and O rings and tried it twice with the same result.

 

[SenorNoob]

With both being low, I'd suspect a blown head gasket between the cylinders...