Unknown misfire

[Thebluestreak]

I’ve got a 2015 2500 with a 6.4 hemi. Not long ago I was driving home from work when I got a check engine light. Simple scan showed cylinder 6 misfire. I put a ignition coil in and cleared the code, and it worked for a small amount of time until it returned. I then checked the plugs, and they are fairly new, I replaced them back in Summer of 23’. I put new injector in cylinder 6 and still show it throwing multiple codes for Cylinder 6 misfire, and multiple cylinder misfire. I’m stuck. I’m worried it could be MDS, but want to check all my boxes before going that far. Any ideas?

 

[Wild one]

I’ve got a 2015 2500 with a 6.4 hemi. Not long ago I was driving home from work when I got a check engine light. Simple scan showed cylinder 6 misfire. I put a ignition coil in and cleared the code, and it worked for a small amount of time until it returned. I then checked the plugs, and they are fairly new, I replaced them back in Summer of 23’. I put new injector in cylinder 6 and still show it throwing multiple codes for Cylinder 6 misfire, and multiple cylinder misfire. I’m stuck. I’m worried it could be MDS, but want to check all my boxes before going that far. Any ideas?

Do a compression test,sometimes when the cams left the chat,it'll show up on a compression test,next step after that would be to pull the valve cover and check rocker arm movement,while somebody cranks the engine over,and you watch the rockers.

 

[Thebluestreak]

Did a compression test and got the following:
Cylinder 1: 105
Cylinder 2: 111
Cylinder 3: 110
Cylinder 4: 95
Cylinder 5: 120
Cylinder 6: 104
Cylinder 7: 105
Cylinder 8: 110

Cylinder 4 I rechecked and even tho it isn’t my trouble code cylinder, it did appear to have less compression. Are the MDS cylinders supposed to have a lower compression?

 

[Wild one]

Did a compression test and got the following:
Cylinder 1: 105
Cylinder 2: 111
Cylinder 3: 110
Cylinder 4: 95
Cylinder 5: 120
Cylinder 6: 104
Cylinder 7: 105
Cylinder 8: 110

Cylinder 4 I rechecked and even tho it isn’t my trouble code cylinder, it did appear to have less compression. Are the MDS cylinders supposed to have a lower compression?

#4 definitely deserves a better look at the rocker arm movement. MDS isn't at play when you're doing a compression test,so all cylinders including the mds cylinders should be pumping roughly the same psi. How many miles are on the engine,as you have a fairly big spread in psi readings.By chance did you pour any oil in the cylinders and re-check them,if they don't change with oil in the cylinders,that pretty well rules out bad rings,and leaves you with valve or camshaft issues.

 

[Thebluestreak]

It has 138k on it, and with the compression test, I just pulled the ASD relay after the truck got to operating temp and bump started it

 

[Wild one]

It has 138k on it, and with the compression test, I just pulled the ASD relay after the truck got to operating temp and bump started it

A better way is to pull the starter relay,and use a remote starter switch to turn the engine over while you're doing a compression test.The key/ignition remains in the off position,so there's no chance of it starting or getting fuel,and you can hold the compression tester in one hand and watch it,while you hit the remote starter switch with the other hand.
You can build or buy a remote starter switch.They don't really take much to build if you decide to build one.Lots of youtube video's like this one,showing how to build a simple remote switch.

HOW TO MAKE A REMOTE STARTER SWITCH - DIY

This video is about making a remote starter switch for testing ,troubleshooting and diagnostic purposes.

www.youtube.com

https://www.amazon.com/OTC-3650-Heavy-Duty-Remote-Starter/dp/B002YKFA3G

 

[jws123]

Make sure your compression tester is accurate have been done over by faulty testers before.

 

[Thebluestreak]

I had help while doing this. I was able to watch the gauge and have someone else start the truck. Within first bump, 2,3, 5 and 8, I noticed within seconds of starting where at 60 and climbed strongly, where the others didn’t build it as fast and seemed sluggish to get to their peak.

 

[Burla]

If you are worried it is mds just leave in tow haul all the time and even stay in 5th, take mds out of the equation. I never heard of mds issue looking like this, but never say never.