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Old 11-03-2013, 06:06 PM
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Misfire on hemi HDS cylindar

Hello all - been going through a lot of the posts here and other places trying to track down a misfire, and wanted to get some opinions on next steps...

06 WK with hemi has started to misfire on cyl. 4 Originally it was missing until it warmed up - about two minutes of driving. Then it started to happen more, and longer, now it happens more than it does not. I think it started after an oil change, after a longer than usual change interval. We went probably about 6000 miles (3000 of which was in one week) and had it changed. Were doing a few vehicles at one time, and did not realize until a few days later that it got 5 quarts of 5w-30 and two of 5w-20 Very shortly after is when the misfire started. Immediately thought it was the oil. The following weekend it was changed, and Mobil 1 5w-20 went in. Following morning it was back. So, since they were due, plugs were changed. (Used the champion coppers) Swapped coils between 2 and 4. Swapped injectors between 2 and 4. Manually activated MDS solenoids and they sounded the same. Trying to decide where to go next (Besides the dealership..)

When it is misfiring sometimes check engine light comes on, sometimes it does not. Usually has a misfire on cyl 4 code. Two or three times I have gotten a MDS reactivation performance cyl 4 code as well (but not usually). Sometimes, and it seems more recently, when it is happening if I stop, shut the engine off for a minute (like long enough to get $20 of gas) it will run fine for a little while. Today for example, I drove a good 15 minutes normally. One night last week I pulled one of the plugs out of 4 after it was happening, wet but clean. I have confirmed wiring from injector and coil to PCM are good. Temp readings coming from sensors look good at all times. Lately I will run it in tow mode so MDS doesnt kick in, and it has been behaving better - and on more than one occasion I have been able to get it to start misfiring by switching out of Tow mode and letting MDS activate.

MDS - I know it works on pressures - deactivated is something like 3psi to the lifter, activated is 15? Does the solenoid regulate the pressure, or is it a function of the oil passage it is following? In other words, could I hear the MDS solenoid clicking on and off but under running conditions it passes more pressure in the off position than it should? Or a weak spring in the solenoid lets the running oil pressure force it open? Could pull the intake and swap two (or replace one) to rule it out. Swap could be done next weekend, replace would be another week out.

Oil - I have read it might take two changes to run normal again. Of course, cheapest thing to try next, but should I be in the clear already? I get paid Weds. so can change it out again this week.

I know a few have gone through this before - anything I am obviously missing?

Thanks guys!
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