'99 Ram 1500, 360

No engine codes

Hello, since i bought my truck if its been sitting for more than a few days it would start up sounding like it was running 7 cylinders. Then it would start popping out the exhaust faster and faster until the 8th would come online and it would smooth right out. That's what i always assumed was happening anyway. Recently however its started being a loud backfire and the engine never totally smooths out. It only backfires for the first ten seconds or so and then it just acts like a very inconsistent miss. The truck drives down the road perfectly and revs up just fine. Its just when its idling, randomly once or twice a second the engine shudders, you can see the whole thing twitch and feel it through the cab. When pulling all the spark plugs the number 8 was quite a bit darker than the rest and had a very strong "burnt" smell to it. I have no check engine light. It seems to be entirely random there is no rythem to it at all.
I've replaced crank sensor, map, IAC, plugs and wires, has a newer cap and rotor. Replaced a couple of injectors i was suspicious of with oem junkyard replacements. Tried unplugging o2 sensors, checked for vacuum leaks as best I can. Unplugged injectors one by one but that makes the engine shudder to the point that I can't tell if the original problem is still there or not. Cleaned the IAC port on the TB. Having alot of trouble finding any other forum threads or reference to an issue like this.
Any help would be really fantastic, thanks

 

What plugs are you using? Are the wires still 7mm? Is the cap one with the brass or aluminum inserts?

 

Plugs are some NGK V powers, not certain past that just what the guy gave me at the parts store. New wires are 7mm yes. As far as the cap and rotor I'm not sure as the P/O put it on. About 1,000 miles ago i took it apart and cleaned all the contacts when trying to track down another issue which ended up being unrelated. Only info I know is it has a blue plastic shell to it

 

How many miles on it? Plenum been fixed?

 

~170,000, P/O didn't mention anything about it having been fixed. I did the test where you plug the pcv and check the driver side breather hose for vacuum. It had slight positive pressure which i was told means the plenum isn't causing a vacuum leak. Not sure how valid that is though

 

Pull the air box off the throttle body, crack the throttle wide open, and take a look down in there. (a bright flashlight helps here....) See how the bottom of the intake looks.

I might be tempted to run a compression test, considering how many miles you have.....

 

Just checked the intake, from what I can see I would say the bottom definitely looks wet, but not pooling. However I can't see the very back so hard to say for sure.
I had thought about running a compression test but figured if it was a mechanical issue it would be more consistent. I don't know if that's an accurate assumption though. Thinking about swapping to a different brand of plugs, maybe I'll try it then

 

Could possibly be a cylinder having an issue (perhaps #8) due to the plenum issue. Also, with that mileage, has the timing chain ever been replaced? A compression check would be helpful.

 

If you think that's a possibility I'll look into getting some plenum gaskets put in (maybe when its not 28 degrees and snowing out). No mention of the timing chain from P/O but no way to know for sure. I did a water pump not long ago and didn't occur to me 'till after that I should have done the chain too.

 

Oh come on. Who doesn't LOVE working outside in the sub-freezing temps????

Yep. Me. Screw that. I'll pay someone to deal with it, if it has to happen RIGHT NOW. Otherwise, it'll wait til the weather is more reasonable.