About a week ago I started my truck up and it started misfiring; previous day ran fine. Codes P0300, P0306, and P0308. Changed the cap, rotor, plugs, wires, and coil. There are no vacuum leaks and the plenum is sealed with no oil in the intake. The misfire is most apparent at 40-55 MPH when it makes the truck shake like mad. If I step on it a bit it gets up to speed just fine but my fuel mileage is suffering and it smells of unburnt fuel when idling. I’m a new here so any advice is good advice, please help.

1999 Ram 1500 RWD 5.9 360

 

Two of the cylinders are next to each other..... not a good sign. Given what you are seeing though, I might suspect a fueling problem, injector stuck open, or shorted to ground.... Might be interesting to check fuel pressure, and see how it is behaving.

 

If it’s a fueling/injector problem what kind of numbers should I expect from the fuel pressure test? Or will it behave erratically?

 

You should see at a minimum of 45 PSI at initial key on. Then around 49 or so engine at idle. Think I would start the engine, see what idle pressure is, then shut off the engine, and see how quickly pressure drops off.

 

Ok I'll check that on my day off. Any other suggestions? I'm going to make a day out of diagnosing.

 

If fuel pressure looks good, do a compression test on the passenger side. Dry first, then wet. See what the numbers look like. (the two cylinders right next to each other make me nervous.....) If you have the gauges/compressor, leak down test would be nice too.

Edit: And just for giggles, verify the plugs wires on on in the correct order..... It may seem to run fine, but, around 45mph is when you get o/d, and tcc, and if you have misfires, they become VERY noticeable. (don't ask me how I know this. )

 

I work at a dealership so I'll have the shop do the compression and leak down test.

 

That's awful convenient.

 

Alrighty, the compression is 20 psi lower in cylinder 6 compared to all others. And during the leak down test they determined that the intake valve on 6 is leaking. It snaps into place if enough compressed air is pushed in but it seems that I have a valve train issue.

And I did switch the wires too, nothing changed at all.

 

Broken valve spring? Wonder if doing a top engine clean would help at all. (de-carbon the combustion chambers.)