Hi, I am new here and already this site has provided me a wealth of info. on my newly purchased 98 Ram. It is my first Dodge and I am enjoying learning about it. It is a 98 4x4 with the 5.9. I just learned I have the Death flash and just bought a used 3715 to take care of it. Truck ran awesome until it developed a heavy skip. It only skips at idle and when you give it any gas, clears right up. This started right after I wound the truck out on the highway with the performance tune. Also continues to throw the po304 code, misfire cylinder 4. Returned truck to stock with no help. List of things I have done:

Replaced all plugs with wires, cap and rotor, routed wires correctly
Seafoam through gas tank and intake
Swapped injector positions to see if code changed cylinders but no
Checked pushrods for straightness and springs and rocker condition
Checked for possible vacuum leaks and found none

Really sounds like it has a vacuum leak but after checking and spraying carb cleaner everywhere, found nothing. Maybe a collapsed lifter???? How would I check for a bad lifter? I have read on the plenum gasket as well and it seems that I have a LITTLE pooling of oil on the passenger side of the intake but drivers side is spotless.

If it is the plenum gasket, do I really need the billet aluminum intake pan or will the stock one with new gaskets be fine? No hotrod here or hardly any offroading. Just a daily driver so not much abuse.... If I replaced the plenum gasket, I hate to remove intake twice if it is a bad lifter but don't want to replace them if the are fine.

Any advice is greatly appreciated and thanks in advance for this great site.

 

edit - welcome to df.

run a compression test and see how #4 compares with the others.
started after "wound the truck out" makes me nervous.

could be a vac leak. i've never had any luck using carb cleaner to try to find one. one thing you can do is temporarily cap off all vac ports at the intake and see if it helps. this elimates all the external hoses and crap, but not the intake, tb, plenum, etc.

 

Well I didn't have time to check all cylinders tonite but compression on number 4 is 125 psi. Number 2 is 150 psi. Then I checked one on the other side, I think 1 or 3 and it was 175 psi. BIG difference! I am getting a little worried myself. It didn't start skipping immediately after my trip on the highway but about 30 min or so after I "saw what it would do". Doesn't seem so smart now, "Let's see what she'll do"

 

175 seems too high. 150 is really good. 125 is ok. i was just hoping it wasn't 0 or 75 or something terrible like that.

check them all and write them down. its a real good diagnostic report to keep for reference down the road.

 

OK, did a full compression test with all spark plugs removed and this is what I got:

1: 160
2: 160
3: 155
4: 140
5: 155
6: 146
7: 148
8: 164

4 seems lower than the rest but isn't as bad as I thought. I did a leakdown test on 4 and I could hear some seepage out of the tailpipe but not a lot. So I know the exhaust valve isn't air tight. On all of the cylinders, I could hear a tiny bit of seepage into the crankcase. Also heard minute amounts of seepage through the throttle body and exhaust on all cylinders but more pronounced on 4.

 

it seems pretty good to me. nothing catastrophic like 0 or 50 or 100. so while the computer is judging a misfire on #4, it doesn't seem like there's a major compression problem, or cracked head, or major valve problem.

so what could it be ?
skipping at idle, clearing up at rpm, P0304 misfire.
i wouldn't think leaking plenum would act like this.

 

Whats left? I have since return the truck back to stock programming and removed the cel. It has not returned however the truck continues to skip and seems to be getting worse. I still think I have a vacuum leak. Before I bought the truck, the module on the front axles that pulls the axles in went bad and the previous owner removed all of the vacuum lines from it and the transfer case.

Which vacuum line took vacuum to the transfer case??

 

I capped off the 2 vacuum ports on both side of the intake and no change. Are there any others I should cap off to eliminate the possibility of an leak? What about the one going to the front of the throttle body? Are there any more on the intake besides the 2 ports on each side?