My 99 ram 1500 with the 318 has been shaking really bad around 45mph the CEL said it was a misfire in cylinder 4 so I did the plugs, plug wires , cap and rotor but the truck still shakes and runs rough did a compression test and all cylinders were at 150 except cylinder 4 was at 90 any ideas what it is

 

Mine was running like crap when my fuel injectors werent all the way seated. For 3 bucks you could get a set of fuel injector O-Rings pull the passenger side rail (cylinders 2 4 6 8) replace O-rings and check it. That was my issue. CEL misfire on 5 and 7 for me. Replaced O-rings made sure they seated down good, CEL gone. Could be more complicated but 3 dollars is a cheap easy fix attempt

 

Thanks I'll get those today and see if it fixes everything

 

Make sure to clear the codes afterwards, and do another compression test. Im no expert on compression, but i know you need it. Just figured Id put my two cents in about my Fuel Injector O-ring Problem. Im sure HeyYou will have another answer for you in approximately 2 hours :P

 

The shake right at 45 mph is BECAUSE of the miss..... that's where you drop into O/D, or TCC engages, or both..... any miss will be dramatically felt right at that speed. (yep, voice of experience)

The low compression on number 4 suggests there are other problems as well. I would suggest doing a leak down test on that cylinder, to determine where the pressure is going. (need compressed air, and a gauge set for that though.....

 

Knew you would know HeyYou

 

I know other ppl have had the same problem and they all say its the fuel pump but how would a bad fuel pump affect just on cylinder

 

I don't think it was the 'same' problem.... maybe the same symptoms...... But, a miss on a cylinder with low compression won't have anything to do with the fuel pump. If you have an injector stuck open, it might wash down the cylinder walls, so your rings don't seal as well as they should... but, not to the point that you are seeing. You have a mechanical problem on number 4. You need to find out what it is, and fix it. Might be as easy as a broken valve spring.

 

I agree with HeyYou 100%. The shaking is caused by the miss. You need to figure out what's wrong with cylinder #4. And a leak-down test would be the easiest way.

If you have a leak-proof way to pressurize an individual cylinder through a plug hole (your air lines and fittings used to pressurize the cylinder through the plug hole must be quiet so as to not mislead you), you could try the following:

If you pressurize #4 and hear air coming from the oil fill hole, it's leaking past the rings.

If you pressurize #4 and hear air coming from the throttle body, it's leaking past the intake valve.

Same goes if you hear it leaking out of the tailpipe, then it would be the exhaust valve it's leaking past.

If none of the above are happening, check the radiator for bubbles forming... would be a bad head gasket.

I had a miss and wasn't too lucky. Mine turned out to be a burned valve because of a cracked cylinder head on #5. Cracked heads are common.

-JT

 

It's either carbon on the valves or a Burnt valve so I'm screwed