Hello to all. I have a 1998 dodge Ram 1500 with 360 magnum. It recently developed a rough idle with check engine light on. After checking it read "misfire cyl #5". I then changed out to a new fuel injector and spark plug then switched plug wire with another and compared cyl #1 compresssion with # 5 and they were 150 lbs but still no change. I then installed a new IAC and TPS and checked vacuum at idle which is 17 and fuel pressure at rail still no help. The engine when hot idles at 650 rpm with the rough idle but driving on the road it feels fine. After cold starts I notice when I hold the throttle at 2k the rpms flucturates up and down untill it warms up. I am thinking, a vacuum leak but I have checked and sprayed everwhere for one. I know about the plenum leak, could it be that with 17 vacuum at idle. Don't for get this engine only has on main coil. I have searched my you know what offf and found this is a common problem. Any help would be appreciated

 

cap and rotor?

 

Fixed the plenum yet? Have a look down your throttle body, see any oil down there?

 

Yea, What condition are they in along with the plugs?

 

I am sorry guys, I forgot to mention that I installed new cap, rotor and wires. This rough idle has been going on for 6 months and I forget all I did after a while.

As I mentioned above, I know about plenum gasket leak and doesn't that cause a vacuum leak? I have 17" of vacuum at idle. Is that good or bad?

 

I dare to ask which port you are measuring vacuum at but I think on these motor all ports are the same

 

I used the small bottom nipple on the passenger side

 

Hello again and may I say that I did a very extensive vacuum test to see if my plenum gasket was leaking and yes it is. You have to be careful of others showing how to on this subject.

I used rubber female and male plugs, not duct tape and closed off every vacuum line or nipple including the brake booster and every outlet coming out of the valve covers (2). In other words every outlet or inlet to the engine with the exception of the dip stick tube and that is where I hooked my vacuum gauge to and found 10" of vacuum in it. When I started the engine I waited and watched at a minute or two to see it slowly climb to 10" at idle.