Last week I installed the Hugh's plenum upgrade kit now I have a check engine light on with a p0306 cylinder misfire code. The truck has a rough idle in park and if you put the truck in drive and before you step on the gas the engine kinda pulses. When you give it gas it runs smooth and evens out. Does this mean I have a leak somewhere coming from the intake manifold causing a lean condition. All help is much appreciated.

 

Did you change the plugs when you did the plenum?

 

No I changed the plugs and the cap rotor and wires before I did the plenum. I put NGKs in then after I did the plenum I put autolite 3923s in. After about 2 hours with the autolite 3923s in it threw the p0306 cylinder 6 misfire. So I changed back to the NGKs it helped the idle and wasn't as rough so I thought I was on the right track. After a few days the code came back. The truck only idles ruff in park and when your at a stop but once you start to take off its smooth and at cruising speeds the truck runs smooth, that's why I was thinking I have an intake leak because at idle it's creating a lean condition but once more fuel is added it evens out at operating speeds.

 

No I changed the plugs and the cap rotor and wires before I did the plenum. I put NGKs in then after I did the plenum I put autolite 3923s in. After about 2 hours with the autolite 3923s in it threw the p0306 cylinder 6 misfire. So I changed back to the NGKs it helped the idle and wasn't as rough so I thought I was on the right track. After a few days the code came back. The truck only idles ruff in park and when your at a stop but once you start to take off its smooth and at cruising speeds the truck runs smooth, that's why I was thinking I have an intake leak because at idle it's creating a lean condition but once more fuel is added it evens out at operating speeds.

 

Do a dry/wet compression test. See what's up.

 

Sorry man I don't want to sound stupid but how do you do that ? Do you need special tools to complete this task? I really appreciate your help man thanks!!

 

No worries. The only stupid question is the one you don't ask, when you don't know.

Just need a compression gauge. if you don't have one, you can prolly rent one from your local parts store.

Basically, you pull all the sparkplugs, disconnect the electrical connector from the coil, and yank the fuel pump relay. That disable the ignition, and fuel systems, so, nothing goes BOOM!

With all the plugs out, then engine will roll over very easily. Just screw the gauge into a cylinder, stomp the gas pedal to the floor, and hold it there, and crank the engine till you hear it hit the compression tester five or six times. (you WILL be able to tell when it does.) Trick is, be consistent. Same number of hits per cylinder. Then, you read the gauge, right down the number (and which cylinder it is on) release the pressure. (little **** on gauge). Pull gauge, put a teaspoonful of oil into the cylinder. Best way is with one of those pump style oil cans. Two pumps is enough. Just make sure it goes in the sparkplug hole. With the heat shields on there, it can be a pain.

Run the test again. Write down the number. Do the same on all cylinders.

Ideally, they should all be north of 150 pounds, and within 10% of each other, on the dry test.

100 PSI is considered the minimum "good" value. Though if they are that low, it's gonna be time for a rebuild in any event. If there is more than 10% difference between the highest, and lowest, also means time for a rebuild. If the numbers jump significantly on the wet test, that means the rings are shot..... and... you guessed it, time for a rebuild.

 

Before you go to the trouble of a compression test...I would re-check the torque on all the intake manifold bolts...when i put in my hughes airgap, a couple of my manifold bolts kept backing out resulting in a misfire that would jump around from 2 - 6 - 8 cylinders...I recheck them every once in a while for peace of mind. my misfires were only happening when the truck was at an idle, when i accelerated, you couldn't tell anything was wrong.

 

I was also wondering when I first did the spark plug wires I didn't know about the tsp bulletin on the spark plug wire routing. So my question is if the spark plug wires aren't routed correctly could this cause a cylinder 6 misfire? Thanks for your help man .

 

It's possible. The rerouting was to help prevent ignition crossfire....