Two dead holes

[Paul G]

LA 360 engine. Two weeks ago, last time I had the Charger out, I noticed the engine sounding a little different on acceleration. Didn't think too much of it then. Drove the car this Friday night and it got worse. Sounds like puffing out the exhaust and feels like a definite misfire on acceleration. The engine seemed to be idling faster than normal too. It is slow to fire up, and died on start up a couple of times. Never did this before, usually starts on a turn of the key. As I drove the car on Sunday the engine was running worse and worse. 

Engine idling, I pulled a plug wire one cylinder at a time. Cylinders 5 and 8 both had no change in idle with the respective plug wire pulled. Good spark on all cylinders, even 5 and 8. loud snaps as the wire came off the plug. I swapped the plugs from the offending cylinders to another cylinder, problem still with cylinders 5 and 8. Didn't swap plug wires since they were delivering good spark when being pulled off of the plug. Did a compression test. They are from 160 to 175 on all cylinders.  

So what can cause dead cylinders when the spark is good and good compression? Flat cam lobes?

[A383Wing]

5 & 8 are in the same plane on the intake, I would suspect a vacuum leak around either cylinder. Maybe power brake booster or PCV hose, maybe carb gasket...pinch off the hoses and see what happens

[c00nhunterjoe]

If no signs of obvious leaks, pull a valve cover and verify proper lift at the rockers.

[Chargerguy74]

EDIT: I missed some info.

[Paul G]

The brake booster gets vacuum from the back of the intake, eddy RPM air gap. That might be a problem if the booster is leaking vacuum. That booster is about 2 years old, makes it a prime canidate for failure. Could explain the faster idle than normal. I would expect two dead cylinders to cause a lower than normal idle speed. None of the small vacuum lines were broken or leaking.

To check for cam lift, we have a dial indicator at work I can use. If I set the dial indicator on the rocker arm, what would keep the lifter from bleeding down under pressure of the valve spring? Should I remove the rocker shafts and get the reading from the push rod somehow?   

[A383Wing]

they will not bleed down that fast so you could not get a good reading

[c00nhunterjoe]

they will not bleed down that fast so you could not get a good reading

Ditto.

Plus, with 2 dead cylinders, if its a cam, it should be fairly obvious when you start spinning it over.

[Paul G]

Found it! As usual, when we put our heads together there is no problem we cant solve. That power booster/master cylinder combo was installed two years ago. Remaned Cardone unit. Look at the pic, the vacuum check valve has multiple taps. The rubber cap on the top tap is broken off. That was the vacuum leak. Right in front of me and I didn't see it.

Today after work I had some time, and the garage was only 100° inside. Did a little trouble shooting. This problem has been gnawing at me since Sunday. I plugged the vacuum line to the booster first thing. Started the engine and right away the engine idle came back to normal. Then I notice the broken vacuum cap. I am so glad I dont have to do any internal work like a cam change to this motor.

[A383Wing]

which is why I said look for leak...cylinder 5 & 8 share the same runners in the manifold....and they are closest to the hose for the booster. You should have been able to hear that leak from that broken cap on the "T" part

[green69rt]

First rule of trouble shooting,  "look around, examine everything closely!"  Then look for other problems. 

Sorry for the jab, so many times I've had problems that were staring me in the face or that I made myself that I just had to blurt it out....

[charge69]

Glad it will be an easy fix!  Vacuum problems can be sooooo frustrating.

[Paul G]

It would be nice to hear anything around that engine. A few years ago I had several ticking lifters. I got a full set of new lifters from Hylift Johnson. They have two styles of hydraulic lifters. Standard and bleed down type. I got the bleed down type. They make a real racket at idle.