67 427 down one cylinder

[Bob H.]

This is a follow up to my slight vibration/missing problem.

I did some checking using my heat gun and found #8 cylinder over 130 degrees less than all others. I pulled the plug wire which had no effect on the idle. I checked for spark at the cap and replaced the wire with no change. I also put a spare plug on the wire to make sure I had fire on the plug. I did a compression check and found it good so I pulled the valve cover and checked the valve adjustment. Perfect. So if I have compression, apparent spark, what else could be wrong?

I know I have to have fuel so I noticed the vaccum for the brakes seems to come off the manifold on that cylinder. I don't seem to have a leak and don't know that would cause this anyway.

Although I am seeing spark, could the distributor cap be a problem? It's new. At higher rpms I can't even tell there is a problem.

[William C.]

Re: 67 427 down one cylinder

change the plug? Have you measured the resistance in the wire? It takes a lot more voltage to fire a plug under compression pressure than outside the engine with atmospheric pressure

[Bob H.]

Re: 67 427 down one cylinder

Hi Bill:

Changed the plug, twice. I used a spare wire I had and noticed no difference. I was beginning to wonder that although the spark looks the same as another cylinder, maybe it is the rotor or cap. Could there be some resistance in the cap?

Bob

[Dick W.]

Re: 67 427 down one cylinder

Vacuum leak will most definitely cause a miss at idle

[Duke W.]

Re: 67 427 down one cylinder

It could be a problem with the cap - just that one cylinder.

I had this problem with the cap on my MBZ, which took a while to isolate. Each terminal has a built in 1K RFI resistor, and when I hooked up a meter and started pushing on the terminals the needle jumped up on one of the terminals. So I bought a new cap - 150 bucks for an OE Bosch!!! But the idle roughness went away and fuel consumption went down along with idle emissions in the CA field test.

There's no resistor in your cap, but there could be a small crack, dirt, contamination, or something that's causing energy leakage and an intermittent misfire.

Also, put a timing light on the "bad" cylinder and check for steady flashes while wiggling around the wires and applying force to the requisite cap terminal.

Worse comes to worse buy a new SMP cap. What are they - 10 bucks?

It could also be that the particular cylinder is just lean due to mixture distribution issues, in which case richening the mixture an eighth to quarter turn should help.

Duke

[Edward J.]

Re: 67 427 down one cylinder

Bob, if you have compression within a acceptiable range of what the book calls for and you have spark, all you need is fuel to complete the cycle,

if the valve cover is still off, check the valve train on that cylinder to see if the rockers have about the same lift for intake and exhaust.

Dicks call on the vaccum leak, that generally will lean out the fuel mixture and raise the cylinder temp.s, Not to say that a leaned out carb. adjustment will not cause a rough idle, because it will.

also checking the engine timing to make sure that its not over advanced, this can cause problems. if you have a timing lightcan be easily checked.