Occasional miss--What would you check?

[ET16]

Originally Posted By: ET16
Well...it was the wires. I changed them today, and the miss is gone!


Well...it looks as if I was wrong. The miss is back. I first thought the wires were bad, and I changed those, and the car seemed to run a little better, but the miss came back. Today I changed the coil and the miss is still there. Changed the front plugs--miss is still there. I'll do the back plugs tomorrow if I can, but I'm at a loss. What would you check next?

 

[CharBaby]

Possible vacuum leak!

 

[ET16]

Originally Posted By: Char Baby
Possible vacuum leak!


Good idea! I'll take a look.

 

[mechtech2]

The miss went away right after new wires were put on.
So the ignition is still a possible culprit.
Maybe a cracked coil?
A little carb cleaner or starting fluid can help find a vacuum leak.
A quick shot at suspected spots will momentarily change the idle.
But don't be fooled by letting any spray get in the normal intake opening, as it will have an effect.

 

[ET16]

Originally Posted By: mechtech2
The miss went away right after new wires were put on.
So the ignition is still a possible culprit.
Maybe a cracked coil?
A little carb cleaner or starting fluid can help find a vacuum leak.
A quick shot at suspected spots will momentarily change the idle.
But don't be fooled by letting any spray get in the normal intake opening, as it will have an effect.


mechtech2,

Thanks for your suggestions.

I changed the coil. Still had the miss. I used to check for vacuum leaks with a propane torch (unlit). Is this considered safe?

Here's a new clue: it seems to occurs when the defroster is on.

 

[mechtech2]

Sure, propane will work great top note a change.
Just aim it carefully.
Any propane in the regular air inlet will cause a change.

Having it worse with the defroster on could be 2 things:
Most cars turn on the AC with the defroster, and this will drag the engine and cause more idle vibration.
Secondarily, you may have a vacuum leak on the passenger compartment side . The vacuum to the doors may leak at any number of connection, lines, valves.

 

[ET16]

It turned out that one of the insulators on one of the back plugs was cracked. I hadn't checked the plugs first because they were changed about a year ago. Mystery solved.

Thanks to everyone who posted suggestions.

 

[CharBaby]

Happy to hear that your found what was causing the "miss".
Well done!

 

[Sayjac]

Excellent thread. Nice of mechtech2 and all others giving an honest opinion/options of where to start and trying to help the OP save some money.

I'd have said wires first especially if they hadn't been changed in a while. As mechtech2 said, even if they weren't the issue, can't hurt to put a new quality set on.

Glad you found the issue and posted the resolution to the problem. Thanks.