Low spark for 93 XLT

[rabulloc]

So a couple of weeks ago, while driving in the mountains some smoke came out of my tailpipe. Only for a minute or so, but enough to get me concerned. I checked the oil and coolant to verify they weren't contaminating each other and they weren't. I went ahead and did a compression check to verfiy the head gasket was in tact, had 180psi on each cylinder, so turned out okay. So I took it to the shop and they did an engine diagnosis and found low spark on 4 and 5, very low. The spark plugs and wires are less than 1 year old, but checked them again anyway, and visually look okay. The shop said it must be the plugs or the wires, but I doubt this since they are so new.

Any suggestions out there?

Thanks in advance,

 

[marragtop]

Switch the wires with different cylinders and see if the problem follows the wire(s).

 

[rabulloc]

-update- tonight I checked the coil pack, and found the secondary resistance was out of spec. So I replaced the coil pack. Everything seems to be running okay. I'm not sure how to tell if there is still low voltage across the plugs or not without taking it into the shop and paying for a engine diagnostic test.

 

[BrooklynBay]

Welcome to this forum! I was going to suggest the coil pack. You could pull off one wire, and test it's spark against a ground (frame). The more expensive way is to use a special test tool from OTC that displays secondary ignition voltage on a digital display. If it's running OK after replacing the bad coil pack, then I wouldn't bother confirming anything else. I think that the problem is fixed. If the plugs, and wires are really old, you might want to replace them anyway.

 

[1IronMike]

I wonder if while going down hill, increased vacuum, automatic transmission, leaking vacuum modulator on transmission? Pull the vacuum line off real quick and check for leakage. That whole weak spark thing sounds kind of iffy to me.