1994 GMC IDLE SURGE

[George T]

Truck quit. Had it towed home. It was a bad distributor module. Replaced the module and now the tach will jump a split second before it idles up. This will happen intermittent. It didn’t do it before I put the module in. Could the new module be bad?

 

[Schurkey]

Module failures can and do happen.

The market is flooded with crappy, bottom-feeder Communist Chinese replacement parts.

Fairly often, the module does not "fail" so much as it's murdered by a defective ignition coil. I would be testing the coil with a spark-tester calibrated for HEI ignition systems, and an ohmmeter across the primary, and secondary windings; and from each winding to ground.

MAYBE what's happening is that the vehicle misfires, and then the computer reacts by increasing the idle speed. So you see the tach "jump" and then the idle speed increases.

How old are the "usual" tune-up parts--cap, rotor, plug wires, plugs, air filter, fuel filter, PCV valve? How long has it been since the initial timing, and the timing advance were verified?

 

[George T]

New cap and rotor, module, all Standard Ignition brand. Timing just set. MSD blaster coil, MSD 6A, MSD wires a few years old. It ran just fine until it quit like turning off the key. It runs fine now except for the intermittent idle surge.

 

[Schurkey]

The Blaster II coil is a piece of crap, at least those made in the more-recent years. High-failure Chinesium. I've owned two MSD spark boxes that failed on me--one was an MSD 5 that broke in '88, went back to MSD for repairs, and then worked good for a long time. The other was an MSD 6T that I had on my '88 K1500 that died, got taken off, and is sitting in a little drawer full of MSD boxes in my shop. The only MSD spark box I'm currently using is a 6AL on my '68 Camino, and it hasn't run in two or three years due to Holley carb problems I'm too lazy to deal with.

I have an ancient Blaster from...1979 (?) that's double the size of the newer ones, and made in this country. Also on the 'Camino.

It's more important than ever to ohm-and-spark-test that coil. How old are the spark plugs? Connect a scan tool, verify all sensors and computer outputs.

 

[George T]

Just fixed this problem. It’s a 16 year old MSD billet distributor and the module was only part of the problem. I ordered a pick up coil/module and replaced the new module and old pick up coil with the MSD combo. That’s a dual terminal set up on a ‘94 GMC C1500.