Hey all, long time reader here. I haven't had much need to post any problems until now. (The 'Search' box really helps!) However, I need y'all's advice on pinning down a running problem on my wife's 1994 S10 Blazer. CPI, Vin W.

Long story short, the truck now has 200K on the clock. After doing a full servicing on the engine when I got it, the engine was running extremely well for about 8 months. Then I had a rough-running problem to sort out. I thought it turned out to be a leaky nut kit (raw fuel everywhere inside the plenum) , so I installed a Dr. Injector rebuilt CPI unit and a new GM nut kit. I also changed both the upper and lower intake manifold gaskets at this time. After the repair, the engine ran much better, but it still doen't idle properly. It 'surges' at idle in roughly 5-second intervals and will almost stall. The truck is still very drivable, but it will ocassionally stall out when coming to a stop at a red light with the extra load of the A/C on the engine.

SO, a friend of mine hooked it up to his Snap-On scanner and we took a look. We found a problem that Ive never seen before. There are no trouble codes present, but the ignition timing is going haywire at idle. What I mean is, I'm sitting there watching the timing curve on the scanner, and the timing advance is toggling between 32 degrees BTDC and 4 degrees ATDC several times a second. As soon as you hit the throttle (give it about 10% or more) the timing advances to 32 degrees, holds steady, and the engine smooths right out except for an occasional 'blip'.

Any ideas what's going on? Im assuming the ECU is getting 'confused' because it isn't recieving a good signal from the distributor? I installed a new dist. cap, rotor, plugs, and wires at 198K. The ignition module inside the distributor failed on my wife a month later, so I replaced it with a new one from BWD. The engine ran normally after the repair until this rough running/idle problem started out of the blue 3 months later. As far as I know, the pickup inside the distributor is years old. I never changed it.

Any help is much appreciated! Sorry for the rant, but I try to be descriptive. Ive replaced most all of the normal wear parts in the system. Now I need to figure out what's going on so Im not just throwing new parts at it. Thanks!