I ran this post about a week and a half ago, and got no replys, 2vmod was nice enough to email me a return, thanks.. 3.1v6 runs great most of the time. I was told that it has a performance chip in it. However, occationally it runs erratic. When it does, if I turn on the heat blower the tachometer falls back to nothing, and the engine missfires even more, at times it dies completely. Turn the blower off, and the engine run smoothly. If I turn the blower back on, most of the time it doesn't affect the tach/missfire. and sometimes it does. I would think it is definately a coil/ignition problem, but caused by what? Fuel does not appear to be an issue, and no other gauges are affected. I think the tach falling back is telling me it is ignition. The blower by the way works fine, and blows plenty of air in all switch positions. The alternator voltage does not seem to drop when this occurs. I really could use a hand.

first off, does it have any trouble codes (is the service engine soon light on)...then also when it misfires does it sound like its missing on only one certain cylinder, or is it random. also, get a actual voltmeter and test the voltage while all this happens...dont trust the one in the dash.
im thinking that its probly ignition too, but you cant rule out fuel yet...there is one injector for every cylinder and if only one goes bad it can cause a cylinder to miss.

One other thing to look at when the RPMs fall when a load is applied to the engine is the IAC. The ECM is supposed to detect the load or decrease in RPM and open the IAC to maintain idle speed. Some accesories feed a signal to the ECM to indicate a need to raise the RPMs like the case of the AC.
As far as the PROM you should pull the ECM and look at the PROM to see if it is a custom or the factory one. My factory PROM has the Delco name on the chip if it helps any.
Good luck,
Ozzy

Thanks for the replys, When this occurs, there is no service engine lights (and it does work). Also it does not appear to be just a single cylinder miss, it feels more like every cylinder is quiting and then restarting (surge). When it runs, it runs flawlessly, I would think that would rule out the prom being faulty. It can run for a day or two without a problem. Again, when it runs well, I can turn on any electrical load without the least bit of a prob.

I havent had this happen before, but could it also be a vaccum leak at the HVAC controls? Just a thought.

Also sounds like maybe the power wire to the blower motor could be nicked, cut, cracked, etc.. causing it to ground out, possibly near the dist.? Another thought.

Check the wires in the engine bay, near the firewall. Start the car, turn the blower motor on, and stat shaking, pulling, jiggling the wires around in the engine bay. See if there is a difference in how the motor is running. Probably going to take alot of trial and error.

Hope that helps, keep us posted.

Thanks again for the replys. StngKlr's advice was taken. I tried moving harnesses around with the engine running and the blower on. I found that if I moved the large harness which heads down the firewall near the right side of the bellhousing, I could recreate the problem. Nothing looked chaffed or near exhaust. But by pulling that harness up and down, it causes the same effect. The grounds in that area are solid, and moving them does not bother. I noticed that the porcelan on the O2 sensor is broken, but I wouldn't think that could cause this? All the wires are encased in wire looms, none hanging out to rub etc. Is there a specific wire in that part of the harness that might cause this symptom? I think I am getting closer.. Any more ideas? Thanks Dave.

I'm starting to think its the small black vaccum wire that goes to the HVAC controls. Its going to bundled with the other 5 or 6 wires that go to the HVAC controls. They enter the firewall at the bottom left from the distributor, going through a gromet. Pull off the wire loom that encases the wires and vaccum line. Inspect for cracks and cuts. Might have to bend it in all directions to find the cut.

Actually I would pull off all of the wire looms that cover wires near the firewall, that way you could check all wires and such. Its easy to replace, and will give the engine compartment a facelift.

Hope that helps, let us know.

I know this has taken a while, but here is the end result. I took Ricks advise, and by moving the large harness on the right side of the engine, I could recreate the problem of the engine missing, and finally quiting. I checked the Helms manual for both heater blower motor circuits, and ignition, and came up with a g118 circuit which is the ground for the blower motor, and an ECM ground. it happens to connect 4 wires to the back of the RH cylinder head, about 2 inches away from the firewall, so you cant see. I felt in back, and could spin those wires back and forth. I hand tightened them about 4 turns of the 1/2" nut. and another 1 turn with a wrench. Evidently the ground noise from the blower motor was feeding back into the ECM, and shutting down the ignition. Probably never tightened when they changed engines. Thanks for the help. The old adage applies once again. check the simple things first. But... without the book, those grounds were impossible to see, they only came out of the harness about 4 inches, and right to that stud on the back of the cylinder head. Thanks again, Dave Roberts