Steve, this symptom sounds to me like you may have a problem with the temp switch that's mounted on the top of the thermostat housing. There's a hose that runs from the air intake tube to this switch, and from there to the common chamber on the intake manifold. If this switch opens you will get that idle "surging" that you describe. The ECM is monitoring the RPM of the engine. If it sees the RPM going over around say 1600 RPM, AND the Throttle Position Switch is on IDLE, the computer will enter the FUEL CUT MODE (cutting off fuel) until the idle goes back to around 1200 RPM, when it turns the fuel back on again.

It does this only when you are driving ???

What about if you pull off the road when it starts this, and just let it idle ?

Does it stop doing this the instant you crack the throttle open again ? (just enough to get the TPS idle switch open?)

It could also indicate an overheating condition, so make sure that you've "burped" any air bubble out of the cooling system. (Does the heater work OK ?)

One thing you can try that might isolate the problem to that switch: Pull the hose from the air tube to the switch and plug it. Plug the hole in the air tube. See if the problem goes away.

If you have a spare one of those switches kicking around, try a different one.

I know that you know you DON'T want to overheat that brandy new motor, so make absolutely sure your cooling system is functioning properly! (Is the T'stat installed correctly?)

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