Surging at idle GM 454, transmission clunk..

[falconbrother]

I know I have seen this discussed before but, I can't find it. What causes a idle surge on the Chevrolet 454 TBI? Seems like it was a simple fix. When the engine is fully warm and we are easing through the campground on arrival the engine is surging at idle. It's nothing more than an minor aggravation, doesn't seem to hurt anything.

Secondly, when I go from park to drive or reverse it engages harder than it should I think. I know it's the little vacuum operated shift module. I bought one.. My question is this. Is this a bolt on item or, does it need adjusting. I asked a "guy" who is a mechanic and he says just bolt it right on, that it's set from the manufacturer. But, I have also read that there is an adjustment procedure that requires special equipment. What's the deal on that?

[falconbrother]

I did check the line to the shift module and it is perfect. I do think it needs to be switched out though. I've been putting that off (laziness, pure laziness).

[enblethen]

It can be the same issue. The IAC if not operating correctly causes a vacuum leak.

[ScottG]

The clunk is a classic issue with those drive trains. It's the yoke of the drive shaft sliding on the output shaft of the trans. GM has a special grease for that area that solves the problem but if you can put up with the noise, it's not really a problem and wont hurt anything.

[rgatijnet1]

Have you checked/replaced ALL of the vacuum lines on your coach? The cost is minimal and if some of the vacuum lines are originals, there is no doubt that they are leaking.

[falconbrother]

I have the 454, Turbo 400 combo.

Thanks

[BurbMan]

Bolt it on and if it doesn't work right take it in and get it adjusted.

[enblethen]

It could be the Idle air control valve (IAC) It is mounted with two small screws on the lower right side of the Throttle body.
Not sure what year your rig is. The THM 400 has a vacuum modulator, the 4L80 doesn't.
You might want to verify what engine transmission you have. The 7.4 TBI has the 4L80 transmission.
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