RPM Surge After WOT, Weird Idle, Worried I’ll damage my engine Options

[drewvw]

Hi All, experiencing a tricky problem and I’m worried I’ll blow the engine if I don’t figure out.

73 1.7, fly cut heads, SSI/aftermarket exhaust, mostly stock otherwise.

Core Symptoms:

- Idle rpm inconsistent. Steady, but it seems to keep changing its mind on whether it’s 400, 1000, 1500. Decides again after you’ve been driving and slowing down for a stoplight or something. Gets a bit better when warm.

- If I floor it, when I change gears the RPMs continue to zoom to redline levels. Can’t say for sure it’s an air leak type of surge, might be sensor or linkage (investigating next) based. Does not seem “floaty” like air if that makes sense, more hard.

I’ve owned the car a long time and it general it runs great. But this has been an issue all summer. Just replaced exhaust and shored up SSI. Pretty Thorough review of vacuum lines, points, sensors, timing, etc. Swapped in another MPS.

Looking for ideas here, tonight was scary with the RPM revs.

Will update as a evaluate more. Thank you are any help!

Drew

[Rand]

Hey Geo, how does a cable cause hunting idle? Sticking for sure, but hunting is not mechanical which would stick. Guess what, we wasted that minute and are back to vacuum. And bonus points for LEAN.

[rjames]

Hey Geo, how does a cable cause hunting idle? Sticking for sure, but hunting is not mechanical which would stick. Guess what, we wasted that minute and are back to vacuum. And bonus points for LEAN.

He said the idle was steady, not hunting, but that it idles at varrying rpms.
That could be a weak throttle body return spring or sticking throttle cable, among other things. When it idles high, can you bring down the rpms by manually pulling on the throttle cable in the engine bay to ensure the butterfly is closed?

Also, what is meant by ‘if I floor it when I change gears the rpms continue to zoom to redline levels’?
I would expect the rpms to go up when you floor it.