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

[rhodyguy]

If you disconnect the throttle cable at the throttle body, does the cable move freely without any binding?

[drewvw]

I've done another round of investigation, here is what I've found. In summary, it appears to be a combination of both the cable issue and an air related leak:

- Went through cable from pedal to throttle body. After doing so, cannot reproduce the "redline RPM with pedal released" condition. So that's good.

- Timing is fine. New points and condenser.

- I am still experiencing an issue with having two idle different idle speeds. Link to a video showing the issue is below. Car will have a very low (400 rpm) idle. If I drive the car OR sit in neutral and hold the car steady at 3500 RPM, when you release the pedal the idle will settle at 2100 rpm. If I put the car in gear and drive at low RPMs usually the idle will reset back to the 400pm level.

https://www.dropbox.com/s/1dahe0hdhb3hr1d/IMG_1002.MOV?dl=0

- If the idle is at the 2100 pm level and I yank/wiggle the thottle cable, no effect.


- Went through all the air hoses again and checked for leaks. Used propane and starting fluid plus visual check. AAR is working fine. Next step is to try and bypass the decel valve.


So getting closer and the car is drivable again but something is clearly wrong and elusive. Thanks again for all input!