You capped the manifold end of what? Hopefully you capped the manifold itself and not the booster hose - b/c capping the booster hose won’t do you any good.
Haha sorry about that. Yes, I capped the manifold itself.
Pumping the brakes on a car working normally will raise the idle a little. It’s normal.
Have you cleaned the throttle body?
Has the valve lash been adjusted in the last 30k miles?
I heard that is is normal but I didn’t think it would cause such a dramatic increase in my idle.
Yeah, I have cleaned the throttle body and installed a new gasket.
Yep, done that too.
Did you ever find a solution? I have a 99 4Runner and I have very similar problems. Idles fine until engine reaches steady state temp. Then, oddly, about every 15-20 seconds the idle will drop and its a crap shoot if it will stop the engine or not.
I have been using seafoam in the fuel tank and crankcase however it just seems to diminish the hiccups at low idle. I think it is just masking the real problem which I have not identified.
I have not found a solution. I do think it’s a vacuum leak but I don’t know where exactly. I’ve pretty much have lived with it for the time being. I can fully understand the crap shoot on whether my car will die while stopped.
I may have to throw a can of Seafoam in the tank next time.
If you can. pull and inspect the plugs yourself. Start with good spark. Check the plugs for bad fuel injection(too lean). check the cap and rotor for deposits. If you have a direct coil then really check the wires. At night with a spray bottle of water give a spritz on the wires and watch for a spark glow. The vacuum may be a false trail. Check the throttle plate for carbon buildup that may make it sticky. Then run down each injector. test for lack of injection or leaks.