I own a 98 ES300, lots of miles (around 250000)
Good grief... Trying not to punch myself in the head... So... had an oil leak situation, replaced valve cover gaskets, still leaking, power washed engine, discovered oil sending unit was leaking. Replaced, no more oil leak. Great right, took it out for a test drive, won't shift into 4th gear, ides around 1500 RMP when warm, and engine now overheats. WHAT IN THE WORLD DID I DO??? I cleaned the throttle body and air control valve, put in a new PCV valve and replaced the bad oil sensor. That was it. Checked vacuum lines and connectors, don't think I missed anything there.To say I'm frustrated is the understatement of the day. New to the forum so don't really know anyone here. Thanks for any thoughts. Don't have much hair left so I don't want to pull anymore out!

All connectors are plugged in? Is the coolant topped off? Do the rad fans spin?

I have double checked all connectors. Unless I'm missing one somewhere, but pretty sure I've got them all. I did add some coolant, however it never got hot prior to this valve cover job. Yes the cooling fans are kicking on. I'm baffled... Upon starting car, RPM is around 1200, when it gets warm it runs up around 1500.

Do you have an IR temperature gun? Does the rad and rad upper hose get hot? Maybe you have a pinched/shorted wire that is making the ECU think the engine temperature is not what it actually is. Also do you have an OBD-II scanner? That will tell you the exact temperature the ECU is reporting.

....make sure the check the two connectors by the coolant cap, single wire is for the gauge cluster 2-wire is for the ECU.

The temperature gauge inside the car is showing that it is running hot. I will double check the wiring and try to get into a part store so I can hook up a scanner to the car. I have wondered about a vapor lock as well since I did take the top radiator hose off in order to do the valve cover gasket replacement.

To confirm, you checked the cold coolant level by removing the pressurized cap and not just the coolant reservoir.
Also, did you try to bleed/burp the system for trapped air?

I'm definitely going to add some additional coolant. Didn't have any pink antifreeze so I added about a quart of distilled water. Will grab some pink antifreeze. we are out of town for the weekend and will get back to it on Monday. Any special thoughts on removing air from the system if there is any?

Ideally use Toyota Long Life (red) or equivalent like Zerex Asian Formula. Not Super Long Life (pink). For bleeding use a funnel like this

I will look for the red. It just has pink in it now and I was just going to top it off.

If it has pink then I would use that.

Can you get that pink antifreeze at most auto part stores? I can't recall where I purchased it last time.

The colour is secondary the type is what you are looking for, which is silicate free HOAT formula with a phosphates. Zerex comes in red and blue not pink. I simply go to any Toyota dealer but some people insist on saving $5-10.

I also have the fast idle issue on my '98 ES300 once it warms up and I shift to Neutral or Park. It's not really an issue while driving around in gear (unless the engine's effort to idle @ 1500-1700 RPM is quietly stressing my transmission while stopped).

OTOH, my engine temp is fine, so I would be surprised if you find these two issues are related.

Temperature the cluster reports is not from the same sensor the computer uses. That is a 2-wire sensor just for the EFI system. Meaning, your cluster can report fine but the computer is getting wrong info.

Topped off coolant and left radiator cap off for a few minutes. A lot of air bubbles and now no more overheating. I also pulled the throttle body off and checked all hoses and sensors. I noticed I failed to tighten the top of the egr tube. I recall needing to loosen the nuts in order to get the bottom nuts on. Apparently I forgot to go back on top and tighten those nuts.. Also drained transmission fluid and replaced filter. Seems to be shifting fine at this point. So all is well however we are still idling around 1200-1500 RMP when engine is warm. Am going to check idle control valve tomorrow and see if there is anything unusual. Thanks again for your help. Scott