The Impossible to Diagnose 3rd Gen T4R Issue
 

I have had an issue with my truck for the past 3 years that has stumped myself and two other "Toyota Specialists" in the area. Bestow upon me me your 4Runner expertise so I can fix this problem!

The Car: 99 T4R SR5 4x4 5VZ-FE

The Issue: ONLY on hot days (80 F and above) when stopped (both in drive or park) the car will suddenly begin to idle a bit rough and then suddenly stall. Without fail, I can start the car immediately afterwards and keep driving like nothing ever happened. Usually when the idle becomes lower and rough, I can give it a bit of gas and things return to normal. When this happens, the car dosen't throw any kind of CEL. Keep in mind, when the temperature is colder than 80F, it runs perfectly. No issues.

The "fixes" so far: Since my workhorse has over 250K on the clock, I have replaced a few old, dry vacuum lines in case they were cracked. Since I do not have the time or space to work on her these days, I took it to a mechanic who cleaned the IACV. I thought this fixed it for a month or so, but the problem resurfaced. Another mechanic looked it over, and told me to continue driving it until it throws a code.

Parts Replaced Recently: All belts, TPS, Plugs, Wires and One Coil Pack.

Please give me any tips you may have, this has been hugely frusterating and I would like to take care of this before warm weather returns.

I Know you had it cleaned..... but I say IAC valve. Also check your ect sensor.

Check the fuel filter?

My 98 Tacoma has done the same thing but only like 3 times.
Once when I was cruisng at 50 mph and had to pull over and restart.
I was leaning toward the computer but my mechanic thinks its vapor lock?