02 toyota sienna runs when pushing on gas but dies when idling

Coming home from a road trip we stopped for gas not 5 minutes later the check engine light started flashing. The car was fine as long as you were puching on the gas but as soon as you slowed to a stop it would either die or shudder until I gave it some more gas. Being stupid I figured that I could probably get it home since It was only 25 miles of freeway driving and I wouldn’t have to stop until I got near my home. I took it to a mechanic the next day and they said that it was my timing belt. The next day I get a call from them telling me my timing belt is fine but they discovered that a valve adjuster shim is missing on cylinder 6 and the adjuster cup is jammed down as well as a crack in the locating pin hole so that the pin is moving causing the cam to shift in relation to the cam gear. They suggested I rebuild or replace the cylinder heads and may need to replace the engine. I know nothing about cars so the above information is gobbledy gook to me. I get the feeling that they don’t really know what they are doing because they wanted an additional $500 to figure out what was wrong with my car. Any other ideas on what this could be?

You drove 25 miles down the freeway with the check engine light flashing? And you’re questioning the mechanic’s judgement?

My advice? Get a rebuilt replacement head. Here’re some for $325.
Hopefully you can get out of this alive.

To add on just a little to mountainbike’s comment, if there is head / valve damage - which is certainly a possibility - yes, it will take at least $500 to figure it out for sure. They will have to pull the head off - lots and lots of labor time.

You’re free to have it towed elsewhere for a second opinion - but they already did put time in on it so you would rightfully owe them some $$ anyway.

As you are learning, you don’t drive with a flashing check engine light. Chances are you will have error codes for one or more cylinder misfires & those will have been cause by low to no compression owing to valve issues.

Was there any subtle, or not so subtle, ticking or tapping noise from the engine before this happened?

A shim coming out of a bucket is usually caused by the valve lash (clearance) becoming excessive. That excessive lash may be due to ignoring any ticking noise caused by failure to inspect and adjust the lash regularly or failure to change the oil often enough. Either one can cause a cam lobe to start going flat and this in turn creates excessive lash. At some point the shim will get jolted out of the bucket.

It’s also possible for a shim to come out if a valve sticks in the valve guide but the odds of this are extremely slim; so slim as to pretty much dismiss the thought. Just pointing out that slim possibility is all.

Offhand, it does sound like another cylinder head could be needed and operating the vehicle with it running poorly is likely what made a possibly simple fix much worse.