I have owned my 2002 SR5 4runner for about three years and I have never had any issues, except one. About a year ago, I smelled oil burning which I traced to a loose valve cover. I tightned the bolts and it stopped. Today, I smelt the same burning smell and I tightened the bolts again. When I started the engine about an hour later, I heard a very distinct clunk like something was knocking in the motor. Now the engine won't run at all but I can hear something clunk very few cycles of the engine (when turning the starter)

Is it possible to overtighten the valve cover to the point that the cam or valves could make contact. What could have happened?

Any ideas would be greatly appreciated.

I don't think the valves can hit the cover on these motors. Only one way to find out, loosen the bolts up a bit and try cranking it over.

If you have a valve cover gasket leak, you should replace the gasket. If you tightened down the bolts even though the gasket is bad, this could lead to the actual valve cover cracking, or the bolts breaking... which my guess is prob what happened since you tightened them down twice and prob over-torqued them. Have you removed the valve cover since your problem? Are the bolts still intact?

Edit: Was it a loose valve cover or was the gasket blown? Usually if you have an oil leak from there, the gasket is blown....

The first thing I did was to back the screws up a bit but nothing changed. I am sure the gasket is blown but it doesn't leak more than a drip or so.

I highly doubt that's the problem.

verify you have air/fuel/spark. How does act when cranking? like normal, just not starting?

There is one other strange symptom. It appears like a small amount of antifreeze is leaking and pooling under the motor. I was wondering if the timing belt broke and this whole thing had nothing to do with the valve cover at all. I pulled back the cover and the belt is in bad shape but still on. Could the knocking be the water pump? Is it possible he belt slipped and all that I need is a new belt and pump?

stupid spam.

I was going to suggest maybe the tensioner or belt let go but that's why I asked how it was cranking for you.

Sounds like something's in the engine if he's hearing clunking upon starting it. OP - clunking like something's bouncing around in the engine, clunking? That's why I was thinking a small piece of bolt broke off from over-tightening the valve cover.

Have you pinpointed where/ what specific area the noise is coming from? ... you should be able to tell if it's the water pump...

The engine could have skipped a few teeth. Replace the belt, pulleys, tensioner, and water pump. You need to anyways.

My guess is the water pump sh*t the bed, taking the TB with it. Pull the timing cover off and check it. Just a coincidence that it happened when you tighten the valve cover bolts.

leaking coolant, burning oil smell, clunking engine won't start.
hmmmm.
check the coolant level/color. check the oil level/color.
crack the oil drain plug and see what comes out.
I'm thinking this might be your head gasket. (and subsequent failures)
Verify where the coolant is leaking from.
hope i'm wrong.

to expand on subsequent failures:
a leaking head gasket could cause the oil burning smell from leaking onto your exhaust(which could be mistake for valve cover)
leaking hg would cause a coolant leak(which could be mistaken for valve cover)
hg failure would cause all sorts of fluid mixing and therefore water pump failure and then tb failure and so on.
failed hg could fill your cylinders with coolant and cause it to not start.
these issues could certainly lead to a spun bearing (read clunk)
again; for your sake I hope that's not the case.