Toyota Avalon 97 smell burnt oil when you start the heater?
Probably valve gasket leak.
Can be done yourself but requires some moderately complicated disassembly.
A fuel leak. A coolant leak, coolant has a sweet smell. An exhaust leak. A burnt clutch. A burnt rubber smell, from a slipping accessory belt/tensioner or rubber contacting the exhaust. A Moldy smell with the A/C on from a dirty/moldy evaporator. An oil leak, if it is leaking near the exhaust manifold, from say the valve cover. A smell from burnt brakes from a faulty brake caliper. A burnt plastic smell, from plastic melting…
Could be several things, is there coolant in the overflow tank? **Don't open cap when hot** does your heat work? your heater core might be clogged? do your fans turn on to cool the engine? Do you have oil? & what does it smell like ? burnt ? or just like old oil I had this problem on a 98 Jeep, it was a clogged heater core, & rusty coolant .... Matt