Check Charcoal canister
under center console
a burnt rubber ball in the cardorater
The Oil filter in under the motor of the truck..
Your fuel system might have a leak if the strong smell is similar to the smell of refuelling. Or the exhaust might have a leak allowing fumes to get in the cab with you. Diesel fuel has a strong oder if burnt in a rich mixture an in under hard acceleration.
Check your fuel pressure regulator , i had the same problem.
locate the top radiator hose.then follow it to the engine. the housing it attaches to under it is where the thermostadt is at.
It should be under the hood on top of the passengers side inner fender.
I had the same problem and it was my cabin air filter under the glove box.
If it came with one it would be located on the passenger side, under dash.
look under you Chevy and locate the catalytic converter. Your 02 sensors are located before and after the catalytic converter on your muffler. See 1 or 2?
To change the thermostat on a 1987 Chevy van, drain the radiator and locate the thermostat under the upper host assembly. Remove the bolts holding the thermostat in and replace it.
It depends on how strong the smell is. Yes, it is harmful. It is a gas. And if can kill bugs it could be harmful for you. It could damage your lungs.