I have an intertherm mobile home heater. Lately I have been smelling exhaust when the heater is heating and the cover to heater is on. So I take it off. It rids the smell. Almost like it is being blocked when the cover is on. Is this smell bad for me? Why am I smelling this exhaust? And how can I fix it? The filters on cover are clean. Are there any other filters that I can change or should check?

Could be vent backdraft or possibly a cracked heat exchanger. Check the outside heater vent for a possible blockage. Over the summer, dirt daubers and wasps can build nests in them. I've seen bird nests too if the cover and screen are missing. You will need a HVAC person to check the heat exchanger. This could be a dangerous situation with Carbon Monoxide, find a fix fast!