Cabin Fumes in a 2000 Focus

I have a 2000 Ford Focus ZTS with about 135,000 miles on it with a fume problem. Once it’s warm, I’m noticing what smells like exhaust fumes in the cabin when stopped or at low speed (not at highway speed.) It runs perfectly otherwise. I took it in a few months ago (before I noticed the correlation with engine temp), and the mechanic could not replicate the problem, and he said there was no exhaust leak, I now suspect this is because he only tested it cold. Before I take it back, I was hoping for some ideas of what the likely source(s) might be and just what kind of repair I might be looking at. Any ideas are appreciated!

It depends on how the exhaust gasses are getting in. I suspect the pipes are in good shape, so check for bad gaskets. The exhaust manifold gasket is suspect. Also, since you have a hatch-back, check the rear hatch weather gasket. Exhaust gasses could be creeping in from the tailpipe back there.

@Aron did your mechanic use an evap/smoke machine to locate exhaust leaks?

Sorry for the delay. I thought I’d get an email alert when there were replies, and I didn’t know there were responses. Thanks for the feedback! That gives me a starting point.
Quick point of clarification, this ZTS is a 4 door sedan.
I don’t remember what the mechanic used, he just said he couldn’t find a leak. I’ve been using the other car in the meantime, but will take it back this month probably. I will update when I have more info. Thanks again for your input!

Oil Leaks That Will Develop Over Time, In The Engine Compartment, Can Sometimes Allow Oil To Drip Onto Hot Exhaust Parts, Burn Off, And Cause Fumes.

Also, coolant fumes can enter the cabin from a leaky heater core, but usually you would notice a problem with windows fogging inside and probably a coolant loss.