I need some advice. I just bought a used 97 Jeep Grand Cherokee Limited. It ran fine but I was told the exhaust manifold was cracked and the VIC said "coolant sensor bad". I took it in for an oil change and to replace the sensor and replace the serpentine belt. The mechanic told me the sensor was fine, but it was running too cool so probably a bad thermostat. Turns out there was no thermostat in the jeep at all and the housing was in bad shape. So they installed a thermostat and a new housing. As soon as I left the shop it started running rough and the "service engine" light came on. I took it right back and they told me it may take 3 cycles of running for the computer to reset itself with the proper levels. I drove it around for a while and now I have bad exhaust fumes inside! Especially when I run the A/C. I know I have a crack in the exhaust manifold but I didn't have the fumes before todays fix. Any ideas what could be causing this?

With the engine running hotter, there are a few things that are different and might have caused the changes you've noticed - since the engine was running too cold, it would have also been running too rich, which would have loaded the oil with fuel contamination (through blow-by from combustion), and that extra fuel it was drawing through the PCV system from the crankcase would have sent the fuel trim readings into the negative. If they didn't remove the battery terminal while they were doing the oil change and the other repairs, the PCM still remembers all that old data and may be having to learn to run without the fuel vapor it was getting through the PCV system from the crankcase. I've seen that on cars before. As for the fumes, the quick answer to that problem is to get the exhaust leak fixed. The exhaust leak can also confuse the fuel system if oxygen is making its way into the exhaust upstream of the O2 sensors, so there are a lot of things to consider there. Get the exhaust leak fixed first. That would be my advice.

Thank you for the information! What you are saying makes alot of sense because before I had the oil changed, I checked the dip stick and it did have a gas smell to it. So you must be correct!! I just don't understand why I would have such strong fumes from the exhaust manifold now but not before they worked on it. Maybe it is running more rich now that it has a thermostat? Or maybe all the checking made the hole in the manifold bigger? Do you think I should unhook the battery to reset anything?

No, just get the exhaust leak fixed...

Thank you!! You're help is very much appreciated!!