Offroad49

My 2001 Jeep Cherokee 4WD 4.0L fuel injected inline six indicates error code P300 and P303 (misfire codes) -- It would be great if anyone on the forum has had this problem and know exactly what the problem is and what needs to be done to fix it. -- (P300 is general misfire code, and P303 is cylinder 3 misfire code)

History: These same codes came up last summer and the shop I took it to claimed to have found the problem and fixed it - for a hefty sum too. (Among other things, they replaced the spark plugs and the electronic ignition coil manifold) - It worked fine during the cold weather all winter, but now that we've had a few hot days again, the problem is back, and I need help to figure it out myself before taking it back to another shop to work on it.

Some useful information: The problem occurs when the weather warms up and the engine gets hot like engines do when using the air conditioner (the temp gauge indicates normal temp at all times once it warms up). It starts fine when the engine is cold and then when it heats up and you turn it off and come back to it a few minutes later, the engine starts but runs rough - like when the idle is set too low on a carburetor engine, and when I try to give it a little gas, it acts like it's going to stall - as if it's getting flooded. If I wait and let it run a while, I can put it in gear and it will move while running rough but not stall. As I slowly speed up, the engine runs fine again - but the check engine light does not go out (of course).

That's the symptoms and additional info. I'd very much appreciate it if anyone can help me with this problem with my 2001 Jeep Cherokee.
Thanks. ---AL

winston427

This is what is commonly called heat soak misfire. Basically after turning the engine off the heat from the exhaust migrates up in the injectors and boils the fuel causing vapor lock. Upon restarting the Jeep you will get a misfire code on one or more cylinders.

One possible cause that is very common on the 00-01 models is an intake/exhaust gasket leak. The very back bolt is known to back out slightly and with the repeated heating and cooling of daily driving the gasket will warp creating a leak. A tell tale sign of this is a slight whistle when letting off of the accelerator. Sometimes a simple snugging up of the bolt will fix the problem if it just began. Or the gasket may need to be replaced.

Had the very same issue on my 01 and tried everything I found while researching the issue, including replacing the injectors and the problems returned. After finding a thread about the whistle I replaced the gasket and never had the problem again.

Offroad49

Thanks Winston427. -- I'll check it out for the whistle. Since it definitely seems to be heat related, it makes sense the exhaust manifold could be involved. I thought about that, but since the thermostat keeps the engine at an optimum operating temperature, the exhaust manifold should be about the same temperature all year around. The problem however doesn't occur in cold and cool weather - only hot weather. -- Is there a tool available for detecting exhaust leaks - like one that can detect exhaust gases? -- Thanks again for your reply.