Restricted Performance and Misfires (Sad in Chicago)
 

After changing the knock sensor, battery, and cleared the codes, my 2003 XJ8 Base overheated then limped home. Now rough idle and restricted performance. Was thinking that I may have to change the valve cover gaskets. Any thoughts???

What codes do you now have? I don't see leaking cam cover gaskets causing overheating, maybe lots air leaks -- but more likely you have something disconnected or not installed correctly from the work you have done. Review every step . . .

Misfires in each plug

I wondered if the overheating might have damaged the gaskets. I also switched out the plastic thermostat housing and thermostat after the overheating.

How is your coolant level? Could you have disturbed a valley coolant hose while replacing the knock sensor, initiating or aggravating a leak? You can look down between the air intake elbows to see if coolant is leaking in the valley.

Could you have disturbed the electrical connections at the engine coolant temperature sensor?

If you can please post all of the diagnostic trouble codes that are present that will help us see the full picture.

Cheers,

Don

The trouble codes are as follows:
P0300,01-08, P1313, 1314,1316 and P1000

Correction... Please omit codes P0304 and P0305

Sounds like you have misfires in 6 of 8 cylinders and excess emissions codes resulting from there. Leaking valve cover gaskets can cause misfires if oil leaks down into the spark plug wells. I would remove all ignition coils and see if the spark plug wells are flooded. If they are siphon any oil out and replace all spark plugs/clean coil boots. You will also need to replace the valve cover gaskets but I agree with the previous poster, overheating rarely causes this. I fear if overheated for too long you may have head damage. A compression test while the plugs are out may be worthwhile but first things first.

As far as the overheating/leaking the two hoses that run under the intake manifold and plastic thermostat tower are known leak points. The water pumps in these cars also had plastic impellers so these could fails as well if not upgraded. The octopus hose behind the engine is also a failure point. Lastly I once had the same overheating problem in my XJ8 even after replacing all these parts, turns out the cooling fan fuses were blown. Check these as well (30 amp fuses).

If these DTCs were not present prior to replacing the knock sensor, you have to suspect they are caused by something that was disturbed during your work or was not properly reassembled.

I asked earlier about your coolant level - is it low? Could you have air pockets in the cooling system? Was the engine coolant temperature sensor disconnected?

If one of your valley hoses is leaking coolant, it could be dripping on electrical connector PI1 on the bulkhead behind the engine. The ground referenced by all 8 coils/ignition modules passes through that connector, and any added resistance could cause misfires in multiple or all cylinders.

Cheers,

Don

Thanks for all of your input..I greatly appreciate your assistance. I've ordered the valve cover gasket set and the intake manifold gasket set. I already purchased the iridium plugs. After I complete the task at hand I'll repost.

Coolant level?

The coolant level is fine

Any chance there are air pockets in the cooling system?