Hello all, my wife came home today with a sad look on her face... I just bought her a 2006 XJ8 with 42,000 on the clock and the check engine light was on. We have put roughly 500 miles on it since we bought it and it just passed NYS inspection last week. I ran a diagnostic with the Actron CP9180 tool and received the P0128 code. I ran the data link and watched the coolant temp which reached 190 degrees and maintained this temp for about fifteen minutes while idling. I also checked the air filter while I was out there and it was filthy… (so much for the mechanic doing the 140 point inspection before purchase) could this lead to, or trigger this code? I ordered a K&N which will be here tomorrow. Oh, and the diagnostics were read without the air filter in it…

My question is...is this something serious or can I clear the code? Needless to say, I'll be returning to the dealership if it returns as it has a warranty.

Thanks in advance,

Paul

 

Coolant sensor on this car is a common failure, but could be a failed thermostat as well. You'll want to get either fixed with not much delay as lots of things feed from the coolant temp. If it were mine, I'd replace both while there, and flush the system and replace with new OAT coolant.

I would send the oiled filter back for a refund, replace the current filter with a new paper filter, any brand. This will be another "popcorn" topic, I expect.

 

Is the K&N going to be a problem? I've always used them and have been satisfied with the performance. Would the sensor/T-stat be a warranty item by any chance? The temp guage as well as the diagnostic maintained the temp but I do understand changing each if already there... dumb question...where is the T-stat? Not used to the engine covers...

 

K&N filters are oil saturated. These Jags are so sensitive and the oil in the filter can play havoc on the MAF.

 

Yes, the MAFS issue is one. The "performance" that you mention is another. The X350 air box and cold air feed, as well as the X308 filter system, is pretty hard to beat. The designers left nothing on the table that an aftermarket product can pick up. This has been informally tested by some techs I know with a dyno of their own. The difference in HP between a clean paper element and no element at all was less than a full HP on a N/A engine (both the 4.0 and the 4.2). So, interjecting a filter with different technology cannot magically produced more HP over no filter at all. This leaves you with the cleanable/reusable feature. OK, I can see that, but the oiling must be done carefully. Too much and you'll load up the MAFS, as George said, too little and you'll be filtering air through a sieve, along with much larger particles passing through to the combustion chambers. The last argument comes with the simple cost tradeoff. How many brand new paper filters can you buy with the cost of one reusable. The paper has benefits here. Just open up, throw away, replace with a new one, no cleaning, no oiling.

To give credit, these filters might be a performance benefit on some cars, especially older models and the clean/reuse does appeal to some; however, engine designers are awfully good these days and awfully hard to beat.

The coolant temp sensor should be a warranty item, but I'm not certain. Probably not the coolant flush though. T-stat and coolant temp sensor are right behind the plastic header tank.

 

a p0128 is coolant below threashold. in other words it is sticking partially open. The design of yours has a rubber seal around the edge of the thermostat that tears and sticks the t stat open alittle.So the ecu sees the coolant not reaching operating temp fast enough, so you get a code. Replace the t stat, forget the sensor, and yes you can flush the system now....beware though the t stat is mutlipieced, has to be assembled then installed with a twisting motion to seat, both to takeout and install

 

i agree. im a land rover tech by trade and we have the same problem on our rover wich use the ajv8. torn/deformed thermostat seal