thanks Don, you are right about the code, It is actually P0172 and P0175. For some reason, i got it mixed up. I will take out and reassemble the whole air intake when I get a chance and see if the DTCs will go away. When I disassemble it, I would also make sure I clean all the plugs that is connected to the throttle body. And check for any vacuum leaks. Hopefully this Cat will mellow down on me!

 

Those are rich codes so very unlikely to be an air leak. Bad MAF, maybe.

As you may have upset things you might try a hard reset to force fuel trim relearn.

 

Thanks JagV8, I replaced a new MAF just a couple week ago. After replacing a new Denso MAF sensor, it threw the code P0175, so I clean the air intake system along with the throttle body, then it threw a P0175 and P0172 couple days ago. However, after cleaning, I didn't do a hard reset, I will try to do that and see. The hard reset is basically disconnecting the +wire and tap it to the body and hold for like 20 seconds right?

 

Yes. You're shorting out the car - not the battery

To reduce the instantaneous current wait a few seconds till you do it.

All with key out of ignition.

 

Thanks JagV8, I just hard reset the car, and clear all codes. Also cleaned the MAF sensor and connector with electronic cleaners. After doing more reading, it appears people are having problem with Fuel injectors and rail related issues too with P0172 and P0175. I wonder if I should look into those areas too. Anyhow, I will keep you guys posted. Thanks you!

 

Hi Jayson,

I don't know if you've downloaded the X350 Workshop Manual but the Powertrain section contains numerous possible causes of the P0172 and P0175 codes in addition to the ones that have been mentioned in this thread: Engine Coolant Temperature sensor (ECT), exhaust leak, and several others. It's worth a look.

Cheers,

Don

 

thanks Don, I did download it. I know it is not an air leak for sure because of the negative short term and long term fuel trim. In term of ECT, when the engine is warmed up and in operation, it is at 95 degree which is normal so i don't think it is that neither. I will look into the exhaust leaks, and fuel injectors to see any leaks. And work on it slowly until the culprit is find. Any advices are deeply appreciated.

 

My recommendation is to not clean the throttle body on these cars. As long as the butterfly is seating properly, cleaning it will make no difference. This is one of those things where the fix can cause problems. The butterfly is moved by a stepper motor which is fairly delicate, don't push it around.

 

Thanks old mots, I read you purchased a new throttle body from JustJag, can you tell me which part # is for my 2004 Xjr because I would like to buy one. In term of cleaning throttle body, it is very controversial, some swear the improvement after cleaning, some say no; however, I did cleaned my. I didn't see any difference. Sometime you just have to take a chance to see if it works.

 

CEL is still on with code P0175. The long term fuel trim is at negative 10 when rpm is at 1900. Short term fuel is -5 at idle. I have checked exhaust bolts, replaced new MAF, cleaned throttle body, check fuel rails and fuel injectors. I even pour one of them Chevron fuel injector cleaner. Another thing i notice in the morning because I start up the car, the upper coolant hose is flat as it having a suction in the hose. My next thought would be the spark plugs, since it have a negative fuel trim at the idle, maybe it is not burning fuel at the get go. Should I replace the spark plugs? Any advice will be appreciated!

 

That sounds like low coolant or an air lock.

I don't see how plugs would be a cause. If they fail to cause a burn then the O2s see air and report lean.

 

I have refilled about a quart of water for the pass 2 month. Is that normal to lose a little water like that? What is the approach to resolve an air lock? I ordered a thermostat and plan on replacing it, in addition to the changing out the coolant. What if the plugs are not burning enough fuel which lead the fuel trim to take away ?

 

It's (meant to be) a sealed system, so no.