Hello everyone I need a little guidance on where to start diagnosing.
Driving into work yesterday I noticed my 2008 xj8 started idling weird when sitting at stop lights. The idle will consistently drop down to 400 rpm and bump back up every 3 seconds or so. I notice the odometer will dim a bit everytime it dips down. I bought a can of MAF sensor cleaner and tried cleaning it a couple of times with no difference. The car does not hesitate or stumble when driving any other speed. I'm thinking maybe the MAF sensor needs replaced but on my way home something else weird happened. My display screen started to flicker on and off for a bit. This concerned me thinking maybe the alternator is on its way out which would be odd because I put a new alternator in and have maybe put 7000 miles on it since then. Below are screen shot of the codes I am pulling when I put my code reader on it which all seem to point to the MAF. Should I start with a new MAF or are their some other tests to run first?

 

P0171 is associated with unmetered air
look for leaks between th MAF and TB inlet
Spray ducting exterior with starting fluid or similar
and look for any effect on idling

See also post by Marty on 9/10/24 on this page

 

What brand Maf is on it if possible I just use Denso or Ngk products

 

It has what I assume is the original denso on it right now. Car has 145k miles currently. I did a little spray testing with starter fluid but I will spend a little more time trying that and see what happens.

 

Try to find the reason for it . Do not drive around you can wreck your catalysers
Had that on a daimle/r xjr engine big hole on lower duct resonator going down between radiator and belts and on a V6 I currently own the airfilter box was not closed and one fixation on the throttlebody was loose

 

Looks like electrical issues with MAF and/or IAT - chafed wire? bad connection? wet?

Check battery in case it really is low voltage (in which case expect false codes).

 

Luckily it is my fun spare car so I don't need to drive it until it is figured out. I'm leaning toward the maf being bad based on the way it is acting but will check for leaks and electrical before I order one.

 

All point to the battery voltage issue/ground fault. It would be the first thing to check. Could be either B+ or ground.

 

So today I checked the battery with a multimeter and with the car off the voltage is definatley low and when running at idle it showed major fluctuation in the voltage. I guess first step will be to have the battery tested and if that tests good maybe looking at a problem with the alternator? Like I said it is only two years old so that would be very frustrating.

 

Charge battery full on low amp. The fluctuating might be gone then.

 

So I am really starting to think the alternator is going out. The battery tested fine according to interstate. Brought it home and ran the car for a while and eventually the hunting idle leveled out but as I was running it the fan kicked on in high gear and now when I start the car the fan immediatatley turns on. The red battery light also turned on and below is the code it is throwing on my reader. P1632.

Are there any fuses or relays I should check first or is this a pretty tell tale sign?

 

Is your serpentine/tensioners/idlers in good condition?

 

Yes they seem to be all in condition and no odd noises. I checked the voltage directly at the alternator while the car is running and only getting 11.4 volts so pretty confident the alternator is Goin. Going to talk to the shop that had replaced it today and see if they can offer anything.

 

So I talked to the shop that had replaced the alternator and they warranty their work for 2 years. Unfortunately I was outside the two years and the part it's self did not offer a warranty. They installed a reman bosche alternator and charged me 500 dollars for it so I thought it was ridiculous the part it's self wasn't warrantied. Since they could not help me I ordered an alternator and got it installed and that solved all my problems.