I just bought a 2001 s-type with ~156K on it two days ago. I drove it for 2 days with no problem however, this morning I had problems with it.

The last two days it was about 35F where I was and this morning it was ~25F. Also, there was a lot of frost on everything because there had been fog overnight.

After it started, the rpm would go up and down. It seemed to fluctuate between normal starting rpm and a very low rpm. I thought it was going to stall but it didn't. After I put it into drive, pressing the gas pedal would result in the engine rpm dropping a lot. It slowly moves forward normally if my foot is off the gas. I waited a few minutes for it to warm up but I had to get to work so I left the car at home.

I had noticed that this vehicle had a pending P1121 Throttle Position Sensor Inconsistent with Mass Air Flow Sensor. Does anyone have any idea what is wrong with this vehicle? Thanks.

 

Also, the battery light goes on when I hit the gas sometimes but goes off again once I release the gas pedal.

 

Suspect battery part-dead.

Pending code is not a worry unless permanent.

 

Greetings, and welcome to the list. Congratulations on your new car. My '02 V6 is a little balky and the RPM will hunt on cold mornings, but it always clears up within 90 seconds or so.

A few thoughts come to mind. The most probable issue is a vacuum leak. This may also explain the code you have, but I'm not sure. Courtesy of Gus, here's a great way to troubleshoot vacuum leaks with a scanner:

JagRepair.com - Jaguar Repair Information Resource

If that process confirms a vacuum leak, report back and we can help with locations of suspect areas.

Take a good look at the accordion hose between the air cleaner and throttle body. A loose clamp or cracked hose could also throw off the MAF.

The battery light is likely normal if the RPM has dropped too low. It just means the alternator isn't charging properly, so don't panic about that if it's due to low RPM.

However, these cars are VERY finicky about marginal batteries. Unless you know the battery is new or nearly so, I'd suggest replacing it as a precaution. One scenario is with the starter engaged, the battery voltage drops quite a bit under load, which is normal. If the voltage stays above a certain level everything is fine. Below a certain level, lots of funky things can happen with various control modules. You'd think they'd recover once the alternator brings the system voltage back up, but this is not always the case. A marginal battery and cold temperatures make it worse, so that's why I'm suggesting a new battery.

A new set of spark plugs may be a good idea, too. Inspect for oil collecting in the spark plug wells, too.

 

Since you did not post what engine it has we are at a disadvantage here?

But change the battery first. Clear all the old codes. Drive the car some and then recheck the codes. The V-6 and the V-8 both have a list of different possible problems.
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It's the 4L V8.

I doubt it's the battery because there's no way it would cause surging and dropping RPM while still being able to start the car repeatedly. Battery fault light can come up when there is an electrical fault unrelated to the battery or when the alternator isn't producing enough voltage due to low RPM as KR has said.

I disconnected the throttle position sensor and started the car in accordance to some diagnostic advice for jaguars I found. It immediately started with a higher rpm and RPM didn't surge or drop repeatedly while idle. I wish I had driven it while it was in this state but it had entered Engine Protection Mode or something when I disconnected the throttle position sensor.

 

A poor or low battery causes many faults - you can read all about them on the forum and all the doubters - so stop dismissing that.

Disconnecting like that is not going to help.

 

Also, I took a look at the fuel trims and they were moving around 0-5% for the long term fuel trims for both banks. Catalytic converter 2 looks like it's close to death because o2 bank 2 sensor 2 is often mirroring . bank 2 sensor 1. Catalytic converter #1 is a little off too but that's not surprising for an old car.

 

Was that with a hot engine?

 

I disagree. I've spent a lot of time working on cars and if you have evidence against a certain prognosis, it is usually not worthwhile to investigate that prognosis until other more likely issues have been dealt with first. You can spend dozens and dozens of hours diagnosing unlikely things as I have done before. I've driven multiple cars as their batteries died. I've driven a car as it's alternator died when I rolled into a driveway. The symptoms shown just don't correspond with a bad battery except for the battery indicator light of course. Right now, other possibilities of fault are much greater.

 

OK, go your own way.

 

Wow, great attitude.

 

Your flying blind. Change the battery and report back.
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Agreed! I am an aviation electronics tech with 28 years of experience. Among other duties, I am specially certified to maintain and calibrate the autopilots that can automatically land a jumbo in zero visibility and stop on the runway centerline. The pilot is there to work the radios, lower the landing gear, and not much else. I don't mean to sound like I'm bragging, but I do know a few things about chasing sparks.

With that said, I still don't fully understand how marginal batteries can wreak such havoc with these cars, but based on what I've read in this forum, they sure can. Thread after thread mentions how a new battery fixed some mysterious problem. My personal hunch, and it's just that, is this has something to do with the long distance to the battery in the trunk and the effect this has on the necessary capacitance a battery adds to the system. There are likely other factors in play, too.

The key point is you don't have to fully understand it. Several well-respected members with LOTS of Jaguar-specific experience have suggested replacing the battery. Call me Mr. Vegas, but I'd be willing to take that gamble.

 

Failing that measure min volts during cranking. (But most people don't have a tool that can.)

 

It's not much more complicated than the result of system voltage reducing below a certain threshold and the electronic component failing to function as per design, even if the voltage recovers seconds later.

Reboot the computer, and as long as the voltage does not drop below the threshold again, all is well.

I spent enough hours in cockpits after arctic-like overnight cold soaks to see great similarities with our Jags. Two or three reboots were sometime required if there was no ground power or APU power available.

 

Ah, then you may know this:

What's the difference between a pilot and God?

 

Wow!

If it were my car I would first have the battery load checked and I say this because you have a pending code and low voltage can cause this. If this code were permanent it would relate to the throttle body and possible the MAFS. If the battery checks good and you can have this done at any AutoZone, Advanced Auto and Sears for free. Low voltage batteries can and do cause fictitious codes on our cars and that is a fact! If the battery checks out to be good it would be cleaning the MAFS with CRC MAFS cleaner and clean the plug for it and the throttle body plug with CRC contact cleaner. Moisture has been known to be a problem with the throttle body plug so I would suggest cleaning them. Now the P1121 is related to the throttle body so be aware the code may come back. Also prior to you cleaning the plugs first disconnect the battery cable these cars a sensitive to plugs being remove when power is present. When you restart the car clear your codes and see if it comes back.

 

LOL!

I know the usual answer, but I used to say 'none- if you listen to a pilot'.

 

Who'd listen to a pilot........