I got an engine light and read the code.. a nice lovely P2187 - lean fuel mixture at an idle. There are no other codes. I don't feel any rough idle or anything, doesn't feel like a loss of power or starting issues. No quick visual inspection showed any glaring issues. Thought maybe a loose gas cap so did that and it's still here. Any ideas where to start?

 

Which engine do you have and how old is the car?

Did the code come on shortly after starting the car, or later whilst driving?

 

Oh meant to mention that. It's a 2013 XF 3.0. I noticed it just shortly after i started driving.

 

Basically its telling you it has the wrong air to fuel ratio in the engine. Since its saying its lean that means there's more air than fuel (it should be 14.7:1). Likely candidates are:

1. vacuum leak: The amount of incoming air is measured by the MAF sensor, if there is a vacuum leak after the MAF sensor then air can get in/out of the system and therefore a different amount of air arrives in the combustion chamber than what the ECU was expecting. Check for vacuum leaks/smoke test the engine.
2. fuel delivery problem: The amount of fuel being injected into the engine is lower than expected. Check fuel the rail pressure for a failing fuel pump, blocked fuel filter, faulty injector.
3. sensor problem: One or more of the MAF, fuel rail pressure or O2 sensors are playing up giving the ECU a duff reading, however I would have expected a rough idle, especially on a cold start if one of these was bust.

I'd check the sensor data with an OBD scanner to see if there are any clues to where the fault might lie in, but gut feel is it's probably a vacuum leak, that seems to be a common thing as these get older and the plastic & rubber hoses deteriorate. Check all the vacuum lines that are connected to the engine after the MAF sensor, I don't know the 3.0l engine very well, but on other engines there are breather and PCV hoses that seem to want to turn to dust after about 8-10 years.

 

Funny, i just started it and it was a pretty rough start with high rpms that eventually settled down. i'll run some scans to see whats up.

 

That would be consistent with a vacuum leak or faulty MAF sensor. The ECU is trying to reach the ideal air:fuel ratio, not too rich or too lean. In general it uses the data from the MAF sensors to know what is going in, and the feedback from the O2 sensors to know what is coming out. It basically works like this:

• Cold start (like from overnight): Since the O2 sensors only work once they are warm, on a cold start the ECU will be in "open loop" mode meaning its relying on the MAF/MAP sensor input to know how much air is going in and its pre-programmed fuel map table to guess the amount of fuel needed. If there is any issue with the amount of air going in, e.g. because of a vacuum leak or a fault with the sensors, the idle will be higher than normal and probably a bit rough because the mix is wrong. If you try to drive it like this, and put your foot down, the engine will probably misfire, the ECU won't know why so it will throw the check engine light and might put the car into RP mode.
• Warm O2 sensors: Once the O2 sensors have warmed up (usually 1-2mins) then the car will go into "closed loop" and it will adjust the fuel in based on the O2 readings, the ECU will now alter the fuel mix based on whether the O2 sensors are telling it there is too much or little fuel. The ECU can manage with minor air leaks and inconsistences as long as it is within its tolerances, but you may have reduced engine performance and efficiency because the mix will be ever so slightly off.

Once you go beyond the tolerances the ECU will allow you will get the too lean/too rich DTCs.

When measuring the ECU data, track the MAF/MAP, fuel rate, fuel pressure, O2 and revs. If you leave the car idling from a cold start and you notice the rev's drop when the O2 sensors kick in as it goes into closed loop then I'd still guess that its a vacuum leak.
If your fuel measurements are off then it could be a fuel delivery problem.

 

Thanks! this is great.

Out of curiosity i have looked for vacuum leaks by spraying carb cleaner near the vacuum hoses etc and seeing if the RPMs increase. I never did that on my jag. Is this advisable?

 

You have to be careful with those kind of liquids, some can attack plastic and rubber, and you have a ton of that around these engines and they're known for getting brittle as they age.

The engine bay is also quite packed so you could easily accidentally pool the liquid in various places, which could lead to a fire or flashing.

I've mostly used a smoke tester, or on occasion propane gas as both are much safer and less likely to do any damage.

 

Don't rely on seeing any RPM change. Instead, watch STFTs.

 

Yes the old trick no longer works very well. You will never see any RPM changes. The reason is the cars computer system can react and adjust itself very rapidly over a wide range of air/fuel ratios. Follow what JagV8 said above. Use the carb spray but monitor the short and long term fuel trims with a scanner while doing that. You mainly want to watch the short term stuff as it reacts the quickest to any fueling changes.
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