Are we talking about the MAF (Mass Air Flow) sensor or MAP (Manifold Air Pressure) sensor?

In my S 3.0, the MAF is by the air clearner, and is a hot resistor type. The MAP is on the upper path of the intake manifold, down stream of the throttle body.

While I can see why an air leak down stream of the throttle body would affect the readings of the MAP and make for a lean condition, I don't see how it would affect the readings of the MAF?

I can see how a leak in the intake tube upstream of the throttle body might give false readings to the MAF, but would not the leak need to be fairly large to do so?

Am I missing out on something here?

 

Meh....MAFs like underwear...sometimes they just need changing........

I spent a week or two looking for leaks to no avail. Had an advantage in having another 3.0L in the stable to swap 'em and see if the problem moved. If you have a friend or neighbor.....with an S-type..
Sorry, I got distracted and never found that XJRGuy tutorial for posting a link....
Heima, IIRC, it s'plains all those issues. In short, the MAF tells the eng control comp how much air is coming in for the burn, the chip then applies an appropriate amout of magic smoke (assuming it hasn't been let out) which determines how much fuel to meter via the injectors. If you spring a leak between the MAF and the cylinders, more air shows up in the cyls downstream of the leak than the computer was told about, causing the O2 sensors in the Cat's to call "lean burn" on it. With both banks lean (171/174) the leak should be before the airstream is split to each side.....or the MAF is lying. In my case, the MAF that was causing the problem was reporting greater airflow at similar rpm/loads than the "good" one. So one would expect "Rich" codes b/c the computer should be scheduling fuel for more air than is showing up...but alas...I got lean codes. See underwear comment above - it provides a saner explanation.

 

As cleaner you can use any stuff that doesn't leave a residue. I've used carb cleaner, others like brake cleaner! Very thin wire so don't be tempted to touch it with even a brush.

You may be looking at something the car fairly rarely commands or a slightly loose connector that rarely jogs apart (*). The former might be some sort of valve (EGR? EVAP? PCV?). The latter.... wow there are a lot of possibilities.

(*) oh, or a chafed harness that rarely cause trouble... hmm, or something else, pah.

Maybe stick with live data - it'll happen eventually. (Or Brutal wins - again lol)

If you only have ONE lean code, it's not likely to be the MAF because you'd expect TWO codes. Any tech know better cos I'm just using logic rather than years of experience.

 

Like JagV8, I've used Carb Cleaner but was surprised you couldn't find a supplier for MAF Cleaner in the UK.

Although listed by Halfords, it's rarely available from them. Here's an eBay UK source that has it available from stock:

CRC AIR SENSOR CLEAN SEBSOR-CLEANER FOR AIR MASS SENSORS 200ML | eBay

A can lasts a very long time!

Graham

 

You can use Halfords electrical contact cleaner for the MAF.
If you use another brand of contact cleaner be absolutely sure there's no lubricant in it.

 

Well cleaned the MAF and It's still the same.
So I think I need to buy a new one, still It's done 202,000 miles/325,000kms

 

Now that's some substantial mileage. Congrats, and if a new MAF sensor is all you need, then count your lucky stars....

 

Cheers, and It's still got the same engine and gearbox it left the factory with.

 

Post 17... what are the LTFTs at idle & 2500rpm, engine warm?

 

Hi,
With hot engine, the LTFTS are both about 7% at 2500rpm and about 3% at idle.

 

Appears not an air leak, if any, worth worrying about.

So... something else. MAF is a suspect particularly with both banks lean. (With only one, something such as a failing cat would be more likely.)

If the codes keep happening, another thing you could do is look at their associated freeze frame data and see what it shows. Do it a few times and look for a pattern.

 

Cheers, what should the LTFTs be at 2500rpm under load?

 

Don't do it under load but near zero.

 

Well after trying to read live at the same time as it go leans with no luck,
I got a good 2nd hand MAF from ebay and fitted it over the weekend, i drove for 2x 45mins today and no RP, so it looks like its sorted.

But then i was fitted it i noticed some coolant sitting on the front of the sump which can only be leaking from the coolant pump, so it looks like thats next to be replaced, still the last coolant pump i done was on a MG ZT V6 which is driven off the cam/timing belt and it took 6 hrs just to get to it, so the jag looks easy.

Thanks to everyone for the help on this, Simon

 

Hi all,

Its back-RP.
I was driving today on a motorway at 70mph on cruise and RP came up on the dash, i had the wife with me so i plugged in my bluetooth diagnostic and the wife watched live data on my phone. Ten mins later RP went off and the wife again was reading the data.
And the readings from the MAF and long term fuels were the same with/without the RP on, but its still going lean.

Any ideas??

 

You have a vacuum leak!

 

Cheers but I have checked it and its fine. Also all seals had been replaced on the inlet manifold. If it was a vacuum leak it would but there all the time but I now get RP/going lean about every 2 days.

 

Is it the same codes? If so I would start all over again looking for the leak. Do you have a place nearby that can perform a smoke test to verify no vacuum leaks? On the complete opposite side of the spectrum see if the intake filter is not clogged or the pipe to the TB is not connected tightly and air is entering at times the pipe has a crack check it. I know you checked it but I would suggest doing it again and I bet you find it. If I told you how many times I overlooked the obvious it would take a day or two. Use this link and take your time.

Link http://jagrepair.com/images/AutoRepairPhotos/P0171.pdf

I should also note a vacuum leak is all the time but will not throw a code every time. It depends on the driving conditions the ambient temperature and general weather conditions.

 

Thanks Gus,
Yes its the same codes PO171,174.
I don't have any place near me who can do a smoke test.
Then RP/codes came up my fuel trims were both down near 8% not 25% like it says in your link, also the vacuum stayed around 18 inches.

So this i can't understand - for RP/engine going lean it can only be caused by -
1- vacuum leak but readings don't drop under 18 inches and fuel trims don't hit 25%.
2- MAF giving out wrong readings, but i have changed this for a good 2nd hand one and again fuel trims don't hit 25%.

I have replaced the IMT O rings and the manifold to head gasket.

I will check all pipes again today.

This is now been happening for the last 15 weeks/3K miles.

Thanks again, Simon.

 

I am not sure but I think your 2002 3.0 has a hose under the manifold and fuel rail. Someone I hope can tell us for sure.

Link JagRepair.com - Jaguar Repair Information Resource

I do want to mention that the vacuum system is not just under the hood it goes back to the gas cap.