Stft
Hi does anyone have any information on the short term fuel trim expected values? Mine seem to be varying wildly even on idle, .I suspect that I have a vacuum leak somewhere, but upto now I cannot find it, the inlet manifold valve assembly seals are new, and there doesn't seem to be any leak from the vacuum takeoff pipes, the airfilter hose looks ok every thing is tight, and yet I am still getting rough running.

 

Normal is less than 7%.
MAF sensors get dirty and hard to diagnose, clean it with brake cleaner, cheaper one. Also spray throttle. Check air filter.

 

You should NEVER clean your MAF with brake cleaner. Use MAF cleaner, same price. Brake cleaner leaves a residue.

 

Any forum, Here, Mustang, Mercedes-Benz, BMW, Ford, Camero, will say the same thing as do numerous mechanic sites. Procedure =

Before going to the store, pull off the air duct between the air filter box and the throttle body to see how the MAF sensor is anchored. If you see Torx screws, buy a Torx tool and a can of CRC Mass Air Flow Sensor Cleaner. Don’t use any other cleaners; they can ruin the MAF. Everything you need is available at CarQuest, Advance and O’Reilly auto parts stores.

With cleaner and tools in hand, locate the MAF sensor in the air duct between the air filter box and the throttle body. Before you remove the sensor, use a digital camera to record the sensor setup and connections for reference later. Carefully remove the sensor from the air duct and disconnect the electrical connector.

Spray 10 to 15 spurts of the cleaner onto the wire or plate. Don’t scrub the parts; you may break the wire or damage the plate. Allow the MAF sensor to dry completely before reinstalling it in the air duct.

 

Strangely enough I've come to the same conclusion this evening, the car runs okay when cold (rich mixture) and then starts to misbehave as it warms up, it feels like it's a lean mixture problem, which means either too much air or too little fuel, when the lambda sensors are disconnected it runs okay, so we can rule out lack of fuel. I cannot find any vacuum leaks so the most probable cause of too much air is unmetered or poorly metered air in the intake. I shall clean the Maf sensor tomorrow and see what happens. It looks like the maf has been changed in the past, it has a mark in the shape of an arrow pointing forward. I assume that crud on the MAF will make it deflect less and under read the air flow which in turn will make the ECU under fuel the manifold thus giving a weak mixture in the same way that a vacuum leak would. If my logic is fundamentally wrong can someone guru please correct me.

 

One other possibility is the TPS.

 

I installed the new inlet manifold seals today and checked the new plugs, fairly white so definitely running lean, tried cleaning the maf sensor as well, it made no difference, so a new denso maf sensor is on order, I suppose one good thing to come out of all of this is by the end of it I'll have lots of new parts on the car.

 

Just seems hard to believe that if it is running that bad there is no light showing on the dash, whether for a misfire, a vacuum leak or the MAF sensor. You would think a code would be thrown.

 

The manifold air intake temperature sensor is reading 5-10 C higher than the ambient air temperature, it hasn't displayed any maf codes at all but I'm running out of ideas as to what the fault could be caused by, if it was either TPS or throttle body surely that fault would still show when the lambda sensor is disconnected, it all appears to hinge on a very lean mixture, the MAF didn't look that dirty to be honest. The whole saga began after some really brisk acceleration followed by foot off the throttle, I did notice a whine from the car whilst the revs were dying down.

 

When was the last time you changed teh fuel filter?

 

Not sure when it's been changed, however fuel pressure never varies much, 52-56 psi. Surely a blocked or slightly blocked fuel filter would lead to a drop in pressure especially when demand is high. Is fuel pressure measured before or after the filter? If it's measured before the filter then that could explain it, I assume that it's measured after the filter near the injectors. This evening driving home from work I even plastered blu-tack around the vacuum takeoff pipes where they exit the manifold just to be sure that they weren't leaking. It made no difference. I really am running out of ideas now.

 

So the MAF sensor came was installed, made a little difference, ordered a new denso lambda sensor installed it this morning huge difference! The stft values on bank 1 now not varying wildly unlike the stft values on bank 2, the car is much more driveable so a new denso ordered for bank 2. The moral of the sorry tale is cheap parts just cost money with no benefit. Thanks to one and all for your input on this matter. I have learned a valuable lesson here.

 

So after spending a silly amount of money on parts Jojo is now a happy ***** cat, it was probably the lambda sensors and I have read all over this forum "don't buy cheap parts". Advice i ignored and paid the price for (pun intended), the upshot is that the new MAF sensor and upstream lambdas will probably outlast the car, I mistakenly thought that lambdas either worked or didn't. I now realize that they can work wrongly give no error codes and cause all sorts of chaos. Jojo is my first Jag I've mainly had Citroen's in the past so it's all a new learning curve. My advice to any Jag newbies is read understand and heed the advice available on this forum.