I’m at my wits end here. The car runs well. Uses too much fuel though. Plugs look normal when I pull them.

Jag specialist shop checked the fuel pressure. It’s good.
Where else can I go?

I did send the injectors out for cleaning, but this problem existed before then.

Coolant Temp Sensor seems to be trading correctly…through the OBD2 port.

What else? Oxygen sensors have been replaced within the last 2 years.

 

Perhaps try buying a used MAF off someplace like Ebay UK and see if that makes a difference?

 

I have one. No change.

 

Vee,

Can you see your AFR via your OBDII software? Or have you got a "colourtune" or equivalent where you can actually see the combustion colour of one of your cylinders to see if it really is running rich? Although you've changed your lambda sensors recently, what type of voltage trace do you see on them via your OBDII? Are they actually running correctly?

Cheers

Paul

 

I haven't dug that deep. The voltage swings so quickly, I didn't think a voltage trace would be helpful without some way to record, or graph, the swings.

 

Vee,

I can't remember which type of OBDII reader you use on your car? Most of the cheap Bluetooth ELM 327 chipset plugins seem to come with a few versions of laptop software on disc which enables you to monitor and map voltage traces on O2 sensors or MAF readings etc. As these plugins only cost about £3, it's a really cheap way to get some OBDII software.

Even the Torque Pro Android App which only costs about £2 is very good at enabling you to monitor O2 voltage readings, MAF measurements, STFT tracing etc. I do most of my diagnosis just using the App.

Cheers

Paul

 

I have several different programs, including the paid version of Torque. There's a delay associated with the reading and the display of the data, which seems slow.

I can certainly read them, by the data provided is choppy.

I also have MoviPro for my Macbook. Same issue. It takes readings at intervals, not constantly, so all I get is a snapshot of a reading every 2-3 seconds or so.

I spoke with the shop, and they believe that it might just have needed a oxygen sensor reorientation? I briefly spoke to them on the phone, so I'll see what happens when I pick up the car later this week.

As always, will report back.

 

I'm kinda pissed because the shop told me they ran the oxygen sensor reorientation and that seemed to fix the overfuelling issue.

Dammit. I have a WDS that could have done that.

 

After dropping down to a LTFT of 8.6, I seem to have not made any more progress...

 

Vee, is that figure in per cent or just a random number shown by your diagnostic tool? And if so, can it be negative?

I am sure you already know this, but incase ypu don't and others wonder: LTFT is monitored on all cars via MAF and lambda sond (aka O2 sensor). It will adjust itself when driving.

It is the adjustment to the fuel injection amount, depending on a lot of factors... Air pressure, temperature, fuel type, engine age, wear, ... Basically, the ECU will adapt over time to the outer
conditions. The adjustment number should be in per cent.

if the figure is positive, it is a too lean mix and the ECU is trying to rich the mixture up. If it is negative, the mixture is too rich, and the engine will try to lean it out again. This can be really well shown on the Jaguar IDIS software... A rufe of thumbs is to try and keep the LTFT to between -5 and +5%. Anything above or below should be checked.

So, Vee, the figure you posted, is that negative or positive? To be running rich it should be negative. If it is positive, the engine is too lean. Depending on what symbol is prior to the number, there are different approaches... Lean: obviously vacume leaks and others. Rich: too much fuel being injected (too higher fuel pressure, leaky injectors or a wrong MAF/lambda sond), and of course others.

 

Presumably there was a sudden (and serious) worsening of fuel economy that prompted your efforts..but since you didn't actually say so, I'm prompted to ask......

How much is too much, and under what conditions?

You've checked/done quite a few things, covering all the usual suspects. It's now down to LTFT of 8.6. We don't know if it's "+" or "-" but, either way, it's just barely out of the optimal +5/-5 range so I'm not sure what can be discovered that would account for a sudden and/or dramatic worsening of fuel economy....if indeed that's the issue we're dealing with.

What I'm driving at, if you haven't guessed, is that perhaps there's nothing to fix?

Having said that, have you noticed anything different in driving the car? Braking or transmission weirdness, for example? Perhaps the poor fuel economy isn't related to engine management at all ?

Cheers
DD

PS- what sort of STFT readings are you getting?

 

I appreciate everyone's help here. The LTFT is definitely positive. I have never seen it go negative, but I assume it could. The STFT do go negative, so I have no reason to believe it's being displayed incorrectly.

I was previously driving the car, getting excellent mpg, and checking the LTFT, noting it was a zero at times, other times, hopping between 1-3%.

Then the decline. I noticed I could barely get to 250 miles on a full tank, whereas before I was getting to 300. Thats when I saw the trim climb up to 10%. I sent the injectors out for cleaning, swapped MAF sensors, cleaned stuff up.

I checked for vacuum leaks, and have found none.

Sent to a shop that previously discovered a leak in my intake manifold gasket, and they could find no leaks either.

My STFT move around all the time, so it's difficult to report.

The car runs just fine. Not sluggish, no transmission issues....just buying more gas.

My last shot is sending the ECU out for service?

 

What kind of condition are the exhaust manifolds in? They are known to crack and that lets air into the exhaust upstream of the O2 sensors so they think the engine is running lean and adds more fuel.

New manifolds are NLA, but a specialist welder should be able to repair them. They need to be preheated and a special high nickel rod used as the filler. Regular welding for steel with have them crack again very soon.

 

i replaces them with new ones about two, maybe theee years ago. I know, I lucked out. No cracks though.

 

What did you use for O2 sensors? Were they the OEM NTK ones? Are they tightened sufficiently to not leak?

 

As written, if it is positive, then the ECU is adding fuel to compensate for a lean mix. Means you have air leaks. It won't be a huge one but it is there or the air flow isn't measured correctly.

How old is the MAF and is it the proper correct part? I.e. Not from an SC or smaller/larger engine?
Have you checked for a vacume leak? A good method is to use brake cleaner - carefully. Just to check for leaks OR a smoke machine with the engine off with a blocked TB.

if it came all of a sudden, there will be a defect somewhere...

 

Whilst I'm thinking of it... Didn't you fit an adjustable FPR? Or was that a different user?

 

I never installed a fuel pressure regulator.

I have tried to find a leak myself, as well as the last shop that was able to resolve a strange idle issue by finding a leak at the intake manifold gasket....curing that issue.

I have a spare MAF. Swapping them makes no difference.

I replaced the oxygen sensors with Bosch branded, but NTK stamped (as are the Denso branded ones), model 13789. I can check to make sure they are still tight. I suppose I have not tried that. They were replaced a while ago... probably 2-3 years ago

 

Do you get signals from the lamda sonds? Does it jump nicely or is it flat? They should always be varying in voltage...

 

They jump around all the time.

Strange update. Reading through many many old posts, I saw one where someone disconnected, then reconnected their MAF to resolve their issue. I tried that, with the engine running and it freaked out my TPS. My idle started revving up and down and I figured I was going to have to reset it.

Since I was at work, I figured I would try unplugging the TPS and plugging it back in. It seemed to do the trick.

Since then, the car seemed like it was getting better mileage... yesterday on the way home I checked the LTFT and it dropped to 7.8!

I will continue to monitor and post results/progress.