Hello guys,

I have just discovered that my both exhaust manifolds are cracked. And what I'm assuming that it could be the reason of low Mpg on gas.

Now at city cruising i have about 18-20 liters/100 km what is abut 16-15 MPG and i think it quite to much.

Duo to low Mpg i recently changed both O2 sensors but that didn't give any improvement.( at that time i didn't know about cracked manifolds, because they're covered).

Also changed thermostat, but also no results.

The temp sensor is also ok.

I plugged the obd scanner and i see the interesting thing on fuel trim
on STFL i see 24,6% on both sensors, while driving, or on idle the result is the same. It only reacts slightly when on idle i give sudden acceleration, then it drops to 0% but sudden goes back to 24,6%.
The LTFL stays constant on -5% , what is quite interesting.

So my gues is that because of a crack in the manifolds the oxygen passes to the exhaust ant the O2 sensor makes the fuel mixture richer.

Any thoughts on this issue are highly appreciated.
Thanks guys.

 

What engine?

I drove a 1995 XJR/6 for many years and typically got 13-14 mpg in city driving and about 20-21 mpg in open road driving....so your reported 'city' fuel economy doesn't sound too bad to me ! However, if you have a standard XJ6, your reported fuel mileage might be a on the low side, yes.

However, yes, cracked manifolds can cause the problem for the you describe: air can be drawn in and picked up by the oxygen sensors, falsifying the readings

Fuel trim should normally be in the -5 to +5 range with -25 to +25 being the extremes

The OBD on X300s doesn't always communicate properly with all scan tools so it can be confusing dealing with abnormal readings. However, your fuel trim readings are at least within the realm of reality so you're probably OK. I used to get FT reading all the way up to +100 which in theory can't happen!

Good luck finding non-cracked manifolds. I was lucky to find some on Ebay but I suspect they are getting scarce nowadays. Perhaps you can still buy new ones?

Cheers
DD

 

I have a 1997 XJ6 3.2 litre sport.

Recently have found and bought some used manifolds, but one from a car with secondary air injection the other one from a car without. Mines are with air injection so i guess i will have to remove air injection too.

 

Your conversion to MPG is not correct. You're getting 11-13 mpg measured in US gallons. Winter driving may be the cause of the very poor consumption. My mid sized SUV is presently returning 23L/100KM in city driving.

 

Yes i miscalculated because was using UK mpg. Us is slightly different.
Anyway 11 mpg is a horrific number and there is a problem obviously somewhere.

 

I believe there are three pieces of information the car processes to fuel the car:
1. MAF sensor
2. Oxygen Sensor
3. Coolant Temperature

Assuming all three of these are providing accurate data AND the ECU is processing them correctly, the only other problem I can suggest is leaky injectors overfuelling? Maybe sparkplugs need some changing?

By replacing the 02 sensors, you've done all you can do there. I doubt cracked exhaust manifolds can cause that much of a problem. The exhaust is under pressure, so there's not much opportunity for enough air to get sucked in to cause that kind of problem.

You checked the coolant temp sensor, so that should be fine.

Even though it could be the MAF, I would start eyeballing the ECU.

 

Well tomorrow i'm sending the car to the shop for the manifold change, probably will remove the secondary air pump with its components since new manifolds do not have the holes for it.

Sad but after your post i feel a little disappointed because i feel somehow that the manifold swap wont change the problem.

Probably youre right about the MAF, because i changed spark plugs, both 02 sensors, the termostat.
Temperatures via obd looks good to me (about 89C degrees on hot engine).
Also have bought a set of coils from US, but haven't managed to changed them yet, although i see its made in china thing.

So whats left is the maf sensor i guess.

Maybe you can provide a guideline on how to check the maf correctly? What should i look for, what parameters should i check first, to determine any fault on it?
Maybe its worth of cleaning it with a special cleaner, or its a deeper problem?
What data should it show on scan tool regarding to maf reading parameters?

Also you mentioned the ecu, you mean the ecu itself could be the problem? Is there any possibility to check it without going to dealer who as a special scantool for old jaguar? ( This is a problem where i live)


Thank you for all your help!

 

Quick update,
Today connected the scanner and i get the readings

maf - 0,04-0,06g/s on idle. Is it normal? How much exactly it should be.
TPS is 11%.

STFL- 24.2% again quick acceleration on pedal, engine revs it drops to 0%, and sudden rises back to 24,2.
While driving its constant 24,2%
LTFL -5%

 

Hi guys, don't forget manifolds can be welded up, I've got a posting on here somewhere regarding my experience with x300 exhaust manifolds.

 

Found Mr Happy's post on manifolds welding here:
https://www.jaguarforums.com/forum/x...ptions-160147/

Don't know how long welded manifolds can last. I guess it also depends on the workmanship of welding. I notice that new manifolds sell relatively cheap in British Parts (@GBP240), but they are no longer in stock now, probably sold out. Many used ones on eBay are also cracked. There have been discussions on coating the manifolds to fix or prevent future cracks, but am I right to say that the reliability of such fix or preventive measure is still non-conclusive. A bunch of enthusiasts has decided to go for custom-made stainless steel exhaust manifolds on their own. In 2014, I started a thread to ascertain interest in a group order for custom-making manifolds in SS but there wasn't sufficient interest at the time. See
https://www.jaguarforums.com/forum/x...s-x300-114521/

 

Well, as Doug mentioned, any atmospheric air inhaled into the exhaust causes the O2 sensors to read lean, which prompts the ECM to mistakenly enrich the air-fuel mixture.

There are at least a couple of mechanisms that can allow unmetered atmospheric air to enter the exhaust through manifold cracks or gasket leaks. One is the venturi or jet vacuum generation effect, in which a moving stream of pressurized gas creates a vacuum across a port or orifice.

Also, if the exhaust valves do not seal perfectly due to wear, carbon buildup or weak springs, then when the pistons descend they create vacuum in their respective exhaust manifold runner, inhaling atmospheric air through any cracks or gasket leaks. The air mixes with the exhaust gases causing the O2 sensors to read lean, sometimes to such a degree that the ECM cannot enrich the fueling sufficiently to achieve stoichiometric AFR. This is why exhaust manifold leaks can trigger lean diagnostic trouble codes like P0171, P0174, P1137 and P1157.

Which leads me to this question for RB_jaguar: Have you scanned your car for any stored DTCs?

Cheers,

Don

 

Don thanks for your answer.

Interesting , but I'm getting no fault codes at all. And no MIL lights illuminating.

 

What that must mean is that thus far the ECM has been able to correct for the lean O2S readings by enriching the fuel mixture, which will definitely have an adverse impact on fuel economy.

 

So took a car today form the shop.
Cracked manifolds changed, air injection removed.

Connected the obd scanner and i'm seeing the same situation in fuel trims.
That means nothing is changed.

I feel very disappointed and kind a lost in my mind about fuel usage.

Any thoughts guys?

 

My consumption is now up to 25L/100KM.

Wanna trade?

 

Take it to a mechanic that has the software to reset your base idle trim.

I didn't know the 3.2 only has two oxygen sensors? (The 4.0 AJ16 engines have four)

Assuming your shop replaced the metal gaskets when the manifolds were changed.

Is the engine running smoothly? No minor vibration or shaking? What is your idle RPM at?

 

The fuel trims stays the same also by driving , its just stuck on 24,2%, regardless idle or not.
I suspect the maf sensor now, if it would be air leaking or oxygen sensors, i guess trims would change regarding to driving conditions.

Of course new gaskets came in place with the manifold change.

There is 2 O2 sensors because is European market car, as i know just US an Canada cars have 4 O2 sensors.

Engine runs smooth and steady, no vibrations. Idle is about 700 rpm.

What do you mean by reprogramming idle fuel trims? is it doable just with Jaguar PDU? And will it change the driving fuel trims also?

 

A Jaguar PDU will reset fuel trims. It will undoubtedly fix your problem, temporarily at least. If you have an issue that is creating the high long term fuel trims, then you will eventually get back there. If you have the means to get that reset, then find a shop and get it done. I thought only the PDU can do this, but apparently WDS can do this too, perhaps IDS. Any experienced Jag shop will know what to do, but you need the software (and hardware).

The MAF is not a known problem with these cars, although you can never rule anything out. I don't know a foolproof way to test it, but it should throw a code if it was doing something off the charts. My service manual does indicate that you should see the following:
1.30V at 700rpm
1.70V at 1,500rpm
2.30V at 3,000rpm
I'm not sure why the readings would change as a result of rpms...I figured it would only swing based on volume of air going through it. I'll continue to see if I can find something more useful.

 

Wouldn't it make sense that the higher the engine rpm, the greater the volume of air being inhaled by the engine and therefore the greater the volume of air being measured by the MAFS?

Cheers,

Don

 

Ha! You're totally right. Duh. I was thinking of air being pushed in, not the effect of the engine pulling in the air!