I have been continuing to work on the 2001 XJ8 base I bought about three months ago.

Where I am: Engine had had secondary tensioner melt down on one side at some point in it's history. Was repaired "partly" and owner put up with very loud clatter when engine was cold. Sounded like something was broken in engine - it was that bad. Test drove the car three time but engine was ALWAYS warm (sneaky seller!).

I had very reputable independent Jaguar shop check engine and was told clatter on warm up was secondary to bent valves and low compression on two cylinders.

He repaired head, valves, etc. on that bank.

Compression is back up explained to me that down stream exhaust O2 sensor picks up too much O2 and leans the air/fuel mixture to that bank. After a minute the ICU picks up a "misfire" because of low air/fuel mixture and bumps the mixture back to normal.



Thanks,

Robert

 

The unburnt oxygen ejected past the O2 sensor will cause the O2 sensor to read a lean mixture, driving the ECU to richen the mixture.

However, because of the low compression in the affected cylinder, the oxygen from that cylinder will never be consumed & will always be read by the O2 sensor as a weak mixture for the whole cylinder bank.

So the ECU will continuously drive the mixture too rich until it slams up against the ECU's own internal limits-causing the restricted performance faults.

Unless you physically stop the unburnt oxygen from leaving the exhaust ports of the affected cylinder & reaching the oxygen sensor, there's nothing you can do to stop this from happening.

As a desperate measure, you could unplug the upstream oxygen sensor for the affected cylinder bank-that would then force the ECU into it's default 'base' fuelling/ignition map with no corrections from the oxygen sensor.

This may work, but it will mean the Check Engine light will always be on & the fuelling for the other 7 cylinders will not be ideal-but the engine may run without going into restricted performance mode. It will just run 'reasonably' using the base maps in the ECU without the feedback correction signals from the oxygen sensors.

 

Yes, but doesn't that mean unburned fuel is liable to damage the catalyst eventually by overheating it?

 

"As a desperate measure, you could unplug the upstream oxygen sensor for the affected cylinder bank-that would then force the ECU into it's default 'base' fuelling/ignition map with no corrections from the oxygen sensor."

Where is that upstream sensor? I notice on the down stream "pipe" that connects the exhaust manifold to the under-carriage muffler pipes there are two O2 sensors. What is this purpose? What do each do and would disconnecting either of these help any?

 

I could well do that, although on all petrol cars the system never operates all the time in closed loop mode-that generally only happens during idling & steady speed cruising, when the air/fuel mixture ratio is held at the Stochiometric value to allow the catalysts to work.

During hard acceleration the system goes out of closed loop mode anyway, as the mixture is enriched for maximum engine power-so there will be unburned fuel going into the catalysts under acceleration & they are designed to cope with periods of operation while not operating under the 14.7:1 Stochiometric ratio.

It is a gamble though & as stated, it's a desperate measure to get the engine to at least run without going into restricted performance mode. The O/P has a cylinder down on compression anyway, so the engine can never be made to operate in a condition that will allow the catalysts to function normally.

So as the catalysts can never operate normally under those conditions, they will always allow illegally high exhaust emissions into the air anyway. If the catalyst subsequently overheats & then fails, the exhaust emissions won't become any higher as the system was not operating in correct emissions mode anyway.

The result could well be the catalyst failing as well, so the only real solution is to repair the engine properly to restore compression. However, the O/P has stated that the cost of this is too high for him at the moment-so the choice is to either spend a large amount of money to repair the engine, or to disable the emissions system with the risk that the catalysts may overheat at a later date in time.

It's not ideal-which is why I stated it as a 'desperate measure' to get the engine to at least run without going into restricted performance mode. The catalysts are unlikely to overheat to destruction on short journeys-only on long journeys or sustained periods of full-throttle.

 

There are 4 O2 sensors on the XJ8-2 for each bank of cylinders. There are 2 catalysts on each side of the engine & they each have a top O2 sensor (upstream) & bottom O2 sensor (downstream).

The upstream sensors are at the top of the 2 catalysts for each bank-they are the main sensors which allow the ECU to control the air/fuel mixture ratio. You can disconnect the electrical plugs for 1 or both of them. The electrical plugs live in a bank just above the flywheel bellhousing-you will need to remove the long central black plastic trim piece at the back of the engine bay by the bulkhead to make space to get your hands down & unplug the upstream O2 sensors.

The downstream sensors are at the bottom of the catalysts-they report the catalyst efficiency to the ECU & do not play any part in altering the air/fuel mixture ratio-so don't bother unplugging these.

The 2 catalysts are vertical, so the upstream sensors are the ones at the top, closest to the exhaust manifolds. The downstream sensors are at the bottom of the catalysts, closer to the exhaust mufflers. Locate the sensors & follow the wires to the bank of 4 plugs above the flywheel bellhousing.

The upstream sensors have grey plugs, and the downstream sensors have black plugs. There's no guarantee that this method will work, but it's easy enough to try as all you have to do is remove that trim piece & get your hands down by the bellhousing to reach the grey plugs.

One of the upstream O2 sensors failed on my XJR & the car continued to run quite well-the edge had gone off the performance though, as the ECU had defaulted to the base fuelling/ignition maps-however, it didn't go into restricted performance mode & continued to idle & run quite well. The Check Engine light came on & the fault code appeared in the OBDII system. Fuel consumption went up by about 15% but the car still drove quite well.

I did fix it though & it runs fine now

 

"There are 4 O2 sensors on the XJ8-2 for each bank of cylinders. There are 2 catalysts on each side of the engine & they each have a top O2 sensor (upstream) & bottom O2 sensor (downstream).

The upstream sensors are at the top of the 2 catalysts for each bank-they are the main sensors which allow the ECU to control the air/fuel mixture ratio. You can disconnect the electrical plugs for 1 or both of them. The electrical plugs live in a bank just above the flywheel bellhousing-you will need to remove the long central black plastic trim piece at the back of the engine bay by the bulkhead to make space to get your hands down & unplug the upstream O2 sensors.

The downstream sensors are at the bottom of the catalysts-they report the catalyst efficiency to the ECU & do not play any part in altering the air/fuel mixture ratio-so don't bother unplugging these."

Red,

Boy, do I appreciate this. Yesterday evening I had the car on my lift and saw the two sensors per bank. I did not even think the thing they were plugged into was the catalyst! I could not figure what the purpose of the dual O2 sensors were. (I am also not sure how they could ever be removed.)

I did locate the plug conections you mentioned while under the car and traced them to their connection points.

BTW, when I test drove the car three times it never went into restricted performance mode. It drove quite fine and DID NOT CLATTER at all. It only clattered when cold. You can hear the knocking/clatter here:



Jump to 1:00 where I start the engine.

Once he rebuilt the valve head on Bank A he must of plugged the upstream O2 sensor back in and thus that took care of the check engine light BUT make the car run worse because of restricted performance.

I will try what you suggested and report back.

I have owned and restored many vehicles over the years doing most of the work on these vehicles myself. This Jaguar is a new animal to me. The engineering is so different than the BMWs, Mercedes and antique American cars (mostly Buicks) that I have worked on.

And FYI, I bought the car for about $ 4200. The Jag mechanic (who is VERY trusted) did work on the car totaling about $2700. So at $ 7000 into the car I am above it's value in this market.

Thanks!

Robert

 

Have you thought about these? They remove the sensor from the gas stream so they work at 'normal' temps, as your's is running cool on 1 pot the stainless cheater may stabilize the reading is relays...

Some fit them after O2 codes when fitting aftermarket cats. It's an issue having low compression, but might be worth $10 if you can live with it, dare I say it, I couldn't! But I do understand your predicament

 

The fundamental problem is that the cylinder with low compression results in the oxygen not being consumed, as the compression is too low to generate a combustible mixture.

So the unburnt oxygen from that cylinder passes through the exhaust system & the O2 sensor reads the excess oxygen as a general weak mixture, so it then tries to compensate by richening the mixture.

However, this will never achieve the desired result as the oxygen in the 'rogue' cylinder will always be present in the exhaust system unless the engine is repaired & compression restored.

By disconnecting the sensor, the ECU will stop trying to compensate for what it thinks is a 'weak' mixture according to the O2 sensor. Instead, it will default to the uncorrected base fuelling maps which allow the engine to run reasonably-and then throw up a fault code for the O2 sensor.

By leaving the O2 sensor electrically connected, the ECU will continually try to compensate for the weak mixture it sees-but it will never be able to do this as the cylinder is not burning the oxygen. so the ECU will eventually 'hit the buffers' regarding mixture enrichment-then it will go into restricted performance mode, which makes driveability much worse than if the ECU defaulted to it's base maps due to an O2 sensor being disconnected.

It may also be worth disconnecting the electrical plug to the fuel injector for the rogue cylinder-there's no point in having the injector squirt fuel into the cylinder when it's got no chance of being burnt & instead just ends up being thrown down the exhaust system. This will probably cause yet another fault code to appear, but provided it doesn't cause the ECU to go into restricted performance mode then it should be ok.

 

I like that option. At least he won't be dumping raw fuel into the exhaust, and the ECU will not be trying to adjust the mixture.

 

Three more lower cost suggestions;
A can of Restore might bring up the compression enough to burn the fuel.
One of the very experienced techs on the forum (Brutal) says that it works.
Buy a known good used motor and install it yourself; about $1500.
Check the valve clearances on that cylinder.

 

Sean,

My O2 sensors are located in a difficult to reach place on a large, cylindrical catalytic converter that comes vertically down from the exhaust manifolds. I have a lift and have looked at the position of the sensors and they look very difficult to remove. For example, some of them have the hex fitting below the surface of the converter wall's outer surface. Not sure what kind of tool to use to remove them. Maybe something like a spark plug remover??? AND area is TIGHT.

Does the O2 cheater in the photo you posted just block off the exhaust?

I have tired disconnecting different combinations of O2 sensors pre and post catalytic converter and nothing seem to improve there.

 

Disconnecting the sensor(s) did not seem to help the problem. It still goes in and out of Restricted Performance mode a lot.

Any comments on the O2 cheaters?

I guess my next step will be to try disconnecting the fuel injector.

 

Greetings!

I have still been trying to work with this Restricted Performance mode issue on my Jag.

It seems to do it less when the tank is low.

Does anybody have ANY idea why this is?

Very curious.

Robert

 

From the symptoms in your last post, I would suspect a problem with vapour recovery system not venting, creating a vacuum in the tank and lowering fuel pressure. When fuel is low, there is enough air volume in the tank to negate the vacuum. Purely a guess, but nobody else has posted. Could try a fuel pressure gauge taped to outside of windscreen, see what happens.
good luck
Pete

 

Pete,

Thanks for the reply. I did something a little different to circumvent the possible vacuum problem and it did not work.

I basically left the gas cap off and used a stubby screwdriver to hold open the gas filler flap. I would assume that this would stop ANY vacuum from being created in the tank.

I am wondering if there is something with the central control module that stops the car from going into Restricted Performance mode when the gas level falls below a certain point.

Robert

 

Turn the cams to where all flat spots align up and flat, use a flat edge to make sure they are perfectly flat and aligned. Its possible you are off a tooth or two, very common.

 

LOL. Aode06: do you know you are responding to an issue that is over 15 months old?

One of the problems with this forum, and all forums, is that problems are hardly ever finally reported as being fixed or still at issue. .I wonder if Robertjag ever got the car fixed?