I’m an XJS owner with the 4.0L AJ6 engine. I thought that I had learned my lesson a while back, but I guess that I did not. The problem with engine issues in the XJS forum is that a good percentage of the owners either own a V12, or they own a car with the AJ16 engine, therefore they have no reason to know about AJ6 engine problems and I get zero responses. I believe that, for engine issues, the XJ40 forum is my XJS forum.

OK, my 1994 XJS 4.0L is losing power as it warms up. Great when cold, really quick and responsive, but after the engine is fully warmed up you no longer get the same response. It’s frustrating because I know how good it felt just a few minutes ago when it was still cold. I want to think that my coolant sensor may be defective and lying to the ECU as it warms up. I thought that I had tested this and the readings were close, but I have to do the test one more time, just to make sure.

I’d like to know if any XJ40 owner here may have experienced this problem before. I’d appreciate any input on this.
Thank you and regards,

Reinaldo

 

Hi Reinaldo,

Welcome to the XJ40 Forum!

I think your instincts about the coolant temperature sensor (CTS) are good. If it has failed at a "cold" reading, it will lead the ECM to continue to apply cold-start fuel enrichment even after the engine is warm.

Often, symptoms like yours are due to multiple issues, so my first question would be, how long has it been since you gave the car a good tuneup: plugs & wires, distributor cap & rotor, air & fuel filters, cleaned the sludge out of the air intake plumbing, cleaned the ground points in the engine bay including the engine ground strap, a bottle of good fuel injector cleaner in a tank of gas, etc.

I can't recall if an XJS has a Vehicle Condition Monitor (VCM) or the equivalent, but if so, have you checked for any stored diagnostic trouble codes (DTCs)?

Thinking of things that affect running more after the engine is hot than when it is cold, the carbon/charcoal canister purge valve can misbehave; the EGR valve can misbehave and the EGR plumbing can become clogged with carbon; the idle air control valve (IACV) can stick open, leading to lean running.

The oxygen sensor is a prime suspect because the ECM ignores it for a period after startup to give the sensor time to heat up to operating temperature (around 600F and above).

An obstructed catalytic converter can lead to power loss.

If the throttle position sensor (TPS) becomes contaminated with oil, it can cause strange symptoms.

A failing fuel pump can provide sufficient fuel pressure at idle, but not at higher revs. A failed fuel pressure regulator (FPR) may not hold the fuel pressure high enough for the engine to run above idle.

A failing ignition coil can act up when hot.

An intermittently failing crankshaft position sensor (CPS) can act up when hot.

I'm sure that not all of these fit your symptoms, but hopefully they'll be food for thought.

Please keep us informed.

Cheers,

Don

 

Thanks, Don, for the thorough reply.
Worn and/or defective components tend to affect the way an engine runs regardless of engine temperature, meaning that if this engine runs perfect when cold those components/conditions may be fine. Most ignition components have been replaced at least during the car's last 4,000 miles. However, being temperature dependent, the coolant temp sensor should be the first suspect for obvious reasons. I think I'll have some time to test the sensor this weekend. BTW, I have a nice electronic fuel press gauge on my a-pillar and pressure is always between 38 and 44 PSI, regardless of engine temp or revs. Also, my trip computer has no codes present.

The other item that may be the next suspect would be the O2 sensor, although I'm not so sure whether this sensor is important when it comes to acceleration, as in WOT, which this is where this difference in response is more pronounced -between cold and warm- I say this because under WOT the ECU is out of the "loop", so the question is how involved is the O2 sensor when things are out of the loop. What feedback signal does the ECU look at in order to determine the amount of fuel it needs to feed to the engine under WOT conditions? Does it look at the O2 feedback at all for this? I may just change the O2 sensor because they do wear out, regardless, so it can't hurt.

It would be interesting to place a variable resistor in place of the coolant sensor after the car has warmed up, set it to the perfect value called out in the manual for around 200 deg. F. The only problem that I may face is that, if I do have parasite resistance in its circuit (typically bad ground connections behind the instrument cluster) the dummy resistor will get me nowhere because that "extra" resistance will make the ECU believe that the engine is still cold (temp sensor's resistance decreases as temp goes up), in which case the ideal would be to also measure sensor resistance as it comes in to the instrument cluster, after the suspect connection(s). For that I will need to pull out that instrument cluster.

Thanks again for your great feedback!

Cheers,

 

You can pick up a coolant sensor for as little as $5 or as much as $20 ...it takes all of 3 minutes to change it - why not just try swapping one out to eliminate the possibility yours is fried?

(The coolant sensor connector can give problems too; heat can make the harness pretty crispy sometimes)

An O2 sensor OTOH is a pricey little devil and requires a bit more "dedication" to change out - Having done both, I know which job I'd choose first, mate!

Anyway before tearing into the sensor in the exhaust, I'd have a look at the o2 plug connection and the o2 sensor heater relay connectors - plus you could try exchanging that relay with a similar relay in the group.

Larry

 

Hi Reinaldo,

You didn't mention in your first post that the problem was most pronounced at WOT. That is helpful.

I still agree that the CTS is your first suspect, but I tend to think that if it were the only problem you would have worse driveability issues at speeds other than WOT.

I would respectfully disagree that worn and/or defective components will always affect engine performance at all temperatures. For example, cold or cracked solder joints in a control module can behave differently when cold and when warm/hot (from engine/exhaust heat, climate control heat in the passenger compartment, or resistance in the electrical circuits). An EGR valve or carbon canister purge valve that is stuck closed may have little effect on the engine when cold and at lower engine speeds, but when the engine is warm and the ECM signals the valve to open and it doesn't, driveability issues can arise. The EGR valve is usually opened only during "high engine combustion temperatures (high loads and engine speeds)." The purge valve is not called upon to open until the coolant temperature is above 93F/34C, and it is supposed to open proportionally more as engine load and speed increase. A throttle position sensor (TPS) contaminated with oil can behave differently when the oil has settled into the bottom of the housing and when the oil has been sloshed onto the resistive traces from the motion of the car. Old ignition coils and oxygen sensors are known to behave differently when cold and hot. We could probably think of numerous other possibilities.

The instrument cluster temperature gauge receives its signal from the coolant temperature sender, a single-wire sensor on the thermostat housing. The CTS is the two-wire sensor on the thermostat housing, and it connects directly to the ECM in the right-hand passenger footwell behind the dash underpanel.

If you don't have them, the following two documents may be very helpful in figuring out what is going on:

AJ6 4.0L Engine Management System / OBDI Diagnostic Guide

Jaguar XJS Electrical Guide 1994

Cheers,

Don

 

Absolutely, I was thinking why am I forcing myself through this challenge before changing a $5 sensor? I think the answer to that is that I always like to get to know what exactly failed because that way you'll always learn something.

I replaced the O2 sensor about 6,000 miles ago (actually, like 7 years ago) and I remember it was not a walk in the park... but, luckily, I do have the best tool made for this specific application (not cheap, either) if it comes to that. I believe that I have relays that will fit this application, although these automotive relays tend to be inexpensive.

Cheers,

 

I am watching this thread as my 92 VDP is doing the same thing. I had to pull over going up a hill and shut down my engine for about 10 minutes before I could attempt the rest of the hill. Check light fired up halfway up the hill too. Like it went into some limp mode. I was also wondering if a TPS could have these symptoms upon heating up. Anyways, hopefully you resolve it as well. Where is the CTS located on the 4.0?

 

On my second post I mention "whether this sensor is important when it comes to acceleration, as in WOT, which is where this difference in response is more pronounced".

I agree in that, a bad CTS would most likely cause more problems than just acceleration, but I don't seem to be experiencing any other related problem, which may indicate that the sensor could be fine, just could....

In regards to cold vs. hot I would agree for this to happen more to components inside the engine bay because the changes in temperature are far more critical there than anywhere else, as in the cabin or trunk. But you're right and I'll go there if the more obvious components don't give me an answer.

BTW, I forgot to mention that I have a wide band air/fuel ratio (AFR) gauge on my A-pillar that has its own Bosh wide band oxygen sensor, which I installed right next to the OEM O2 sensor. This is a very accurate system that displays instant AFR. While driving, the indication typically stays in the vicinity of stoichiometric, i.e. 14.7:1, oscillating between 14.4:1 to 15.2:1, which is inside the loop and this tells me that in normal driving, things appear to be going the way they're supposed to. So, I have just tested AFR at WOT acceleration as in cold AFR vs. hot AFR. The gauge read 12.5:1 cold, which is good WOT AFR for a cold engine but it read 10.3:1 after it warmed up, which is insanely rich. This is confirming that the ECU is feeding the engine excessive fuel when the car warms up because the CTS is probably stuck somewhere at a "cold engine" resistance value and all this is doing is just confirming what we already suspected: that the CTS is prime suspect. 10.3:1 AFR is a guaranteed sharp drop in torque/HP. The only thing is that how can this excessively rich charge not be captured and/or reported via a fault code by the OBD system?

Thanks again for the responses!

More to come

 

Both coolant temp sensors (temp gauge's and ECU's) are next to each other near the front/left of the head (if looking from inside the car). The gauge sensor is a single spade male contact at its top and the ECU sensor has 2 spade contacts at the top surrounded by a brownish square plastic connector frame.
I'm not sure about the TPS on the XJ-40 (i.e. AJ6 engine) 'cause, luckily, I have not had problems with it, myself, so far. However, on my X308 XJR I had similar problems and I had to buy a new throttle body assy at no less than $3K after a long and painful history of "limp-home-mode" tortures of 50 miles or more for several months. I sure hope that this won't be your problem. - How does power feel when the engine is still cold, is it normal?

Cheers,

 

Thanks for the reply. It's a new to me car but it seems ok, a little sluggish but not really sure. It's a tow yard auction find and trying to get her back to normal. I did all the basic tune up stuff plus coil and did the rear wheel and diff output shaft bearings, just to also find out the rear shocks are shot as well plus got the top bushing thumps. I did see there is a local auto shop that has the CTS in stock for around $25. I may just replace it as I am sure it's original to the car. Check engine light still on the dash, without a way to hook up my OBD2 meter, how do you check what it means?

 

Oops, disregard. Found the way to check codes and clearing in my Haynes manual. Also, location and testing of the CTS.

 

Ok, update. Had a code 44 (fuel fail). Replaced the CTS and pigtail. Unplugged battery to reset CL. Tried to start and it wouldn't idle (computer relearning?). Even warmed it up, it barely stayed running but I took it for a test drive. It died at every stop. Raised idle and that seemed to sort out that issue.

Like I said earlier, upon warm up, I would pretty much lose all power. This CTS change definitely helped but still seems like it should have more power. Verified Trans fluid color and level looked fine.

 

Hi MPDano,

Fuel Fail 44 indicates a problem with the oxygen sensor (O2S) circuit, stuck either rich or lean. See the diagnostic flowchart for FF44 in the Engine Management System manual which you can download here:

Jaguar AJ6 4.0L Engine Management System / OBDI Diagnostic Guide

Where the guide calls to use PDU (the contemporaneous Jaguar diagnostic system), you can instead use a volt-ohmmeter to measure the resistance across the O2S heater wires and to backprobe the O2S signal wires and monitor the voltage with the engine running, which should constantly switch between approximately 1V and 4V.

If you determine that the O2S is at fault, I highly recommend that you spend the money on a correct Bosch with the correct electrical connector installed at the factory. The "universal" sensors are less expensive, but it is easy to mess up the installation of the electrical connector. The sensors on the XJ40 draw ambient air through the narrow passages between the stranded wire conductors, and if the electrical connector is not installed perfectly, this flow of air may be blocked. The Internet is full of accounts of folks who have had problems with sensors after installing the connector themselves.

Cheers,

Don

 

Ha ha, I was just looking at the O2 sensor at OReillys when I got the email that you replied. Bosch 13313

I take there is only 1 "upstream" O2 and no downstream one.

https://www.oreillyauto.com/detail/b...as?q=o2+sensor

Leo

 

There is only one O2S on an XJ40 and it is at the downstream end of the first catalyst, which is part of the downpipe from the exhaust manifold. There is a second catalyst further downstream, under the floorpan.

Here are some photos from our '93 (sorry the large images aren't working, but you can see my thumbnails and descriptions):

Jaguar XJ40 O2S Part 1 of 2
Jaguar XJ40 O2S Part 2 of 2

Cheers,

Don

 

Thanks Don. Ordered one.

 

Replaced my O2 Sensor after work. What a pain. Ended up dropping the downpipes and cutting the top off the sensor to get a socket on it. Installed new O2 and put 2 of the 4 downpipe bolts back as I think I stripped one of the studs. Got too dark and I needed to move the car so I hooked it up. The one single small plug was really rusty, I bet that was part of the problem it not getting a good connection. Anyways, gave it neighborhood test drive when warmed up and it seems way better. I may be expecting too much from this 4.0

Going to get a smaller nut and get the other 2 nuts back on this weekend. Then need to replace the valve cover gasket and rear shocks. Never ending.

 

That seems to have resolved it. Took it for a test run and climbed some hills and was able to downshift and speed up. I guess mine was a combo of bad sensors. CTS and the O2 Sensors. Thanks all for the help.

 

Ok, this thread's initiator here. I replaced the system's temp sensor and it now accelerates as fast fully warmed up as it does when engine is cold.
Should've done it long ago. Very happy

 

10 bucks well spent then

Larry
Day17