hi i have a 95 xjs; and when driving about 100-120 km/h without accelerating or decelerating, the engine, only when its hot, has some minor hickups. not as big as a spark plug not working but smaller hickups. ive measured the short and LTFT values; and the short ones are about +2 %, and the LTFT is stuck at -89%, and does not move.
also i cannot get the car less thirsty than 18 mpg, even if i try very hard.
is this a MAF issue? or maybe a vacuum leak? please help... for the rest it runs very smooth, just when not on load and at 100 km/h. if i put it on cruise control its very noticable.


thanx, pim

 

Six or twelve cylinder?
Any codes (DTCs)?
How many miles on car?
How many miles on spark plugs?
Any oil in plug wells?
What scanner are you using?

 

its a 4.0 six, no codes, 90000 miles, 500 miles on spark plugs and coils(changed due to this problem but no solution) no oil; im using a obd2 scanner app; car scanner.
today i measured again; and the long term fuel trims were at -100%, constantly withou moving.

Just a nice shot

This is a screenshot of the car scanner app
The short term fuel trims were between +2 an +7% or somerhing like that.

when i unplugged the second sensor on the intake manifold; the one thats closest to the driver; there was NO difference at all in driving; and the problem persisted. LTFT syayed at -100%. Which one is the sensor that reads the LTFT?

thanx, pim

 

Do you have access to another scanner?
With LTFTs at -100% you should be getting rich codes.

 

If no codes then I reckon the tool is not working properly.

+1 on another.

 

i tried 2 different apps; both of them give same readings. But i also saw that there is no data read from sensor b1s1 and b2s1. That is what the app says. Where are these situated; id like to clean and test them... thanx

 

Before you get too deep (and disruptive) into what is an early version of OBD2, I suggest you address the symptom you originally described.
That is, check transmission fluid level. It sounds like the locking torque converter may be toggling in and out. Does it behave differently if you cruise in a lower gear?
If you should explore oxygen sensors, be sure to keep track of which sensor plug attaches to which harness plug. Don't mix them up!

 

Your apps / device are CLEARLY NOT working.

You will likely destroy the sensors if you try to remove them and they cost a lot more than a working tool.

 

I think this will be better with it's own thread in the XJS section.

 

What you might be running into is the torment of the 1995 OBDII system.

I drove a '95 for years and struggled with driveability issues.....and struggled with OBDII readings that made no sense, a reluctance to set troubles codes in some cases yet constantly setting (what were apparently) phantom or erroneous codes in other cases. In particular I struggled with fuel trim readings that usually were 'fixed' or, when not fixed, were often wayyyyy off the normal scale (which is typically considered 0-25%)

I couldn't rely on the OBDII as either an accurate indicator of a problem nor proof that any repair attempts were effective.

There's some degree of consensus, even among Jag techs, that the 1995 system was A) not fully developed and B) often didn't play well with non-proprietary scan tools.

I wish I could offer something more helpful.

Cheers
DD

 

Listen to Doug.....
While the 1995 AJ16 engine system is 'technically' OBDII, the system was not fully implemented. The OBDII mandate did not actually begin until 1996.
Lots of things [PIDS] were not reliable. LTFT liked to remain at 75-100% even on a flawless engine. MAF readings were two or three decimal points off as well.
Your tool IS NOT FAULTY.
The O2 sensors are titanium dioxide, differing from almost any other engine. The only way to assess them is with the proprietary diagnostic tool, PDU or WDS.

Good luck!

 

ok that is good information; i will not try to unscrew the o2 sensors; everything including the wiring does look very good so i'd rather not ruin them.
i will look at the transmission fluid level; in the beginning i did think it shifted to a next gear; until i realised there's not a gear left so it might be that the torque convector is toggling in and out. i will keep you posted.
thanx, pim

 

hi today i checked the two sensors at the exhaust manifolds; and when i simply unplugged them; there was no change in behaviour of the engine (when at idle, i did not test drive it), i did see a change in the STFT voltage in the app, although it didn't give me a percentage reading. aren't these the LTFT sensors?

i also checked the transmission fluid level, and it was low. because all shops were closed; i couldnt top it up, will do that tomorrow. hope that will solve the hickups when at constant 100 km/h.
also when i unplugged the air temperature sensor on the air intake manifold, there was no change in idle. is this, and the behaviour of the two exhaust manifolds as it is supposed to be?

thanx, pim

 

There are no long term fuel trim sensors.

Basically the short term fuel trim number is just a result of how much fuel the ECU has to add/reduce based on what the engine is doing while reading the oxygen sensors in the present.

Think of the long term fuel trim is just a historical average of what the ECU has been doing. For the 95, as long as Banks 1 and 2 are equal, there’s not much more info to be gained. If they’re not equal, you have an exhaust leak or clog.

A reset of the LTFT would have zero impact on anything your car does. It’s only an after the fact calculation based on STFT.

 

Unplugging the oxygen sensors will force the ECU to go open loop, where it fuels the engine based on a preset program, which is rather inefficient on fuel.

Unplugging the air temp sensor should prevent the car from going closed loop, so you’d be back to square one above.

Either way, you wouldn’t notice a change, the engine might actually appear to run smoother, but you’d smell a gassier exhaust and you’ll be running to the gas station more often.

 

ok thank you vee; i was looking for the answer of these questions because of the problem i have of the engine hickups when cruising 60 mph, without accelerating or decelerating. when there's minimum power needed; the engine hicks; as if sometimes a cilinder is not firing. but not very noticable. it only does it when the engine is warm.

PatrickO told me to check the transmission fluid; and it is kind of low. will top up tomorrow; but to be honest; it more feels like a gasoline problem in stead of a transmission problem.

thanx, pim

 

Might be a coil beginning to fail

Also, a torque converter locking or unlocking can sometimes feel like a slight misfire. At times you can have the engine speed/engine load/road speed conspire against you. If the hiccough goes away by driving a couple MPH slower or faster, it might suggest that you're feeling the TCC

Cheers
DD

 

hi i topped up the transmission fluid; but unfortunately the problem persisted.

i unplugged the 2 o2 sensors from the exhaust manifold; drove it and the hickups were gone... so what does this tell you guys??

thanx, pim

 

As I mentioned before, by unplugging the oxygen sensors, the engine will be running very rich because it is being fuelled according to a preset program, not actual driving conditions.

I've had the oxygen sensors get "lazy" on my car. You could start by replacing the oxygen sensors....

 

Hi vee; i read your answer about the o2 sensors; and that the engine is running rich without it; does that mean my engine with these sensors might run lean? That would be strange, because now, with sensors i cant get it more economic than 18 mpg; . I thought the aj16 would be capable of doing 23-24 mpg when driven like a grandma does. is there another way of testing the sensors?
or might it be a vaccuum problem somewhere?

thanx again