98 XJ8 216k. The part-load breather orifice was clogged and there was oil in the intake arm and throttle body. I unclogged the orifice and cleaned up the arm and the throttle body. Now it is doing what is captured in the below video. It ran fine before I did any of this. The video is in park, but it is also doing this in drive and reverse, which is causing the tranny to act up. No codes.

I tap the gas pedal only twice in the video. The first time is to show what happens at or below 1500rpm and the second time is to show what it does if revved over 1500. I am only on the gas for about a second or less each time, everything else is happening without any further driver inputs. In terms of throttle readings, my OBDII software only shows absolute throttle position (which is the same as before). A hard reset did not help.

 

Hanging idle? Typically the result of a vacuum leak, or an air leak behind the MAF....unless you cleaned everything out real good with TB Cleaner and there's still some pooled somewhere in there like in the manifold, etc...

From the few times that I've had to disconnect my battery, the car ran real funky for a few minutes before it re-learned the idle, and the transmission took longer to get back to normal...around 150 miles of commuting but with only a handful of stop-starts each day.

 

Thanks Jaggin, those were my first thoughts, but I don't see how I could of introduced a new vacuum leak. I have taken it all apart a few times to double check and to verify that there is no carb cleaner getting sucked into the intake. It is totally clean.

I have driven it a little over 40 miles, with multiple starts and stops, and the problem has not changed at all in any way, I would think any excess carb cleaner would be gone by now. I was thinking that I may have disturbed the TPS or done something else to the throttle body.

When it is coming down from under 1500rpm it looks like a typical hanging idle. From above 1500 it is a little different because it comes down and then bounces back up before hanging for a while. I am nervous about continued driving with this condition because it confuses the tranny and there is a harsh thunk when I come close to a stop and it starts revving itself up again. I can usually avoid this by popping it into neutral just before it starts revving itself back up.

I will go back out and check fuel trims in a few minutes.

 

I think I must of screwed up the TPS

Fuel trims are fine. Plus, they do not go negative when it revs itself (I think they would if it was sucking carb cleaner into the intake.) To me, this rules out a vacuum leak or excess carb cleaner.

I noticed it seems to keep the rpms a little high while I am driving it. I viewed absolute throttle position while driving and I believe it points to the problem. If I put it in drive and take my foot off the brake it feels normal, rpms are normal, and absolute throttle position is zero. If I rev it a little before taking my foot off the gas the rpms stay a little high, the car pulls more then usual and absolute throttle position remains between 3.9 and 4.3%.

The throttle cable is good and the pedal is not getting caught on the carpet. I am going to do some more tests in park in the morning. I think I must have damaged or thrown in off the TPS when I was manually opening and closing the throttle plate while cleaning it.

I don't like the idea of messing with the TPS, but if I do not get any other suggestions I will try adjusting it in the morning because I am out of ideas. (except that I might have done something else to the throttle body)

 

The 98 has a vacuum hose on the throttle body for the cruise control. Is it still connected properly?

 

If that is the hose at the top of the throttle body with the elbow connection, then yes, it is good. Thanks for the suggestion.

 

If you have not tried the old "reboot", then do so. Otherwise, you need to really look good at what you might have dislodged or split like the gasket ring where the plenum meets the TB, and the full load breather tube connections.

 

Did you look closely at the part load breather tube? I broke mine when I took it off to clean the breather.

 

Thanks for the ideas. I have taken it back apart and checked everything a couple of more times this morning, everything looks good (including both breathers and the plenum gasket). I am relatively certain that this is not a vacuum issue because my fuel trims are fine.

When looking at absolute throttle position on the laptop it seems to track the problem perfectly. It is always between 3.9 and 4.3% when I am experiencing the symptoms. When the idle returns to normal it always goes right back to 0%.

I tried a few adjustments with the idle control screw, but I was unable to make any improvements. I have done 2 hard resets since yesterday.

Unless there is something else I could of done to the throttle body or the throttle body plate, I think my next step is to try and adjust the TPS. I have removed the cover and adjustment looks pretty straight forward.

I am going to take an hour break and then try the adjustment if I don't get any other suggestions.

 

p0507 just popped up. That seems consistent with what is going on, does it point towards the TPS or something else?

 

*OHHH, you opened and cleaned the throttle blade manually? Crap, I didn't catch that part. You're never supposed to do that.

I remember when fly-by-wire throttles were introduced in some of the German cars and someone in my company did a decarbon service and opened it manually, and the car had to go back to the dealership to have the throttle reprogrammed.

I've been told it's harmful to the actuators, and if you actually power the key on and use the gas pedal to open the throttle and feel it with your fingertips, you can feel it vibrating, like it's checking input 10x per second. I fear you may need a throttle reprogram or a new throttle assembly.

Try this first though... (I have ZERO proof that this will work, but having spent the better part of the last 10 years working on cars, this is what I would do)...

With the ignition off, reset the idle screw to its factory position as close as you can possibly get it. You're better having it a touch too low than a touch too high, because the computer can pick the idle up with the IAC valve. What it *can't* do is lower the idle with a screw that's too far open.

Power the ignition on ONLY and don't touch the gas pedal. Give it a second, then power the ignition back off again. Now disconnect the battery and leave it disconnected for maybe 20 or 30 minutes, long enough to hard boot it. Now reconnect it. Power the key On. Power the key Off. Power the key On again. With the Key in the On/Run position but with the engine OFF, slowly depress the gas pedal all the way to the floor. Hold it there for a second, then slowly release it all the way off. Power the key Off. Power the key On, give it a second, and Start the engine.

Now see what it does. If I were trying to "teach" the computer what 0% is and what 100% is, that is how I would do it.

 

Thanks you very much Jaggin. You just turned around a bad day for me. The worst part is that I remembered about the fly by wire throttle bodies not being opened manually when I read what you said; next time I will try to remember before I do something stupid.

If someone else has this same issue, this is what I did:

-turned it to the run position for a few seconds, then turned it off (did this twice)
-turned it to the run position and slowly pushed the pedal all the way, and then slowly released it all the way, while still in the run position I repeated the same pedal movement
-turned it off, then I think I put it in the run position again and then turned it off and fired it (but I might have just fired it)

Problem solved

Thanks again Jaggin

 

I wasn't so sure it would work, but I figured moving the blade manually may have caused the computer to lose what its "default" position is.

So I thought about how you retrain auto-up/auto-down power windows, and figured why shouldn't that work for a throttle body as well.

Happy for ya.

 

216k wow! glad to see you got it fixed.

 

I am glad it got fixed, but a few points:
The AJ26 engine IS NOT "throttle by wire"- it has a mechanical throttle linkage (the AJ27 is). That said, it does have redundant TPS pots, so if the stated method did indeed fix it, and it seems it did, then there must be some safety check and lockout of the two signals that the reset fixed.

 

I should have been more specific... You are correct, there is a mechanical linkage, and it *seems* to be tied into some sort of fly-by-wire type of system...it's pretty unusual. I know that I cleaned mine with the key powered on and the pedal held to the floor with a prop-rod, and as I wiped the throttle blade gently with a lint-free towel and TB-Safe cleaner, I could feel it constantly vibrating, as if it were linked to a servo that was a two-way, active feedback system or something.

I figured maybe it was a primarily drive-by-wire system with a redundant, mechanical linkage for backup...or some other sort of dual-system.

I'm not an expert on Jaguars, so please, correct me where I'm wrong. Knowledge is power.

 

Actually, neither of us was completely right (or wrong!).
The AJ26 engine has a "electonic throttle with mechanical guard". Essentially, the ECU can command a position, but no greater than what the pedal allows, AND, to your original point, it can shutdown for a problem.

According to JTIS:

1998 XJ RANGE - Intake Air Distribution and Filtering - 303-12
Engine shut-down
Normal mode occurs when the ECM uses the mechanical and monitoring arrangement of the throttle valve to control throttle opening. The ECM does not permit driver demand to be exceeded but it can be restricted to allow for such features as stability! traction control, or engine power limitation.
The ECM determines engine idle speed by controlling the throttle valve motor to vary the blade angle between the non-adjustable preset limits of the mechanical guard and the throttle valve motor.
Cruise mode is engaged as a result of the ECM calculating and controlling the required throttle valve opening via the vacuum system. When the driver releases the throttle pedal, the input shaft disengages from the mechanical guard or the vacuum actuator pulls the guard away from the throttle valve. The throttle pedal will feel light should it be pushed again to accelerate (pressing the pedal further will re-engage the input shaft with the mechanical guard and restore normal feel).
The ECM utilizes sensors to monitor the relative positions of the mechanical guard and throttle valve and adjusts them to maintain the set cruise speed.
Mechanical guard mode permits full mechanical operation of the throttle if the ECM detects that a problem has been encountered with the throttle valve position sensor, dc motor, associated harnesses ! connectors or the ECM.
Fixed idle mode occurs when any two of the three sensors (two input shaft sensors and the mechanical guard sensor) fail. The ECM will assume values which represent a blade angle of approximately 2,5° and 1200 rpm (unloaded) maximum engine speed.
Redundancy mode occurs when any one of the three sensors (two input shaft sensors and the mechanical guard sensor) fails. The operational pair will be deemed to be safe to continue without intervention, but cruise will be inhibited.
Full authority mode is invoked when a mechanical guard failure occurs which indicates that the guard is stuck fully open. The red warning lamp will be lit and road speed will be limited to 120 kph.
Engine shut-down mode will occur following multiple failures, such as mechanical guard mode following full authority mode (or vice versa) or the throttle blade sticks.

So, I bet your analysis of what happened was spot on!

 

That makes some sense for my issue Ross. (deviating from original post) Great result!

My cruise has gone intermittent. It may work once or may not. If the cruise button is left depressed/on, the Failsafe will come on. So this points to the TPS related part of the JTIS

Thoughts Please?

 

Ross, great and useful info there. So many automotive forums I've found....people spend their time arguing eachother, instead of working collaboratively to resolve an issue or learn something new.

As you stated, neither of us were 100% wrong, nor 100% right.. I knew there had to be *some* kind of fly-by-wire type of setup when I felt the throttle blade "buzzing" away because nothing else could do that except a servo, and when I engage the cruise control and release my pedal, I can feel the pedal return with my foot even though the speed is maintained...

Neil...are you able to pull data with a scan tool?

I would pull the TPS voltage or percentage, and if possible, put your tool into graph mode and with the Key On Engine Off, depress slowly from 0% to 100% and back again, noting peak percentage or voltage, and verify that you have a steady pyramid on the graph. That will at least tell us that the TPS data being reported to the computer is correct...

I'm still hoping that no damage was done to the servo.....

 

Unfortunately, an OBD scan tool only reports one reading from the TB. And there are five sensors involved; one "mechanical guard", two for pedal position and two for throttle position. The pairs of pots operate in opposite polarity - onje moves from positive to negative hile the othe moves from neagative to positive. It might be better to use a good old voltmeter for testing.

BTW, Jaggin - your MY 01 does use "fly by wire" with a mechanical override for failsafe.