Somewhere around 40 is where the engine braking comes into play. Above that, let off the gas too long and the engine will start slowing the car.

 

Hi Woo5ie, thanks for the comments. I ran manual Saabs in the 70's, which coasted in overrun, and I thought that was a good idea (my 'green' contribution). Ever since, particularly with automatics, I've felt the need to let them coast on a straight run when possible. So, since 2006 with my XK8 and the last year with my XK I have regularly slotted it into neutral when I no longer need to push the car. As the XK8 was happy (?) to be subjected to this for 10 years I had assumed that I could use the XK likewise! For 11 months this was the case. These problems only materialized in the last month or so.

 

Nigel, you are in luck. I did this test last week for another board member who also has the misapprehension that putting a car in neutral is safe. Its not for many reason, and in England they would never give us a diver's license if we did that on the test.

In any case, when you are at 30 and you put it in N, you are only dropping if 400 rps or so. At 70 if you drop it in N, you are asking your engine management to jump in quickly and stabilize the situation from 2500rpm. Here is exactly where it is failing you or worn out.

It is the same reason if you take your foot off the gas at 70, the autotrans doesnt automatically drop it in N. They could do what you are doing in the software, but they dont for good reason. And believe me there is an army of engineers trying to find a fraction of extra fuel efficiency.

 

Hi Q&C - I did not make any reference to safety aspects; obviously without a built-in freewheel it takes a fraction of a second to slot it back into drive, but on empty roads, downhill, if I'm not in a hurry...
The early Saabs that I drove, in England, had the built-in freewheel which effectively disconnected engine from drivetrain on overrun.

The issue of engine dying when I chose to drop into neutral is annoying but easy to overcome. Your suggestion of magnitude of rpm drop sounds plausible, any idea which sensor failure could be causing this?
The engine dying when I brake and turn sharply is a more serious issue as the power steering turns off just when it's needed. I'll have to experiment and see if this is also rpm-drop related.

 

Got home last night, disconnected, shorted for over a minute, reconnected and went for a short drive - sufficient to observe that the issues are still there. Thanks for the suggestion anyway.

 

QC and I have different engines.

His maintains a higher RPM when rolling in neutral, mine does no. Mine drops to the same idle it does at a stop light.

At 70MPH, I would not go down a long downhill in neutral, as I suspect the transmission fluid pump is driven by engine RPM. The transmission is still spinning at speed, yet the pump is not providing corresponding pressure.

I've never actually checked how the system works to see how the pump is driven. Cars that are designed to automatically disconnect will have a design to ensure there is still plenty of flow in this situation. If instead it is driven by the output, or an auxiliary, then it isn't an issue.

Aside from the possibility of less flow at idle, there isn't going to be a ton of bad from doing what you're doing. For a lot of situations though, you will get better MPG throttle closed

On the down hills at speed, I just let the car slow down and make it up again on the up hill.

Given that I do not expect most XK's will not be desirable at 200K+ miles, in the big picture I don't really see it being an issue whether you do this or do not do this. Big picture, you had your last car for ten years, you should be fine having this one for ten years, and what worked for your last should work for this.

 

Different for your 5.0 from my 4.2. Mine drops to regular idle. Yours evidently keeps the RPM higher. The higher RPM when rolling would keep better transmission fluid flow if it is engine input side driven.

I mostly(and still do) go to neutral in stop/go situations, and take the stop/go out of it by not stopping.

Without knowing how the transmission fluid circulation pump is driven I wouldn't want to do this for sustained long downhills at interstate speed.

 

Hi,

Here is the logic why putting a car in neutral is a bad idea on many fronts. And why your automatic transmission doesnt do it automatically for you. And why its dangerous.

There are so many illogical downsides that one has to seriously reconsider why they do it in the first place.

A jet landing on an aircraft carrier has to increase engine speed to full throttle, so as to be prepared for any change.

Conversely; a car that has been taken out of gear on the interstate, has diminished steering and braking because accessories are turning at standstill speeds. Add to that that it will take you 4x longer to get back in gear and up to engine speed to swerve to avoid an animal on a collision course.

Here is something important regarding the last point that can save your life.
Momentum only travels in straight lines. A ball or a car rolling downhill will lose 100% of its velocity at 90 degrees of deviation to the angle of travel. And 50% at 45degrees. What means is that if you swerve 22.5degs left and then 22.5degs right, you will go from 70mph to 35mph, regardless of the car you are in. You could be on a flying carpet and those are the laws of physics.

You are doubling if not tripling wear compared to an engine traveling at a steady rpm- which practically has no wear. Not only are you creating stop and go conditions, when load is abruptly disconnected from an engine, the residual energy has nowhere to go. Like punching a punching bag that is abruptly removed.

When you re-engage the transmission, you have needless wear. The shift knob is also going to wear out faster.

AND the Idle Air Controller will also wear at a 2X+ rate.

 

+1

 

Ok, got the ELM327 set up with Torque Pro this afternoon, No Faults in the ECM.
Thought maybe it wasn't working but the performance gauges all work great. Went out to prompt engine stalling, which it did, but still no codes.
By the by, contrary to my expectations, the engine will restart by pressing Start/Stop twice, without any need for Brake pedal depression.
I'm beginning to wonder if it isn't a physical issue rather than an electrical one!

 

By way of an example, a sudden air leak during starting/idle can cause the engine to cut out.

In my experience on a jag of this sort of era you'd get a flight data recorder code (P1582) with the details (which a typical tool won't show you, sadly).

However, if (say) the ignition appeared to the car to have been switched off then that code would probably not be stored because all would appear normal.

 

Funny, this popped up in my youtube feed.

 

+1. I don't get the point of shifting to neutral either.

 

In my 4.2XK owner's manual there is a section suggesting situations where the owner should shift to neutral while using the car.

It may be different for 5.0 XK owners.

 

Yes- it says

"N = Neutral - Disconnects drive to the wheels. Use the parking brake when stopping temporarily."

Nothing to do with what you're doing.

 

I don't understand. I have real concern that my engine is cutting out, causing loss of power steering when braking into a turn. I will not permit my wife to drive with the car like this as the steering becomes very heavy. Coincidentally this also happens if I engage neutral while travelling in a straight line.
I have asked for help/suggestions with determining the cause of this issue but the predominant responses in this thread seem to suggest that I am an idiot for engaging neutral on the Interstate. I had honestly expected better!

 

Nigel,

Although it was said by others, I can elaborate to explain.

There are many instances where a car would naturally stall. Such as those very scenarios you described. Where the engine needs more throttle than you are giving it. In those instances the car's computer takes over via a device called the Idle Air Controller and increases the throttle on your behalf.

We believe that it is buggered up or the inputs its receiving as to how much air is coming in and where the throttle position is could also be wrong.
For instance, you car need the throttle plate to be open 5% to remain idling, if the throttle position sensor is sending a message that it is in the 5% position when its at 0, the IAC will do nothing and the car will die. Or if the TPS is correct and the IAC is malfunctioning the car will die. Also in this equation is the mass air flow sensor.

Do the following, clean the MAF, it needs to be done anyway, clean the throttle plate, it needs it too. Then if probs persist look into IAC.

 

Yes, your topic has been hijacked by silly banter about shifting to neutral while in motion, I apologize.

Back on track, under no circumstance should the engine quit until commanded to do so by the driver. Other than going after what's already been suggested, I'm stumped.

 

Just to be perfectly clear you are saying the engine turns off (0 RPM on the tach) at the conditions you describe. Do you have to restart the engine or does it restart by itself. If this is the case and you do not have an engine fault code then your best move would be to take it to a dealer and have them run their Jaguar diagnostics.
By the way do we assume you are a hyper-mile'er type . Somehow I don't think these transmissions are made that way.

 

Sorry if I missed them but what are the fuel trims, especially the LTFTs:
1. at idle (hot engine, parked)
2. just before/at a stall (probably need a passenger to watch them)

Trying to figure out if it's a (perhaps sudden) air leak.

I think you should have P1582 and it may well be worth decoding.