[xj40] Engine , gearbox or torque converter?

I really don�t know if this is an engine, gearbox, or torque
converter problem on a V12 Auto 1988.
When traveling with no throttle above 40 MPH when hot and 30MPH
when cold the car exerts a breaking effect (ie is slows down
almost as if the car had changed down a gear) once 30 (or 40 mph)
has been reached the car free wheels as normal. The point at which
this occurs is very precise. If I put the car into neutral while
getting this effect the rev counter is oscillating between 300 and
1500 rpm as soon at the minimum effect speed is reached the car
idles as normal.
Holding the box in 1st or 2nd has the same effect accept that the
speed at which it occurs are lower.
Only other clue (which may be related) is that when the car is
starting (not being driven) from cold the engine oscillates between
300 and 1500 rpm (until it is warm).
Have checked the governor gear and that is fine–
richardv12
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In reply to a message from richardv12 sent Sun 27 Mar 2005:

the idle speed should be around 700rpm and if I was you i
start by checking that you don’t have a vaccum leak some
wheres. and a point to note its not a good idea to put an
auto box into neutral while its moving.
macg–
The original message included these comments:

this occurs is very precise. If I put the car into neutral while
getting this effect the rev counter is oscillating between 300 and
1500 rpm as soon at the minimum effect speed is reached the car
idles as normal.


'92 3.2 sov lhd (blue car) '85 bmw 323 lhd (black car) '88 m
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In a message dated 3/27/2005 6:37:13 AM Eastern Standard Time, “richardv12” richard@1stfuture.co.uk writes:

I really don�t know if this is an engine, gearbox, or torque
converter problem on a �V12 Auto 1988.
When traveling with no throttle above 40 MPH when hot and 30MPH
when cold the car exerts �a breaking effect (ie is slows down
almost as if the car had changed down a gear)
If I put the car into neutral while
getting this effect the rev counter is oscillating between 300 and
1500 rpm as soon at the minimum effect speed is reached the car
idles as normal.
Only other clue (which may be related) is that when the car is
starting (not being driven) from cold the engine oscillates between
300 and 1500 rpm (until it is warm).

Hi Richard:

You might want to clarify what model & transmission you have – is it an XJ-S with the 3-speed GM TH400? I didn’t think the V12 was put in the 1988 XJ40.

If it IS the TH400, the only thing I can think of that might cause BOTH a transmission shift problem and idle hunt would be the vacuum hose from the rear of the right intake manifold to the modulator mounted on the right front side of the transmission. If this hose is loose, or has a hole in it, your shift points are affected.

If the idle speed is hunting between normal & 2k RPM, you definitely have a vacuum leak. If you have white smoke coming from the right exhaust pipe, you may have a ruptured modulator diaphragm allowing transmission fluid to be sucked into the right intake. Check the vacuum hose for transmission fluid.

George Balthrop, Clifton, VA USA
85 & 89 XJ-S Coupes; 89 XJ40 VDP

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