[xj-s] 90 V12 loses power under load when warm

XJ-S
1

When my 1990 XJS V12 gets warm it starts to miss under
acceleration, the more I drive it the worse it gets until it will
hardly go 50 mph. When first starting out is seems fine. In
neutral it will reve easily and smoothly to over 5000 rpm. In the
first few minutes it will accelerate without missing, to over 5000
rpm, or close to 100 mph before the shift. But as I drive and the
temp goes up to the N, it gets very balky when you try to
accelerate over 3000 rpm, eventually even 2000 rpm. The car is new
to me, but has new thermostats and supposedly new plugs, but I’m
not sure what else if anything. Could the temp sender on the left
side (I think it sends signal to the fuel control) be the problem?
It acts like fuel starvation, but maybe not. It will get so bad
that if you barely push on the throttle, it will immediately balk,
like the fuel or ignition is just shut off. As soon as you let up
on the throttle it runs smooth. The point at where it will run
smooth though, keeps getting slower.–
apache2003p
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2

First off, put the year, model and engine info with your name. Use
the ‘Edit your profile’ to do that.

Next, take your distributor cap off and see if there are any telltale
markings on the inside of the cap. At any rate, clean it good. Then
check the ventilation of the distributor. There are two nipples at
the back side, base of the distributor. Follow the hoses and make
sure they are clear by blowing in the end that has the little air
filter. There should be no restriction whatsoever and the air should
exit into the left hand air cleaner box, the round hole behind the
air filter.

If not remedied then go for starved fuel. The thermostat on the left
side and the O2 sensors, both sides. You should use contact cleaner
from Radio Shack on all three connectors. Generously spray them and
then hook them back up. Don’t worry about excess remaining. The
cleaner is both a dielectric and a lubricant and eliminates contact
resistance.

Next, you want to check out the entire cooling system. Even in hot/
warm weather for summer, the temp should stay below the N in the
evenings when temps drop into the hi seventies.On May 12, 2012, at 11:01 AM, apache2003p wrote:

When my 1990 XJS V12 gets warm it starts to miss under
acceleration, the more I drive it the worse it gets until it will
hardly go 50mph. When first starting out is seems fine. But as I
drive and the temp goes up to the N, it gets very balky when you
try to accelerate over 3000 rpm, eventually even 2000 rpm. The car
is new to me, but has new thermostats and supposedly new plugs, but
I’m not sure what else if anything.

It acts like fuel starvation, but maybe not. It will get so bad
that if you barely push on the throttle, it will immediately balk,
like the fuel or ignition is just shut off. As soon as you let up
on the throttle it runs smooth. The point at where it will run
smooth though, keeps getting slower.

As long as idle shows no firing miss at all, absolutely no change,
hot or cold!, fuel is the best choice for the problem.

Ptipon
Modesto/CA, 90 XJS-V12 conv, United States

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3

In reply to a message from Paul Tipon sent Sat 12 May 2012:

Thanks, I’ll try all that & get back to you to let you know results.–
Larry Philyaw 1990 XJS V12 Conv
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4

In reply to a message from apache2003p sent Sat 12 May 2012:

The ignition system on all cars abhors heat. your symptoims tell
mme you are loosing cylinders as the enmgine and underbonnett heat
goes up. The resistance on old HT leads goes up with heat. Coils go
weak or just out with heat.

If these HT leads have been in that car for long, bet on it, they
are toast.

Run the car at night and see if you get a light show under the
bonnett.

I am a stickler for sound ignition producing s fat blue spark under
any condition at the plugs. Nothing less will suffice.

Carl–
Carl Hutchins 1983 Jaguar XJ6 with LT1 and 1994 Jeep Grand
Walnut Creek, California, United States
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5

In reply to a message from cadjag sent Mon 14 May 2012:

An update on the missing/lack of power problem:
First inspected Distributor cap and rotor. Both looked good, no
signs of arcing, dust, etc. Distributor contacts & rotor cleaned
up (didn’t need much) with emery cloth. Vent tubes on distributor
base clear & functioning properly. Thermostat on left side cleaned
with contact cleaner, O2 sensors didn’t want to come out, but I
don’t see how dirty sensors would cause fuel starvation. I would
think they would make it run rich. Plus, it runs smooth when cool
or not under load. After car warms up, right side exhaust is hot,
so apparently is not the classic Marelli failure. Pulled a couple
of plugs from each bank, (Motorcraft AGSF 22C) and other than the
fact they were installed by a gorilla with a 2’ breaker bar and no
anti-seize, were a nice grayish brown, no residue.

This morning I re-tested it with the following results:
Ambient temp 75F
Idle after starting (in park) 1150 rpm. After about 1 min, 1300
rpm. Revs easily to 6000 rpm (no load) without missing or
hesitation.
After coupl minute warm up (gage just off C) very good acceleration
up to about 4500 (75 mph in 2nd) then some minor missing to 5000.
After 8 min, gage just below N, missing now starts at 3000 rpm.
Heat gun showed Rt cat 340F, left cat 350F.
Drove in 2nd at 4000, for several minutes, runs smooth until
pressure applied to throttle, then will miss. by ‘‘feathering’’
throttle could coax it up to 4500. Eventually got gage up to
barely above N, could coax it to 90 (3500) in 3rd, but any pressure
to accelerate would cause missing. Sudden pressure on throttle
would almost turn the engine off. Almost like timing was suddenly
too far advanced.
Drove it for 30 minutes, and when stopped, left hose to thermostat
housing measured 190F, rt hose 195F, back of radiator 150F.

cadjag may have something with the leads and/or coil getting hot
and degrading spark. I will try tonight to see if I have a light
show. Meanwhile, all suggestions from you V12 sleuths are welcome.
As I am still learning about this car, I don’t know if there is any
relation to my problem but the cruise does not work (bellows checks
ok), AC does not work (have not looked into that yet) and
the ‘‘rochester’’ valve is probably shot since my fuel gage is stuck
in one place & I have 3 gallon less tank capacity than the book
says I should have.–
The original message included these comments:

The ignition system on all cars abhors heat. your symptoims tell
mme you are loosing cylinders as the enmgine and underbonnett heat

–
Larry Philyaw 1990 XJS V12 Conv
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6

Drove in 2nd at 4000, for several minutes, runs smooth until
pressure applied to throttle, then will miss. by ‘‘feathering’’
throttle could coax it up to 4500. … but any pressure to
accelerate would cause missing. Sudden pressure on throttle would
almost turn the engine off. Almost like timing was suddenly too
far advanced.

Due to the throttle sensitivity, I would say that it seems to be
definitely throttle sensitive, air sensitive, not timing. Also with
the fact that feathering the throttle gets higher rpms. Definitely
sounds fuel related.

When you feather the throttle and get higher rpms, can you hold the
throttle constant and keep the speed/rpms constant? If not that is
also a fuel problem, not ignition.

Meanwhile, all suggestions from you V12 sleuths are welcome. As I
am still learning about this car, I don’t know if there is any
relation to my problem but the cruise does not work (bellows checks
ok), AC does not work (have not looked into that yet) and the
‘‘rochester’’ valve is probably shot since my fuel gage is stuck in
one place & I have 3 gallon less tank capacity than the book says I
should have.

Don’t know if these could contribute or not.

Ptipon
Modesto/CA, 90 XJS-V12 conv, United States

// please trim quoted text to context onlyOn May 15, 2012, at 8:43 AM, apache2003p wrote:

7

In reply to a message from Paul Tipon sent Tue 15 May 2012:

Paul, it does seem to be very air-sensitive. When it’s at it’s
worse, when you press on the trhottle it’s as though the smallest
increase in manifold pressure just cuts the fuel off – or at least
way back. However, I can’t figure why a failed valve would only
cause this when hot, but then, I don’t know what all those valves
do, exactly. It will hold the rpm once you attain it, but will
drop immediately upon attempted acceleration. I am pretty sure the
HT leads and coils are original equipment, and I can see that being
a problem when things get hot under the hood, but you wouldn’t
think it would just shut the engine down, like what happens when
it’s really warm & has been driven for a while.
Perhaps I have more than one problem here.–
The original message included these comments:

Due to the throttle sensitivity, I would say that it seems to be
definitely throttle sensitive, air sensitive, not timing. Also with
the fact that feathering the throttle gets higher rpms. Definitely
sounds fuel related.
When you feather the throttle and get higher rpms, can you hold the
throttle constant and keep the speed/rpms constant? If not that is
also a fuel problem, not ignition.

–
Larry Philyaw 1990 XJS V12 Conv
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8

In reply to a message from Paul Tipon sent Tue 15 May 2012:

Paul, it does seem to be very air-sensitive. When it’s at it’s
worse, when you press on the trhottle it’s as though the smallest
increase in manifold pressure just cuts the fuel off – or at least
way back. … It will hold the rpm once you attain it, but will
drop immediately upon attempted acceleration.

This is definitely a fuel problem. The ECU senses a fast throttle
change and also adds more gas for any kind of acceleration. This
sounds like the problem.

You can check the ECU function by turning the ignition on without the
engine running and quickly twist the capstan in the middle of the
engine that has the two rods on it that go to the butterflys on
either side of the engine. When you do this you should hear all of
the injectors pulse each time you do a quick twist which simulates
increased throttle.

If you don’t get this function then you will have to check out the
throttle position sensor under the capstan. You can search the
archives on how to test and calibrate that by looking for TPS
subjects, diagnosis, repair, replacement, etc.

Ptipon
Modesto/CA, 90 XJS-V12 conv, United States

// please trim quoted text to context onlyOn May 15, 2012, at 9:54 AM, apache2003p wrote:

9

In reply to a message from Paul Tipon sent Tue 15 May 2012:

I just tried that, and almost immediately upon opening the throttle
I hear the click. I can check the throttle position sensor,
anyway. Is there a reason this would only happen when it is hot?–
The original message included these comments:

This is definitely a fuel problem. The ECU senses a fast throttle
change and also adds more gas for any kind of acceleration. This
sounds like the problem.
You can check the ECU function by turning the ignition on without the
engine running and quickly twist the capstan in the middle of the
engine that has the two rods on it that go to the butterflys on
either side of the engine. When you do this you should hear all of
the injectors pulse each time you do a quick twist which simulates
increased throttle.

–
Larry Philyaw 1990 XJS V12 Conv
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10

In reply to a message from Paul Tipon sent Tue 15 May 2012:

I just tried that, and almost immediately upon opening the throttle
I hear the click. I can check the throttle position sensor,
anyway. Is there a reason this would only happen when it is hot?

That I don’t know. Looks like we are down to the either the vacuum
sense going to the ECU and/or the temp sensor on the left hand B
bank. That sensor is temp/fuel related.

Lets see if someone else chirps in. Preferrably George Balthrop.

Ptipon
Modesto/CA, 90 XJS-V12 conv, United States

// please trim quoted text to context onlyOn May 15, 2012, at 12:13 PM, apache2003p wrote:

11

In reply to a message from Paul Tipon sent Tue 15 May 2012:

Paul,
Just tested the TPS, .12v red wire idle, 4.58v wot; 5.0v green
wire, and .02v yellow. I realize the idle voltage is too low, but
couldn’t find any info on how to calibrate it. Nor do I understand
why that would be causing the symtoms I have, but then there is a
huge amount of stuff I don’t understand about the systems.
Is the temp sensor the one located on the left thermostat housing?
Is there a way I can test it?
Which vacuum sensor feeds the ECU? And is it testable?
Thanks.–
The original message included these comments:

That I don’t know. Looks like we are down to the either the vacuum
sense going to the ECU and/or the temp sensor on the left hand B
bank. That sensor is temp/fuel related.

–
Larry Philyaw 1990 XJS V12 Conv
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12

In reply to a message from Paul Tipon sent Tue 15 May 2012:

Paul,
Just tested the TPS, .12v red wire idle, 4.58v wot; 5.0v green
wire, and .02v yellow. I realize the idle voltage is too low, but
couldn’t find any info on how to calibrate it. Nor do I understand
why that would be causing the symtoms I have, but then there is a
huge amount of stuff I don’t understand about the systems.
What you do is loosen the pot from the mounting plate and rotate it.
I set it at idle for .32 to .36volts between the red & yellow leads
and then tighten it down. You can also check once everything is
together by sliding the meter leads into the connectors or use some
paper clips slid in. It is also good to check it with everything
assembled and on the engine.

It’s possible that with a low starting voltage, the voltage at WOT
may not be as high as it needs to be. Once again, check this with
everything on the engine ready to run.

Is the temp sensor the one located on the left thermostat housing?
Yes

Is there a way I can test it?
If you check the archives there are resistances per coolant temp.
Then you can go to Radio Shack and get some resistors to place across
the connector that will then give the correct resistance at temp. Of
course you can always measure across the sensor, engine cold and then
hot to see what you get.

Which vacuum sensor feeds the ECU? And is it testable?
Don’t know what to do to test this, never have.

Ptipon
Modesto/CA, 90 XJS-V12 conv, United States

// please trim quoted text to context onlyOn May 15, 2012, at 1:23 PM, apache2003p wrote:

13

In reply to a message from Paul Tipon sent Tue 15 May 2012:

You set the TPS to .32-.36 volts measuring across the red and
yellow wires.

You can check the coolant temp sensor by measuring the
resistance and comparing the reading to a chart in the
service manual.

Also if it is starving for fuel the obvious thing to start
with is fuel pressure. You need to measure fuel pressure when
the problem occurs. if it starts dropping you have a definite
supply problem. Could be as simple as a partially clogged
fuel filter.–
Greg 1985 XJS HE DANA rear
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14

In reply to a message from Greg Edge sent Tue 15 May 2012:

Do you get a nice, smooth sweep up in voltage as you move the TPS?
The temperature-dependent thing has me wondering if due to normal
thermal expansion the TPS wiper isn’t losing contact as it warms
up. Check your TPS voltages both cool and when it’s acting up.–
Darel
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15

In reply to a message from cadjag sent Mon 14 May 2012:

Well, even though distributor cap & rotor, & plugs looked good, I
replaced them all, plus the HT leads. Also replaced the coils
while I was in there. The problem persists, will run and rev good
until it starts to warm up. As temp gets up to Normal, doesn’t
want to accelerate under load.

A new symptom is that when it starts acting up pretty good, I can
hold the rpm at 4000 in nuetral (running smooth but under no load)
and with the throttle perfectly still, it will suddenly start
backing off the revs, sometimes almost like you shut the ignition
off.

I ran it until problem started, then put ice on ignition amps to
cool them, and it seemed to help, although didn’t completely cure
the problem. I guess I will try replacing the amps (ouch!).

After that, I suppose there is the TPS, which testing showed right
on when at WOT, but low at idle. I just can’t see how that would
relate to a heat-sensitive issue.–
The original message included these comments:

The ignition system on all cars abhors heat. your symptoims tell
mme you are loosing cylinders as the enmgine and underbonnett heat
goes up. The resistance on old HT leads goes up with heat. Coils go
weak or just out with heat.
If these HT leads have been in that car for long, bet on it, they
are toast.

–
Larry Philyaw 1990 XJS V12 Conv
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16

In reply to a message from apache2003p sent Tue 29 May 2012:

I also have a 90 and had a sismlar problem turned out to be the cts
coolant temp swith has differet ohms or resistance reading as temp
warms up and sends signal to the fueling computer car was running
way to rich and flooding it ws almost like it was running on 6
cylinders.There are two of these temp switches on these cars one on
the left water rail at thermostat and one on the right the one
signals ignition and timing ecu the other the fueling ecu in trunk
the one that caused my problem was the one on right if you are
seated in the car or passangers side the oem sensor was green with
injector type connector you can buy either of them for less than
30.00 bucks Motorcars ltd or Welsh enterprises both stock them they
are two diffierent parts and different part numbers .I can get the
part numbers if you need as I have a parts manual.let me know.may
not be your problem but has the same symptons that mine had 35.00
switch solved it.–
T Boy
Raleigh NC, United States
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17

In reply to a message from T Boy sent Tue 29 May 2012:

They are coolant temp sensors not switches. Switches would be
on and off. Sensors will have a varying resistance depending
on temp. You can test the sensors by comparing them to a
chart in the repair manual.–
Greg 1985 XJS HE DANA rear
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18

In reply to a message from Greg Edge sent Wed 30 May 2012:

Yes Greg is right didn’t realize I called them swithes they are
sensors but two different parts, different part nubmers, even
though the ressitance to temps are very similar.–
The original message included these comments:

They are coolant temp sensors not switches. Switches would be

–
T Boy
Raleigh NC, United States
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