[v12-engine] Cold start problem

Richard Dowling, Melbourne, Australia. 1979 coupe + HE V12 + manual;
1989 convertible; 2003 XJ350.> You are right, I must check if the signal from the oxygen

sensors and the TPS are arriving to the ECU intact and that
the heating element works OK (I have installed heated oxygen
sensors).

Will do that and report back.

Aristides


The sensors only work over a limited A/F range, I think typically +/-
30% of stoichiometric.
If you have a misfire in a cylinder, or a mixture that is more than
30% from ideal, the sensor output is not reliable.
You need be careful about interpreting the sensor signal with an
engine not running properly.

Richard Dowling, Melbourne, Australia. 1979 coupe + HE V12 + manual;
1989 convertible; 2003 XJ350.

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With a misfire (no spark, plenty of fuel) you would think an O2 sensor would
show rich. In fact it will show lean. This is because all the O2 is still
present (as it hasn’t burned). This is counter intuitive and does your head
in when trying to diagnose these things.

The misfire therefore makes the ECU overfuel even more (runs the remaining
cylinders rich).

If the engine is running normally, the O2 does what you’d expect. When the
engine isn’t running normally, the O2 does the opposite of what you’d
expect!

Took me a while to figure this out, chasing my tail.-----Original Message-----

If you have a misfire in a cylinder, or a mixture that is more than
30% from ideal, the sensor output is not reliable.
You need be careful about interpreting the sensor signal with an
engine not running properly.

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In reply to a message from Aristides sent Sat 23 Jan 2016:

hello there!
a thought because your problem happened’‘all at once’’ and
‘‘popping at the intakes’’.
check your connections for the engine harness not only at
the cts but at the firewall header.it’s possible that
corrosion has reared it’s ugly head.
check the grounds for the engine to cradle and cradle to body.
check the rotor and also see if any carbon tracking is
happening under the cap.
any slack in the distributor shaft?
vacuum leak to the computer caused by dry rot?
these are the easy things.it only becomes a pain from those
points on.
that’s all I can think of at the moment.always an adventure
these things are!
=dok=–
thewytchdoktor=v12 fun!/94 xjs 6 litre/ s=k.log w
Winchester Virginia, United States
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In reply to a message from Mark Eaton sent Mon 25 Jan 2016:

I had a very interminttent problem some years ago, and after
trying everything else, I looked in my 6CU ECU. I found four
electrolytic caps. I obtained the closest replacements
(Philips brand, as per originals) and replaced them. I’ve
had no trouble since. This is a 1984 Series III with Middle
East origin (no cats or oxygen sensors or air injection).
I’ve done the same repair to a few other ECUs from Jaguars
and Land/Range Rovers, it’s worth a try, the capacitors are
very inexpensive.–
The original message included these comments:

From photos I have seen, it doesn’t look like there are any electrolytic
capacitors in the ECU. That is good news and indicates the quality of the
original design.
Electrolytic capacitors leak with age and reduce capacitance as the
electrolyte evaporates or leaks …


Pete_M
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In reply to a message from Pete_M sent Thu 25 Feb 2016:

Pete,
Thanks for the suggestion .
I am waiting for a replacement ECU, but I will try that on
the meantime.

Doc,
Yes, will check the engine ground, you never know…

Richard,
The TPS and Oxygen sensors signals are OK, I get the same
measurements at the sensors as I get at the ECU plug.

On the other hand, I opened the TPS and the carbon
resistance was worn in one place.
I repaired it but did not change anything.

I will post the TPS repair separately as it could be that
some might find it helpful…

Thank you all for you replies.

Best regards
Aristides–
The original message included these comments:

In reply to a message from Mark Eaton sent Mon 25 Jan 2016:
I had a very interminttent problem some years ago, and after
trying everything else, I looked in my 6CU ECU. I found four
electrolytic caps. I obtained the closest replacements
(Philips brand, as per originals) and replaced them. I’ve
had no trouble since. This is a 1984 Series III with Middle


Aristides Balanos, 1986 XJ V12,VDP - www.e-psychi.com/jaguar
Roussillon, Provence, France
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In reply to a message from Aristides sent Mon 25 Jan 2016:

Update…

It has been a while, but problem solved !
It was a bad ECU…

I decided to go with a 16cu and I found a pristine DAC 6335
from a later North America XJS.
The engine started right away !!

I can report that so far with the 16cu the engine starts
somewhat easier, literately in half a turn, and the idle
seems to be smoother.
Haven’t taken the car out yet as I am doing other repairs
and upgrades, but will report back.

The only drawback is that my feedback monitor does not work
any more as the 16cu has a pulse output (the 6cu is analogue).

Any ideas on how to set the idle mixture - base fuel map ?

Thanks again for all your help.
Aristides–
The original message included these comments:

I think that Kirby you are right, my ECU is giving up.
I have now the ECU in the bench trying to find obvious
faults, but no luck so far.
The question is where to go from now… 6cu or 16cu?
And there are many 16cu variants
DAC 6335
DAC 6337
DAC 6338
DAC 4585
The current one is a 6cu DAC 3586 and the car is a 1986
Canadian market.


Aristides Balanos, 1986 XJ V12,VDP - www.e-psychi.com/jaguar
Roussillon, Provence, France
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The only drawback is that my feedback monitor does not work
any more as the 16cu has a pulse output (the 6cu is analogue).

Yeah, that’s not true. They are both digital, the 16CU just has a faster
processor and higher-resolution fuelling maps.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 18 Apr 2016 at 23:46, Aristides wrote:

In reply to a message from Kirbert sent Wed 20 Apr 2016:

So are we saying that any car with a 6CU would be improved
by a 16CU plug and play swap?–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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So are we saying that any car with a 6CU would be improved
by a 16CU plug and play swap?

Yes.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 20 Apr 2016 at 0:48, PeterCrespin wrote:

In reply to a message from Kirbert sent Wed 20 Apr 2016:

Thanks. Was the 16CU introduced at the same time they
deleted cold start injectors? (Having just spent money
replacing mine…)–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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Was the 16CU introduced at the same time they
deleted cold start injectors? (Having just spent money
replacing mine…)

Nooooo… The cold start injectors were deleted WAAAY earlier. My '83
didn’t have them, although it still had the holes in the intake manifolds for
them, covered with little blank-off covers.

Where did you even FIND cold start injectors? They were supposedly
removed from earlier cars by a recall. I’m surprised anyone carries them.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 20 Apr 2016 at 7:02, PeterCrespin wrote:

In reply to a message from Kirbert sent Wed 20 Apr 2016:

One Volvo, one BMW IIRC - both from eBay by Bosh number?
It was a year ago. Inlet fuel stub slightly wrong angle on
one but it should serve on my twin-rail '79.–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
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