[v12-engine] re:[V12-Engine] Poor Gas Mileage - Problem Found

V12-Engine
1

I posted a longish message about my gas mileage being reduced after
doing extensive/expensive work on my V12. I mentioned that I suspected
and tested the air temp sensor and the coolant temp sensor. I requested
the ohm/degree table for the coolant sensor.

Confirmed ECU water temp sensor not open or shorted
(I need a table of proper ohm values for this sensor)

Sean replied:

I don’t know if the sensors used on the Marelli are the same
as on the late model Lucas Digital P, but if they are, this
is supposed to be about the same as the air temp.

He is correct.

The reason I thought they were different is that during my stove top
test last week I was getting resistances in the multi-K ohm range. That
resistance range, per the ROM, translates to 100 degrees below zero. I
retested this same sensor last Saturday, but this time the results were
totally random throughout the temp range. The sensor was bad, but had
not failed to the point that the engine could not run. I reinstalled
the sensor (have to mail order the replacement) so I could drive the
car. Sunday while doing some wiring work I had to remove that sensor’s
plug; when I did so the top of the sensor separated from the body and
came out with the plug. Much of the broken surface was dirty,
indicating that it had been partially cracked for a long time. I
installed a resistor in the plug to tell the computer that the engine is
at 185 degrees F. This morning as I drove to work my gas mileage was
above 16 mpg for the first time since I got the car.

I thank each of you who offered suggestions and ideas as I worked on
this problem!

Don Neff
1991 XJ-S V12 Conv. (with improved gas mileage!)

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2

At 08:14 2000-08-28 -0700, Donald F Neff wrote:

[snip]

He is correct.

Whoo-hoo! What’s the next question, and how much is it for, Regis?

came out with the plug. Much of the broken surface was dirty,
indicating that it had been partially cracked for a long time. I

Hmm, that’s probably why it was erratic, eh?

installed a resistor in the plug to tell the computer that the engine is
at 185 degrees F. This morning as I drove to work my gas mileage was
above 16 mpg for the first time since I got the car.

Drat, after reading the top part of your message (“reinstalled the
sensor”), I was about to suggest that you get a resistor (or resistors with
a series resistance) with a value close to what the car’s operating
temperature should be (185’F = ~280 ohms, 195’F = ~240 ohms, at least with
the Lucas Digital P – isn’t it a gas that the ROM doesn’t tell you the
resistance value for typical operating temperature?). Solder these to an
injector-style plug (if that’s what the Marelli uses to connect to the
sensor) which should be available at a parts shop, and connect to that
instead of your dead sensor. A short bit of shrink tubing (common in
electrical stuff) over the resistors and solder joints will keep it from
shorting out against anything, or you can just use a wiretie to wiretie the
sensor wire to something such that this temporary fix won’t ground against
anything.

I’d have done this whether or not I was presented with a broken sensor
part. In fact, since the ECU-side plug is female, you could manage to
solder the resistors to some male spade connectors ground down to the
appropriate width, and it’d probably hold (if you couldn’t dig up a male
injector plug). I forget what the size is of the spade connectors that are
just below the 0.25" variety, but they’re probably pretty close to what
you’d use to start.

It is a given that the car will run poorly while you warm up (though
certainly it can’t be any worse than the all-over-the-place readings from
the failed unit), but once it is at operating temperature, it shouldn’t
notice the difference from being connected to a good sensor.

— http://jaguar.professional.org/
Sean Straw '88 Jaguar XJSC 5.3L V12 (LHD) ‘Black Cat’ 63K
Marin County, California '85 Jaguar XJS 5.3L V12 (LHD) ‘Bad Kitty’ 210K
'69 Buick GranSport 455 V8 324K

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