[v12-engine] abrupt change to overly rich fueling- part III

Some of you have been following this thread series, and I have a
next installment for you to ponder. The car has me on the run,
right now, diagnostically.

One weekend ago, I replaced both the coolant and inlet air
temperature sensors for the fuel ECU. The quick verdict: no change
in rich running behavior.

This weekend, I tore the insulating jacketing off the harnesses
feeding both sensors, and snaked this back to the rear of the
engine, where it comes together with the TPS. Looked OK. I then
went to the fuel ECU, and pulled the AMP connector and did a
resistance check for both the coolant and incoming air sensors, and
then, went to the sensors themselves, disconnected the harnesses,
and read out the values. These match; I made several measurements
as the car cooled down, and every time, the readings at the ECU end
of the harness matched those at the sensor terminals.

I have verified that the full load vacuum switch functions; I
pulled the vacuum line off and sucked on it, and heard the switch
actuate, and I also confirmed that the switch was actuating by
measuring continutity.

I DID find one thing: the WOT switch at the capstan had been
installed per the S57 diagram, and this seemed incorrect when I
investigated. I chose the terminals on the capstan switch that
caused the engine speed to increase when the switch lever is
depressed. This is clearly adding fuel when the switch is
depressed.

I think that was PART of the problem, but not all of it. Moreover,
the fueling is getting almost to the point of stalling the car out,
right now.

In order to do more diagnostic work, I had to change the plugs for
a fresh set (second set this month). The set I pulled were
uniformly, to a plug, coked up heavily. Barely was running in this
condition.

With new plugs aboard, it starts sharply, right away. But, on a 30
minute test drive, my trip computer indicated that the average
mileage I was experienceing was about 7 mpg. The absolute highest
number I saw (for a scant few moments) was 13. I believe these
figures in part because the exhaust reeks of fuel, and on this
particular run, I could actually see the fuel gage dropping as I
drove.

Along with the second set of plugs, I of course readjusted the
throttle linkage (I always pull the throttle tower to do the rear
plugs, and you can never depend on this going back in exactly the
same position with respect to the throttle links). I also verified
that the TPS voltage was reading 0.36v at the closed position.

Also, I pulled out a spare 16CU ECU from an '89 (also a Marelli
car, the one from which it had been pulled). This made no
difference at all, but, to be fair, I tested this on the car before
changing the plugs again.

So, I am left with two possibilities: 1) fuel pressure is too high,
and 2) something’s amiss with the manifold vacuum signal to the ECU.

Anyone think of anything else I’ve missed?

I need a beer, now.

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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In reply to a message from mike90 sent Sun 27 May 2012:

I think a while back it was suggested (by me?) that the vacuum
line to the ECU is the usual culprit here. Just get a length of
tube, and run it from the manifold back to the trunk to the ECU,
don’t bother trying to feed it inside the car. You should get
immediate results just running in the driveway, but you could tape
the hose down if you want to drive it. Just leave the bonnet
unlatched, and figure a way that the boot lid doesn’t crimp it.

Dave
Atlanta 1994 XJS 6.0L–
Penfold99
Atlanta, GA, United States
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In reply to a message from Penfold99 sent Sun 27 May 2012:

Easy enough to rule out fuel pressure. Hook up a gauge and
measure it.–
Greg 1985 XJS HE DANA rear
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In reply to a message from mike90 sent Sun 27 May 2012:

If the vacuum line back to the ECU checks out ok, and if fuel
pressure checks out ok, I think I would check the O2 sensors. Maybe
even disconnect them and run in open loop for a test.

SD Faircloth
www.jaguarfuelinjectorservice.com–
The original message included these comments:

Anyone think of anything else I’ve missed?


www.jaguarfuelinjectorservice.com
Jacksonville, Florida, United States
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In reply to a message from Penfold99 sent Sun 27 May 2012:

Dave:

Thanks for the suggestion: I will try this next.

The FPR possibility doesn’t get much traction with me right now,
because the ECU has no idea what the rail pressure is. If it were
in O2 feedback mode, it might adjust to compensate for other than
nominal rail pressure, but right now, I am running the ECU with the
O2 feedback inhibit line grounded.

What I am seeing on the trip computer’s instantaneous mileage read
seems to track the actual engine behavior. This is why I am
rejecting the FPR theory (that, and the fact that I had replaced
the FPRs about 12-13K mi. ago.

Thanks (again?) for the tip on the vacuum sense to the ECU.

Will report when I have had the time to try this out.

-M–
Mike, 1990 5.3 XJS Conv., 5-speed, SE-ECU, TT Extractors
Lakewood, OH, United States
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