Good used 6cu ECU and throttle potentiometer?

Kirbert, I am leaning towards the coax being good. But I did find the wires leading from the distributor to the ign amp looking pretty bad. It appears to have a coax look to it also except just wrapping both wires. I can see 2 burn spots through to the coax part and the wires are stiff inside. The do have continuity though. I get a spark still with them but tried new wires to see if there was a difference in spark. It appears to be the same but I need to replace that wire set anyway.

There is one thing I don’t understand on my fuel relay in the trunk, the orange wire is grounded and leaves the pump running all the time. There is no 2 second shut off when I turn the key on. I am not sure which owner did this or why. I haven’t found the timer switch that runs the fuel pump yet and haven’t really looked for it yet because of the current issue.

There is one other question I have too. The fuel gets cooled by running through an A/C line to help with vapor lock. On my car it is on the left hand bank and the fuel runs through it after it runs through the engine and fuel rails. Why wouldn’t it be run the other way to cool the fuel before it goes through the engine rails and regulators? Just curious.

Thanks

Russ

Thanks Steve. I am currently watching a 16ecu on another site. I hope to borrow one from another person that replied here first to see if it might cure this problem. I was wonder though about the “enhanced” ECU you bought on ebay.

The number you gave says that it might not be for the same model as mine but when you plugged the enhanced ecu in, what kind of difference could you feel or notice? Mine was with a package that included the butterflies enlarged a half inch, the runners in the manifold made a bit longer and of course the enhanced tune for that.

I hope he has an apprentice or will at least let this information out so another can continue his work. It would be a shame to lose that knowledge. He seems to need an apprentice anyway the way he was talking about being overloaded with work.

Thanks

Russ

I appreciate the heads up with the ECU. I am going to borrow one from one of the guys here that offered to let me use one of his for testing purposes. I guess the coax problem is big but the testing shows it as good and the wire itself is in pretty good shape for being in the engine compartment so long and compared to the other very brittle and cracked wires I have attempted to replace. It would be nice just to have a complete new engine wire loom with all the correct plug ins and wrapping. Oh well.

Thanks

Russ

Russ:
36CU is the last ECU model. Used in the 6.0L V12s. Much different than yours, but the idea is the same.
I had my TBs bored by an outfit in Florida. AJ6Engineering provided the remapped ECU (I bought a stock 36CU off of eBay). The car feels “peppier” but you’d appreciate the difference only if you know the car well.

In any case, with all the mods you already have, if you prove by swapping a good known 16CU that the one you currently have is caput, you should get it repaired.

Good luck,
Steve

I plan on getting mine fixed if it is what is causing the car to not run. If the other ECU I get works then I will send mine it back to Roger. Hope he is still able to work on them.

Russ

will check tomorrow, almost certain they are 16ecu’s. I will only sell if I have 2 left as I want to keep a spare. email jrinaman@zoominternet.net

But I did find the
wires leading from the distributor to the ign amp looking pretty bad.
It appears to have a coax look to it also except just wrapping both
wires.

Color me confused. IIRC, the only wires from ign amp to distributor are the
two wires to the pickup, one red and one blue I think. Not coax.

I can see 2 burn spots through to the coax part and the wires
are stiff inside.

Not stiff either. In fact, they are a special type of wire intended to be very
flexible, because inside the distributor they have to flex to allow the vacuum
advance mechanism to move freely.

I get a spark still
with them but tried new wires to see if there was a difference in
spark.

Clearly thinking of something different, as the wires to the pickup are part of
the pickup, not easily replaceable separately.

There is one thing I don’t understand on my fuel relay in the trunk,
the orange wire is grounded and leaves the pump running all the time.
There is no 2 second shut off when I turn the key on. I am not sure
which owner did this or why.

You have a 6CU. The circuit in the 6CU that keeps the fuel relay energized
fails routinely, they have probably all failed by now. The proper fix is to ship
it off to Bywater for repairs, but the cheap fix is to just ground that orange
wire. It only makes a difference if you crash the car; the circuit is a safety
feature that’s supposed to shut off the fuel pump if the ignition is on but the
engine has stopped running.

I haven’t found the timer switch that
runs the fuel pump yet and haven’t really looked for it yet because of
the current issue.

Inside the ECU.

There is one other question I have too. The fuel gets cooled by
running through an A/C line to help with vapor lock. On my car it is
on the left hand bank and the fuel runs through it after it runs
through the engine and fuel rails. Why wouldn’t it be run the other
way to cool the fuel before it goes through the engine rails and
regulators?

Yeah, you’re not the only one who wonders that. But that’s the way it works.
Its only intended purpose is to prevent the fuel in the tank from getting too
hot.

– Kirbert

Not to take this thread off-track, but 6 years into my XJS ownership, I still wonder why Jaguar deleted the fuel cooler for the 6.0L altogether?
Notice that on top of all the heat that the returned fuel will bring into the tank, I also have two submerged fuel pumps, which also produce heat when they run. One is activated only above 2800 rpm, the other one runs all the time.

Steve

Let me try to describe the distributor wires. The wires out of the distributor are soft and pliable and maybe used to be red and blue but close enough. It plugs into another set on my car that plugs into the ign amp. Its just over foot long wire, the one I mention as being coaxial in nature. I has a sheath of woven mesh around it that is grounded to one of the bolts holding the ign amp to the manifold. That wire set is looking rough with the outer part exposed in a place or two. It was running on the floor of the v between the banks and obviously it gets hot there. I was just mentioning it more as another wire set that heat has made hard and brittle. I did check for continuity though and it is fine. I made a set to takes its place fro testing and still the same spark as with the original wires. This was more I guess meant as an fyi thing.

I tried starting fluid but with the runners so long and bent at an upward angle all I get are some fumes in it and it really showed no signs of wanting to start. It would hit a few times but not like it would on a downdraft carburetor. I keep checking the spark with a sparkplug connected to a wire, grounding it and turning it over. There is an excellent spark.

Interesting about the circuit in the 6cu that fails routinely, makes me wonder about the rest of the 6cu and the car not running. I might have a borrowed 16ecu coming from another guy that has been making comments. I hope so.

Thanks. If the car runs when I plug it in then I can send it back to you so you will have spares. If not, well then I will buy it from you just in case I fried it. Unless you want me to buy one of them either way.

Thanks

Russ

I still wonder why Jaguar deleted the fuel cooler for the 6.0L
altogether?

According to Bywater, the fuel cooler is an emissions device. At a time when
carmakers were having to change paint formulations because the paint on
the car would offgas enough volatile compounds to flunk an EPA emissions
test, Jaguar’s problem was that the fuel in the tank would get hot enough that
the vapors would overwhelm the tank vent system.

I agree, I don’t see how the fuel in the tank isn’t going to get hot after the fuel
cooler was deleted. The only conclusion I can come to is that somehow it’s
no longer a problem. One possibility is that the tank vent system is so much
better that it can now handle the vapors from a hot tank. Another possibility
is that the formulation of the gasoline has now changed, and the summer
blends no longer vaporize as easily as they did in the 1980’s.

The worst condition has to be right after hot shutdown. Whenever the
engine is running, one has to presume that the engine itself can suck up
whatever vapors are coming out of the tank, and the EFI system can adjust
the mixture to compensate. When the car is cold, the carbon canister should
absorb whatever vapors are coming off a cold tank. But after a hot
shutdown, the fuel in the tank is hot and the engine isn’t sucking vapors.
How does it deal? Vastly bigger/better carbon canister? A tank that can
hold more than 2 psi pressure before venting? I dunno.

– Kirbert

So you have spark & fuel? When you turn on the ignition and open the throttle do you hear the injectors click? Have you put a timing light on it?

There was an issue with my 83 XJ-S that took me 3 months to figure out. It would start & idle ok but as soon as you opened the throttle it would start hating life and would not rev above ~1200rpm. What it ended up being was the throttle position sensor. At idle it made contact ok but would go open as soon as you cracked the throttle. Take you TPS and an analog VOM on the ohms scale and see if the resistance is nice and smooth and doesn’t go open somewhere along its path.

Rich

I do get a signal to the injectors with the key on and opening the throttle full. Apparently that was one suggestion from another contributor here. When I pull the positive wire off the coil and just touch it to it a few times I can hear the injectors clicking too. I have timed the engine with a timing light.

My car will not start at all. I did take a VOM and without knowing what to look for there was a connection of the 3 wires that showed a rise on resistance to 1 or short. I think it started at .4 or .5 ohms when I tested it closed. I am getting another TPS soon and will throw that at it too.

I have another ecu coming and hopefully early this coming week. Running out of things to check without taking the whole loom apart.

Thanks for more things to look for though.

What I was looking for was if you have spark with the engine turning over. The easy way to tell is with a timing light on the high voltage wire from the coil tower to the center of the distributor cap. If there’s no spark there it simplifies the search.

Rich

Rich

Oh yeah, there is a good spark at the plugs. I have old plugs that I put into the wires, and make sure they are grounded and a nice blue spark. Yeah, if there was no spark it would be a much easier problem to fix! Good power to the injectors too.

Thanks

Russ

Kirbert. I did clean the plugs on the power resistor and thought I cleaned it enough to possibly eliminate that problem. I did get another ecu to try but it still does the same, nothing. I went back to the power resistor and wiggled the wires some and now it seems to want to fire on a few cylinders. If I unplug it then the engine does not try to fire at all.

What do you think of that? Could it be weak enough to partially fire or is it feeding something else that uses the resistor? It does the same with both ecu’s.

Thanks

Russ

I went back to
the power resistor and wiggled the wires some and now it seems to want
to fire on a few cylinders.

Which wires did you wiggle? Seems like a direction to follow up.

If I unplug it then the engine does not
try to fire at all.

Well, duh.

Remember that the resistor pack is only used for the “hold” function of the
injectors. The “open” function bypasses it. If one worked and the other
didn’t, I’d expect some funky symptoms reflecting the injectors trying to open
or just opening and quickly reshutting rather than staying open.

Also remember that faults might not just be the connectors. The resistor
pack itself might be a dud.

– Kirbert

Well I was just stating the obvious about unplugging the resistor pack. Cleaning and spreading the male pins open a bit more has made a difference in it trying to start. I also did the same to the male pins that plug into the injectors just behind the resistor pack. It seems as if 60 to 70% of the engine was trying to fire. As I was turning it over it seemed like it just needed a few more cylinders. I am now led to think that the resistor pack may be another item to replace.

It almost felt like there was water in the pickup tank but after draining it again, I looked inside and found the pre-filter to be in good shape and did not find any water in the bottom of it. I have a resistor pack on the way.

Russ

Oh, the wiggled wires were the wires that plugged into the resistor pack. I am just wondering if the resistor pack is just a connection now but not performing its duty or if it might be partially working.

Just things to ponder for me while the car sits.

Russ

If you go into the archives here on the olde list you should be able to find much discussion on the TPS unit. I see new units for sale at some of thre Jag parts suppliers. Also, a Ford TPS can be substituted and modified (the Connector shaft to fit). The 16 cu unit is an easy inexpensive exchange, and well worth the price. The 6cu unit had solder joint problems with age but can be fixed, again the archives has discussion and pics on this repair. Better to send to Rogerbywater for permanent fix of the 6cu. unit. I don’t know where you are located but some of the later units past the 16 will work, up to Diznmandos that should make a rocket with the mapping,etc. Best, JW