So, 2 years of going over things under the hood. cleaned and replaced parts. ( pictures of the ordeal in my album in profile section.

Anyhow, was having trouble getting idle down, so took off the large bore throttle bodies and put the original back on. The idle is back down to 650 rpm, great right, well yes and no. I had he vacuum hose that runs from the ECU to the balance pipe disconnected and plugged, hooked it back up to the ECU and idle goes up to 900 rpm.

So the question of the day, does that hose need to be hooked up to the ECU even tho the engine runs fine and no hesitation or sputter when revved up. have not had the Cat out to test run on the road yet as I have a few parts to put back on, Hood, grill, spoiler and such.

But, I see no reason that it needs to be connected if all is running fine.

 

Daverb,

Without that line you won't have any adjustment to the injector duration by the ECU. That vacuum line is the primary ECU input. It should idle without it on default mapping, but she won't drive like that.

 

Without that vac hose the ECU "SHOULD" overfuel like crazy.

The MAP sensor, inside the ECU is simplistically, LO vac more fuel, HI vac less fuel.

If that engine is running "sweet" with it not connected, you got other issues.

With the large bore throttle bodies, you will need to set the stops at ABOUT 0.001" or less.

I am assuming the AAV is closing 100% here???????

The TPS is set at 0.34 +/- 0.01v at the idle position??

HANG ON

1) You said that the ECU hose is "disconnected from the balance pipe and plugged", RIGHT???.

2) Do you mean the hose is plugged or the spigot of the balance pipe is plugged, or as we say capped.

IF, and I mean IF, the answer is the hose is plugged, and the spigot is uncapped, then YES it will run, coz that spigot is a MASSIVE vac leak, that is basically adding more air to the super rich fuel the ECU is supplying.

 

I should have said it is capped, when I uncap/remove the the cap I can feel the vacuum, and it idles up, cap it off and it idles just fine. TPS is set right, I removed the large bore throttles, and put the original smaller bore throttle bodies back on set at .002. The AAV is new and set correct as I tested it.

 

OK.

I get back to the priginal statement. without taht vac metering the MAP inside teh ECU, it would be at some predetermine setting.

One I had here years ago, after an engine fire. All sorted, started it up, ran RICH, and I mean smoked the shed out in seconds.

Days later, I was still at the starting post. For some reason I removed the vac hose at the spigot, and at the ECU, and attempted to blow thru it, NOPE. Blocked. After much solvents and compressed air, a wad of fire foam "popped" out the end. BUGGA. Reconnected the hose, sweet as.

That is why I know that the vac hose and the vac supply, is required by that ECU to run that engine as designed. The fact yours is opposite is "special".

There is an idle fuel potentiometer on that ECU. At the risk of grabbing at straws, it may be at the end of its 14 click adjustment, and for some wierd reason is allowing that engine to run without overfueliing. Mine would belch black smoke if I had it at one end of the adjustment, and stall out eventually. I ran mine at 3 clicks IN from the other end, so I know it does have a huge affect on fuel metering at idle. It does ZERO about 0.45v +/- of the TPS. It only alters the "idle fuel map". My adjustments above were ALL done with that vac hose connected.

 

I am a bit lost. For clarity are you saying:

1. You have fitted the original sized throttle bodies? Y / N, if YES:
2. With them fitted AND the ECU pipe from the crosspipe connected to the ECU, what does the idle do?
3. With them fitted AND the ECU pipe from the crosspipe DISconnected AT the ECU end AND plugged AT the ECU END of it, what does the idle do?
4. As 3 above, BUT with the crosspipe spigot blocked off, what does the idle do?
5. What does the idle do with the crosspipe spigot AND the ECU vac pipe spigot blocked off?

If 3 and 4 promote a change, then you probably have a leak in the crosspipe to ECU pipe somewhere. It is also possible that the gizmo that is reading the vacuum inside the ECU is somehow going wrong. You MUST have a vac signal to the ECU for the engine to run properly. Finally, what is the oxygen sensor/catalytic converter situation? Are the sensors working for sure?

If you get the idle fine with the original OEM sized throttle bodies, but it goes up with the larger throttle bodies, then the larger throttle bodies have a leak in their spindles, or they are not able to close properly (they are not symmetrical I seem to remember) or are adjusted too far open.
Greg

 

Beauty sleep does wonders.

Have you installed the air cleaner backings AND the 4 bolts each side that hold it them in place, OR are you tempting fate and firing it up with those bolts/filter assemlies missing.

Those bolts are NOT blind holes, sooooo, another huge vac leak that will reek havoc.

I still cannot get past it not overfueling without a vac signal to that internal MAP.

I am just off on a 1200km drive for a few days, so I will check back when I find a signal in our bush.

ENJOY.

 

Both air boxes attached with all for bolts, gaskets in placed also. air filters installed to. that is what is puzzling me, when I place my finger over the hose in the trunk I can feel a suction/vacuum and idle drops to 650, take my finger off, I can here the idle go up and a "Hissing sound" or like one. even when connected to the ECU, the idle goes up, I have tried 2 different ECUs/ Guess I could hook up my vacuum tester and see what I get for a reading, but not sure what a normal reading should be. I will also have to make a tool (know how on that) and see what an adjustment will do and recheck the AAV idle adjustment. Guess while at it, recheck the new hose I put on to make sure I did not put a small hole in it.

a bit late so hope all my typing makes sense but thanks guys for giving me some ideas, and your help. Let you know how somethings turn out

 

Have you tried attaching a hose to ECU and sucking on that. Might be a problem with the vacuum sensor in the unit. I imagine you should not be able to suck air through it.

 

Haven' thought of that, thanks for the idea

 

There was a T piece in that vacuum line in some cars for fuel tank vent, l am not familiar with which cars and it is a real long shot but might be worth checking.

 

Just a thought: With everything connected up properly, can you get the idle down to the correct RPM by further adjusting the throttle bodies?
Greg

 

OK, that hose from the balance pipe to the ECU is made up of a hose in the engine bay, then a metal pipe that travels through the "hump" alongside the prop shaft, and enters the boot (trunk), and then a hose twists its way around the bits to attach to the ECU.

Some markets have a plastic cannister in this hose, adjacent to the battery cable, these cannisters crack and leak vacuam, and make a hissing noise.

Inside the ECU travelling from the other side of the external hose spigot, is another vac hose that attaches to the MAP, this hose splits with age, BUT, to have this on 2 ECU's is odd at best.

The metal pipe under the car can be damaged when the unknown use a lever in the wrong place to align the transmission mount, oops.

Being a 1984, I reckon it has what is known as a 6CU ECU, and they have their own special issues, hence the swap to the 16CU unit, which is a plug and play retro.

With that hose attached to th ECU, and applying vac at the engine bay end (sucking on it), it should hold vac. If it does, then it is deemed OK all the way to the MAP. If it dont, it needs fixing until it does.

 

The one I have in the car now is a 16CU, will have to check the hose inside and perform the checks you menyion. may be a few days before I get back on it, thanks

 

Is it the CORRECT 16U? There are 16U versions for Lucas ignition systems, and 16U versions for Marelli, and they are not interchangeable. The engine will run, but badly, if the wrong one is installed. You need to go by the Jaguar part number to be sure which you have.

 

the ECU is the correct one. I got the correct number a couple of years ago from a write up here, but do not remember who did the thread. Do know it was one of the more season, knowledgeable

 

Thanks to all for the ideas on what to check over, all is running great now. After checking over suggestions the following was found

1. rechecked the TPS, it was set good when installed but then was showing way low. by that I mean .15. pulled the turntable to reset and found the red wire insulation near the TPS had a split, and the green had a slight amount o bare wire, (this was the RED TPS) I bought used with the TurnTable. luckily I had another TPS of the old style, set it up (APITA) but got it

2.Had installed a NOS AAV, when I tested it, it was closing off, but for some reason I believe it wasn't when on the car (newer type of antifreeze these day maybe). Took my old one, tested and adjusted, installed, working now.

3. Did not notice until last night, one hose on the balance pipe was not sealed properly, just had to tighten clamp a bit.

The XJS now is idling at 700, and the engine sounds great, just have to test run it but when reving up in the garage, no hesitation, sputter, or lag in it.

So again, Thanks for all the help

 

One hell of a lesson here for all of us. Practically always, it is the basics not being right that causes the V12 to act up, and carefully rechecking them is the best thing to do. In other words we must all hearken to the wise words of the Great XJS Prophet Grant Francis.
Also, in general, and like you I have learned the hard way, second hand electrical and electronic parts are juist not worth the savings they might make, in a V12 's case.


Really well done and congratulations for sorting it out. Great feeling when they are right, and you have done it yourself, isn't it?
Greg

 

It sure is a great feeling of accomplishment since I having done this much work since the 70's. as far as the used part, it did cost me $75 (US), but now I at least have a set up for the new Red TPS

 

Well done.

I have said it thousands of times:

These are SIMPLE cars/engines, its the nut behind the wheel that complicates things.

WELL DONE again.

I find DRINK keeps it simple, even on the 2 new beasts in our fleet.