Hello all. I've been working on 1976 XJ12L now for awhile. Kinda a family thing since it has low miles on it. But the question I have is that Bank A seems to have weaker exhaust than Banks B, which is firm, any ideas why this might be? Also on Bank A, there is moisture in the air filter coming from the Air Valve(I think) on the manifold, any ideas why? So much moisture it's starting to rust the inside of the air filter holder. This is the side with the air pump and thinking this might have something to do with it. I have replaced a lot of stuff on it lately, new head gasket, intake gaskets, spark plugs, catalytic converters hollowed out, radiator cleaned, water pump checked, compression good. Idle doesn't adjust very well either. It seems to run good otherwise. Any help on what the problem might be would be great. Thanks.

 

Firstly, welcome aboard.

When time permits please do an ntro in the "New members intro is a must" area.

Tou are not saying where you are from, but the fact you have "Cats" on a 1976 XJ12, I will guess the US of A????.

Reading what you have already done my suggestions are:

Cam timing is out. Since you did 1 head gasket, did you set both camshafts after the event?. Did the Plastic chain tentioner survive?.

The moisture from that "Extra air valve" is NOT common. I think?? your market has that valve as part of the warm up extra air on a timer set up. We had them here on the HE, and I remove them. I do not know where all that could be coming from on a PreHE, but, they are a vapour creating engine when cold.

The fact the "idle does NOT adjust so well", is classic AAV jammed, and that can give wierd exhaust behaviour on the "B" bank mainly, as in kind of "thumpy" when the hand is placed over the pipe. Once that valve shuts 100% (and it must do this when the engine is at operating temp) that exhaust settles down.

Slightly retarded timing could also create that exhaust issue. Try adjusting the timing by ear in the advance direction JUST A SMALL AMOUNT. That is the distributor moved in a "clockwise" direction when looking down at the top.

I will leave at that for the time being.

 

Hey Thanks for the reply. And yes I'm in the US.
I actually did both head gaskets since I was there kinda thing. I didn't break the timing chain tensioner plastic thingy but did have some problem getting it set back down. But pretty sure it's down.
I took out the AAV and put it in water about 190 deg, operating temp for engine. It moved but I could never get it to close all the way, it was pretty close though about 1/16th inch or less. I lubed it up the best I could and put it back on. There is a vacuum leak sound coming from that area but concluded it's the adjusting screw. Maybe the AAV valve is my problem. Thanks.

 

Here ya go.

AAV rebuild procedure.doc

Check the large vac hose on that AAV, particularly the 90deg one on top, as it is known to split on the underside.

 

Thanks. I'll give it my best. I'll keep in touch, but might be awhile. Busy next couple of weeks.

 

The valve inside AAV. Almost closed but has a notch at the top.


Got to the AAV today and the procedure for rebuilding it was really helpful. I cleaned it and it seems to be moving better but stops before it fully closes and the top part. I included a picture of the inside of the valve. Sorry if it's a large picture. The rebuild document didn't show the top notch, the picture had it straight across. The valve doesn't go all the way to the top, it stops where the flat spot should be. Is this for emission's in US? The car is "Federal". I'm thinking it's working how it should or should I hammer it down till it closes? Help again. Thanks.

 

California? since you running cats .... welcome to the smog club


The best
Debbie

 

Nope.

That valve MUST close 100% when at operating temp, and simply boiling it may not do it in some cases.

I use a "hot air gun" set on low, and a good dose of common sense, and if they close 100% thats it.

The emissions for each market is outside of that valve.

"Tapping" the "tophat" as I call it, is about the only way to acheive this some times. This will slow the "Cold idle speed" as it is lowered into the outer casing, but that is a good thing in MY opinion.

Some, well most, I have rebuilt, end up with the "tophat" about 2 - 5mm below the top edge of the outer casing. This gives a cold idle of approx 950, and since it then closes 100%, the "hot idle" adjuster actually works as designed.

If that valve fails to close 100%, the engine will get confused, and all sorts of havoc will be going on, and that depends on the emissions junk fitted, and then how much of that junk is actually still working, and the "hot idle" adjuster screw simply does ZIP.

 

Hello.
It fixed the AAV valve to close all the way. I tapped the top hat down and put some heat on it and it close. Put it back on the car and warmed her up. With 3 turns out on the idle screw I was getting 650 rpm at warm up temp. It really didn't like warming up with low rpm.
I turned the screw in and it dropped in rpm. Screw it out and got 700 rpm max. I figure the hole is all the way open now.
It ran better before. Seems like it wants more air. Might give it what it wants.


I did notice that the bypass has a hole that enters the cylinder and the "top hat" had a solid bar along the bottom of it. When I first removed the top hat, that solid bar had some carbon on it that looked as if the bypass hole had been somewhat blocked at the top. So maybe by putting the top hat down more, I really blocked the bypass hole now? Any thoughts on this? Maybe measure it up and make sure this hole doesn't get blocked at all?


This Jag has only 23,000 miles on it and know the previous owner didn't mess with much.


Thanks again and anything would be helpful.

 

OK.

PreHE needs some special treatment.

The 2 throttle discs do get coated in "goo", and that affects idle seriously. Remove the 2 air filters, get a solvent soaked rag (petrol is ok if thats all you got), and seriously wipe out those 2 throttle bodies.

Now check the air gap of both throttle disc. The factory calls for 0.002" each. I set them at 0.003". Measure this gap at the top of the disc (12 o'clock as we say), and take your time with this, as it is critical. CAREFULLY loosen the small locknut on the throttle stop arm, and rotate the screw inwards, so as to "jam" the feeler strip between the disc edge and the nice clean throttle bore, then rotate it outwards SLOWLY, whilst keeping some pulling tention on the feeler, and as soon as that strip "releases" STOP. You may do this many times, so just stay calm. Relock the locknut and check again this clearence, as it can move with the locknut tightening. Keep at it, it is not hard, just time consuming, and is the"base setting" for the whole shooting match.

Once this is done, check the 2 crossrods are set eaually. again, not hard, just time consuming.

Once done, warm the engine again, and reset the idle as needed. The V12 will give about 1200rpm, if you are blessed, with that idle bleed screw wound out about 8 turns. Some of those adjuster have fine threads, some have a coarser thread, its just what it is.

If that "hat" is too far down, then the cold idle will be slow. These valves take time to close 100%, and the temp needle is usually getting to about halfway to the N before actual 100% closed is achieved.

Next item I would check is the timing, as it may be retarded, and that will give low idle, sluggish rev rise, etc. Your market may have the vac retard, OR the vac advance, OR the vac retard/advance system, so that is where it gets crazy. I would initially loosen the 13mm locknut down the front of the distributor, and VERY CAREFULLY using a blade screwdriver (engine running at operating temp) turn that screw in such a direction that the distributor moves in a "clockwise" direction, about 2mm. This will advance the timing about 5deg, and you will/should hear the engine note "sweeten up" as the timing is advanced. This advance will also increase the idle speed, and sharpen up the rev rise of the engine. So many of these engines are running retarded, and this is a simple "time by ear" method of sorting it quickly. Too much advance is no good, so just ease it around and listen. You dont need to be a rocket scientist to hear any engine "sweeten up", and the V12 is such a sweet engine when running a little bit of added advance, particulalry the PreHE. Please do NOT go any further than that 2mm +/- a tad I mentioned above as that amount is MORE THAN ENOUGH, and it is easy to put it back where it was if you achieve nothing by it.

 

I have set the butterflies on the throttle to .002 before. The sticker under the hood says, .002in and 10 deg at 750 rpm for timing. It does have a vacuum advance and I disconnect it and plug it when timing. I've retimed her a couple of times, all ignition has been redone.
I'll set the butterfly flaps out a little more then give it a little more advance at see what happens.
Thanks.

 

I would leave the vac attached, as that is how you are going to drive it anyway.

I find that the 0.003" is just a tad better than the 0.002", at least on our S2 V12.

Our HE's dont run that AAV, and that simplifies things a lot. Idle speed is set with those 2 butterfly stops, and it never changes. A simple solenoid gives the extra air for cold start just fine, and when its switched off there is no vac leakage.

 

Adjusted those butterfly out somewhat. I always feel like I don't use feeler gauges correctly so I found that half a turn out after close seemed to be close to .003, maybe alittle more. Did this to both sides. Advanced timing up 2 degrees. After warm up I was getting 800 rpm. I was able to screw in the idle and get 750 rpm, readjusted timing again to 12 degrees.
She's running tops now. And probably leave it this way.
It still has some weaker exhaust on Bank A and I didn't see moisture in the air filter on that bank either.
I think my problem is fixed. Must of been the timing and AAV.
I've been working on this car for 3years now getting it to run and driveable so I can't wait to enjoy her. Zoom Zoom.
Thanks some much for your help, Great advice.

 

Good to hear.

I will open a fresh JD and have celebratory drink in your name.

 

I love people are talking about 1976 XJ12! I'm looking forward to start my engine! Francis I love your knowledge, Ben 3 after e years I understand your are very excited too.