I have a XJ12 H.E. and struggle with a rough idle when the engine is hot.

Since i have the idle screw all in to reach a correct idle speed i am looking for vacuum leaks. But the only way the engine gets air is over the PCV Valve....and that quite a lot. When i block the PCV the engine stalls.

Now i changed the PCV to a new one...wich was rediculous expensive. And it changed nothing.
Now my question: is it normal that it draws so much air through the pcv at idle?

the butterflys are set correctly and i checked every vacuum connection with a vacuum pump and all of them create and hold vacuum.

 

That list gets long and messy, The Emissions for the market the car was built for, and/or lives in, plays along also.

Things I have found over too many years.
PCV system, YES, and it can be via the oil cap seal, chaain rubber plug (A Bank front edge)
Brake booster diaphragm. Hoses also, and the plastic elbow in the booster body.
The 90deg elbow on top of the AAV, they crack without human intervention.
Over run valves, oh boy, remove and plate the end on the manifold.
Vac pipe and associated hoses UNDER the car that go to the ECU. RARE, but I have had 2.
Injector seals, the lower PITA ones.
Inlet gaskets, common.Blanking cap on ONE of the spigots at the rear of the A Bank Inlet.
Emission junk, we have zero, but some later Lucas cars had a little, and they all had issues until it was all cleaned up.
Some markets have an Extra Air Valve, Solenoid inserted into the A Bank air cleaner backing plate, they fail, rare, but it happens, and will raise idle by about 200rpm.'
OF COURSE, the AAV is reknown for jamming, and if that sucker does NOT close 100%, you are doomed. More common than people will admit, fact.

With that screw closed, and an air tight engine, idle is :about 300 ish, but every engine is different. Mine were ALL set at 1 1/2 turns OUT from zero, and that was as close as scary to what I wanted.

 

Thanks for your quick reply.

The PCV is openly connected over the Air Filter Case so a crack in the metal filter and its pipes would not effect this. I still dont understand how this can create a negative pressure in the crankcase but well - it seems to be the right setup.


as written, i checked all vacuum connections including brake servo and they hold vacuum.

injector seals are new and i tested these including the manifold seals with brake cleaner.

overrun valves is a good point. i will have a look on those thank you.

 

My bet is the AAV is not closing 100%. They are a service item, every 2 to 3 years, daily driver or garage queen.

Attached PDF to assist.
I also have "my" removal in PDF if required.

Thats why many delete them, and go with a solnoid, switched inside the car, simple and trouble free.

 

The AAV was not closing completly, i rebuilt and tested it.
Still have my troubles with it because the rebuild kit did not fit my AAV - meaning it never fully opens.
That results in a rather low idle when cold (about 900). But since i usually dont drive when its really cold outside thats a topic for the future me.
It definetly fully closes when hot, i testet it befor installation.

 

OK, then you have a search and find in your future.

Never heard off a kit for them, and PreHE and HE, same, same down here.
Some report issues with the wax bulb, never had that,

As I said, emissions stuff, too old now, and ineffective anyway, None of mine have emissions stuff, zero.
Over run valves, oh boy, Jag fitted them, deleted them, refitted them, throughout the life of the 5.3 engine. I remove them, problem solves.

I suggest blocking ONE vac item at a time, see what changes.

OOPS, forgot something, age in a mongrel.
On the underside of the RH? throttle body are 2 vac ports. The larger one is used for Vac to the Cruise system, it can leak. The other port may be used, emissions junk, or capped, another possible leak. Check the LH side, why not?

The hoses, or whatever people call them that go TO each vac operated item are highly suspect, so replace hem, ONE HOSE AT A TIME, as Vac diagrams are basically useless.

 

I have been recommissioning my HE V12 after about 6 years storage. I used the AAV rebuild procedure above as mine was stuck open, and produced the surging at idle. The AAV is not perfect but works ok. I tapped the top outlet in about 1mm to lower the idle. If the idle screw does not lower the idle sufficiently, I wonder if putting a sleeve or bushing into the hole that feeds the idle bypass might help.
The next job was to get the heater working (A/C is empty). I found that the vacuum feed to the heater control had a split in the hose on the Tee fitting that splits the vacuum from the manifold to the reservoir and the heater controls. It was hidden behind the wiring loom. This could also have been contributing to the high idle speed. I repaired that and the car ran much better.
I wasn't getting any movement in the heater servo, so made up a switch to power it manually. I got it to move and cycled the servo full hot/full cold several times. When I reconnected the amplifier to it, now it works and I have a heater again. I can give details if anyone wants to make their own switch. Now to get the A/C working again!

Pete M

 

Ok here is a thing:

I did not find (or search) the vacuum leak because i did not have the chance to start and warm it up. But i did change the plugs since i had the hood off and the ac compressor out.

i noticed that cylinder 6 on the right bank is significant darker than the rest:


the left bank are all like 1-5 on the right.

Now this raises the question: How would you interprete this plug? Just rich running? Not running? Burning oil?

And also: why do i have misfire on both banks (noticeable at the exhaust) and not jist on the right?

 

Looks like oil to me.

 

1. Perfectly normal looking set of plugs from a V12 that has been babied all its life rather than being thrashed, which they should be.
2. To be 100% sure of getting rid of misfires you need to: change plugs, HT leads, Dizzy rotor, Dizzy cap, make sure the vac capsule is working, amplifier, amplifier loom and coil loom, new coil, cleanup all earths you an find, get rid of your air leak, clean and service the injectors, etc etc. Then give the car a damn good thrashing to the red line several times and some high rev high load driving. Then if you have cats, these may be partially blocked and the O2 sensors may be malfunctioning.

These are all things that are causes in an unused/barely engine as old as yours is. Work though the list and eventually you will get there!

 

I think all plugs except nr. 6 on the right side are "normal". or would you say that this is still in the range? There is no real other sign of burning oil, i would rather suspect a leaking injector. I did have all of them out, cleaned them, changed filters, put new seals and caps on. But i just testet if they have a decent spray pattern - dont have the equipment for a real test.

I actually completed your list except the "italian tune up". and i have to look for vacuum leaks again as discribed above.

Maybe i will do the italian tune up with a car following to see if there is blue smoke at any time.

 

Ya need a beer or 3, really.

A V12 that is idled, run around the block, while looking/listening for something odd, will ALWAYS do that. Greg is SPOT ON, they are a DRIVERS engine, and no one here will ever change that.
PreHE even worse.
A house rule I have from my Father (Jag man seriously), DO NOT START THAT CAR IF YOU DONT INTEND TO DRIVE IT FOR 30 MINUTES OR MORE, and he was TOUGH. My 2 kids learnt that rule, sometimes easier than others, but I did no budge, and the same today, even with the Poor Mans Bentley.


It sounds like you are chasing :Freddy Fiddler: and his patch ups over many years. FRUSTRATING for sure, but ONE system at a tome, look twice, fix once is a good system. These are SIMPLE engines, only coplicated by messy work ethics.

IF you drive less than optimal, install "P" spark plugs. I use NGK BP6EF. The "P" puts the spark out directly into the Bang Juice, as apposed to part covered up inside the cone of the plug, and it does make a huge difference, especially with the "quality", haha, fuel were are forced to use these day, as apposed to 1954 when this engine was designed.

My 1st one took just on 1 year to sort, a 1976 PreHE XJ12, and now sits at 860k kms (OZ is a BIG place), and is as sweet as the day I finished it.

You will sort it, and rtime is the key, so dont drop the bundle, please.

 

So here is the story about a post that wasnt intended to help me did help my by solving a problem i didnt knew i had:

Today i thought i take it for a long drive with the new plugs and when then have a search for the vacuum leak. With the plugs i also fixed a fuel leak in the trunk.
I didnt even start it - i noticed another fuel leak on a differen connection.
I am fighting these leaks since half a year. Always on another connection. I thought its either stupid me who cant connect fuel hoses or the bad fuel hose quality. But today i remembered a post about a bad inlet pressure regulator.
I measured the fuel pressure at the injectors while standing and it looked fine - but i did not measure it befor the inlet regulator. So i thought, screw it - i put out the inlet pressure regulator befor the drive.

And wow did that transform the car. The fuel leaks and smell are all gone and it feels like the power has doubled. I never drove a V12 befor and everyone told me it does not feel like 300hp because of the transmission - but wow that was a lame duck conpared to now. Also it idles faster (950rpm hot in P) and a bit smoother.
Also: i did never have temperature issues but it always reacted to power with a higher temperature - thats gone too.

Anyway i am not yet happy with the idle so i went home, put the air filters off and blocked the overrun valves first. There is no airflow over them unless in overrun. This seems ok.

Next i blocked the AAV - there is almost no airflow over it. ehat seems reasonable since the screw is almost all in. There is no noticeable change in revs when blocking the AAV.
Next i blocked the PCV-Valve wich results in a drop to 400rpm.
That means there has to be a huge air leak somewhere. so i pulled all vacuum hoses off one by one and blocked the port. All except the one under the Right bank. Non of these had an effect. I will get the one under the right bank off and block it when its cooled down.
I also sprayed all injectors and all other connections with brake cleaner - no effect.

One thing i noticed: The left bank seem to be way hotter than the right one. I have no explanation for that. the water temperature is at 85-90°C while driving and at 95-100°C after long idle and the electric fan comes on once in a while.

 

They can be frustrating, for sure.

The heat thing I have noticed, and moved on. The RH (A) has a huge engine fan drafting that side, the LH (B) basically has nothing. Also, the heat inside the engine bay of any V12, is horrendous, hence the constant chatter about wires, hoses, sensors, etc all have9ng approx?? a 10 year life span.

 

thats good to hear.

Would you bin the inlet regulator or replace it?

And then the search for the vacuum leak continues. I will check if i can stall the engine by blocking the inlet of the air filters - that would rule out all vacuum hoses and sealings of the manifold. I somehow suspect the PCV, i read somewhere that it should effect the idle for max 100rpm, not 400.
Its new, but the other post also hat the problem with a new one....they replaced it with a cheap one from another car.

 

Bin the FPR, more trouble than they ever tried so solve for a select few markets, USA mainly.

The PCV valve has a looooong discussion re the spring setting inside the thing, PITA on a good day. Memory again, some here used a Ford V8, 351 I think?????? and claimed it did the job. I deleted that system 100%, and rotated the Pig Snout 180 deg ish, and used a suitable fitting in the snout, and a hose dropped down and zip tied to ???dunno down below, maybe the sway bar, and removed that stupid capsule filter totally. NEVER had oily film in that hose, or anywhere else.

NOT a good modification IF you have emission testing, we dont.

Not sure on the 100rpm, as that PCV has a huge hole, so 400 would be my best guess.

 

What state is the injector loom in?

 

Hey Greg,

the Injector loom is still on my list of things to make new, but so far the pressure regulator made the biggest difference

And when i block the throttle bodies, the PCV and the AAV and the engine is still running, there has to be a vacuum leak somewhere, so i will sort this out first.

 

One thing left off my 1st list.

The Inlet manifold gasket/s, are a cardboard type medium, they last about 10 years.
BUT
During that time, they settle and the Manifold/s come loose, NOT to the feel, but air leaks will find that path. CAREFULLY tighten the 24 nuts, and see if anything changes.

 

I made the manifold gaskets new when i serviced the injectors. I did retighten them several times since they settle.

Since one water hose built a scary baloon i changed the two top hoses today befor continuing with my search. I also changed the PCV Valve to a Ford one. I noticed that this one almost closes on high vacuum.

On cold start it then idled at 600rpm, so i put the idle screw out quite a bit to get it up to 1000rpm. Then when hot i had to turn it back in a bit, but still its about 4 turns out instead of all in to have it idle at 800rpm fully warm.

I then tried and blocked both air filter inlets - and it kept running at about 400rpm. And i heared a hiss at the rear of right bank - so here is my leak. I didnt manage to locate it, i need a second set of ears next time but i now know: i have a vacuum leak and a way to find it.

I also tried to disconnect the injector from the one cylinder where the plug was darker and it did change the occusional misses to harmonic misses. If i disconnect another injector i get harmonic misses + occusional misses. So i think this injector is leaking and i will replace it when i get hold of a good rebuilt and tested single injector.

Afterwards i noticed a rather large oil leak on the floor. I mean it always made some drops at the front but today there was a hand sized puddle under the left rear side of the engine. I suspect the banjo joint at the oil filter or at the one at the rear of the left camshaft. But this is new. Do you think thats from the new PCV valve? but the pcv cant effect the banjo connections, only the crankshaft seals.