decided to test the vacuum today on my 88' xjs as it has been starting but dying after a couple minutes. First I tested in back at the ecu. Started it up and only got to 5 inHG... gave it some throttle and it dropped to zero. Went up from and attached it... it got 7 inHG before dropping to zero when given throttle. So ahh this is real bad, yes? Where are the most common or largest leakers of vacuum?

 

I recall "normal" vacuum as being around 15-20 but it depends on altitude, but obviously dropping to zero is what you would expect..

A lot of the small hoses are the same material that rots. The whole engine bay is hot, so you may just have to do the old wd40 or starter fluid trick to hunt for a leak.

 

14" about normal for these engines.

You could have low vacuum due to a leak....it would have to be a pretty big one, I'd think. And you'd likely have a high idle.

Or the low vacuum might be that the engine isn't running right.

Cheers
DD

 

yeah I had read 15-20hg. trying to track down a leak is always fun and usually does make a car run like crap. Hopefully the engine can stay running long enough to find a leak with starting fluid.

I don't have a high idle... prob 500rpms. It's like the chicken or the egg? Is the car running bad because of poor vacuum or is the low vacuum caused by a bad running engine. Ha

 

For the vast majority of engines, yes. The Jag V12 doesn't develop that much. I don't know why



Of course you'll want (and need) to trace and repair any leaks but if the idle is normal you might not have a leak at all......unless someone has been there before you and masked the problem by adjusting the AAV way low to compensate. The AAV itself is simply a controlled vacuum leak for all intents and purposes.

On this type of F.I. system any incoming air from a vacuum leak has the same result as opening the throttle. Neither the engine nor the F.I. system 'knows' if air is coming in thru the throttles or if it's coming in thru a vacuum leak.

Cheers
DD

 

Depending on model, the Jag V12 has a vacuum regulator that limits vacuum to 10 in hg. That figure may be 11 in hg, too early for me.

edit: duh, ECU pulls vacuum from the manifold. Like I said too early for me. Back to my coffee.

edit again: Regardless, I'm pulling 18in hg here and I don't know of any reason any properly running engine would only pull 14in hg.

 

the aav is out pretty far. So many things that could be with a poor running engine. When I apply throttle the car wants to fall dead on it's face... couldn't a leak cause it to dump too much fuel and want to flood? I'm at the point where I feel I'm getting too much fuel because the car usually takes an hour to restart

 

Too much fuel:

- pressure regulator on B bank working properly? Correct one fitted?
- injectors opening correctly?
- temperature sensors reading right (saying too cold)?
- air temperature sensor malfunction?
- AAV closed too far? Turn in inwards and slowly turn it out with the engine running until you get your idle straight
- throttle body butterfly valves set up correctly?
- idle switch set up correctly?

A vacume leak would more so cause a too leaner mix rather than a rich one (causing it to flood). Iirc the XJ-S V12 doesn't have much of an adaptive ECU, apart from the lambda adaption on cat-models. It runs off of a fixed mapping, with tiny alterations for water temperature and air temperature (IIRC). Everything else is basically mechanical or via vacume hose.

 

no gas in the vacuum line of fpr. Held vacuum. Injectors all clicking, I had cleaned and tested them all. Maybe my rebuilt injector harness is to blame but all injectors are clicking so I don't think it's the issue. Air temp sensor unsure of. Throttle body's I'm going to look at today. Tps is good. I'm going to install a fuel pressure gauge asap to see if the pump is to blame. New filter put in but didn't check the sump filter.

 

Check the B bank side idle switch... It is mounted on the rear of the intake, above the AAV.

 

Ok I'll check that... adjusted the air gap on the pickup.. it was very close to the starwheel. And hooked up the mityvac to the vacuum advance... nothing. This is me blowing air into the distributor

https://streamable.com/ag5ef

 

You say TPS is good, is it also correctly set?

 

yep did that a couple weeks ago.

Put it all back together again. And it idles decent and if I slowedly give it gas it will climb the rpms and I can hold it at like 2-3k. If I floor the throttle then it won't climb and makes a burrrr noise. It smokes out the back like a son of a gun and now the atf coated catalytic converter smokes. The whole thing is worrying.

 

There's a MAP sensor inside the ECU that takes its reading from a long vacuum pipe that goes to the balance tube at the rear of the engine. Your low vacuum is being interpreted by the ECU as open throttle and heavy engine load....and is adding extra fuel accordingly. The MAP sensor is the main player is fueling.

The fuel pressure regulator will also increase fuel pressure a bit is response to low vacuum.

Cheers
DD

 

I don't think you'll find one on a '88....which should have Lucas ignition.

Cheers
DD

 

Good ! But you need to check pressure to actually prove that it's working correctly


You're probably right and clicking is a good sign....but injector operation is measured in milliseconds so if an injector is staying open too long you probably couldn't tell just from the clicking.

Everything is worth checking but the air temp sensor is just a trimmer. If the engine is running well you can disconnect it and hardly notice any difference to the the way the engine runs. I wouldn't suspect it in this case, personally.


Good!


Or the regulator. Pressure test is a great idea.

Cheers
DD

 

So so basically it's getting too much fuel? I'm not too familiar with how an engine sounds when getting too much fuel... but the sound it makes when I open it up all the way can only be described as putting your face in a pool and blowing out air...burbblebuebleburble. And also this is worrying because it seems the cats are heating up real fast and seems like a good way to catch fire?

yeah no switch it's a Lucas

 

Might depend where the leak is, exactly, but a vacuum leak typically would not change the fueling or mixture

The engine and ECU will respond to incoming air from a vacuum leak just the same way as they respond to incoming air from an open throttle. The ECU will adjust fueling to match, just as though the throttle was opened. More air is more air, be it from an open throttle or a vacuum leak.

If these engines used air flow meter or air flow sensor, it would be a different story. In those cases incoming air is measured and the ECU uses that measurement to adjust fueling. So, with an AFM or MAF system a vacuum leak would be 'unmetered air'.....air coming in that isn't measured and the ECU doesn't know about, which is a problem.

Cheers
DD

 

so really the idle should be faster with a leak? This is all new to me. The only other older car I have is an 86' 944.. 1/3 the cylinders and runs like a Swiss timepiece. It had a vacuum leak and ran like a truck because of it... but didn't give me the fear I was killing it slowly. As a complete amateur the v12 fascinates me and is frustrating me.

 

Right.

Think of the Aux Air Valve. It increases idle speed on a cold engine by letting more air in. It might be called an intentional, controlled vacuum leak. Or, if you leave the air cleaner housing bolts out the idle speed will be very high...because you're letting more air in. Meanwhile, the ECU adjusts fueling to match....because it doesn't know or care if the air is coming in through the throttles or via some other source.



In a case like this diagnosis is more about 'process of elimination' than anything else, IMO. Choose a stepping-off point (fuel pressure is as good as any, IMO) and go from there. If that checks out, move on.

Cheers
DD