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my workshop worked on a rough idle problem of my car (XJ-SC 5.3 V12 from 1987) and exchanged - besides some other parts - my AAV.
The rough idle is much better now. But the car has a very strange cold start behaviour now: (i) for the first 30-45 seconds it has a very low rpm of 400 (the engine nearly dies and its not possible to start driving); (ii) then for the next minutes the rpm goes to 600 rpm, its possible to start driving, but when I start with too much pressure on the pedal the engine dies too; (iii) after 5-10 minutes with a warm engine the rpm is 750 and everything ist smooth and perfect.
The workshop gave me the first response, that they tried several adjustments of the AAV´s setting screw and the adjustment at the moment is the best compromise.
But following my research on the AAV´s funktion, the AAV´s adjustment screw should control only the warm engine idle and it has no influence on the cold idle; is that correct?
And isnt´it the main feature of the AAV that it increases the idle speed IMMEDIATELY after the start to 1000 rpm and then -- the warmer the engine gets -- the idle goes down to 750 (while these 750 can be adjusted with the setting screw)?
Is it possible that my new AAV is defective and does not open at all during the cold start?
Thanks in advance
Hendrik
Last edited by DocBottster; 06-22-2021 at 08:02 AM.
Hi
The asdjustment screw is controlling how much air by-passes the AAV With a hot engine it is used to set the ides speed, but it will also influence the total amount of air @ cold and thereby influence the cold idle.
The amout of air that passes the AAV should depend on engine temperature: cold engine --more air--> higher idle RPM, , hot engine --less ai--> lower idle RPM. So indeed your AAV does not seem to do what it is supposed to do.
Car is back in the workshop and they say, it‘s a new Jaguar AAV and they already checked it when they installed it. In their opinion the AAV gives sufficient air during the cold start, but for any reason the engineManagement does not react to that with the perfect amount of fuel.
And they think my new AAV still has a positive effect, but just not in the first 45 seconds and then later not very much.
In their opinion finding the causing defect on the engine-Management would take a lot of time and money and it’s not worth that, as the car runs beautiful when it’s warm.
What do you think? Give up and give the car the extra time for it‘s „own“ cold start procedure?
Check for Vacuum leaks, especially on the Elbow that goes into the AAV as this can have a marked effect on the idle (check for vacuum leaks everywhere)
I would lay $ on the AAV is not opening as designed when cold, thus not enough air being drawn in.
As you are finding, once the engine gets some warmth in it, the idle settles to where it should for the HOT setting.
Relying on people is always hard, and accepted to a degree.
I would be removing the Intake hose, air cleaner TO AAV, using a mirror, look up the AAV intake, observe the COLD spec opening, I reckon the thing will be shut, or very close to shut.
If that is not possible, ask them to remove the AAV, give it to you, take a snap of the Inlet, as I have shown in that Doco, and lets look at it.
Ask them HOW they tested it, listen to the explanation. It might sound sensible, it may sound of "spin", as we say.
Remember, just because its NEW, does mean it is GOOD, and after 55 years in Spare Parts I know that for FACT.
Vac leaks as Alex has mentioned, are very valid, BUT they will reek idle havoc once its at operating temp, and you do not have that, so AAV is my bet.
Last edited by Grant Francis; 06-29-2021 at 05:35 AM.
Car is back in the workshop and they say, it‘s a new Jaguar AAV and they already checked it when they installed it. In their opinion the AAV gives sufficient air during the cold start, but for any reason the engineManagement does not react to that with the perfect amount of fuel.
And they think my new AAV still has a positive effect, but just not in the first 45 seconds and then later not very much.
In their opinion finding the causing defect on the engine-Management would take a lot of time and money and it’s not worth that, as the car runs beautiful when it’s warm.
Ask if they've checked the coolant temp sensor....which plays a vital role in cold engine starting and running. It's simple resistance-vs-temperature test. For that matter, they're not expensive and are easy to replace....so replacing one on the basis of "give it it try" isn't painful.
If the CTS is giving a skewed reading the engine will never be quite right when cold. Over or under fueling when cold can result in low idle speeds and poor driveabilty....even if the AAV is OK
Check the vac hose on the cross/balance pipe at the back of the engine bay... It goes to the ECU in the trunk. It's easy to leave off and forget when installing if they removed it for any reason during the AAV install...
And, I'm sorry, do away with them service folks... That's jus me...
I´ll get the car back next week and the first thing is to check the AAV cold opening with a mirror. Next thing will be the cross pipe and then the coolent temp sensor (it was recently changed, but still wort checking).
My workshop just sent me these picture of the AAV with cold and hot engine. The AAV does seem to open when cold, but maybe not wide enough.(?)
What confuses me the most is the totally different design of my AAV´s aperture (compared to the photos in Grant´s document). Is that really an AAV for a 1987 XJS V12?
That is open for sure, but not much, so cold extra air will be minimal at best. That is the way its been assembled, as per my write up, and adjusting that could be awkward at best. Basically the :"op hat" section needs to come out of the main housing about 2mm.
HOT seems to be good.
Decisions, decisions. Do you have the old AAV?, maybe worth rebuilding/cleaning it and see what happens.
What do you think about the design? I that even the correct part?
There were several variations. I think the different aperture shapes were designed to give different high idle speeds depending on market or emissions requirements. For example, 1100 RPM for xxx-emission package and maybe 1250 for yyy-emissions package.
I'd think any of the choices would work as a replacement...so long as it is working properly.
Well, closed looks closed. Doesn't look like there is much to worry about at that end of things... But, open doesn't look open and that is what your symptoms seem to be agreeing with.
Hmm. Do you have the old AAV? Maybe service it and reuse?
If the wax ball gets old and doesn't function to completely seal off the inlet due to age and time, it's similar to a thermostat that way, there is a group that sells a kit to replace the wax ball section in an old AAV. I've seen it but it's a google search to find.
Expensive new parts are a PITA cuz although it's tempting to cut a part open to see what's going on - basically you OWN it at that point. I would be tempted to take a fine flat head screwdriver and carefully pry down on the AAV piston and see if (even if it won't remain down) there is more down (up?) travel potential. Wait,,, do I have the piston travel backwards, lol? Well, you know what I mean, I hope.
Of the dozens I have refurbished over the years, I have noted many different "cut outs" in the mouth.
Yours would be be least amount of opening "cut outs" I have ever seen.
Looking at the cut out (in the housing) for the extra air at cold, it seems that the opening at the bottom gets much bigger, or is supposed to be a part of the inlet, but the cylinder is maybe NOT going down far enough to allow that.
These things can sit on shelves for years - decades even.
Is is possible to apply a bit of force in the down direction? Could it simply be stuck, unable to go down fully, maybe on a little bit of leftover casting in the manufacturer process, or be cockeyed or both? Stuck on old lube that's gone waxy? Looks like the design is that the cylinder "*should*" go down further.
*EDIT* Just beware! I mean if ya took a screwdriver and get it to go DOWN that added 2, 3, 4mm - it MIGHT not come back up!? Be careful.
My workshop now says the small opening is a result of the high temperatures in Germany at the moment; the AAV needs lower tempertures to fully open. We have 25° C here at the moment (which is 77° F)...
I think I´ll have my original part refurbished and give it a try. If the car reacts the same way with the refurbished part, the issue definitely is somewhere else. Any idea who does these refurbs in Europe?
My workshop now says the small opening is a result of the high temperatures in Germany at the moment; the AAV needs lower tempertures to fully open. We have 25° C here at the moment (which is 77° F)...
I think I´ll have my original part refurbished and give it a try. If the car reacts the same way with the refurbished part, the issue definitely is somewhere else. Any idea who does these refurbs in Europe?
I don't agree with the mechanic... It's been in the upper 90s here. If 77 degrees shuts or partly shuts an XJS AAV, I'll be a monkeys uncle...
06-22-2021, 08:00 AM
V12 - Is my new AAV working?
