I am still chasing a problem with my XJ6 S3 4.2. I have recently acquired a good second hand AFM (from Paul Novak, thanks Paul) and installed it. Assuming that this AFM was properly adjusted in it’s previous car (or at least my PO hasn’t had a chance to get his hands on it) There has been no real change in my problem except the car is idling very high now. (around 1600 RPM). I have noticed that as the car gets closer to operating temperature there is a click (I am sure I heard this) and then the car starts to cycle it revs up and down. I tried disconnecting the micro switch on the throttle body and shorting the leads together which then sorts out the cycling. Should I assume that this switch is faulty? I have only replaced this switch about 6 months ago but am now wondering if it is just not suitable for the hot environment of the engine bay. It was sourced new from a reputable Jag parts supplier here in australia. So assuming the switch is faulty can I just leave the leads shorted together indefinately or should I replace it again.
I am still not convinced that my vacuum system is working properly. I have just purchased a vacuum gauge and would like some help on how to test the vacuum. My car has an abbreviated vacuum system.
The distributor is connected to one of the spigots on the throttle body the other spigot is blocked off. The fuel pressure regulator is connected to the fuel temp sensor which is also connected to the manifold.
All the other spigots ( 1 on the manifold about half way along, 1 under the manifold are blocked off. The heater and a/c are not connected at present.
When I connect the vacuum gauge to the throttle body on the unused spigot I get no reading at idle but when I rev the motor the vacuum gauge gives a reading. I am not sure of the correct method or position to use this gauge. And how to interpret the results.
Can anyone please give me some pointers please?
The switch is okay, Con.
At an idle of 1600 it will definitely cut the injectors until they fall below, I think, 1400, when the throttle is closed. This results in the sawing action. Your idle is set way high. Maybe your AAV is stuck open, pull off the hose from above and peer in, move the slide, clean it (if you see a very small hole that is alright, it should move freely, etc.). This is the most likely solution.
The spigot on the throttle is ported vacuum (we debated this once, Frank…?) and will only show something with the throttle cracked open.
If it is not the AAV it should simply be a question of setting the idle at the air distribution block.
High idle is exclusively tied to high air ingress, Con - air leak or idle items settings. The AFM reed switch only control the fuel pump - irrelevant to high idle or rpm cycling…
Check throttle gap (0,002") and that the throttle cable is not hanging up and has a slight slack - and abuts the tiny throttle stop screw.
Check that the AAV slide is fully closed with the engine hot.
Check if the idle responds to idle screw adjustments.
Cycling between 1600 and what rpms? A fault with the overrun valve (in the air distribution block) may cause both high idle and cycling. And an overrun cut-out switch fault at the throttle pedestal/body may cause cycling - but at different rev limits. With the throttle closed above 1600; fuelling is cut - but resumes as revs drops to 1400 rpms. Again rising the rpms - repeating cycling. Disconnecting the switch disables the overrun fuel cut-off - eliminating this cause of cycling. With the overrun switch reconnected; slightly applying throttle should eliminate the cycling (rising the idle slightly).
The basic cause is to high idle - see above…
This spigot may be used for ‘ported vacuum’ - used with ‘US’ ign set-up. The other spigot, giving manifold vacuum is likely used for AC vacuum etc…
The vacuum gauge is a very useful diagnostic tool - but requires some interpretations.
In your case; connect the gauge to the hoses, in turn, at the pressure regulator, at the distributor and then at the vacuum reservoir. All readings should be the same; manifold vacuum. Which nominally should be some 18" Hg, and steady - with the engine idling. Report readings…
‘0’ reading at the ‘vacant’ throttle body spigot is normal, but must be blocked if unused. Indeed; with suspected vacuum leaks vacuum spigots/hoses should be checked. Incidentally; the spigot under the manifold is a drain…
In short; pursue the reason for the high idle - and rectify…
xj6 85 Sov Europe (UK/NZ)
Hi Frank, I suspect that the PO has probably fiddled with mixture control and the new AFM is closer to normal than the original. The car was still running quite rich as evidenced by the sooty but not oily plugs. I think thwy will be much better from now on. I also suspect that he may have done this to “adjust” the idle without really understanding what he was really adjusting. I am working on cleaning the Air Distribution Block and the Auxillary Air Valve. I also want to check the viability of the hoses associated with those parts.
Is the fuel pressure regulator correctly connected to the fuel temperature sensor? Can I bypass this and connect it straight to the manifold?
David thanks for your input. I now seem to remember that we have discussed this before and you put me right then too. I really have to start writing this stuff down.
Thank you both
Yes, you can bypass it. But the cycling is from the (no-) throttle switch plus the high idle. I had that when I replaced my air distribution block.
Could well be! No worries.
I think you should tear into the air distribution block if the overrun valve inside is stuck open. Before that and especially with the PO story have you already bottomed out the idle air screw on said block? Pain to get to but as long as there’s adjustment the PO might have messed it up and then used the idle mixture bypass on the AFM to bog it down.
I suggest it must be metered air. Not a leak but bypassing the throttle provided the new AFM does the same.
A bit of soot when idling cold should be very normal though.
The high idle might relate to the overrun valve in the air distribution block, Con…
If not carefully noted; the valve can easily be reverse mounted. Also, while in the air distribution block; inspect idle screw and it’s seat - a displaced seat may increase idle and and render idle adjustment impossible…
The ‘fuel temperature sensor’ is just a valve that dumps fuel pressure regulator vacuum at high fuel temperature. This raises fuel rail pressure, increasing fuelling - basically to compensate fro the lesser density of hot fuel?
Indeed, if the sensor has failed the engine will run (very) rich. And since hot fuel is a seldom occurring feature; bypassing/omitting the sensor is harmless - and may indeed be beneficial.
As David also says; the cause of the cycling is likely caused by high idle - which is unrelated to the fuel temp sensor or any adjustment of the AFM. It’s simply too much air to the engine…
As an aside; the mixture adjustments, AFM bypass, has a limited range - but adjustments should be done with a CO meter. A crude setting is to bottom the bypass screw and back off some 2 - 4 turns…
xj6 85 Sov Europe (UK/NZ)
Lots to chew on. The CoVid shut down in South Australia has weirdly resulted in my work levels increasing, so once again, time has become rather short in supply. Include in that, the wet winter and things on the Jag are going a bit slow but I now have a plan for the next steps to take in getting this beast running properly.
I will post with the results of these things as they are completed
“Incidentally; the spigot under the manifold is a drain…“
Frank- Interesting! I just redid/rerouted all my vac lines on 86 S3 and as I was finishing up I was feeling around under the intake area from above (no lift).
I was surprised to find what felt like a spigot under there the had a bit of vac hose with a screw inserted in end. I suspect a PO or service person could not figure its use and thought to block a vac leak
What exactly is the drain for??
Sometimes there is fuel or oily condensation in the manifold. The drain makes life easier then.
I also had a piece of hose with a sheet metal screw there.
A way to clear a flooded engine???
Yep I"ve got the same plug. The car doesn’t run well without it. I think mine blew off originally during a back fire in the early days of getting this car running. I fitted another plug after noticing a largish fuel leak and finding it coming from there. I think Frank pointed out that it needed a plug. So I made a new one and the motor has been running since although not as well as it could.
As David says, Randy - condensation in the manifold…
However, when open; it is a minor vacuum leak - though that can be countered by suitable mixture adjustments. Though, frankly, it’s value as a drain is somewhat dubious…
xj6 85 Sov Europe (UK/NZ)
Ok ,so I know where the idle adjust screw is on the air distribution block but I don’t know how to get an Allen key in there to adjust it. I know that there is some sort of bending required to make this accessable but can any one tell me what bending is required and where on the Allen key. Also is the Allen key size the same as the idle mixture socket on the AFM, 5.5mm? I have bent one Allen key up but it doesn’t seem to be right as I still can’t get it into the socket. A picture of the key would be great.
Negative, Con - the idle adjust screw is 5 mm hex, or the imperial equivalent…
Access to the idle screw is indeed difficult, and listers have used various solutions. Mine is a socket with the hex, of suitable length, glued in…
xj6 85 Sov Europe (UK/NZ)
Thanks Frank, As usual I posted then searched the archives and found a good article by Doug Dwer and Alex Cannara with all the info I need.
Got some fabricating to do.
Here is a what I have done on my Series III XJ6s over the past 20 years to adjust the idle using the Air Distribution Block. I cut off about a two inch piece from a 7/32" Allen wrench and put it into the Air Distribution Block as shown circled in the attached picture and leave it there. Then when I want to adjust the idle I rotate the Allen wrench with a small 7/32" crescent wrench. It is always fiddley to get the crescent wrench onto the Allen wrench, but that is part of the fun.
Now the important step is to tie a 9" piece of wire onto the end of the crescent wrench before trying to make the adjustment. This will save you a LOT of time because YOU WILL DROP THE CRESCENT WRENCH while making the adjustment and you don’t want to lose the crescent wrench. There is no need to ask me why I know that this is important. In fact in addition to the 9" of wire I also put some blue painters tape on the end of the wire to help in seeing the crescent wrench when dropped.
BTW, I took this picture of the Allen wrench in place in the engine that I transplanted into my former 1984 XJ6 Vanden Plas last year specifically to share with others like you who wondered about this.
Good tip, thank you.
I tapped in a suitable torx bit and use a wrench on that, yet it still is a bit of a pain.
At the emissions testing the guy gave me a very funny look when I turned up the idle a little so he would be happy with the CO values…
well I think I have found what why the idle speed was so high. I have adjusted the idle down to a lower 900 RPM. (still not at spec idle but much lower than before.) I turned the idle mixture back to zero as suggested by Alex Canarra in another thread and checked the timing. It’s sitting around 32 degrees with the vacuum advance disconnected which seems a bit high but the car is running ok at that so I left it there. Then I readjusted the idle mixture 2 full turns in the anti CW direction and left it there. So time for a test drive into reverse and the car just cut out. Started up straight away and tried again and cut out again. Hmmm! Is it too rich or is something else going on? With the higher idle speed the car didn’t do this. I am open to suggestions
Advanced timing will indeed rise your idle, but in the long term also kill your pistons…
You should adjust it to the factory specs.
Isn’t this just guesswork?
You might expect to pick up another 20 degrees as the distributor weights do their work - which is much to high. Think Con should come back to specification timing. Sounds a bit like the distributor has gone on a little excursion. Paul.