That’s right!
In a manner of speaking the idle adjustment can be viewed as a small, controlled leak…which can be masked by a larger leak elsewhere.
Cheers
DD
I’ll be anxious to see what happens after I clean it up this weekend. I
have still been experiencing some challenges. There seems to be a spot
when the Temp gauge is registering 60-70 deg C that it will stutter
badly when trying to accelerate from a stop. Even putting it in N and
trodding on the accelerator, it doesn’t want to run. If I can get it up
to 2,000 rpm or so it smooths out. It’s almost like the EFI system is
getting irregular feedback from some sensor. But once the engine reaches
operating temperature the problem totally disappears.
**
Does it actually run and respond normally before it reaches the critical point, Matt - and does it always misbehave at that temp…?
Nothing special happens around that temperature - the sensor’s, CTS, input should change smoothly. And apart from the air temp sensor (with minimal effect) on the AFM there is no specific temp sensor inputs. If consistent; the first step is of course to check resistance of the CTS at that temp - should be around 500 ohm. If there is a ‘break’ the engine will overfuel excessively, and misfire.
If the fault is intermittent; the CTS should be disconnected and the plug checked for continuity. Resistance between the wires should be around 2000 ohms, but more importantly - there should be no resistance variations when plug is manipulated. Changing the CTS has no effect if the plug is faulty. These are the conventional test…
Another possibility. As the coolant heats up and expands, coolant pressure may force coolant past a leaking top radiator hose (hose itself or bad clamping) - and drip on to the distributor, causing misfiring. As coolant leaks out, pressure is reduced, the distributor dries out - and the engine is back to normal. Check top hose and hose clamp - area can also be supervised during warm-up…
It may also be a problem with the ECU itself - which would be unusual, but so are your symptoms…
Frank
xj6 85 Sov Europe (UK/NZ)
**
Update: I spent some time last weekend removing the AFM, AAV, and Idle Throttle Body. Cleaned everything up and replaced the big hose connecting the AFM to the elbow with a new one. Nothing was especially dirty. The only surprise was that thermostat looking thing inside the Idle chamber. Wasn’t expecting that. It was protruding into the idle chamber and that’s how I put it back. Was a little challenging getting it back together, seemed like it should have been the other way (sticking down into the manifold).
The AAV is now opening to the correct amount but I am still not getting the 200rpm increase when cold. It seems like it’s a touch better than before but still will not accept throttle advances consistently until the temp reaches normal operating temp. It can start out fine, running normally for a few blocks, then wham…all of sudden at a stop light, will not accept any throttle without choking and nearly dying.
I did check the distributor vacuum advance and that is working properly. I no longer have a timing light so that is still on the to-do list.
No coolant leaks near the distributor. It’s dry as a bone.
I decided to order another CTS and swap it out. Looks like some more sleuthing is in order. When I swap out the CTS I’ll take a closer look at the plug and verify its electrical worthiness.
BTW: what is the electronic module attached to the bottom of the throttle body? I was unable to get my elbow broken loose so I could access the throttle plate. Don’t have a replacement for that narrow rubber bit joining the elbow to the throttle body and was afraid I would destroy it trying to get it off.
Matt
**
To clarify, Matt…
…the ‘idle throttle body/idle chamber’ housing the idle screw is formally the ‘air distribution block’ - did you specifically check the idle screw and its seat? The ‘thermostat looking thing’ in the air distribution block is the overrun valve. Its purpose is to open during high manifold vacuum (above 22,2" Hg) which will occur under overrun with closed throttle. It is important that the valve is working correctly and is fitted the right way around…
The behaviour when stopping during warm-up and not taking throttle inconsistently before warmed up, remains odd - a well behaved xk should respond perfectly to throttle changes at all temps. It may be the AAV coming ‘off the boil’ too early - or not increase the cold idle enough. What is the actual idle rpms at start-up?
With the throttle body cleaned up and the throttle gap correctly set; the only anomaly is the non-response to the idle screw. Which may be a factor - and should be resolved, before pursuing other matters. As said before; a possible cause of non-response to idle adjustments is an air leak - which doesn’t quite fit with perfect hot engine response…
…unless you have the O2 sensor. The Lambda system will compensate for an air leak - when the O2 sensor is hot, which takes some warm-up time. The O2 sensor should be disconnected when pursuing a possible fuelling error…
Frank
xj6 85 Sov Europe (UK/NZ)
**
[quote=“Matt_Grimes_XJ-S, post:24, topic:351768, full:true”]
BTW: what is the electronic module attached to the bottom of the throttle body? [/quote]
It’s a throttle switch.
I’m still on my first cup of coffee so others might come along to correct me, but…
As I recall the switch and wiring is arranged differently for different markets but on USA cars it simply richens the mixture under heavy engine load; only two of the three wiring connections are used. On some other markets it provides richer mixture at idle. Something like that.
Cheers
DD
I did completely remove the idle screw and thoroughly cleaned the inside
of the block a week ago. Today I replaced the replaced the temp sensor
but I’m not overly hopeful that will fix anything. I did test it in a
saucepan on the stove and recorded the resistance from 22 - 90 deg C.
When I started the motor it ran about 650 to 700 rpm. I disconnected the
AAV electrical feed and was rewarded with a significantly higher idle. I
left it that way for awhile and the idle stayed high. When I reattached
the connector it took only a few moments before it started idling down.
I’m becoming suspicious that the AAV is closing up the air port too swiftly.
Just to be clear I have not yet been able to get to the throttle plate
to set the gap.
I’ll drive it to work in the morning and see how it does.
Matt
1 Like
About a year or so ago you recommended I replace the AAV, which I did, along with a new coil for good measure. The car has been running well…(1986 XJ6 US ). Last Sunday we drove it fifty miles to an area where I parked it for several hours. When we returned to the car, when I tried to start it up, engine would run for a second, then die. On the third try, I did the unforgivable before turning the key. I depressed a accelerator pedal half way. This time the car started right up, and I revved it up to 1500 RPM for about ten seconds. When I took my foot off the pedal, the engine ran fine. In fact, it ran fine all the way home. The next day, we took the car out for a shorter drive, making several stops in different places. This time the car performed flawlessly. What could have called the strange stalling hiccup the day before ? So far, so good, but I am still curious, as I’d prefer not to worry about this happening again any time soon. Since I replaced the AAV not too long ago, should I just put some contact cleaner on the electric plug on the AAV, or just leave well enough alone?
Mel R.
**
Fully depressing the pedal is indeed a ‘last ditch’ remedy for a non-starter, Mel…
On carbed engines, cranking WOT is used when the engine has been flooded. On EFI versions it seems more like an unscrambling of a confused ECU. The drivers’ manuals advices this as a remedial action - adding that if the this restores normal operation; ‘no further action is required’.
The cause of this anomaly is uncertain - it’s regarded as an inexplicable ECU occasional malfunction - however, there is also a second consideration. As the engine starts to cool down, its starting requirements, fuel and air, begins to change. This is normally countered by the CTS sensing and the gradual opening of the AAV. But while this works fine fully cold and fully warm; the AAV especially is not necessarily fully ‘synchronised’ with temp changes and engine demands at intermediate temps. It’s not unusual to experience low idle after a 20 min shopping stop…
In your case; the AAV may have ‘hung up’ closed - and with too little air, the engine has to little air to idle properly. Check that the AAV slide moves smoothly when manipulated…?
Frank
xj6 85 Sov Europe (UK/NZ)
**
Since I purchased this AAV used from a breaker here in the US who specializes in series 3 XJ6 cars, I will assume the part is ‘less than perfect.’ For the moment, I’ll go with the drivers’ manual advice of ‘no further action required.’. Thanks again for your sage advice, which I believe offered the added benefit of lowering my blood pressure today…
Mel R.
**
Idle rpms is a combination of idle screw, throttle gap and, during warm-up; the AAV, Matt…
To some extent the idle screw and the throttle gap are complementary - the idle screw compensating for some deviations in throttle gasp setting. The idle mentioned, 650 -700 (cold in neutral?), is then too low for comfort - but the reaction to electric disconnection of the AAV is not in itself necessarily ‘wrong’. The slide opening reacts both to engine heat and the electric heating - both affecting the bimetallic spring that operates the slide…
…in principle, after starting the Jaguar expect you to drive on - and opening the throttle allows the necessary air entering the engine, independent of the AAV. The AAV is primarily a starting aid, to counter initial cold engine drag - which very quickly drops as lubrication gets going, and the inside of the engine quickly warms up. Without the electric heating the AAV responds only to coolant temp - which rises very slowly. The electric heating sort of better matches the inside of the engine…
However, your electric heating seems to ‘overreact’ a bit - which may relate to the rather small opening reported on your AAV. But in any case; measure the resistance on the electric connections on the AAV - it should show either 22 or 33 ohms - depending on type fitted. If the resistance is much lower, the heating coil warms up the bimetallic spring too quickly - prematurely closing the AAV. The only drawback with running the electric disconnected is that the idle will stay high for a long time - which is immaterial while driving…
In all this; you still need to ensure that idle responds to idle screw adjustments. Did you check that the idle screw seat was present and correct…? If it is not an idle screw fault, something else is wrong, which may affect engine behaviour…
As an aside; idle is related to the amount of air entering the engine - a CTS fault only affects mixture. While meeting the precise specs listed for the CTS temp/resistance is desirable - some deviation will not noticeably affect engine behaviour…
Frank
xj6 85 Sov Europe (UK/NZ)
**
Coming home from work on the motorway the other evening I was startled
by the Jaguar suddenly “roaring”. Everything seemed to be ok aside from
the noise so I proceeded the few remaining miles home. After some
cursory inspection I determined that the cooling fan clutch had seized
up. The fan itself survived the ordeal, I saw no evidence of any
cracking or missing bits. The fan clutch however has severe cracking
around the bearing and I am fortunate that it did not explode and make a
mess of the radiator. Phew…dodged that bullet. A replacement turned
out to be challenging as all the parts stores were out but eventually I
found a store that was able to bring one in the next day. Replaced it
last night; a fiddly, annoying job but not too awful.
I purchased a vacuum gauge, compression gauge, and timing light on the
way home this evening. The new air hose will hopefully arrive by the end
of the week and I will finally be able to remove the elbow and access
the throttle body to check the gap.
I measured the resistance of the AAV (cold) and it is 40 ohms.
The last inspection of the AAV (after cleaning it up) showed it fully
open now when cold. BTW…where does the air hose leaving the back of
the AAV go? It’s rather hard to tell. It would be good to verify that
the hose is in good shape. I remember on my V12 the hose running from
the AAV looped under the throttle body and had gotten very soft from oil
seepage. It would collapse under vacuum (then release) yielding a very
humorous “surging” when the engine was cold. Had a devil of time
figuring that one out.
Regarding the idle seat, I saw nothing in the air distribution block
that I would consider a “seat”. I simply cleaned up the interior of the
block and removed the idle screw and cleaned it up putting some
anti-seize on the threads to make it easier to turn. I’m guessing the
“seat” is molded into the block? If not then apparently I don’t have one.
All of my testing efforts so far have yielded inconclusive results. I
will attempt to resolve the last few remaining questions this weekend;
- checking the timing
- verifying the throttle plate gap
- measuring the compression
- measuring the engine vacuum
I am curious about the overrun valve. At present, when I rev the motor
by stabbing at the gas pedal, it will rev up some but then drop clear
down to 500 rpm before settling back to the set idle speed. Is this the
sort of thing (overrun) you are referring to?
Matt
1985 XJ6 - Colorado, USA
**
Lucky escape with the fan clutch, Matt - no harm done then…
With the AAV opening more when cold - has the cold idle increased as it should? And have you verified that the AAV is properly closed with the engine hot? The 40 ohm resistance of the AAV is too high - this will likely slow down the closing of the AAV, which is of little consequence, though implies that the heating coil is incorrect. More to the point; is the car now behaving as it should during warm-up driving…?
As for the idle screw seat; does the engine now respond properly to idle adjustments with the engine hot and the AAV closed?
The hose underneath the AAV is connected at the throttle body - it’s function is just to bypass the throttle butterfly. If the hose is leaking, or disconnected, it will be a vacuum leak…
Functioning properly the overrun valve will open whenever manifold vacuum drops below 22,2" Hg. Manifold vacuum depends on the amount of air the engine requires (which depends on rpms) and the air available (which depends on throttle opening). ‘Overrun’ relates to engine braking - high rpms with the throttle closed, which creates high manifold vacuum. Without the extra air supplied by the overrun valve, the mixture might go too lean, causing irritating backfires…
As you blip the throttle, rpms rise - and as the throttle is released, manifold vacuum will increase. Increased manifold vacuum allows less air into the engine, causing loss of power; rpms drop and the rpms may ‘overshoot’ below idle - before settling at the set idle rpms. It’s all related to engine inertia and engine management response times - and is not abnormal. The vacuum created by smallish rpms drop usually do not operate the overrun valve…
Frank
xj6 85 Sov Europe (UK/NZ)
Things are improving now. I had to cut the old “rubber band” off to get to the throttle body. The inside of the throttle body and butterfly were very dirty. I cleaned them up and proceeded to check the gap. Turns out there was no gap. The “stop” wasn’t even touching the arm. Go figure. So, I set the gap and adjusted the “stop”.
Also found a very short, oil rotted vacuum hose exiting the throttle body to a tee, which I replaced. Put it back together, started up the motor and was rewarded with a cold idle of 1,500 rpm. A bit too high, however, turning in the idle screw had no effect on the idle. Apparently, the throttle gap was not quite right. Perhaps I should have done it after warming up the engine. Backed off the stop a bit and then was able to utilize the idle screw. The whole process seems a bit fiddly but I am confident I’ll reach a happy medium soon.
It definitely idles smoother now. I need to drive it a few more times to know if the problem is completely solved. It did behave better this morning. I still plan to check the compression one of these days when I get home from work.
1 Like
**
Any attempt to adjust idle without the engine being fully warmed up is bound to fail, Matt - it won’t react properly to the idle screw due to the large amount of air via the AAV…
Do reset the throttle gap to the proper value - however fiddly; and a clean throttle body is important. With the throttle gap too narrow, the engine will ‘overreact’ when the throttle is released - the idle screw bypass provides too little air for good transition between open and closed throttle. And reset idle with the engine properly warm - ensuring that the AAV is properly closed with the engine warm. While the throttle gap is not overly critical; the idle screw set is not ‘a happy medium’ in relation to the throttle gap - the throttle gap is a constant, idle to be adjusted by idle screw only…
I think you are out of woods - though finer points may still require some attention. Good work…
frank
xj6 85 Sov Europe (UK/NZ)
**
Well the problem is permanently solved now (for me) although not in the way I expected. I received an offer I could not refuse this weekend and have sold the car. The cold idle situation had improved to around 90% proper functioning but would still stumble around a certain temperature range. I can only suspect that replacing the AAV might have totally solved the problem but now it will up to someone else to finish it.
Thanks for all the input! It has been remarkably valuable and this group remains my go-to source for Jag wisdom.
So, I am now Jag-less. Which will be ok for awhile. I am probably done with the Series III and XJ-S models. I am thinking my next Jag acquisition will be a couple of decades newer.
Matt
Matt been reading your posts on all the issues you had with your jag! I just want you to know, if you had not had problems with your jag then we all would not have learned anything. sometimes I know my self and sure others don’t really want to put there 2 cents in, because some of us are newbies. thou reading others issues helps others to understand
how certain systems on jags work. and yes this is also my go-to place for jags. There are quite a few experienced brain boxes on this site. that’s why we can come to here with our issues and someone always guide us in the right direction. Good luck on our next find matt.
cheers
cheers