Speaking of Idle Adjustment

XJ
1

After a significant relocation, and 6 months in the garage, I’ve gotten my 85 Series III barn find out and started driving it again. It sat idle in the garage because the climate control system was stuck in full cold mode. Not too nice in the winter! Finally got that repaired thanks to Gary Crosby and a rebuilt Servo Motor. Now I am chasing several other problems.

The idle is doing two things that I don’t like:
a. When the engine is cold it tends to bog and die when you try to accelerate from a dead stop. Rather embarrassing at stop lights. Based on the FAQ article, I have started diagnostics on the Aux Air Valve and after removing the hose, find its little valve is closed when the engine is hot. Have not checked when cold yet. Will try and do that in the morning. Anything else that could be affecting this?

b. The idle sits at 1,000 rpm in Park but drops to around 500 rpm when the transmission is put into gear, and then adjust up to 750 rpm. I can adjust the idle high using the allen screw but when put into gear it always settles back to 750 rpm so it seems to be controlled by something else. Is there anyway to tweak the idle speed while it’s in gear?

Matt Grimes
Back in Colorado Springs, CO
1985 Series III (Barn Find)

2

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Firstly, Matt; idle is always set with the engine hot, in neutral/park and AC off…

Due to ‘feedback’ from the torque converter in-gear idle tends towards 750 rpm - if idle is increased, the resistance of the torque converter also increases. And is reduced if idle is lowered - all within reason, of course, but 750 rpms in gear is fair enough anyway. However, the difference between idle in neutral and idle in gear is an indication of engine state - brand new the spec idle was 800 rpms, giving an idle of some 750 in gear. The point of this is; what is your idle in neutral, engine hot…?

As for the AAV; it should be about half open with the engine cold - gradually going to ‘closed’ as the engine heats up. The idle adjusted hot; the cold idle will be about 200 - 400 rpms higher with the engine cold. The reason for the enhanced cold idle is ‘cold engine drag’ - due to higher friction etc with cold engines. Check AAV slide as you intend…

There should of course be no engine hesitation at any engine temperature. A specific fault is overfuelling during cold acceleration - watch for distinct exhaust smoke. This fault is rectified by a simple modification, using a capacitor - see ‘cap-mod’…

If there is no smoking during cold take-off, it gets more elusive - but check idle rpms and relation to the AAV as an initial diagnosis…

Frank
xj6 85 Sov Europe (UK/NZ)
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[quote=“Matt_Grimes_XJ-S, post:1, topic:351768, full:true”]
The idle is doing two things that I don’t like:
a. When the engine is cold it tends to bog and die when you try to accelerate from a dead stop.[/quote]

My first thought is “under-fueling”.

I would begin with something very easy. Check the coolant temp sensor, as it’s the primary player in fattening-up the fuel mixture for proper cold running. Resistance values are in the FAQ section.

Or, if testing doesn’t appeal…just replace it. Only $20 or so and 5 minutes labor. This is one item where taking a chance is painless.

Cheers
DD

4

Hi Doug,

When I pulled it out of the barn (sitting for 15 years) I went through
and replaced a number of things right away. The coolant temp sensor
was one of those things along with hoses, thermostat, coolant, and
heater control valve. I did check the resistance values last night and
it was spot on when the engine was warm. I was running late this
morning so did not get to check things out with a cold engine.

It did it to me again this morning at the first stop light I came to.
Died when I tried to accelerate and after restarting; coughed,
snorted, and didn’t want to run at all but finally cleared it’s throat
and was fine again.

Many things I have found to be in a “stuck” condition. The fuel
injectors, the brakes, one fuel tank sender fixed itself after a few
hundred miles (the other I had to remove, disassemble, and clean),
door locks/switches, etc. Hazards of a barn find I guess…oh well,
eventually will get it all sorted out.

Matt

5

Frank,
Good info…thanks!

I deliberately set the idle (with engine hot, in park, and a/c off) a
little high; at 1,000 rpm. I can adjust it up/down using the idle
screw on the throttle body behind and below the AFM. I had done so in
the hope that some of that increased idle would carry over to the
in-gear state. Which of course, it did not. I’ll go ahead and drop it
back to 800 rpm.

Was unable to check the AAV this morning as I was running late for
work.

The capacitor idea is worth looking into. I’ve not been looking in the
rear view mirror to check for smoke but will add that to my list.

I’ve also checked for vacuum leaks but all seems well in that
department.

I have been suspecting that it is running “rich” given the smell of
the exhaust and somewhat laggard performance. As I mentioned to Doug,
the car sat for 15 years in a barn and I’ve finding the need to
disassemble and clean a variety of items. I think the AFM and AAV may
need to get added to the list.

Matt

6

Next easy thing to check:

Is the ducting between the air flow meter and the throttle body secure and non-leaking?

Cheers
DD

7

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Definitely check the ducting as Doug suggests, Matt…

Ducting sealing may be intermittent, and cause changes in mixture - carefully check the rubber parts for sign of deterioration…

The consistency of symptoms is an important element in diagnosis. If symptoms come and go willy-nilly it is of course a sign of an intermittent fault - and finding the fault can only be properly diagnosed while the fault is present …:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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8

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Increasing the idle to compensate for an 'underperforming ’ engine is common enough, Matt - sort of Band-Aid applied properly to a wound. Sometimes improving the engine performance costs a lot of money…:slight_smile:

After setting the idle hot - what was the idle with the engine cold? The AAV, working properly, should increase the idle by some 200+ rpms. Theoretically; if the AAV closes prematurely, the hot idle set may be insufficient to run the cold engine - do check AAV cold. If it is half open cold and closed when hot it is likely OK. It is also pertinent to register the engine temp shown when the problem occur.

It’s also pertinent to look at the tacho; if it suddenly drops to zero while the engine is turning, there is an ignition problem - the tacho should follow engine rpms. It is prudent to cast a wide net.

Also, while inspecting the air ducting; clean out the throttle body and verify throttle gap set to spec - 0.002". Also check throttle linkage for smooth operation - a hanging throttle may interfere with idle consistency. Verify that there is the necessary slack in the throttle cable to ensure the throttle position does not interfere with idle…

Do verify that the CTS’s resistance is to spec at other than hot some (300 ohms) - as Doug says, it is a main player in fuelling, and should be in the Mohm range witht he engine cold. Unless the CTS temp/resistance curve adheres to specs, there will be fuelling anomalies.

The AFM is very reliable, but may develop flat spots in the resistor unit - this may cause hesitation or felling anomalies at certain revs, usually round idle flap position. There is particular test for this condition - but with a possible fuelling problem; do disconnect the O2 sensor while pursuing the problem. Disconnecting the O2 sensor should not affect engine behaviour at any time - but if the disconnecting the O2 sensor causes changed behaviour, there is an underlying fuel problem…

A crude test; remove air filter and with the engine idling hot gently push in the AFM flap - if fuelling is correct, the engine will instantly falter even with fractional flap movement. If fuelling is lean, the rpms will slightly rise…

Whatever the fault, it is likely easily fixed - it is finding it that is the difficult part…:slight_smile:

Incidentally; did you do a compression test? It will indicate the state of the engine - which is the fundamental starting point…

Frank
xj6 85 Sov Europe (UK/NZ)
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9

Lot of things to look at. I’ll have to reserve most of it for this
weekend.

The AAV was open about 1/4 inch (5mm) at most this morning. Looks like
the primary culprit. It seems to move freely when assisted with a
screwdriver but is spring loaded and snaps back to it mostly closed
position. Can the AAV be disassembled and cleaned? Or just soaked?

I see that the AAV has an electrical connection. What is the
electrical function of the AAV? On my old V-12 the AAV was purely a
thermo-mechanical device if I recall correctly.

Have never seen the Tach drop to zero.

10

That sounds normal to me. The AAV is never fully open even at sub-zero temperatures.
The AAV has a heating element wound around a bimetallic strip inside. As the bimetallic strip heats up it bends and this rotates a cutout brass disk in the air path - producing the open/close action. I suspect they either needed the AAV to close faster than the warming engine temperature allows for. Or engine temperature alone doesn’t produce enough movement and the strip has to be heated to higher temperatures still by the element. The brass disk still only goes through about 1/8 turn of rotation at most.
The AAV can be disassembled, but you’ll have to drill the rivets to do it, and then find suitable bolts or rivets to reassemble.
A spray of WD40 can help if it’s gummed up but yours sounds like it is moving.
If soaked it may be difficult to get all the cleaner solution out afterwards.

11

So does the ECU open the AAV based on the reading from the coolant
temp sensor by applying voltage to the AV heating element? Then when
the coolant temp reaches a certain point it drops the voltage thus
allowing the stip to cool off and close the brass disk. Or, I suppose
it could be a linear process. Doesn’t have to be an on/off scenario.

12

It’s not that clever Matt. The heater element is powered off the fuel pump relay and is ‘on’ whenever the pump is running.
As such its independant of the ECU - though the ECU will be adjusting fuelling to match the extra air that either is or isn’t getting through the AAV.
The AAV is simply a bypass of the throttle butterfly that allows extra air into the engine when cold - equivalent to just touching the pedal enough to open the butterfly slightly. This will be sensed by the airflow meter, and the ECU will fuel accordingly.
In addition to this the ECU has it’s own temperature sensor and independantly uses this to enrich the mixture in a cold engine. The combined effect is that the engine gets extra air and extra fuel to keep it idling (at higher RPM) when cold.
Once the engine is hot and can idle around 750 RPM the ECU leans out the mixture and relies on the AAV to be closed - otherwise the idle will be high (but the ECU has no way of ‘knowing’).

According to Roger Bywater on his AJ6 Engineering site on the AAV. “The valve is mounted so that it follows the temperature of the engine structure but the rate of response is trimmed to suit the engine requirement by a heater coil around the bimetallic strip.”

Here is a link to some photos I put on the old Jag-Lovers site

http://www.jag-lovers.org/snaps/snap_view.php3?id=1275308875

Regards,

Andy

13

So, I had it backwards then, the voltage to the heating element (and
engine temperature rise) must close the disk.

The pictures are helpful Andy. Mine did look about that far open this
morning at room temperature. But I’m certainly not getting that 200rpm
increase.

I’ll have to look into the items suggested by Doug and Frank. I
suppose it wouldn’t hurt to remove and clean the ECU connector as
well.

14

A little more data but no real answers. Adjusted the idle screw down to 800 rpm (hot) and nudged the flap in the AFM, rpm’s dropped slightly. Checked the idle w/o the oxygen sensor connected. No difference. Lowering the idle to 800 results in a much poorer throttle response, just barely touching the gas pedal results in sudden rpm drop and the engine nearly dies. I had to increase the idle back to 1,000 rpm to eliminate that problem.

Putting it in gear still delivers a 400 rpm drop but it then settles at 750 rpm.

Engaging the A/C compressor causes an additional 200-300 rpm drop. This seems wrong…I thought engaging the compressor was supposed to be compensated for.

At this point I think the best course of action is to pull it all apart and clean it up, check the throttle plate gap, adjust cable tensions, inspect for air leaks, and see if we can get back to a “ground-zero” state.

15

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Andy gives a comprehensive outline of AAV function, Matt…

The ‘cold’ opening depends on ambient air, but there is nor real standards set, I’m not sure that 5 mm cold is adequate. The degree of opening of course regulate the increase in cold idle - though it is a bit more complicated. While the AAV opens more as temp drops - engine drag also increases, the net increase in idle rpms will only increase marginally from ‘normal’. You will also notice that after starting, idle rpms will rise for a time as engine friction tends to drop quicker than the AAV is closing.

The AAV is principally a cold starting and idling device - as Andy says; opening the throttle will exceed the AAV capacity. So while driving, the AAV is sort of redundant - but when stopping, like for a traffic light, the AAV ensures adequate idle if the engine is not warm enough. Also, as the engine cools down during parking, the AAV will gradually open - ensuring correct extra air according to engine temp. But, as Andy says, the AAV is not perfectly synchronised with engine temps…

As the AAV slide moves and resets freely when manipulated, it is assumed to be OK - but to me 5 mm opening seems a bit little. The AAV can only be opened by drilling out the 4 corner rivets - and reassembled using screws. But I’m not sure it is a cure for your problems, though apart from cleaning the opening can also be reset…

Frank
xj6 85 Sov Europe (UK/NZ)
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Your ‘best course of action’ is likely as stated, Matt…

But, if you have not already done so; you should definitely run a compression test - to ensure that the engine condition sin not part of your problem. On the xk engine it is a simple test - and is unambiguous, and pursuing other faults when the engine itself is impaired is not entirely fruitful. Checking compression should be the first step in reviving an engine…

The poor initial throttle response may relate to the throttle body/throttle gap, but other causes is not to be disregarded. Have you checked ignition timing and distributor response to vacuum - easily done by sucking on the dist vacuum hose and listening to movements inside? Unless the dist responds properly to vacuum changes as the throttle is opened and the ign is correctly set - the engine may not respond correctly to throttle movement…

You should also connect a vacuum gauge to the dist’s vacuum hose and read vacuum with the engine idling. This will also clarify if you have the ‘US’ or ‘European’ set-up - the ign timing is set differently. Manifold vacuum should also be checked, vacuum gauge connected to the fuel regulator vacuum hose - nominal manifold vacuum is around 18" Hg for a good engine. These are straight forward tests - with ‘objective’ answers…

Keep your idle at 1000 rpms for the time being - engine power increases with rpms, and gives the engine a ‘head start’ when throttle is opened.

Like the torque converter there is a feedback with the AC compressor. Crudely; one would expect 750 rpms in gear and around 550 in gear with the AC engaged - the compressor use a lot of the engine’s power. Some versions of ‘our’ Jaguars did have AC compensators, a simple air valve opening when AC was turned ‘on’ - however, it was not generally fitted. Most newer cars do have it, so idle is the same with or without the AC - you don’t…:)…

As an aside, while engine power comes from burning petrol - but the amount of petrol that can be burnt depends on the amount of air available; 1 part petrol to 14,7 parts air, by weight. The amount of air entering the cylinders depends on manifold vacuum - but the overall capacity for the engine to ingest air relates to engine rpms. The faster the engine turns the more air - and the more petrol can be burnt to increase power, and vice versa…

Frank
xj6 85 Sov Europe (UK/NZ)
**

17

Well, it turns out the idle adjustment screw was more out of whack than I realized. Once I closed it all the way down and then backed it out to about 1,00 rpm the problem went away! I can back the screw out way beyond that point but the idle does not really increase. I did not realize there was such a narrow range of adjustment there. I’ll still go though and clean it up, but I’ll wait for my new “hose” between the AFM and Elbow to get here before embarking on that project.

Thanks for all the technical advice…it’s why I love this forum!

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There is a fault with the idle screw, Matt - usually caused by a faulty/displaced screw seat…

To remedy; the air distribution block must be removed and the seat inspected. The idle screw is designed to provide continuous adjustment of idle rpms - somewhere around 30/40 rpms per half turn. Basically, the idle air is provided by the throttle gap, 0.002", and ,while cold, the AAV - the idle screw providing the precise setting of the idle. It is difficult to get a setting of the throttle gap that gives the desired idle - adjustments here is very fiddly…

Of course, if you are satisfied with idle and engine performance, there is no urgency about rectifying the idle screw fault - but it is defective…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

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Years and years ago I was faced with an idle screw that didn’t seem to do anything. Lucky me, the air distribution block simply needed a good de-gunking of the passageways.

Cheers
DD

20

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Which of course may be the case for Matt as well, Doug…:slight_smile:

A vacuum leak will also interfere with idle screw reactions, but I’m not sure to what extent…

Frank
xj6 85 Sov Europe (UK/NZ)
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