XJ6 Series 3 Cycling on acceleration when cold (amende

(Con Saris) #1

This thread continues of from the last which was “Red Diode Pack Modification”. So the car stutters a bit when cold but as it warms up it gets better. I have been chasing the cause of this fault for some time but other problems turned up that had to be rectified before I could continue the fault finding.
I have: Checked and/or replaced the following ignition parts- the ignition module, the coil, the wiring between the module and coil and the spark plugs. I have removed an aftermarket alarm system. I have checked the CTS and removed and cleaned the injectors and checked them for proper operation. I have drained and flushed the fuel tanks and replaced the fuel pump which was causing a fuel shortfall at the manifold.
I have been waiting for a fuel pressure gauge to arrive and today it finally got here. So I am now ready to do the testing but a not exactly sure how to go about the actual tests. I know I need to test for raw and manifold pressure so where do I connect things up to? The gauge I have, has 2 hoses on it so to test the raw pressure do I connect 1 hose to the fuel inlet to the rail and the other to the rail itself? The for the manifold pressure do I remove the hose for the cold start injector and then just replace it with the 2 hoses from the gauge? Once connected, I remove the -ve wire from the coil and cranking the motor will give me the relevant pressure? Do I leave the gauge inplace once the pressure has been measured to see if there is a drop in pressure over a period of time or do I need to crank occasionally? Appreciate any help on this please

XJ6 S3 fuel tank breather?
(Frank Andersen) #2

Read ‘capmod’ in the Archives, Con…

Briefly; the engine stutters when accelerating cold due to a fault in the AFM - presumably due to arcing, causing fuelling errors. It is completely cured by fitting a small capacitor across two wires at the AFM…

You may notice some exhaust smoke while accelerating, confirming the diagnosis. It is a fairly common fault; peel back the rubber boot - the ‘capmod’ may already have been fitted, small capacitor soldered on - and has failed…

The symptoms should completely disappear as soon as the engine is warmed up - so hot problems still warrants the fuel pressure tests…

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #3

Here’s the cap mod



(Con Saris) #4

Hello everyone, finally got back to working on the Jag today. Checked out the AFM plug and discovered that the Capmod had been done before but fitted a new high temp 100uF 63V capacitor in case. Unfortunately it made little difference. In fact none at all.:unamused:. So now on to fuel pressure testing. I believe the correct way to do this is to remove the hose from the cold start injector and put the inlet and outlet hises from the gauge in it’s place? Do I need to do the same at the place where the fuel enters the fuel rail?

(David Jauch) #5

I know that you made another thread (oops, you didn’t) but you either have some flow measurement device or a gauge to compare pressures. Figure that out.
Then the fuel is a fluid, so it is a hydraulic system and the pressures are the same everywhere. All you need to do is remove the cold start injector hose (after depressurization) and put the single connection from the correct gauge in there. That pressure, with engine running, is all you need :slightly_smiling_face:

(Frank Andersen) #6

Did you ensure correct polarity of the capacitor when soldering it on, Con…?

And I’m not sure whether the 63V rating rather than the 25V has any bearing? (I think not - but polarity likely is…)

The procedure for testing the fuel pressure is as you believe. However, the difference from spec must be pretty narrow to only affect cold situations, but it is still relevant to test it…

xj6 85 Sov Europe (UK/NZ)

(AndyBlakey) #7

Yes, polarity is critical. The capacitor will have its polarity marked on it.
The voltage rating between a 25V and 63V capacitor is unlikely to make any difference in this application.


(David Jauch) #8

Yes, and if reversed by accident I’d definitely solder a new one in, not the mispolarized.

(Con Saris) #9

Checked and double checked. The negative side of the capacitor is closest to the front of the motor.
Pretty sure the voltage rating will have no real effect on any thing. If the voltage rating was too little for instance, 10V, there is a good chance that it would go bang! One thing that I have noticed since replacing the capacitor is that the car is idling up around 1000rpm even after warming up for a while. Befrore the change it started up at around that and then dropped to around 700rpm after a short time. Not sure if this is relevant or not. Pressure test will be forthcomng.

(Frank Andersen) #10

That’s indeed the correct polarity, Con…

As your symptoms - ‘stuttering’ during cold acceleration and perfect engine behaviour when hot - is perfect for the capmod, I’m a bit concerned. There is not really any other fault that readily fits the bill…

Possibilities; a dud capacitor - did you check it before installing? The soldering damaged the cap - did you use a heat sink while soldering? Particularly if the soldering took some time…

As the capmod was already done there should of course be no confusion on wires used; pink (front) for neg and pink/green for pos - just for the record…:slight_smile:

When starting, with a properly functioning, cold engine drag will initially slow idle - which will the rise to some 200 - 400 rpms above hot idle set as the engine loosens up. While the AAV is heated by engine heat; there is also an electric heating coil to more quickly close the AAV - ‘after a short time’ as you say. Basically, this set-up is based on the driver moving off immediately after starting - and the open throttle renders the AAV unnecessary.

The capmod, and work on it, should have no effect on AAV function. But you could remove the electric plug on the AAV and verify that there is a resistance of 22 or 33 ohms. Just to verify that the heating coil is intact…

As an aside; too much or too little fuel gives the same engine misbehaviour - misfiring. The only difference is that with fat misfiring there is usually visible smoke from the exhaust…

xj6 85 Sov Europe (UK/NZ)

(Con Saris) #11

Hi Frank,
I have just put the fuel pressure gauge on the car and I have a pressure of .25Mpa or 36 psi so I’m guessing that rules the fuel pressure out. Once I turned the motor off the pressure held for at least the ten minutes that I was out there looking at the gauge. I have spent a little time trying to tie down exactly what is happening with the stutter. While the car is relatively cold, on a slow increase in revs similar to slowly pulling away from the curb or traffic lights the revs increase to about 1400rpm and then start to oscillate down to 400rpm and back up to 1400 and then back to 400 as long as I keep the accellerator at that position. If I add more fuel it will go past this point and rev freely as long as I keep the fuel on. As the car warms up the stutter is actually still there but not as noticeable. That’s why I thought it had disappeared. The car is definately idling faster since the cap mod up around 1200rpm. How can I drop the idling revs?
The resistance on the AAV pins on my car are measuring at 34 ohms, so I think that is close enough to ok.

(Frank Andersen) #12

Fuel pressure good, Con - it’s not the problem. Your much improved description means engine is not hesitating or stuttering - it is cycling. Which has a simple explanation on the ‘European’ set-up…:slight_smile:

In this set-up the ECU cuts fuelling below 1400 rpms when the throttle is closed - ie, when you release the throttle at speed fuel is cut to save fuel. Fuelling is then resumed when the rpms drops below 1400 rpms…

This is effected by a microswitch bolted on at the throttle linkages. If this switch malfunctions or is loose/maladjusted; you get the symptoms described.

Remedial action; localize switch and check/adjust that it is activated with the throttle closed. I don’t trust my memory, and the wiring diagrams are imperfect - so you have to do some groundwork.

Ie, I don’t know if the switch is ‘make’ or ‘break’ with the throttle closed - which decides if a test/ temporary fix can be done by disconnecting the wires or shoring them; try either…:slight_smile:

Obviously, if the switch is just fallen off it’s perch you just reattach and adjust. Or change the switch is defective. It’s not a vital function; engine will behave perfectly if the switch is bypassed…

xj6 85 Sov Europe (UK/NZ)

(David Jauch) #13

Yellow wiring. Standard microswitch on throttle body. Should just click with throttle closed. Sounds worth checking, was just what I was thinking earlier but as you were I waited.
Mine got maladjusted recently and it see-sawed in about half-second intervals downhill when cold and 1400 was the

Do you know where the idle air set screw is?

(Con Saris) #14

Hi David, Located the Mico switch today and on first observations it does look out of proper adjustment. Will fix that later, sooner rather than later. Not sure about the idler adjust screw. I will have to study the manual for that one.
Clearly I haven’t helped myself on this problem and it was only the other day that I decided to try and isolate the symptons. Glad I did, sounds like I may have finally have a cure, with the help of the good folk here on Jaglovers.

(David Jauch) #15

Good news! I adjusted mine so that when I released the throttle I could hear the switch click, just click and then slightly more so it is definitive.
The idle air screw is there to fine tune the idle speed and it is on the air distribution block. Hard to get at (what were they thinking!) and keep track of the turns you do. One or two for a start maybe.
The block is below the AFM on the plenum chamber, and an aluminum part. It bypasses the throttle and doesn’t alter mixture. If you find a hex key that fits, leave it in there for the time being so you can try adjusting hot.


(Frank Andersen) #16

You can bypass the switch by either disconnecting or shorting the wires for testing, Con - but solve the issue now. Properly adjusted the ‘off/on’ switching of fuel on overruns is imperceptible - and slightly increases engine braking…

As an aside; the idle screw is a 5,5 mm hex - and the difficult access (indeed; what were they thinking, David) has provoked various solutions, as seen in Archives…

xj6 85 Sov Europe (UK/NZ)

(Con Saris) #17

Hi Frank and David, I think I have found the idler adjuster. I have attached a pic. Is it down the hole shown at the back of the AFM in the pic? Just below the fuel rail or is that the mixture adjust screw? I couldn’t actually find a decent diagram in the manual.


(David Jauch) #18

No, that is the bypass screw for the mixture at idle. It bypasses the barn door metering flap, and has the most impact at idle thus it is set to the required richness (with exhaust testing, goal is 3%CO per Jaguar). The distribution block is down deeper, bear with me…

(David Jauch) #19

… there it is.
Sliced open to show the overrun valve about whose existence I learned from Frank.

Don’t mind the surroundings, the intake is also cut on the right. Your idle set screw is in top right corner of the contraption.

(Con Saris) #20

Hi all,
Tried adjusting the throttle switch today. More issues.:neutral_face:. Found the bottom terminal on the microswitch was in fact broken so there was no contact or maybe continuous contact with the other terminal not sure which really as I didn’t actually do a continuity check before starting. So new switch ordered. This switch looks like it has open contacts normally and then closed when actuated so when the throttle is off the switch is actuated. I have left the wires off to test but are now having problems getting the blasted thing started again. Have done the ignition testing, checked that all the injectors are firing and have power at the plugs. Fuel is again the problem. I have just installed a couple of fuel filters between the fuel tanks and the change over solenoid (which I removed and cleaned) and am not sure if they are the problem. The fuel seems to flow slower than I would have thought from the tank to the pump. I have removed the bronze filters but left the magnets in place to see if that would make any difference. It is better but still not the free flow that I would expect. I have started another thread with regatds to the existence of breathers on the tanks. Any thoughts appreciated,
PS Idle adjustment on hold.