Random hot start issue Series3

Hi I have a series 3 what has recently happened is after a short stop such as buying fuel the car will not start appears to be fuel everything under the bonnet appears fine checked voltage at coil etc, took a punt and replaced the fuel pump and made a new earth lead for the pump and soldered a new end on the positive wire. When it happens if I wait around 10 minutes the car will just start right up with no evidence of any issue at all, it is random so its been hard to diagnose worst part is being stuck at a servo blocking the pumps. Could be just luck but when it happened this morning I kept switching tanks as I cranked and it started cant see how that should make a difference.

Did you check the high voltage side of the coil? I’ve had failing ignition coils act like that.

I have had it start then just slowly loose revs and stall pretty sure its fuel,had a signal from the coil checked that wire next maybe the relay just hopeing some one has had similar. did it agin at lunch time pulled up, quite warm here today turned it off and went to the bank say 10 min went to restart nothing but this time after a wait of another 5 min it restarted but only just and gradually picked up and from then fine. I think its just going to be one I will have to tough out its happening more and more but totally unpredictable.

Some questions, Simon; what is your hot and cold idle…?

In either situation; if the engine starts; does it respond to gas pedal inputs - or just die? Also; when starting, hot or cold; if you touch the gas pedal while cranking the engine may baulk or refuse to start - did you?

Extra air is provide by the AAV with the engine cold, raising idle by 200 - 400 rpms, gradually dropping as engine warms up. With the engine warm, and then stopping for shopping; the AAV gradually resets as the engine starts to cool. If the AAV is out of step with actual engine temp at restart; the engine cannot idle - and if it cannot idle it cannot start. Waiting a bit may reset matters as engine/AAV cools further - and the engine fires up. And then gradually increase rpms as engine warms up…

Hot/cold idle rpms is a fair test of AAV function - a non-functioning AAV may cause problems as described…

That the engine starts and then gradually dies may imply that the fuel pump stops running after the initial ‘crank’ - or fails to get fuel.

If the fuel line from one tank is blocked; changing tank selection will restore fuel flow - and the engine starts. A sometimes problem is debris in the tanks, which randomly blocks fuel flow. It may happen on either or both tanks; debris blocks the in-tank filter of the selected tank due to pump suction - engine falters/dies. Selecting the other tank instantly restores engine. Meanwhile; debris falls away from the filter in the unused tank - ready for a another changeover…

The point is that this is random - and may repeat at odd intervals/situation, and may increase in frequency, particularly with low fuel levels. The initial solution is to open tank drains to drain of debris/water.

The fuel pump is triggered by two separate circuits. One directly from the ign key to ‘crank’ - the other, via a diode pack, by a reed switch in the AFM. This switch triggers the relay as the AFM flap opens as the engine draws air, ie runs.

If the switch fails, the pump stops as ign key is returned to ‘run’ position. The engine will run for a while on residual rail fuel then gradually die…

To test; bypass the fuel relay; remove relay and jump between the white and white/green wire on the relay base. The pump will now run whenever the ign is turned ‘on’ - and you can run the car in this configuration while you check out other possibilities.

Or; this is a heat related issue; interfering either with the coil or the ignition amplifier when these parts are hot. Nominally, the engine will then fail to start hot, or quit as the engine warms up.

To test; check for sparking with a spare spark plug connected to any plug lead. While cranking; you should have sparking, and if so; the engine will start unless the fault is elsewhere…

The gist of this is that a wide variety of fault will cause fairly similar symptoms. So you have to grind through the lot in turn - with an open mind…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

Have you checked so that you do not have a vacuum in the fuel tanks

How about heat soak? Tank vacuum should be more noticeable at full throttle, at least I think that

Not obvious. I had similar problems. Sometimes when I stopped, the car did not want to start and I waited 10-15 minutes then it went on as nothing had happened. It’s easy to just investigate. When the car does not want to start, you just open the fuel cap and hear if there is any wooch. Never stopped when I drove.

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Wow thanks all, the idle is a steady 500rpm hot or cold sometimes a little lower cold until I give it a litle rev then settles down would say its slightly flat if I drive straight off cold but only for 30 sec or so then faultles, both tanks have been cleaned recently and no debris in the fuel line so going from the description perhaps the AAV is at fault its strange only recently purchased the car and drove it 1900k home and faultless until I spoke to a memeber at my club who described exactly the same fault with his car he hasnt been able to fix i think he jinxed me passed it on his car is probably fine now, just swaped out the relay this morning as an easy thing to try.:thinking:

Oh & I actually had it happen when I had a test light handy and had a pulse to the computer and correct voltage at coil etc although I didnt check for high voltage spark

I assume this is the idle in neutral, Simon…

In which case your AAV is not working properly. To overcome cold engine drag (basically friction), the engine needs enhanced idle during warm-up - which is provided by the AAV

Disconnect the air hose on the AAV and look inside. A slide opens and closes with engine temps; with a cold engine, the slide should be about half open. If the slide is closed; use a small screwdriver to prise it open.

It is spring loaded (bimetallic spring), and due to accumulated gunk over time the slide may stick. Manipulating it may free it; it should spring back to previous position when prised - and should be closed with the engine hot…

This is an initial action, and when/if the slide is freed; reset the hot idle to 750+ with the idle screw - ensuring the slide is closed when doing so.

Lack of AAV assistance and the low idle is the cause of the lazy ‘drive off’ you describe - after some half a minute is loose enough to cause no problems. Working properly, the Jaguar should start, and drive off, with no hesitation - hot or cold…

Mind you; the malfunctioning AAV may not explain all your symptoms - put verifying/remedying it is essential for progress…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

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