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XJS 4.0 AJ16 misfires...less as it warms up.

XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

XJS 4.0 AJ16 misfires...less as it warms up.

Old 06-21-2019, 05:08 PM
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Default XJS 4.0 AJ16 misfires...less as it warms up.

Hi folks.
I'm after a little help diagnosing what is causing a misfire on our 6cyl
Always cranks and starts without trouble, even from cold...nice smooth idle. no check engine lights.
When driving off cold, there is no real power as from 1000 rpm up I get a sort of misfire. It struggles to get to 20mph.
As it warms up, you end up being able to rev strongly and smoothly up to ~3500rpm before the misfire starts. You can get 70+mph then.
When it misfires...the tacho can be seen to fall...bouncing around down to about halfway.

Does someone recognise these symptoms? Googling etc doesn't come up with a good match to what may be the reason.

The car was running fine 2 years ago before having a new suspension sub chassis and brake work done.
A year ago, we got the car back, and it ran even worse than it does now, with a CEL light on and codes FF14 and FF16

It sat from then until now ( mostly out in the rain) when we started a couple of weeks ago to seriously look at the problem.
Took us a while to realise it was in LIMP mode... only when downloading the 140 page pdf did we cotton on to this.There's not much Information on the net about limp mode...what sensors will trigger it... and that it stays there until codes are erased. And the net erroneously suggests that codes can be cleared by leaving the battery off... but this is NOT the case for this model with the 15CU ECU at least.
OK, so we chased round for a while looking for FF14 coolant temp sensor error and FF16 air temp sensor, and testing carefully all the various sensors, the readings for them and the signals from the TPS, CPS, O2 all of which come up ok.

I am working on the assumption that the garage (not a classic Jag specialist) maybe fired up the engine at some point with a few sensors off, tripping the CEL and limp mode. What else they may have inadvertantly done...who knows. Everything else appears ok as far as we can ascertain.

Once limp mode was cleared the very severe limiting misfire that mode gives was removed, but the lesser poor running is still there as described above.... no popping backfires, just a severe stumbling.... and the tacho dropping.

Parts changed.
3 different ECU . All seem good now we know...
New HT leads and distributor (but not the rotor arm)
New fuel filter, cleaned the lines , new fuel. checked and made sure injectors weren't clogged etc.

Scoped the CPS and 02 signals. Look really clean
Confirmed meter voltages for the TPS, CTS ATS were within range... and that expected voltages were on the other pins of the ECUs yellow connector.

What else should we be looking at then please?
Can it be a fuel starvation / fuel pump issue... that runs out of fuel pressure at 1000 when cold....and upto 3000 when hot? How can we test for the pressure?
Grounds seem ok.... and seem to meter out ok...but how critical is this?

What else can make/ trigger the ignition to do this misfire/ stutter?
Can some of the lesser fine trims / compensations trigger the ignition to do this?

Any help gratefully accepted work out what to test next ... or change...without wanting to swap out all sorts of parts just for the sake of it..

Old 06-21-2019, 06:46 PM
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You said new ht leads and distributor but the AJ16 is coil on plug. I suspect you have an AJ 6 and based upon all the symptoms you have indicated I would try a new coil.
Old 06-21-2019, 09:06 PM
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3 different ECUís ? Why? This all sounds like a case of poor diagnosis. If the car is sitting out in the rain, water could be getting into the wrong places like the ECU pins. Please specify the year of your vehicle
Old 06-22-2019, 01:05 AM
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Hi Sgt Bilko

As Spike said, AJ16 Engine is Coil over Spark Plugs and so I think that I would start by removing each one of those Spark Plugs together with its Coil and seeing if they are 'Coked up' and then Testing each of them for a Spark you can see

Rather than using something like a Spark Plug Tester

If you find a Bad Spark Plug that isn't Sparking like it should, then try it with one of the other Coils and see if that makes any difference, that way you should be able to see if you have got a Bad Coil

I had an even worse problem on mine a 95 4.0L XJS with AJ16 Engine that refused to Start no matter what I did, until I tried every trick in the book and the very last one worked!

So if you have a look at my Thread, you hopefully may pick up some ideas that could help you

Its all there including lots of Photos, including Spark Plug Testing and Coil Removal/Ecu Removal and Cleaning the Pins/ Compression Testing/Removing the Fuel Rail and Replacing all the Injectors and Testing the Old ones/Removing and Replacing the 'Crank Position Sensor' (which only takes a few minutes to do!)

Trying Every Trick In The Book To Get My AJ16 Engine Started (Full Write Up Including Lots Of Photos)
Old 06-22-2019, 03:51 AM
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The AJ16 does not use a 15CU ECM. Itís a GEMS 6.

Please let us know what the car is and sure
someone will help you.

You are most definitely not driving an AJ16.
Old 06-22-2019, 04:01 AM
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Thanks for the help so far..

First Correct it is an AJ6 not a 16. It is a 1992 MY

It has a single coil, with normal distributor

We have noticed that the coil gets hot. This morning we tried preheating the coil with a hairdryer and right from the off there was more performance. 30mph was possible instead of 10mph !
This week at 16deg cold start temps it wasn't too bad... but trying to run the engine mid winter was terrible...with the car / coil at 2 deg.

That's a fairly cheap item, so going to get one on asap.

I also forgot to mention that we have had new plugs.... and they have all been coming out sooty black.

Old 06-22-2019, 06:41 PM
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New coil made no difference

Hopefully here is a video of the problem....

The engine is quite warm and revs freely up to nearly 4000 before the misfire starts. When driving / under load the misfire will start at 2500 perhaps... and when cold...1000-1500.

Reading through the technical book again... and trying to work out what is the most likely...
I get 2 possibilities..

Not enough fuel pressure or quantity of fuel.


from page 62... "If an engine misfires accompanied by an erratic tacho then suspect poor crankshaft sensor "

The first of these seems the more likely.
When you get fuel starvation... is this the sort of symptom you will get?

Old 06-23-2019, 05:33 PM
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It’s not the fuel pump, the crank sensor is the culprit.
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Old 07-11-2019, 11:22 AM
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Yippee. Back up and running. It was indeed the crank sensor. Butter smooth all the way.

Thanks guys..
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