Intermittent misfire

Hi all,
Since I got the car (AJ16 3.0) I get a strange intermittent misfire.
The car starts well and then the revs go down and settle at around 700. That’s the time when the engine has a very little shake and if you go near the exhaust, you notice that the running isn’t smooth and the intermittent misfire.
It seems to feel smoother, but not totally, when I select D and hold the brake pedal.
The car pulls well and accelerates smoothly through the rev range and never misses a bit.

I did the following:
Battery reset procedure (when a car gets a new battery and the ecu needs to “learn”), new air filter, cleaned MAF, throttle body, and idle stepper motor (it was all filthy), changed to Champion RC12Ycc plugs, changed to new coils and used premium fuel and injector cleaning additives.
These procedures had zero effect on the idle of the engine but it still pulls well.
I’m due to replace the fuel filter soon, but the pipe from the filter to engine is corroded and I’m waiting for a new hose to replace a small section.
From experience in other cars, an intermittent misfire is usual electrical, but this one is baffling me as no lights on the dash and I don’t have a decent code reader.
I though of vacuum leak, but that would cause a consistent lumpiness.
It’s not the crankshaft position sensor either because, as pointed out by a former Jaguar engineer in another topic, it only affects the starting of the engine.
Any ideas of what else could be?
I spent already a bit on this car and to be honest, I’m ready to part with it. There other issues too and I feel that I bought a lemon.

Did you take a look at the exhaust manifolds? Maybe a small crack that closes when warm?

I haven’t looked at them yet to be fair. Can’t hear the usual manifold tick noise, but I’ve heard that in these cars it’s harder to track a manifold crack

How many miles on the car? How long have you owned the car?

You mention misfire. If you have a random misfire you should get a MIL light with a P300 code. That would be followed by a P030N code where N is the cylinder. Your problem does not appear to be a misfire.

Take a look at the Idle Issues Thread I created. A couple of other things:

  1. Get a cheap stethoscope and listen to each of the fuel injectors as you rotate the throttle. You should hear a consistent clicking pattern on each cylinder. If you don’t, report back.
  2. Remove & clean the EGR Valve and EGR Sensor.
  3. Did you remove the throttle body to clean it? Look at the idle thread where I found damage to the bellows that goes on the throttle body.
  4. If that stepper motor is original - consider changing it. The SNG aftermarket one is good and inexpensive. My experience is it is impossible to test.
  5. When you turn the ignition key on (engine off) does the MIL light come on? i.e. is the bulb working?
  6. When you start the car does the secondary air system come on briefly?

Btw - if your car is a 3.0 that could be the problem. You’ve lost a cylinder. Most AJ16s are 4.0s…just Kidding. :grin:

PS. CPS only affects starting - Hmmm - Not my experience. But should not be your problem as it sounds like an idle issue.


Thank you very much for your input and questions.

It has 113k miles and I’ve had it for 4 months.
No lights in the dash at all but the EML stays on before I start the engine, so the light does work.
My car is Uk spec and it does not come with EGR.
When I start the car, I can hear a electric pump noise which shuts off when the idle settles.
The throttle body was removed and cleaned as well at the stepper motor. All looked intact after being thoroughly cleaned.
Andy Stodart, the former Jaguar XJR engineer says the CPS only affects starting and the sensor is pretty much redundant when the engine is running. He made this point very clear, so I have no reasons to doubt him.

I’ll get a stethoscope and check the injectors. They do look a bit rusty outside actually.

Ok. That’s pretty low mileage for these cars. Mine has 275k.
One of the key sensors that can cause issues is the Coolant Temperature Sensor. The resistance can be tested at different temperatures but they are very cheap and it’s not a bad idea to change it if you don’t know when it was changed. When I bought my car it had 87k on the odometer. MPG was 16/19 US. I changed the mostly inexpensive key sensors and within 2 weeks of owning the car MPG was at 19/23 US which is spec and where it’s been ever since. So that shows what impact they can have on performance.

My recent issue on idling/stalling looks like it’s turned out to be either the airflow leak at the bellows (possible), sensor connector cleaning (doubtful) or the TPS (my bet). The TPS can be tested by back probing the connectors - carefully! - and watching the voltage change as the throttle is applied. Mine had never been changed. Fixing a problem without pinpointing the cause is not ideal but I can only verify the old TPS when I have an analog meter - something I intend to do.

These cars do need love as many will point out. But you can also go lots of miles without issues once you eliminate a problem like this. My wife has driven the car 60 miles per day 5 days per week since 2009.

You should buy a cheap OBDII reader. You can see the fueling system change from Open Loop to Closed Loop when the ECM takes over fueling with input from all the key sensors. You can monitor the coolant temperature and see the fuel trims and much more, not to mention see any codes if they are set.

When I cleaned the throttle body I also turned the unit upside down and thoroughly sprayed the TPS and the spindle to which the TPS is attached. Oil can drip down through the TB to the TPS it seems.

As a matter of interest what mpg are you experiencing?


The CPS is absolutely required for running not just starting.

The cam position sensor which resides in the distributor of old is what is only required during starting.

Yes, you are absolutely. Sorry, my confusion.

That’s a good point, I’ll look into the coolant temp sensor.
I have just under 20mpg Imperial, but that includes a bit of city traffic and short trips. Occasionally I do a good motorway blast and mpg improves.
Acceleration is good and there are no flat spots or hesitations.
Working on the car today I noticed that there’s a slight blow on the underfloor catalytic converter (as well as a hole in the wheel arch when I poked it with with a screw driver) so not sure if that has an influence.

Take a look at this document. When I’ve had misfires I’ve always believed I’ve gotten a code. But now I don’t believe that’s the case. Code generation is based on threshold limits in the ECM.

X300 Misfires.pdf (295.5 KB)

My understanding is that standard OBDII is for the CKPS to sense momentary slowdowns in the crank rotation speed and for the ECM to identify the cylinder from the CPS as it senses when each piston is at TDC on the compression stroke.

Take a careful look at the connectors that connect to the coils. They get brittle and possibly loose even when the male/female housings seems tightly connected. Also the wires are 18 ga. I believe and get frayed. I’ve just ordered new connectors and wiring as I intend to peel the harness back and repair them all. While I appear to have solved the idle/stalling issue I had I’m still not getting a perfectly smooth idle and I have had a P0300 code.

Last thing to check is for corrosion on the ECU pins. Pain to get to, but not difficult. It’s a common fault. If you see any kind of greenish aqua corrosion, you may have found your problem.

You may also want to replace the oxygen sensors as well. With age, they would do something like this as well.