Hey all - 1997 Jaguar XK8 4.0L V8 owner here with 100,000k miles. I am new to this “being a mechanic as a hobby”.

I have check engine lights blinking constantly and the car shakes when idle and also when moving 1-5 mph (but runs smooth and nicely once I start picking up speed and higher rpms, so weird..). I had an error reading “Cylinder 1 misfire detected”. Because of that, I changed 8 spark plugs and 8 coil packs off the bat (watched DIY YouTube videos and read...a lot to do this myself). The car runs slightly better but still shakes when idle/low speed. Also, I get the “engine fail” and “poor performance” light, but it literally comes on and goes off randomly, so strange.

Question: What do you think this shaking only idle/low speeds? Why do the poor performance and check engine/fail signals go on and off randomly?

 

Keith,

Welcome to the wonderful world of home auto DIY. Your car will appreciate all your efforts, and you’ll quickly build up a decent tool set and working knowledge that might surprise you.

Firstly, your car runs fine at speed but idles badly. You get intermittent CEL and warnings but do these seem to go away at speed? Let us know what DTC Pxxxx you see as sometimes the Jag specific codes are different.

One thing about this car, a misfire will cause a flashing CEL on the dash. Only if it’s a consistent misfire, will the CEL stay on.

You’ve done the coils and plugs so let’s look elsewhere.

Mostly think 3 things - air, fuel spark.

Air - is air filter clean? MAF sensor clean? Throttle body butterfly clean? Cylinder 1 is furthest from the throttle body- where the air comes into the manifold.

Fuel - do you have fuel pressure at rail? There’s a bicycle tyre valve on the fuel rail. Put a towel over it before you press the valve ( with a screw driver). You should get a violent spurt. Has the fuel filter been changed?

Spark - use an in line bulb tester on the circuit to the spark plug to see if it’s firing. Test Cylinder 1 first as that seems to be suspect.

 

Ouch!

Welcome to the forums Keith,

Throwing parts at a Jaguar soon becomes VERY expensive and a code reader is your best friend. Having determined the “Cylinder 1 misfire detected” error, the next step should have been to swap the coil from Cylinder 1 with any other and see if the error code moved to that cylinder. If it does then this confirms the coil is faulty. If it doesn't then it could be Cylinder 1 plug or harness.

You haven't wasted your money by replacing all coils and plugs BUT the frustration is this hasn't resolved the shaking. It has reduced "spark" as favourite from the "air / fuel / spark" possibile causes but there could still be a harness/connector fault to Cylinder 1.

Fuel delivery issues are generally a failing pump or a blocked filter. Check at the Schrader Valve on the fuel rail as David described.

Air issues is either too much or too little in the fuel / air mixture. This is measured by the MAF and calculated by the ECM. Air filter blockage will restrict it but the MAF should compensate. Splits in the plastic trunk connecting the MAF to the throttle body will allow too much air and weaken the mixture.

Throttle Body does require occasional cleaning to ensure the butterfly closes fully and opens smoothly. Cable also needs checking to ensure no slack. TB's are a known weakness and this is usually a failing/failed TPS (Throttle Position Sensor).

The fact the vehicle runs smoothly once above 5 mph tends to discount other sensors at fault in the engine management system.

Don't forget the number one favourite for weird electrical/electronic behaviour in the XK8 - make sure the battery is good and fully charged.

Graham

 

I had a vibration at idle that turned out to be a bad transmission mount and a bad motor mount- both known failure items.