I had an engine warning light on the other day that my local repair shop told me it was a misfire on start, they cleared the code and it hasn’t come back yet.

The car does seem to rev high when I start it and its cold. Sometimes up to 3000 RPM. If you wait or put it in gear it will drop to about 600-700.

I'd been running ethanol free fuel in my car, due to a vibration I have at about 1200 RPM while driving. The garage told me to run the ethanol free previously and it seems to help with the vibration. Recently I changed jobs and the ethanol free fuel station is a little out of my way so I've reverted to buying premium gas from regular stations. My local repair shop suggested that this switch back to regular stations may have caused the engine warning light.

They also ran some tests on my oxygen sensor while it was in. They said it was within tolerances, but barely.

So my question is, is it the cheaper (but still premium) gas, the oxygen sensor or something else that’s causing the misfire/high idle?

 

Mine rev's high when it starts. It takes 30 seconds or so to settle down. A little annoying programming to be sure, but I suppose it does help things get all warmed up.

 

Bad O2 sensors certainly can cause wobbly idling, but I doubt that's the case here. Just after cold-start the O2 sensor isn't involved - the system is on what's known as "open-loop", where the fueling is controlled by internal ECU map tables, and corrections like auto-choke are applied. Does the idle speed revert to normal or near-normal once it gets a little heat into it? I think this is more likely to be caused by some sort or air leak around the intake somewhere, and/or a faulty throttle position sensor.

 

It will eventually settle to a normal RPM (600-700) given a minute or so. If I put it in gear it will settle down straight away.

I've also noticed the RPM needle bouncing very very slightly when driving. Although this is hardly noticible and doesnt seem to cause any issues. Would this also be related to a possible air leak around the throttle body?

Would an air leak be responsible for the misfire or am I looking elsewhere for that?

 

It is entirely possible. Unmetered air like that can bamboozle the computer and cause sudden lean-ness and sputtering, as can dirty/blocked injectors.

 

Thanks, I'll look into this some more.

I've just seen the name of your car, it made me laugh

 

Your problem could be a minor air leak in the intake system, somewhere between the mass airflow sensor and throttle body. Such a leak will be noticed on cold start by higher rpm, like 2k-3k, maybe some surging. As the motor warms the symptoms will disappear.

You can easily check connection tightness to all visible clamps to intake tubing under the hood. If that does not cure the problem, then advise going deeper into the intake system. The air filter(s) are hidden underneath the front fender inner cover. The XKR has filters underneath both fenders. Once you have removed the inner cover (you will have to remove the wheel), then you can easily see the mass airflow sensor. It is the only thing on the intake that has wires connecting to it. Do check the seal connection above the mass airflow sensor. This is a likely leak area.
Some mechanics have vacuum leak detectors, which consist of small smoke producers - and the smoke is drawn into any vacuum leak.

While you are at it, remove the mass airflow sensor (2 screws) and give it a short spray with mass airflow sensor cleaner, which you can find at most auto parts stores.

 

The vibration you have might be caused by the exhaust. There's a fix for that in adding two extra supports between exhaust and gearbox.

This vibration typically exists around 1200 - 1300 rpm.

Add-on repairs as per ref bulletin JTB 00088 31.91.07 - 0.90 N22 42 XR8 55909
arts: Bracket Mount JC2P 13477 & JC2P 13478

 

Since I've posted this I've had the throttle body removed and cleaned, new spark plugs, a new battery (ecus were reporting low voltage), and new coil packs on cylinders 8 & 2 (apparently these were where the misfire was).

...and I still have a car that hunts for rpm and still throws codes when I start it. I've spent over $1000 so far and I'm not sure what to do now.

I'm not even getting through a tank of gas before the car is in to see the doc again. Today the light came back on, I took it to AutoZone and they read the codes, cylinder 7 & 8 misfiring now. Do I put a new coil pack on 7? I found a video of somebody doing this on an xk8 and I think I can manage to do it myself this time.

Is there anything else that I can look for?

Is the fact that cylinder 8 is still misfiring trying to tell me something?

 

So... I got a new coil pack and put it on 7. No change.

I did also get a code scanner, so now I can read the codes myself. What I've got is...

P0301
P0302
P0307
P0308
P0300
P0316
P0442
P0430

I feel like I've spent quite some money getting new sparks and coils when I didn't need to, but c'est la vie.

My next question, is are p0430 & p0442 related to the misfire or some thing else.