Missfiring except with Premium
 

W210, 2002 Engine - M113.940

Generally the engine runs fine at 115,000 miles, and I have used regular gas for 14,000

When I accelerate leaving a parking lot, the check engine light comes on.

Check Engine Light
P0306 Cylinder 6 Misfire Detected
P0307 Cylinder 7 Misfire Detected
P0300 Multiple Cylinder Misfire Detected

I reset and waited, the CEL comes on again

So, I tried Premium Gas, and poof the CEL stopped coming on. So I went back to Regular gas, and wham-mo, the CEL came on again.

I'm guessing the fuel injectors need replaced. What do you think?

What would a type of gas have to do with the CEL???

Thanks for the help.

Doesn't the booklet specifically state that you should use premium unleaded fuel anyways?

You will get pinging on acceleration if you are running unleaded regular. As already stated it calls for premium.

Replace the spark plugs. If the check engine light continues replace the coils on 6 & 7 or swap the coils to other cylinders to see if the misfire follows the coil.

Fuel of different octanes have different ignition characteristics. My guess is that 6 & 7 have weak/marginal ignition that just squeak by on the burn characteristic of premium/higher octane. Run the proper octane from a top-tier brand in the car.

No I don't think it is injector related although 14,000 miles of regular has not been keeping them as clean as premium.

Good luck!!!

Great (sad)b news
 

Yes - I finally got to doing this test. Switched the coil packs between #5 and #7.

Magic - the CEL reported that the misfires (from P0307 to P0305) moved from #7 to #5.

I think we have the root cause.

Thanks so much for your patience.

Comment: I have changed the spark plugs and the wires, but missed changing the coil packs.

So - replacing the coil packs now is fair.

Suggestion to All Reading this. Changing the coil packs when you change the plugs (I was penny wise pound foolish)

This is a M113.940 engine, Model: E430 Year: 2002

I never understand the logic of running regular in the m113 engine and other modern engines that call for premium. Of course your car runs fine on regular. You have knock sensors that adjust the timing, but your fuel economy goes down during the process.......and there goes the few bucks that you saved at the pump.

I suspect that the engine management system is detecting detonation and doesn't have enough control authority to retard the spark sufficiently to eliminate the detonation under load.

...seems to me that rather than replacing a bunch of what are probably pretty expensive parts, just use premium fuel as specified by DB., and if the CEL and codes go away you're done.

Duke

Does it really affects the gas milage?
 

I seem to get 29 MPG either way.

Thanks for the suggestion. I am going forward with premium from now on.

You have the best suggestiin - The 'computer'
 

I was dumbfounded when the misfire would stay on cylinder 7 but 'float' among the other cylinders. (3, 7 and 8 were the most common) But your suggestion of the 'computer' not adjusting properly makes all the sense.

If it was truly a hardware issue, why would it not stay at the same spot.

Duke2.6 you have the best diagnosis!!!!! It's the computer!!!

I just could not justify buying a new car, just cause of an error that happens occasionally.

So, path forward....
Stick with Premium fuel and enjoy the car!!!!!

(As a side note, I recently tried Premium without ethanol and got horrid gas millage. It was just a one time shot, but ...)

Thank you all for your thought (prayers) and suggestions.

My car experience is better because of communities like this.

Been using Premium - and now it happend again
 

Running premium for about 700 miles, and the check engine light came on again.

:-(

P0305; P0307 (two); P0308; P0101, P0128

Read the codes, but did not reset them. Poof, next time car started, the CEL was off.

This is beyond my skill, planning on saying hello to the dealer, cor a proper diagnosis.

I'm guessing, the minimum is the coil packs.