****I inadvertantly posted this discussion in the wrong area and Carolyn D’Aquila was kind enough to email the original post and response (which I copied and pasted below) so as to be able to continue the discussion:
I saw you posted your question to the Car Talk Community, inside the discussion of another question. I wanted to let you know I am removing your post from the discussion, and copying it below:
Wow! I must say I’m flabbergasted at my recent problem with my 98 BMW 540i…
I have a 98 BMW 540i with 305k on it now. I bought the car with 290k and I must say have been really pleased with the way it runs until now. It started missing so I went to Autozone and after they connected a “box” to diagnose it, they told me number 7 was missing due to either a plug, coil or injector. I swapped coils with number 6, replaced the plug and went back to have the box hooked up again—still number 7. I put Techron in to no avail. Here’s what crazy to me: it started out smooth at idle then missing, then smooth again cycling in this manner. It would miss when I would accelerate and drive it. It progressed from those characteristics to steadily missing for a good while and now seems to come and go but is more prevalent than not. So it’s back to missing while idling but when I accelerate sometimes it doesn’t miss and sometimes it goes back to steadily missing!
Thank you all in advance.
Please return to community.cartalk.com and log in and click on the large red rectangular button that says “Ask a Question” on the right-hand side of the screen underneath the cartoonish-looking Car Talk Community logo. You will be taken to screen that will allow you to create your own discussion. I only refer you there to ensure you get the proper amount of attention for your question.
You did get one response from the user ok4450, who wrote:
I would be perfectly willing to be that if you run a compression test on that BWM you are going to find some problems. Even having one cylinder down low can cause a problem like this. (rough idle that goes away when the engine is revved)
Caddyman is 100% correct about the cylinder balance test used by shops and the use of a mechanical compression gauge.
If you have any questions, please do not hesitate to write.