Dumbfounded


#1

****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:

Hello,

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!
Dumbfounded…

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.

Sincerely,

Carolyn D’Aquila


#2

So what then is the least expensive most cost effective course of action?

Also, any experience with Marvel Mystery Oil Products?

Thank you.


#3

As ok4450 said, you need to have a compression test. Any car with 290k could be worn out.


#4

Yep, got it—just thinking ahead…

Thanks.


#5

At 305k if you’re working hard to avoid an engine rebuild, you will need to perform a detailed diagnosis: compression test, wet compression test, and leakdown test to determine whether the problem is rings or valves (most likely at this mileage) or something else (always possible). In any event, solving one problem may not be the best solution since many other problems are waiting in the wings. An engine rebuild or replacement is likely to be nearly the same cost as solving any single problem now and hoping for later.

Marvel Mystery Oil or other “magical” products are merely stopgap measures. As the engine adds miles, you will either need to face rebuild or just put up with leaks, misses, poor performance, and possible catastrophic failure. Achieving 300k+ miles is a tribute to good engineering and good fortune, but there aren’t many success stories that go much beyond 300k miles without major reconstructive surgery.


#6

If your maintenance schedule in your owners manual lists a valve lash adjustment (or valve adjustment), then i would do this first. A compression test may show a bad cylinder, but that may be corrected if the low compression was due to the exhaust valve lash being less than zero, not allowing the valve to close completely.

Valve lash varies with the temp of the engine, so when it is at its minimum, the rough idle will be intermittent and get worse with time. It will also burn the valve if allowed to go on too long.