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I've noticed this for about 3 mos. After the car has been coasting, say in the 30mph range, if I slightly accelerate, it will often catch a point where it I'll hear about a 2 second hum and vibration...the RPM will take a brief drop, then shoot right back up.

Now, if I accelerate a little firmer, it never happens. It has also done it a few times on the interstate around 65mph.

It doesn't seem to correlate with a shift point, though it could and I may not be feeling it.

Just didn't know if it's something I should be checking on the front end or if it's just a quirk.

Thanks!
 

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I have what i assume is the exact same issue, just when coasting along or maintaining a speed around 30-40mph.
Little dip in revs by about 100-200 RPM and what I can only describe as a short farting noise under the car.
Seems to have been occurring less often since having engine and tranny mounts done.

Would be interested in hearing what the forum think this is...
 

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Hello All,
So I think my 97 E320 has this same problem. No noise when accelerating or when actually coasting with foot off the gas, but the humm is very noticeable when cruising at a constant speed on level ground. It sounds like rolling over the rumble strips on the edge of a highway. If I come off the gas, or start to accellerate it immediately goes away. It doesn't happen when climbing a hill because I guess that is putting a load on the drive-line similar to accelerating. That's why I think and fear a trans problem. Has anyone got any updates or info for this issue?
Thanks,
Joe
 

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my problem has become slightly worse, in that at ~1900RPM the engine revs drop in a pulsing way.

Also recently noticed that there is a whack of black electrical insulating tape at the spark plug end of the ignition wire.

My logic is one of the two plugs in the cylinder is missing at this particular speed due to vibration.

1900rpm/12 = 158.333 (divide by 12 because: 6 cylinders firing on every second up stroke means firing every 12th)

158.33/60 = 2.68 opportunities for misfire per second.

2.68 about the rate the revs pulse up and down per second.

So i'm replacing the plugs and wires on thursday, will post updates then!
 

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Hello All,
So I think my 97 E320 has this same problem. No noise when accelerating or when actually coasting with foot off the gas, but the humm is very noticeable when cruising at a constant speed on level ground. It sounds like rolling over the rumble strips on the edge of a highway. If I come off the gas, or start to accellerate it immediately goes away. It doesn't happen when climbing a hill because I guess that is putting a load on the drive-line similar to accelerating. That's why I think and fear a trans problem. Has anyone got any updates or info for this issue?
Thanks,
Joe
Check your flex discs and driveshaft bearing. Your problem is most likely not engine related and I doubt strongly whether it is gearbox related.
 

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Evening,

Just to update on my previous post.

I changed my spark plugs and ignition cables (WD-40 is the best thing on this earth), everything appears to be back to normal, no dip in revs, no surging etc. I swapped the coil pack over to the next cylinder just in case, so if the problem rears its head then in theory the code should move to the next cylinder.

Just FYI, with the new cables and plugs, the engine feels great! More power, better response, transmission seems much happier, throttle is more sensitive.

Also perhaps worth noting is the ignition cables I bought are fine, but have some minor issues: protective plastic ridged sheath over the main cable isn't cut to length accurately, cable lengths aren't all spot on as one would hope. They were made by BBT automotive in Germany, kinda wish I bought OEM but never mind.

Off to do my rear accumulators tonight!
 
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