what spins at 1/10th of the engine speed?

[Vikas]

For years, I can feel rhythmic but extremely faint "thumping" when the van (1999 Odyssey) is idling. I have used stop watch to find its frequency and it is about 60 per minute or approximately 1/8th or 1/10th of idle speed (750 rpm). It was there before timing belt and spark plug change. There are no codes and it does not feel like a misfire. This van never had its EGR system serviced. I have gotten the EGR code twice in the 12 years.

Any ideas?
- Vikas

 

[Boomer]

Something in the auto transmission??

 

[spasm3]

perhaps on of the pulleys? ps or water pump?

 

[SWS]

I have a 2000 Odyssey, and sometimes I think I have the same thing ocurring while at stop lights. I agree with spasm3, but also comment that the timing belt might have a defect that 'bumps' when it goes over the tensioner (that happened in another car of mine), but if it has been doing it for years then it should have gotton much worse by now. Another idea is to check the motor mounts (I'm told they will fail about now) - maybe the engine is rocking a little? Another thought is that the the engine is actually 'surging' a little in some kinda EFI hunt for the the correct idle speed, but that would not be a 'thump'. Does it do it for you in Park or Drive? Is the A/C on or off?

 

[chrome]

Is the AC compressor running at the same time? I have noted on some cars, when AC engages, there's a rhythmic "vibration" that is close to the frequency you are experiencing. Just my 2 cents.

 

[expat]

A road wheel if you are in low gear ;-)

 

[tomcat27]

I'm thinkin belt....

 

[Vikas]

Does it with AC off; engine cold or hot; in park and in neutral. It is difficult to notice in gear.

It has been doing for many years and did not change after the timing belt and spark plug replacement.

- Vikas

 

[dwendt44]

There's nothing that's engine driven that runs at LESS than engine RPM.
Some A/C compressor pullys are about the same size as the damper, but I've yet to see one that's larger.
If it were the belt, it would change over time. Either gotten worse or better.
Anywhere close to time to change the belt? Check the tensioner carefully if you do.

 

[mechtech2]

Yank the serpentine [or whatever] drive belts and see if it does this. This will show if it is a pulley or whatever in the accessory drive stuff.
It is safe to do this for a short time - a few minutes.
This is a good start.

But to answer your question, the thing that spins at 1/10 normal speed is a woman's brain as they yak on a cell phone while driving an SUV!

 

[Vikas]

The serpentine belt along with the pulleys were changed during the timing belt change. That did not change the behavior.

Besides, I am not aware of any pulley which has larger diameter than the crankshaft pulley. All other pulleys run *faster* than the engine.

Engine does not visibly shake nor does the engine rpm change during the thump.

Apart from some gears in transmission, I can not think of anything in else that would be spinning that low.

- Vikas

 

[Craig in Canada]

I'll agree with the assessment so far that it's something in the belt system. At 1/10th, however, I would eye the belt itself. You say it's already replaced and the sound continues, which is unfortunate. I wonder about one out-of-spec pulley making a thump when the seam in the belt passes over it. If you want, find the diameter of your crank pulley and the length of your belt and see if the math is close.

Is this a dealer/OES belt, or aftermarket? I'd still try running without the belt to see that the noise goes away and then keep chasing. I've had "slower" period, very light squeak that have been something wrong with the serpentine belt myself, but never a "thump".

 

[Virtuoso]

I had a Dodge Avenger that would "thump" through it's charcoal canister at idle. I don't remember ever feeling it, per say, but I do remember hearing it, a lot.

Perhaps not your problem, but I don't think it's your belt. If it was a pully, you'd feel it more than once every second. And if you replaced the belt, you can pretty much rule that out, as you have.

I am wondering if it's not something in the transmission as well. It's pump should be going steady at idle.

 

[440Magnum]

Its possibly a harmonic vibration between two parts turning at different speeds, not something actually turning at 1/10 engine speed. Say alternator and radiator fan, something like that.

There are also other things totally unrelated to engine speed that make noises on modern cars. Evaporative emission purge solenoids, for example, often cycle about once per second.

 

[artbuc]

Originally Posted By: 440Magnum
Its possibly a harmonic vibration between two parts turning at different speeds, not something actually turning at 1/10 engine speed. Say alternator and radiator fan, something like that.

There are also other things totally unrelated to engine speed that make noises on modern cars. Evaporative emission purge solenoids, for example, often cycle about once per second.

+1

 

[mechtech2]

Pulleys will spin fast, but the bearings inside rotate at much less speed. They roll on the ball bearings. So a bad spot will take more than one outside rotation to re occur.

Very much like a car front wheel bearing that is faulty - the noise is NOT at rotation speed, but 1/2 or less.

 

[Astro14]

While nothing rotates at that low a frequency relative to engine speed, the accesory belt passes over the same spot about that often.

I agree with the guys who say start with the serpentine belt...

Got a stethoscope?

 

[Vikas]

Actually that is a great idea! I do have a stethoscope.

 

[The_Eric]

Just remove the belt and see if the noise continues. If it does, look else where. If not, then look into what it turns/the belt itself.

 

[mattd]

I know on the 3.0 honda engines there is a bulliten to use a taper shim kit for the idler pulley on the timing belt. The mating surface on the block is uneven, and will cause the edge of the belt/pulley to rub against the back of the front engine mount, causing a "chip/thump" type noise at idle, however it is mostly noticed when the engine is warm, and at idle. Not sure if this applies to the 3.5L or not.