Very minor issue but I'm stumped

Post by **JRL** » 15 Dec 2011, 07:55

As is my VOLVO tech
This is my 99XC I built for the winter. (91K miles)
Great car, runs well except for one minor glitch.
After it warms up it will idle a bit weird.
It will sit there and all of a sudden I feel this very small vibration, the tach will fluctuate all of 50-75 RPM and I can feel the throbbing in my read end (mine)! Stops and starts, stops and starts....(&*#^^*#%
It runs well, very smooth with no issues, just this very slight idle BS.
A "normal" person would probably not even notice this
All 3 motor mounts are new, (OEM) T belt and breather box is new, plugs are new and a new ETM
(To give an example it feels more like it has a poly top bushing when idling in gear)
It has been on a full scanner, everything is nominal from 02 sensor operation to MAF readings, air-fuel ratio, etc
STUMPED!

Do we just wait until it becomes worse?

shegarty · Post by **shegarty** » 15 Dec 2011, 10:03

have you checked to make sure the exhaust is not touching the frame/body along its length - since you've changed all the mounts it might be worth a look

Post by **fazool** » 15 Dec 2011, 10:11

I used to design vibration dampers for engines (we call them harmonic balancers in the US).

I don't think thats your issue but check this:

When it vibrates, look at the RPM.

Now rev the engine to exactly DOUBLE that (or drive at double that RPM.

e.g. if its vibrating at 900rpm rev it to 1800

The magnitude of the vibration will be different but if you have a vibration or shudder then its running harmonics and you MAY want to check your balancer for breakdown of the rubber part.

Post by **JRL** » 15 Dec 2011, 11:06

It's not at a specific, exact RPM, I said the idle fluctuates (VERY slightly) when it does this
I've only seen one balancer go bad on all these cars I've dealt with and it had 50,000 more miles than this one has

Post by **JRL** » 15 Dec 2011, 11:11

shegarty wrote:have you checked to make sure the exhaust is not touching the frame/body along its length - since you've changed all the mounts it might be worth a look

No, but it doesn't feel like that and the exhaust is solid. Besides a touching exhaust should not change the RPM.
I said the second I give it gas, no vibrations, and if that were the case it should
The best way to describe it it is as I said above, it feels like it has a poly top mount but not even as bad as that feels like!

Post by **rspi** » 15 Dec 2011, 20:48

What octane fuel are you using?

Post by **JRL** » 15 Dec 2011, 21:15

Either 89 or 91 depending on my mood
Remember I've owned at least 75 of this generation Volvo (most being AWD)

Post by **rspi** » 15 Dec 2011, 21:18

I was thinking that maybe your knock sensors were tweeking the timing. Toss some octane boost in it or run some 93 for a tank or two and see if it changes.

Post by **jimmy57** » 15 Dec 2011, 21:22

Does it have the shallow sump angle gear and no vibration damper fitted?
I am unsure of that detail on 99's. I can't recall when the deep sump appeared and damper was omitted.
I have the same cyclic throbbing on my 98 that is shallow sump and has the stud head bolts for the damper to bolt to but no damper. I have done all three mounts which made everything else better but actual brought this on. Or maybe the new mounts took away the other harsher stuff and let this be felt.

driesbabe · Post by **driesbabe** » 15 Dec 2011, 22:47

Is the fan coming on and putting a slight load on the alternator. The ECU then compensates for the extra load. The slight change in RPM will change the harmonic and a slight vibration is felt.
My 2 c