Out third 740 was a Turbo - it seemed to vibrate a bit at idle.

I asked my dealer about it and they told me to put some Techron injector cleaner in it.

I was puzzled about that as I had asked about cleaner a few years before and was told that I do not need it,

I had a friend who had sold cars at that dealer and he told me that after a change of management, the dealer had lost their service rating - and lost their bonus. Volvo was giving properly rated dealers a $1000 bonus for every new car they sold if they had an excellent rating - in this case 1.4 million dollars at the end of the previous year.

He told me that they put injector cleaner in my car every time they serviced it.

If you have not used it once a year, it would not be abad idea to try it.

I get my Techtron at auto zone and put some in usually before a long trip.

BTW - the old management opened a new dealership about 1 mile away from the old one, and put them out of business in about 1 year. They now have three Volvo dealerships.

Hi,

I do not normally hang out on the 900’s but the 240’s tab is running without many posted problems.

From your observational references of feeling a vibrations you might be having more of a rumble than a vibrational of motions or an actually banging against something.

I have read where a bad crankshaft harmonic balancer can suffer slippage on red blocks!
This could make the attached engine accessories to do a stop and go shimmy thing for a rumbling. It might worth making a mark across the two halves with a paint line so you can see if it is shifting.
The line should be painted vertically on one side of the front circumference.


The THEORIES, that I have ONLY READ about in dealing with harmonics was referencing large reciprocating aircraft engines, Especially, on the early “radial” engines.
They were really marvelous mechanical masses! Large bores and strokes!
The forces were recognized in the early 1900’s to be detrimental.
This caused a whole new world of research about dynamic balancing to become absolutely necessary.
In our case it’s on a lot smaller scale but still consideration needed with dynamic inertial of rotational masses and not so much reciprocating.
We have to remember that it’s a four cylinder one sided engine. It has its own unnatural balance when compared to opposing cylinders or even a “v” type configurations
The harmonic balancer is made to to reduce torsional twisting of the crankshaft when passing through certain speed ranges with the rods and counterweights in motion.
The rubber center is sort of a disconnect dampener to absorb stresses induced along the crankshaft that may transmit out to the accessories.

In my observation, of our rubber mounted engine accessories, I think the Volvo engineering carried things a wee bit to far and lost rigidity of alignment of the pulleys even while still new.
I would look around into something going on on the front of the engine especially if the A/C is used.

If I’m out of the ball park of reasonable thought here, well, the 900’s tab was just a visit, anyway!

Phil

So based on what you say, it seems it's not overly rpm dependent other than being more noticeable at idle. The transmission isn't in gear and you're not in motion with the drive shaft turning. There are no real harmonic sweet spots where the vibration is notably worse or suddenly goes away?

I assume it's not rocking around from slight misfires due to fouled plugs or loose/broken ignition wires, even a bad distributor cap. It's really easy for the contacts in the plug boots to become a bit weak and lose their firm grip on the plug ends. Such ignition issues when they start are often most notable at idle, especially when cold.

A sagging tranny mount can be a source of noise, but is most often first noticed going over speed bumps or when the engine is torqued during acceleration.

Is there by any chance a bit of a rattle in the vibration? Thoughts that come to mind are a loose warm air shroud on the exhaust manifold. The bolts holding the two pieces sometimes work their way a bit loose from all the heat expansion. Another possibility for rattling sound might be a cracked/loose/warped heat shield above the cat, but that's most often noticeable in motion. I periodically have a problem with that heat sheild as one of mine is a bit cracked. I occasionally have to bend that heat shield away from the chassis after running over old ladies in crosswalks when their walking sticks and umbrellas get caught up there. On turbos there is a heat shield on the firewall behind the exhaust that could conceivable vibrate against the firewall, but I've not had a problem and the NAs don't have those shields in any case and agin you'd probaby have to be in motion for it to vibrate. There's also often a metal shield or plates attached around the pipes where they join just ahead of the cat. That's a real good source for a rattle when it comes loose. Others mentioned the forward clamp at the bottom of the down pipes that, when present, can also crack or become loose and make a mystery noise.

This is one of those cases where a video might help others diagnose.

I just had a chuckle thinking of all the ghost electric cars that make zero sound to alert nearby pedestrians they're starting into motion. You don't exactly have that problem, do you, so maybe consider it a safety feature until you can figure out the source.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Dear tedv,

Hope you're well. Does the vibration change: (a) when the car is moving; (b) when engine speed increases? (c) when the car is in neutral vs. in drive?

A worn-out transmission mount might be a cause. The driveshaft center support bearing or mount could also be a cause.

When motor mounts are new, there's about a 1/2" (12.7 mm) gap between the oil pan's underside and the engine cross-member. If that gap is, say, 1/8", then the motor mounts should be replaced, regardless of age.

Hope this helps.

Yours faithfully,

Spook