I was stopped at a traffic light and while waiting, noticed that every once in a while the engine hood would start vibrating in the center. When I got home, I popped the hood and had a look at the center rib under the hood skin. I never noticed this before; but, on my car the hood skin is not attached at all to the center rib and in the middle (front to back) there is about a 1/8 " gap between the hood skin and the top of the reenforcing rib. The hood skin is attached around the periphery; but, is completely unsupported in the middle. With the hood closed, you can press on the center of the hood with you fingers and it will deflect downward about 1/8" before contacting the center rib. The hood was never damaged and there are no signs that the hood was ever bonded to the rib.

Are other people's 140s like this?

No Dynamat damper pads on the hood. I did run a bead of clear seam sealant down both sides of the center rib. This has eliminated the original first mode vibration with a maxima in the center of the hood. I now have second harmonic oscillations with the maxima midway between the center rib and the outside edge; but, the magnitude of the displacement is much, much smaller. I may try an acoustic blanket under the hood just because its a B20E and sounds like a bag of hammers and nails. But, I expect that the resonant frequency of the hood skin is quite a bit lower than the acoustic noise generated by the engine so I don't expect that an acoustic blanket would make a huge difference.

I would use some auto body sealant and randomly squeeze the sealant Inbetween the gap. My 1991 Saab 900s has the middle rib gap and has from the factory, Body sealant ramdomly Spaced.

Do you have vibration dampener or noise insulation under the hood? If not that would likely eliminate the vibration. Dynamat?

What year is it? Got a photo?

The newer the model, the more poorly they are made. The B18 cars are the nicest, the bonnet has an extra brace roughly where your problem is.

It is a 1971.

As described by others, there are traces of something that looks like an adhesive on the inside of the flat part of the center reenforcing rib where it would attach to the hood skin - if it were meant to be attached. The car was repainted and the hood had most of the original paint removed. The only reason I can spot the residual adhesive is that it is actually inside the rib and couldn't be removed. I could spot the stuff through the 1/8 gap between the rib and the hood skin.

Perhaps I will go back to the body guy who did the work on my car and get his thoughts on a suitable bonding agent.

I bought some stuff at the auto body supply house called Transtar Ultra Flex Seam Sealer that looks like it would work in a situation like that. Comes in a few different colors. Personally I like the rubber wedgie idea. What about getting some of that hood insulation ad cut the pieces large enough to tuck between England and skin?

My 1800 has big spaces there like 1/4".
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

The gap between the top of the reenforcing rib and the bottom of the skin is at most 1/8 ". Most of the gap is much smaller. Just enough to allow a little movement in the hood skin when you are at idle. I thought about trying some thin closed cell foam tape as a flexible spacer. The tape has an adhesive backing which makes jamming it into the space a problem unless I leave the protective cover on the adhesive. Also, the thinnest tape I could find was 3/16" which means that it will be pushing the hood up slightly in the tight spots.

I looked up the Transtar product. Did you use the seam sealer or the seam sealer and adhesive? The seam sealer has a work time of 3 minutes which given that I am somewhat clutzy is probably way to fast. The seam sealer and adhesive has a work time of 10 minutes which is probably more in the doable category for me. Was the Transtar toolable? I have used some sealants that tack so quickly that there is no way that you can tool the sealant if you need to do a little clean up. I have used a special polyurethane seam sealer on other parts of the car that was toolable and easier to clean up (at least compared to a lot of polyurethane sealants).

Since the objectionable symptom is the hood skin vibrating vertically at certain conditions of engine idle, before pondering a permanent fix why not try a temporary one to discover what gets rid of the vibration?

Improvise some shims of multiple card-stock paper or non-corrugated cardboard that fit tightly enough to not fall out in temporary use. Then drive around to test whether the hood vibration is gone.

Tech explanation: It's likely the vertical engine vibration at idle and the natural vertical vibration of the unsupported hood are in resonance. Adding firm contact of the stiffening rib below will raise the hood's vibration frequency higher than before and avoid resonance with the engine motion at idle.

Yes, just like pressing a guitar string down on the 12th fret you suppress the fundamental mode of vibration and only allow the 2nd and higher modes. I filled in the gap between the supporting rib and the bottom of the hood skin with a clear sealer which remains flexible (took a while to dry). The flexibility has the double advantage of introducing a small amount of viscoelastic dampening. Constraining the hood in the center has effectively eliminated the fundamental mode of vibration across the width of the hood; but, now on occasion I get 2nd mode vibrations - vibration peaks that occur 1/2 way between the outside edge of the hood and the center rib. The displacement of these vibrations is much smaller and only occurs briefly when accelerating. I probably would not have noticed them if I weren't looking for them.

I had previously made the comment that the gap between my hood skin and the rib was small. I don't know what planet I was on when I made that observation as the gap between the rib and the hood skin was more like 1/4" in the center of the rib. I debated whether I should press the hood down so that it was in contact along the length of the hood while the adhesive set (essentially eliminating the gap). Because the hood profile seemed to be correct with the gap I elected not to do this and just filled the gap with sealant and allowed it to set.

"...Did you use the seam sealer or the seam sealer and adhesive?..."

Just sealer #4167 (which is black). I only used it once and I don't recall if I felt pinched for time. However, I just looked at the dried up chunk on the tip of the applicator tube and can see it that it dried fairly solid. Maybe more solid than you would want.

Just checked my '69 - which has a slightly later hood. The rib is solidly attached to the skin-no deflection at all. -- Dave

It is possible that a fastening system in the center of the hood would show as some type of flaw in the hood.

This construction is similar to doors, in that the skin is only attached around the perimeter.

I would try a few shims made from various types of semi-solid foam, or possibly styrofoam.

My 1975 244 DL did this.

The sheet metal skin detached from the hood-supporting structure underneath.

It appears some kind of adhesive was used at the factory to glue together using occasion wads of some kind of glue.

I cleaned out the dried glue chunks, what remained. The owner of the shop I was working for had some stuff in a caulking dispenser made for it. It smelled strongly of solvents and was heat proof.

http://board.moparts.org/ubbthreads/ubbthreads.php/topics/695873/gluing-the-hood-frame-to-the-hood-skin.html

I'm unsure what you'd use. Use a search engine and search more for a solution?

It would help to know what the Volvo factory used. The stuff is not flexible. Probably why it detaches in time. The flat hoods on 140/240 seem to get it the worst.

Maybe ask an auto body shop? Some NAPA stores are dedicated as auto paint and body shops.

http://www.3m.com/3M/en_US/Automotive/Solutions/

Probably do not want to weld it. There may be a reason Volvo uses glue as part of front end impact protection and crumpling zones and such.

Hope that helps.

dud.
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