Strange knocking

[johnnym]

I have just replaced the motor mounts on my 86 Grad, and it has caused a strange side effect: the car now has a knocking noise coming from the engine. It is not the exhaust rattling on the bottom, as it is ryhthmical and increases with revs. It is not driveshaft, as it happens when stationary. I can hear it most in the car, where it seems to be coming from the footwells- when I listen to the car running with the bonnet open, I can't hear it!

I have read elsewhere that trapped exhaust gasses can cause something like this, but don' really know how to check for it.

I had to remove the plenum, and many of the air tubes when i did the motor mounts. I also found a small earth braided strap loose below the plenum today, which I figure should attach somewhere. I also recently replaced the oil & filter, but used standard oil.

The big issue is that I am in the military and have just been posted at pretty short notice, so the car is starting on Ebay tonight, and I want to get this fixed. Any help would be much appreciated!

[vf31rhill]

What does your oil pressure look like? It is hard to say, but one of the possibilities is a loose connecting rod bearing. The sound you describe, rising and falling with the revs, sounds like such. You don't want this and there is no way the actions you took would cause something like this, other than the oil and filter change. If there was very high viscocity oil in the engine, and you replaced with low viscocity oil, it is remotely possible for a borderline bearing to show up, which would have shown up sooner or later anyway.
What are your cold and hot gauge readings?
Robert

[johnnym]

Oil pressure is fine-drops slightly when hot but the same as every other Alfa engine I have known. I put 20W50 oil in, as usual, but this was a couple of months back and although I haven't done too many miles in it, I haven't noticed anything. I also should mention that the engine seems to be 'spitting' more than usual since the new motor mounts. I just did a check and there was one small vacuum pipe adrift off the plenum, whcih I replaced.

Oh, and the knock is not noticeable when you are driving- just at idle and pulling away.

[ghnl]

Is the brace under the intake plenum securely attached?

[johnnym]

I have just tried the engine with the adrift pipe re-attached, and the noise seems to have stopped, and she runs much better. I think (according to my manual) that it was the vacuum sensor pipe. Could this have been all it was?

I have also discovered a 13mm bolt that was 'spare' at the end of yesterday's motor mount fitting.... which I seem to remember used to attach the brace to intake plenum. This can't be helping the rattling situation! I will fix it tomorrow and let you know if that is it.

[vf31rhill]

""Oh, and the knock is not noticeable when you are driving- just at idle and pulling away""

This is a good sign. Also the oil prssure readings. Loose connecting rod bearings show up loudest under acceleration under load.

Nothing to do but get back under the car and start doing a close visual inspection, pushing and pulling on everything remotely possible until you find the culprit. Since you installed new motor mounts, you need to look at everything else which raising the engine might have placed in new alignment (or mis-alignment).
Robert

[ghnl]

Here is a photo of the underside of the intake plenum - shows the brace quite clearly. Stolen, umm, borrowed from this thread: engine mount travails



Note that the cross piece is inside a rubber bushing.

And below is a picture of a long dead project Spider (not mine): note the brace also attaches near the cylinder head and at the motor mount. The fuel rail is bolted to it, too.