Can you check your motor mount with a jack?

99xj

Doing the exhaust manifold and expected the mount to be bad due to age but a jack under the oil pan produces no play as the engine is lifted. Is this reliable or should I toss a new one in while I'm here?

Heat and age destroy the rubber compound of the mounts. They sag down.
They can also tear or separate.
On dealer mounts, when they are new, you cannot stick your pinky between the upper metal part of the mount and the rubber compound. After 10-15 years and lots of miles, you sure can. I'd expect the jack method to test for tear or separation, but not so much for the effects of age.

If in doubt, replace, beware of the dirt cheapo mounts, I bought one once, and it rapidly failed, had to replace in under 2000 miles. So get name brand. Mine sagged till it was metal on metal!

Warning!
Mounts have torn such that the engine will lurch forward, diggjng the metal fan blades into the radiator. A very expensive repair.

If they are old, and your down there anyway doing work, then replace them with a good brand.

I ended up going with urethane mounts, I wheel, and dont care about extra noise or vibration, I wanted rigidity.

As a quick check, I grab the valve cover on the passengers side and try to rock the engine left and right. Then I use a flashlight and look at the rubber for deterioration.
If you do replace the mounts use some Loctite for the bolts on the engine side of the mount.
Most of the issues I've had were on the passengers side mount. You'd think the exhaust side would be the worse, hasn't worked that way for me. I've found numerous broken and missing bolts on the passengers side mount, Renix and HO.

md21722 said:

... On dealer mounts, when they are new, you cannot stick your pinky between the upper metal part of the mount and the rubber compound. After 10-15 years and lots of miles, you sure can. ...

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Nice tip.

8Mud said:

As a quick check, I grab the valve cover on the passengers side and try to rock the engine left and right. Then I use a flashlight and look at the rubber for deterioration.
If you do replace the mounts use some Loctite for the bolts on the engine side of the mount.
Most of the issues I've had were on the passengers side mount. You'd think the exhaust side would be the worse, hasn't worked that way for me. I've found numerous broken and missing bolts on the passengers side mount, Renix and HO.

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So you are able to actually move the motor by that method? What about having someone free rev the motor?

maxbraketorque said:

Nice tip.



So you are able to actually move the motor by that method? What about having someone free rev the motor?

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That will work also, when I try to rock the engine I put some oomph into it. You'll notice the movement and likely hear the clack if the rubber is shot or the bolts are broken.

The bolts that hold the mount to the block are the real weak point. Broken, missing, and loose bolts are pretty darned common.

It's a 99 with stock mounts and so you would think it would be shot but as far as I can tell it looks like the new one. Even with the weight of the engine on the rubber there probably is only an eighth of an inch Gap. Maybe a quarter but there's no way you could even get half your pinky in there. I have raised the engine up with a jack and there doesn't appear to be any play in the mount. The rubber looks completely fine, there is no cracking. If I wiggle the engine there is a little bit of wiggle and the rubber Mount itself but I think it's supposed to do that.

It's all exposed it would be very easy to change it out. Is it possible after 20 years it's still just fine?

I just checked my 2000 XJ, and I can still get a pinky between the U and the rubber. Currently has 114K miles. I don't think it was driven off road hard at all before I owned it.