violent engine shake over low speed bumps

a friend of mine has a custom built rod ( not sure of all details) and when he lifts off the gas at slow speed over ripple bumps or train tracks the engine goes into a violent back and forth shake. the steering wheel isnt moving and the car isnt hopping around but the engine is rocking back and forth like crazy. he has to slow right down and then it stops.
he is not the first owner and the previous owner says it never happened to them.
i know his front suspension is leaf spring, drop axle and monotube shocks and are all pretty new, he tried swapping engine mounts but it didnt make a difference. any ideas?

 

Did he check the tranny mount and possibly the drive line for a dent or bad u-joints?

 

strange, I would check the u-joints and all the mounts (trans and engine) first, if nothing there maybe check the rearend bearings.

 

apparently trans mounts are fine.

 

engine...? knowing what mounts might help
trans...? knowing what mounts may help
pictures...? seeing how the assembly was mounted might help
any more you can give will help..
my assumption with the info given is bad or broken engine mounts (and or) trans mounts bad, ripped (if the rubber type)..or very loose..
also check the bolts to see if they are tight or broken off..


sounds to me like the fuckers not tight..big time (engine and trans assembly to the frame)

if all that checks out than move on to drive line balance or bends

hook up to the engine with your engine puller and start lifting it., (The engine)
see if any of the mounts are ripped or broken

 

Sounds like it might be electrical, loose ground or poor connection somewhere.

 

What kind of trans ?

I'm assuming this happening under deceleration without the use of the brakes and only happens going over bumps ?

If the above is true, it could be the rear wheels are unloading going over the bumps and in turn unloading the engine mounts in relation to the rear wheels loading and unloading as it goes over the bumps.

Tell him to try the same stretch of road while holding the clutch in and using the brake to slow and he can see if it still does it and maybe narrow it down a little.

Good luck

 

It sure sounds like bad motor mounts or maybe a design problem. I have seen cases where they used the front mounts on a SBC and a center mount under the trans that let the engine rock back and forth.

 

bad mounts and a driveshaft that's bottomed out in the trans.

 

Yep. obvious to me. Maybe if he jacked it up and got under there and found out what's loose instead of calling the hamb?

 

All previous owners say that. Obvious things -- motor/trans mounts, drive line angularity problem. Ask him if the problem starts when the front wheels go over the bump, or not until the rear wheels go over the bump. Makes a difference.

Bob

 

appprently from what he can see everything is tight so hes running out of ideas. the engine is a 392 hemi and a tremec 5 spd tranny

 

front wheels

 

Tell him to approach a bump that's known to cause the problem, at a speed known to cause the problem, but coasting with engine off and in neutral. What happens?

Bob

 

what mounts?

crossmember tight?

any rubber in them mounts?...is it torn?

 

only happens when he is is going slow and lifts the throttle while going over washboard bumps or train tracks.

 

another very good point...

 

just put new SBC ones in. had stock hemi mounts before. same issue

 

check to see if the trans rubber is torn, or if the drive shaft is bottoming out into the trans as said above..also check the tightness of the trans crossmember to frame..and those SBC rubber mounts have been known to be shit right out of the box..or soon after..check em good to see if they are ripped..or seperated

with all that shakin going on..there should be some wear someplace that you can see..bolts that you think are tight because you cant turn them any more, but are just bottomed out in their holes? and not really tight on the assembly that they are ment to hold tight? (if you get what im saying?)
even check your suspension members to be sure they are all tight and right
if you put your lift on the engine and start to pull it until your suspension is unloaded..does the engine and trans keep coming up?

 

I bet the whole frame is doing the shaking, and I also bet the 4 link or wishbone or whatever system being used to locate the front and rear axles is faulty.

 

My thought too. That's why I suggested having him coast, engine off, over bumps known to cause the problem. The engine shake thing seems less likely the more I think about it, and I've probably already thought about it too much.

Bob

 

Me too, sounds like the frame is twisting and flexing. JMO. Lippy

 

Yep...helping a surrogate troubleshoot someone else's problem is not how I usually start my day.

 

Like bobscogin and adjustablejohnsons and lippy say, There's a lotta 'could be'................. Hell, 'it could be' that the frame is flimsey as spaghetti, but
"your friend" still has to 'look' to see......... or call a mechanic.

Side note; A road test before buyin' is a no-brainer.

 

I hope he is putting a wrench on the mount bolts and testing how tight that they actually are. You can't just look at them and tell. I had a similar problem. Everything looked nice and tight. I felt like a dumb ass when I put a wrench on them only to find that I had left them hand tight and never got back to finally torquing them down.

Mine were the Ford and Hurst style style rubber biscuits style mounts. They allow a lot of movement when not torqued down unlike a 60s style factory side motor mount.

My engine and trans would move back and forth enough to put the fan into the radiator. It sure looked tight. I ended up making anti-chatter rods like Ford did with this style mount.

If like you are saying that the engine is moving more than the chassis it is mounted in then it has to be the mounts. IMHO

 

hmmmmm?

 

Flux capacitor?

 

Sounds like what a stock unboxed Model A frame would do with an 850lb Hemi bolted to it.

 

I think he was trying to hint that the OP didn't give enough information for us to give any relevant help.

 

good point..a picture of this beast would be in order..

and if the facts are that this thing isnt built right to have the weight of this power pant in it..than its time to pull it and start over, before someone gets hurt.

can we see what we are all talking about here?
if not ,, than it all just comes down to mental masterbation trying to determine what is going on with this set up