Severe scuttle shake off throttle

[billm]

I have a modified rear wheel drive Ford Escort MK2 and it has severe scuttle shake whenever I lift off the throttle while still in gear.

Typical scenario:
I am accelerating down the road to say 60 mph, lets say in 5th gear although it could be any gear, and all is fine
Car pulls well, no shaking, no noises, no clonking
As I am catching traffic up ahead I lift off the throttle, still in 5th gear, to slowly close the gap ... and the scuttle shake begins
The shake feels like it's coming from the front, the engine, but that could just be where the noise is appearing and not what's causing it
Remember this is a rear wheel drive car with the engine in traditional direction, ie not transverse.

It has done this since I bought it but due to the number of times it gets used a year I have ignored it, but I'd like to cure the problem as it makes long distance driving a pain.

I had a gearbox guy look at it and we brainstormed what it could be and eliminated what it couldn't be :

It isn't the engine missing, as it never misses on tick over, on acceleration or when cruising
It isn't the clutch (it has a race drive plate with no springs) as once its in gear (and the clutch does not slip) then the clutch is basically not being used and just passing the energy through to the gearbox input shaft
It isn't the gearbox as it happens in all gears, and when in top gear it's straight through anyway

1) It could be weak engine mounts but they look ok although hard to tell unless tested with a pry bar
2) It could be the crossmember that the engine mounts are connected to but that would mean its very loosely fastened to the chassis and rocking
3) It could be the propshaft Universal joint (there's only 1, the other end being splined) but from years of numerous Spitfire ownerships they clonk on drive take up as well as power off and unless really gone when they rattle/vibrate just before they fall to bits. There is no centre propshaft joint it's 1 piece
4) It could be play / lash in the Diff
5) It could be the rear axle mounts
6) It could be the rear gearbox mount

The gearbox guy defined it as "it's when you lift off the throttle, it's as though the rear of the car is pushing the front, instead of vice-versa, like on over-run, and it's fighting against the engine as the engine revs drop"
If I press the clutch in when off the throttle, there is No scuttle shake -presumably as the rear is no-longer fighting a slowing down engine

Anyone had this before on a longitudinal RWD setup ? This isn't a standard Escort MK2, its a highly modified Turbo road car with lots of non-standard parts.

Any ideas what it could be and how I can eliminate each of the possibilities to narrow it down would be most appreciated.

thanks

 

[carmanmemoranda]

I'd be looking at the axle, mounts, anti tramp bars, diff, uj, mostly.

Almost certainly that area, but exactly what I dunno

 

[Burnerman]

Does your car have a Type 49 or another Ford AVO shell, and is it suitably braced at the strut tops?
I would imagine heavy duty engine mountings are needed too.
John

 

[billm]

Does your car have a Type 49 or another Ford AVO shell, and is it suitably braced at the strut tops?
I would imagine heavy duty engine mountings are needed too.
John

No it isn't, standard shell with no additional bracing but with rose jointed strut tops and adjustable TCA's. Bodywork is fine, If the strut tops were moving it'd crack the paint wouldn't it ? which it hasn't and this is just normal road use only not race or rally

 

[billm]

I'd be looking at the axle, mounts, anti tramp bars, diff, uj, mostly.

Almost certainly that area, but exactly what I dunno

I agree, has to be something in that area ? Dya know if diff lash be measured in situ or is it a diff out job .. that would be my last check anyway

 

[Burnerman]

To be fair - and I mean absolutely no disrespect here - you are pushing maybe 200+ BHP though a body shell designed for 70 BHP or whatever so there must be some sort of payback.
To me it sounds as if the rear axle is 'winding up' and it needs to be kept in check......consider fitting a panhard rod kit if one is available. This may be more important if you are running a limited slip diff.
Also the gearbox and engine mounts need to be beefed up - I wouldn't mind betting the motor moves about a fair bit when throttling off.
Are you using a Zetec or Duratec mill in the car?
John

 

[Harry Bloomfield]

Going back a long time, to when I drove a company escort, it had a similar issue - I traced it to the front tracking being well out.

 

[billm]

Hi John, you're right it has a fair bit of bhp, It's running a 2.0 Pinto, Turbo with fuel injection. And to be honest it only goes out a couple of times a year which is why I've never done anything about it before.

I can't see the engine moving more than normal when static and revving but then it's not under load trying to push the rear wheels. But all these things, engine mounts, axle etc are almost impossible to see if there is excess play when using the car and the shake occurs. If I could see what's moving that's half the battle & then I can address it. It's pointless running it up on a 2 post lift as the tyres need to be on the road.

I need a rolling road I can lie under .. as if lol

I think the first step is to test every rubber mount with a pry bar and look for anything obvious
thanks

 

[Mottie]

When I built a MK1 with a 2.0 and a 5 speed box, I had to fabricate my own gearbox mountings. The tolerances are very tight. Perhaps something is juuust catching the bodywork on the overrun or the prop yoke is too far in or out of the tail shaft housing causing a vibration?

 

[Nozzle]

I can't see the engine moving more than normal when static and revving but then it's not under load trying to push the rear wheels. But all these things, engine mounts, axle etc are almost impossible to see if there is excess play when using the car and the shake occurs. If I could see what's moving that's half the battle & then I can address it. It's pointless running it up on a 2 post lift as the tyres need to be on the road.

I need a rolling road I can lie under .. as if lol

Mount a go-pro or similar in the engine bay, go for a drive?

Nozzle

 

[Old Salt]

Post number 2, I had a TR6 with loads of vibration, turned out to be the prop shaft, but any area of play could be causing yours because of the power output. Had a Cortina where a rear tyre was delaminating that only showed up when being balanced on the car and using a strobe light revealed the tread lifting,although whilst stationary it looked perfect. A new tyre and all was good.

Dont discount a number of small amounts of play adding up to combine and check the crankshaft pulley/damper.

 

[Belle427]

Certainly sounds like some suitable strengthening may be required, try some simple things first such as a strut brace/s.