lrpman

Vacum box on a 1984?? Have the hard copy shop manuel and it is refered to but where the heck is it? Looking at that as a possible source for the "Shudder" when on the power.

pcar928fan

Is it between the drivers footwell and the front wheel???? I thought something vacuumy was there...

jpitman2

There is a cruise control box (cylinder shaped thing, pointed top) behind LF wheel rear splash shield, and there is a vac tank in there too IIRC. Easy way to eliminate this as a problem is to disconnect the vacuum line going over there and plug it.
jp 83 Euro S AT 52k

Hilton

Yep, rear side of the left front wheel well is the vacuum reservoir (and the cruise actuator, a which also takes a vacuum feed - so check its connected/sealed too).

The reservoir links via a small blue check valve to the brake booster, and also a splitter that feeds the HVAC.

Landseer

Believe it or not, my shudder was bad ground.

lrpman

Better description of the "SHUDDER"
Stop on the gas and you get a SHAKE, VIBRATION, SHUDDER lift and coast and it goes away 100%.
STOP, Stomp The Old Pedal, and you get a big Shudder, lift and smooth.
My first GUESS was the "Drive Shaft" vibration.

Landseer

Ok. Got it. Mine was a ground. Actually motor missing. Got at it by searching old posts for driveline issues. Found from Steve Cattaneo, the transmission expert, a post that suggested bad shudder could be engine missing. Went so far as to partially disassemble the transmission before redirecting.

Yours may indeed be driveline though since you had some rear renovation going on.
.

lrpman

Engine e is smooth, The Left half shaft has been replaced, the car was hit on the right rear.
The Right half shaft was rotated and checked and had no WOBBLE at all.
I am looking like crazy in the manual for HOW the engine transmits to the transmission. Looking for an intermediate support point that could give this shudder.

Landseer

Mine felt like driveline 100%.

Had no a clue it was an engine miss.

Only happened under heavy load.

jpitman2

I had a misfire in a car once when cold - cleared in about 10mins running. Isolated it to Cyl 3. Changed cap, stayed there. changed plug, stayed there. changed lead, stayed there....therefore problem was inside dizzy. Clearance between the reluctor pickup and the magnets in the rotating thing was at high end of spec - closed it up .002", problem went away. One weak magnet in the set.
jp 83 Euro S AT 52k

WallyP

Could be engine - it can be really hard to tell sometimes.

The level of voltage required to punch a spark across the plug gap varies quite a bit with cylinder pressure. An ignition system that can fire a plug at one-third throttle might fail completely at full throttle, when you pack more fuel/air mix (an insulator) in the plug gap.

If you are looking at the transmission info when you see the reference to the "vacuum box" they could well be talking about the vacuum modulator on the left side of the transmission...

lrpman

Thanks Wally, I THINK you might be correct since ALL the reference is the in trans section.
I will do another ignition test since the ONLY time I get it is under load. My 1982 burns rubber under full throttle. The 1984 BURNS GAS only.
So I am thinking of doing to this what I did to the XJ6 put in an Accell Race coil and see if that cures the problem. I have about 4 of them laying around so no problem.

dr bob

Coil failures are typically complete failures. There's a possibility that there's a "weak" coil but it's in the tiny fractions of a percent. If it's weak it's always weak, so you should see it as a yellow spark (vs blue when good) in any cranking test. Most "weak coil" problems are solved by all the WYAIT maintenance stuff done concurrently, like new wires/caps/rotors, and properly-gapped new plugs.

On other cars, I've cured trans problems with plugs and wires. My V-6 Explorer would drop out of overdrive and sometimes into third when towing the boats up hills. On my first 911, a late 60's S, I could fix an erratic clutch with new plugs. Ignition is often the ultimate cure for wierd drivelime problems.

lrpman

[QUOTE=dr bob;7287063]Coil failures are typically complete failures. There's a possibility that there's a "weak" coil but it's in the tiny fractions of a percent. If it's weak it's always weak, so you should QUOTE]
Agree 100% but if I go to a LOT hotter coil this SHOULD give me a better spark period further up the rpm curve.
I have a spare MSD which is in the mix also.
The car is more than worth the effort.