Power train problem - when your car slow down with no reason

[GiuG]

Another little problem, Guys! My 75 Vette is automatic. After the mechanic is not gliding any more... when at a speed you release the throttle pedal, the car suppose to glide for a while. Now is like something hold her back something brake her (feels like you have immediately a lower gear). I need to check if they pull too much the parking brake, but seem something related to the power train. The mechanic dismount the differential to check it, and mount the new monospring suspension. Now the car needs more throttle to move from hold, sometime I hear 3 consecutive bump coming under my sit, like some little parts clashing to each other, with a subtle rotating saw sound in background. They put a COMMA 1580w 90 gl-5 mineral into the Diff. ___ and a Castrol classic 20w50 in the Engine. This is what I know, Any clue?

 

[teamzr1]

You check for a vacuum line issue to the tranny ?
Did they screw with the torque converter ?
Is that not too heavy of an engine oil ?
Possible U-joint bad or they did not tighten them correctly or sway bar end links ?

 

[Forrest Gump]

Does the car coast if you go into neutral? If it is slowing down without drive engaged then it might point to a seized brake.
Note also It is essential put in one bottle of GM additive into the differential otherwise it will bind-up and may account for the bumps you are hearing from under your seat (where the diff is) This stuff….

 

[Chuffer]

As Forrest Gump said , you must have the correct limited slip differential additive in the diff oil and then do at least 8 to 10 figure of eights slowly in an empty car park or open flat space to work it in to the LSD before driving very far in it . As for engine oil , Mobil 1 5w 30 is by far the best choice for a C3 .

 

[Forrest Gump]

The "Classic" branded oils generally have the essential zinc content that is important for these old engines, reducing valve-train wear.
Castrol Classic appears to have 800ppm zinc content which is okay.
Valvoline VR1 20w/50 is a popular choice for C3 owners. This has something like 1400ppm zinc content.

 

[GiuG]

I love you guys! I'm taking note of all of this stuff and forward to the mechanic hope he can use all of these precious hints. For now one you are the Corvette A-team the C-Team thanks!!!

You check for a vacuum line issue to the tranny ?
Did they screw with the torque converter ?
Is that not too heavy of an engine oil ?
Possible U-joint bad or they did not tighten them correctly or sway bar end links ?

I doubt they checked all this stuff they are not American car lover, they try their best, but they are good in fixing classic small engine British car and most of them are not sportive car, the only sportive cars are Porche and Ferrari from the 80'. To be honest they hate my C3, they say, she is a troublemaker

 

[Chuffer]

I love you guys! I'm taking note of all of this stuff and forward to the mechanic hope he can use all of these precious hints. For now one you are the Corvette A-team the C-Team thanks!!!

I doubt they checked all this stuff they are not American car lover, they try their best, but they are good in fixing classic small engine British car and most of them are not sportive car, the only sportive cars are Porche and Ferrari from the 80'. To be honest they hate my C3, they say, she is a troublemaker

All cars can be trouble if you have no knowledge or understanding of them . If you are not mechanically minded yourself and prepared to research the the technical aspects of your Vette then you really need to find an Amercan / Corvette specialist garage in your area .There must be someone around surely .

 

[GiuG]

Just to keep you all in loop on what I discovered, the the sound of 3 weird bumps under the sits was the exhaust tubes not well tide up, bouncing under the hull specially when the engine is in idle. The slowdown effect was apparently one of the rear calipers that was too tide up around the disk, the mechanic needs to release it a bit to make the drive smoother and not refrained, plus the differential liking badly from the main seals.

 

[Roscobbc]

I love you guys! I'm taking note of all of this stuff and forward to the mechanic hope he can use all of these precious hints. For now one you are the Corvette A-team the C-Team thanks!!!

I doubt they checked all this stuff they are not American car lover, they try their best, but they are good in fixing classic small engine British car and most of them are not sportive car, the only sportive cars are Porche and Ferrari from the 80'. To be honest they hate my C3, they say, she is a troublemaker

Sounds like you need to find a mechanic with American car experience, and one specifically with some earlier Corvette knowledge although really there is nothing too difficult or different enough for any half decent mechanic to understand and maintain.

 

[Roscobbc]

Just to keep you all in loop on what I discovered, the the sound of 3 weird bumps under the sits was the exhaust tubes not well tide up, bouncing under the hull specially when the engine is in idle. The slowdown effect was apparently one of the rear calipers that was too tide up around the disk, the mechanic needs to release it a bit to make the drive smoother and not refrained, plus the differential liking badly from the main seals.

Releasing the rear caliper pistons isn't really a long term fix........sounds to me that if it is brake related you'll ideally need to have the rear calipers (at the very least) overhauled. No big deal, simply new pistons with 'o' ring seals on all 4 corners, clean-out the calipers and remove 'high-spots', perhaps replace brake pads if worn down (but with hi-performance pads) andcheck-out the discs for excessive wear and 'run-out' and bleed through new brake fluid.