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phil400
Cochise

Canada
456 Posts

Posted - 10 Sep 2017 :  5:05:11 PM  Show Profile  Reply with Quote
All of a sudden my rpm gauge is off, by about 300 rpm, for instance for years it has turned judder the 3K mark at 70MPH, now it's showing 3250, what would cause this out if the blue?

It's the factory gauge.

78T/A 4-SPEED Marty blue 400/milled 6x-4's/268H,comp roller tip rockers/ performer Q-jet/RA manifolds 2.25" exhaust with x-pipe/ mine since '99 3rd & final owner as long as I'm alive
"sometimes I do what I want to do,rest of the time I do what I have to"

Steve C.
Crazy Horse

1675 Posts

Posted - 10 Sep 2017 :  7:24:43 PM  Show Profile  Reply with Quote
Related ?

http://www.howtoalmanac.com/kevin/projects/automotive/tachfix.htm


When tachometers start to go bad they read high, like 1200 RPM at idle, and get worse as the RPM's increase.

http://www.73-87.com/7387garage/interior/factory%20tachometers.htm

More...

https://www.corvetteforum.com/forums/c4-general-discussion/2662597-high-reading-tach-repair.html

.

Edited by - Steve C. on 10 Sep 2017 7:30:32 PM
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Phil
The Great White Buffalo

USA
7219 Posts

Posted - 10 Sep 2017 :  11:17:45 PM  Show Profile  Reply with Quote
More noticeable at highway speed or only noticeable at highway speed? Any chance your trans is slipping?

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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phil400
Cochise

Canada
456 Posts

Posted - 11 Sep 2017 :  07:40:09 AM  Show Profile  Reply with Quote
More noticeable at highway speeds. No dice on slipping, 4 speed car, and the engine sound doesn't reflect rpm reading

78T/A 4-SPEED Marty blue 400/milled 6x-4's/268H,comp roller tip rockers/ performer Q-jet/RA manifolds 2.25" exhaust with x-pipe/ mine since '99 3rd & final owner as long as I'm alive
"sometimes I do what I want to do,rest of the time I do what I have to"
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Cobrabill
Talking Dog

Aruba
3128 Posts

Posted - 11 Sep 2017 :  8:53:13 PM  Show Profile  Reply with Quote
hook up another tach and verify readings.

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