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Why does it rev up and down?

Tiny
SHADY420
  • MEMBER
  • 1993 HONDA PRELUDE
  • 93,000 MILES
1993 Honda Prelude I Have Just Changed The Water Pump And Timing Belt, Thermostat, Radiator, Both Radiator Hoses All Other Belts Around A Hundred Miles Ago. My Dash Temp Gauge Always Stays At Middle After Warming Up. But Drive It For While Put In Park the idle goes up and down from 1000 to 2500 rpms.
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Friday, May 4th, 2012 AT 1:03 AM

8 Replies

Tiny
FIXITMR
  • MECHANIC
  • 9,990 POSTS
Sounds like vacuum problem. Maybe hooked some line wrong/loose.
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Friday, May 4th, 2012 AT 1:07 AM
Tiny
SHADY420
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  • 6 POSTS
Had all the vacuum lines replaced and double checked.
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Friday, May 4th, 2012 AT 1:11 AM
Tiny
DJCL
  • MECHANIC
  • 767 POSTS
Make shore there is no air in the cooling system and that its not low on coolant. It this car is low or has traped air in the cooling system the idle will rise and fall quickly.
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Friday, May 4th, 2012 AT 1:32 AM
Tiny
SHADY420
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  • 6 POSTS
I bleed the coolant a few times and coolant level is fine. Do you think it could be the thermostat. I bought the cheapest one from AutoZone. Maybe I need a Honda thermostat or maybe get a gates racing low temp thermostat
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Friday, May 4th, 2012 AT 1:37 AM
Tiny
SHADY420
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  • 6 POSTS
Or do you think it could be the tps sensor?
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Friday, May 4th, 2012 AT 1:38 AM
Tiny
DJCL
  • MECHANIC
  • 767 POSTS
Check and see if there are codes in the computer if the engine temp is ok the thermostat is probalay ok
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Friday, May 4th, 2012 AT 1:42 AM
Tiny
FIXITMR
  • MECHANIC
  • 9,990 POSTS
Is your IAC coolant controlled? Any air in it/blockage will cause idle problems.
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Friday, May 4th, 2012 AT 1:53 AM
Tiny
BWIMPMNBOI
  • MEMBER
  • 3 POSTS
*Shady420
The reason why your prelude is reving up & down is you have a bad fast idle control valve. From there inside that is a plastic piece that has been loosened over time there for, when your car has warmed up & sitting in nutruel it revs up & down from what you desribed. Easy fix, take off the fast idle control valve (3 bolts) look inside & turn the white plastic piece with a flat head srew driver until it is tight. Re-install & it will no longer rev. If it still revs but revs up to 1200 rpms replace the fast idle control valve. Ebay it! $25.00 for used one I have a 95 Honda Prelude Si 5spd. I had the same problem. Whenever warmed up It would rev up & down to 2500 rpms from 700. Honda's are kinda known for this problem. Easy fix! Took me 15 mins & the part coasted me $25.00 used. Any questions feel free to ask.
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Monday, May 7th, 2012 AT 4:47 AM

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