Small RPM cycle at idle

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Neu
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Small RPM cycle at idle

Post by Neu »

Hey y'all,

i tried searching but couldn't find anything related.

I have a mild rpm cycle at both cold and warm idle. My idle sits around 1100rpm once warmed up. Occasionally it will "cycle" about 100-200rpms. They're quick little cycles.

Any ideas? It's probably an easy fix.

Thanks.
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dlb
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Re: Small RPM cycle at idle

Post by dlb »

Have you checked all the vacuum diaphragms to make sure they hold vac? I can't remember which (if any) have vac directed to them at idle, maybe the upper one on the dizzy? Probably a good idea to check them all while you're at it though.

Long ago, I remember 4wdChico had a rpm cycling issue and it wound up being related to the charcoal canister. There was a whole thread about it but I can't find it now, just this post where he talks about that same problem:

viewtopic.php?p=27748#p27748
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Petros
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Re: Small RPM cycle at idle

Post by Petros »

sounds like a vac leak. Visually look over all of your vac lines that they are routed correctly and there is no visable cuts or damage. Than check all your vac lines and gaskets for a leak using stater fluid or carb spray, give it little short puffs at various places while it is running, listen for an temporary increase in rpm. usually that will find it.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Neu
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Re: Small RPM cycle at idle

Post by Neu »

After going over everything with some carb cleaner the only leak I found was around my master cylinder/brake booster mating surface. I made a new gasket for that and there doesn't appear to be a leak anymore. The RPM cycle, so far, is gone. My idle is also pretty consistently at 900-1000rpm now.
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Petros
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Re: Small RPM cycle at idle

Post by Petros »

that is unusual, there should not be any vacuum at that connection. perhaps the brake diaphragm has a leak? if the brake booster still works, I would not worry about it too much, just keep it in mind if it becomes more difficult to stop the car (takes more brake pedal effort).
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Neu
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Re: Small RPM cycle at idle

Post by Neu »

Nah the booster seems to work fine.

I still plan on replacing a lot of the brake system anyway so I may look into replacing the booster.
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Petros
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Re: Small RPM cycle at idle

Post by Petros »

you might consider getting a first gen MR2 master and booster to replace the factory one. you get more braking power, and the booster/master combination will bolt right in. you have to locate them from the wrecking yard, new, if you can find them, are pretty costly.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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Neu
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Re: Small RPM cycle at idle

Post by Neu »

The RPM cycle has pretty much gone away. It's happened maybe twice since I fixed that leak.

I'll keep an eye out for the mr2 booster/master.
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