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Old 05-27-2018, 05:09 AM   #11
Deerhurst
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The wax solenoid controls your idle speed. It uses coolant to pull a plunger in to lower the idle speed. The piece the plunger hits must be adjusted properly to have proper hot and cold idle speeds. It does weird stuff. Mine is 32 years old and is starting to give me random high and low idle. Sometimes is 700RPm hot and sometimes it's 1200RPM when hot. Cold is usually at least 1500RPM right now. Sometimes is 1200RPM.



I'm not sure how high it can get your idle unless the piece the plunger piece gets moved out of position



Check all the vacuum lines. Maybe try pulling codes. Might get lucky.
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Old 05-27-2018, 05:53 AM   #12
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I don't think the wax solenoid would cause something like that. Even with it open full bore the fast idle maxes out at about 1500 RPM, provided the manual idle speed hasn't been messed with or anything. I wonder if one of the injectors is stuck open or the fuel pressure regulator has failed. Even with a massive vacuum leak I think it would still need enough fuel to make it rev up.
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Old 05-27-2018, 10:34 AM   #13
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Quote:
Originally Posted by Deerhurst View Post
Mine is 32 years old and is starting to give me random high and low idle. Sometimes is 700RPm hot and sometimes it's 1200RPM when hot. Cold is usually at least 1500RPM right now. Sometimes is 1200RPM.
mine has been doing something close to what you describe. when the temp is below 65, when the engine is cold started, my idle is whacked. it starts off really high. 2500 for several minutes. as the engine starts to come to operating temp. the idle starts surging from 750ish to 2000 non stop. it is up and down. it is loping very close to how it acted when the cylinder temp switch was broken. but after it warmed up with that broken switch, it was normal. i replaced that switch and it never gave me a problem again until now.

with daytime temps in the high 80s/low 90s with heat index readings in the 100s, 65 degree dew points, 40 to 70 percent humidity, it is completely normal. cold engine idle is about 1500 and settles down in about 3 minutes to 650 to 800 depending if the AC is running.

what really pisses me of is it didnt start this until i had to replace the water pump earlier this year. all 3 timing marks are dead on the money. i have played with the timing ten degrees either way and if i retard the timing enough, the idle jumping stops. BUT, it is so choked back it wont climb a hill. with it advanced, it runs like a scalded dog but has serious idle problems no matter where the ambient air temp is sitting. i have it sitting just a hair under the specified timing and it is fine for now.

im stuckin fumped. guess i need to pull the seat out and check the codes.


edit........maybe i need to double check the cylinder temp switch. i do not recall damaging it when the water pump and belt were replaced this last time, but it never hurts to be sure.
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Old 05-27-2018, 01:45 PM   #14
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Since you said you just washed this thing before the problems started, take a look around the intake and air filter and make sure you didn't pressure wash a hose off. There is one that goes under the air filter, too.
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Old 05-27-2018, 03:39 PM   #15
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Check the TB linkage, when the engine is off, look at the linkge to see if the throttle body butterfly is in the fully closed position (position of the linkage when the gas pedal is not being depressed at all), if the butterfly in the TB apears to be open the wax element may be bad and is preventing the butterfly from closing to the normal idle position.


There is a small adjustment screw with a lock nut on the bracket the wax element plunger engages, you can loosen that lock nut and adjust the screw so the linkage can close all the way. At least you can do this to see if the idle drops to normal or until you can replace the wax element.


Also may be a good idea to use some TB cleaner spray and an old tooth brush to clean the inside of the TB and the edges of the butterfly, sometimes gunk builds up on the edge and will cause the butterfly to stick.
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Old 05-27-2018, 03:48 PM   #16
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Left red arrow points to adjusting screw that contacts plunger of wax element (also known as thermal element), top downward facing red arrow points to plunger of wax element, diagram from 89 FSM:


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Old 05-27-2018, 04:34 PM   #17
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I would look back to SBJ's post on all this...

I had a problem (up and down idle all the time) with my old 1989 truck and I never got rid of it, but it's true (as SBJ says) that anything related strictly to the wax solenoid, etc maxes out revs at around 1,500. And if you turn down the idle screw enough, the engine will just die. So...you turn it UP to the point where it will run....but the idle will lope up and down.

If this engine is just revving up and staying there at max revolutions, it probably isn't the idle solenoid. Something else is wrong here. Injector stuck open, fuel regulator bad, etc as SBJ suggests.

We should get this straight: When you start this truck, it just revs up to max, like it is floored, and just STAYS there? Screaming along at max revolutions? Not the solenoid I would think. Something is just wacko and causing fuel to be fired down that TB in mass quantities.
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Old 05-27-2018, 09:15 PM   #18
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If an injector is just dumping fuel it should stall. My sentra was doing that one time, injector was bad and car kept stalling.
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Old 05-27-2018, 10:01 PM   #19
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Yeah...we need some clarification on the extent of the revolutions. Just running up high, or actually full bore frickin' throttle going here? I mean the kind of full throttle where the engine is screaming, and you shut that sucker down before the engine tosses a rod on you.
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