RPM Fluctuations

C

capt512

Senior Member
Hey Everyone,

So I have had a light miss on and off for a while now (sometimes it will idle smooth as a glass table, other times a noticeable miss).

Anyways today I was driving back from court at aprox. 55MPH and noticed that the RPMs were fluctuating. When I took my foot off the gas the car wasn't really slowing and in fact the RPMs were going up 200 or so and back down every 2 seconds or so. This is all while my foot is off both pedals...the car wasn't changing gear, staying in the same gear but the rpms were going up and down every 2 seconds or so. At lower speeds its not noticeable.

This doesn't seem right...thoughts?
 
The ECU is trying to compensate for something and is revving the engine. First thing to check for is vacuum leaks.
 
Clean out your IAC, MAF and make sure your intake tube is tight. If none of that helps, you may wanna check the voltage on your TPS sensor. Chances are the IAC is sticking and causing a surge.
Its the silver cylinder on the side of the throttlebody with a plug sticking out of it held by 2 bolts. Remove and clean out with carb cleaner and apply some penetrating oil. Reinstall and see how it works. Recently my IAC gasket was ripped and it caused the idle to hang up a little and caused hard starts when warm.
 
Yeah IAC was my thought too as the car is also not real smooth on slow acceleration from a stop, it seems to go straight from 600RPM (at stop) to like 900/1000 when starting off, can't slowly move through rpm range. And it hangs when I left of the gas the rpms go quickly down to 1000 then wait a second and continue to fall. However I replaced the IAC not that long ago (less than 5K miles), cleaned MAF (anybody know what the voltage should be on it?) What should the voltage be on TPS?
 
capt512 said:
Yeah IAC was my thought too as the car is also not real smooth on slow acceleration from a stop, it seems to go straight from 600RPM (at stop) to like 900/1000 when starting off, can't slowly move through rpm range. And it hangs when I left of the gas the rpms go quickly down to 1000 then wait a second and continue to fall. However I replaced the IAC not that long ago (less than 5K miles), cleaned MAF (anybody know what the voltage should be on it?) What should the voltage be on TPS?
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MAF Im not certain, the TPS is .098-1 volt at idle and about 5 volts at WOT.
Was your IAC aftermarket or a motorcraft part?
 
thats what I can't exactly remember, I want to say it was motorcraft as most of the parts I have bought are...but I'm not 100% it could have been maybe delco or similar...
 
Did you ever find the cause....I just had my car dyno'd and it's doing the exact same thing. When I let off the gas at around 2000 RPM, the RPMs will drop to 1700, surge back to 1900, drop to 1600, surge to 1800, etc. until about 1500 RPM it stops and drops to idle speed. I'm wondering could it be tune related, or do I need to start looking elsewhere?
 
robertmee said:
Did you ever find the cause....I just had my car dyno'd and it's doing the exact same thing. When I let off the gas at around 2000 RPM, the RPMs will drop to 1700, surge back to 1900, drop to 1600, surge to 1800, etc. until about 1500 RPM it stops and drops to idle speed. I'm wondering could it be tune related, or do I need to start looking elsewhere?
Click to expand...

Clean the throttle body, MAF, IAC & TPS. Using the appropriate cleaner for each. Just did it to my MM and it runs great.
 
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fyi mine still works a little squirelly, even after cleaning everything etc. I have just given up on it for now as its tolerable and I have other things money needs to go towards. At times I will get a bad miss with mine in park, bouncing 3-400 rpms at times, but it is rare. Then other times it will run right. However its always slow to respond when you mash the pedal from a stop, as in I have to calculate pulling out more with it as there is a delay til takeoff. Almost like an electronic throttle.
 
capt512 said:
fyi mine still works a little squirelly, even after cleaning everything etc. I have just given up on it for now as its tolerable and I have other things money needs to go towards. At times I will get a bad miss with mine in park, bouncing 3-400 rpms at times, but it is rare. Then other times it will run right. However its always slow to respond when you mash the pedal from a stop, as in I have to calculate pulling out more with it as there is a delay til takeoff. Almost like an electronic throttle.
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Sounds like you might have a flaky COP.
 
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