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Discussion Starter · #1 ·
My 2008 4.7L Tundra has what I consider a Safety Issue, because when stopped at light or in traffic, it will, after a few (20) seconds, rev up from a reasonable 600 RPM to 900 RPM for no apparent reason. After staying at 900 RPM for a few (20) seconds, it will go back to 600 RPM. It will run through this routine 2 or three times in a long (2 minute) light cycle. If if don't have my foot planted firmly on the brake my truck jumps ahead suddenly because 900 RPM is enough to really get it moving in low gear.
The issue is there with engine fully warmed up, summer and winter (VT) and with AC manually turned off. The problem has been around since at least when the odometer read 5000 miles. I contacted the Dealer's Service Manager at that point. He said it was the AC compressor switching in. I don't buy that theory. Why would the turned off AC compressor switch in 3 times in 2 minutes when it is below 20 deg. F outside? All my experience with AC compressors switching in cause a drop in RPM's rather than an increase!
Anyone have a fix for this issue? :(
 

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Is your air direction switch in any of the DEFROST positions?
Any defrost position cycles the A/C compressor (you can't stop it) to deliver cool dry air to the windshield.
Try setting the selector to dash vents only and see if you still have this issue.

Steve
 

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Discussion Starter · #3 ·
Steve...
I will try this, but defrost with cool air? We usually see hot air coming out of the defroster to melt the frost and vaporize the condensation here in VT and any place I ever lived. Can it be than it sends the air through the AC to take out the moisture before sending it through the heater core to warm it up?

Rhaski
 

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Typically, especially on late-model vehicles, any setting that directs air to the windshield is interpreted by the vehicle controller electronics as a "defrost" setting, regardless of your temperature setting, and any "defrost" setting uses the A/C. In the winter, if you're driving around with the temp up high and sending air to the windshield to melt the snow, the A/C still cycles, it's just overwhelmed by the heat setting and so not very noticeable. The A/C runs so that the air to the windshield is de-humidified; one of the reasons defrost setting clears the inside of the windshield so well.
Try it.

Steve
 

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AC compressor removes humidity thus its used in all temps.

AC compressors do cause a drag which is compensated for by the
ECM thus the fluctuation in idle speed. when it cycles off, the engine instantly has a decreased load and thus will idle up a hair before the ecm readjusts it. just keep your foot on the brake. alot of us have noticed it, its not just in this truck either. our yukon does it noticably as well.
 

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I have this same problem with my 2006 Tundra, I've pushed the a/c button on/off turned fan high/low, turned lights on/off to try to replicate problem and it doesn't seem to be causing the issue. One thing I've noticed is I can hear a clicking sound behind the vent switches when the idle is surging so I am going to pull dash apart and see what's clicking on/off. Ive even replaced my alternator because of this issue since it surges my amp meter also but this did not have any effect.
 

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This sort of thing can actually happen if you have serviced your air filter and the maf sensor is unplugged for any reason, or if it isn't plugged in all the way. That or you can take a look at your idle air controller. That could be bad, which could also cause your idle to go up and down. Also, try cleaning your maf sensor on your airbox. Use electrical component spray and nothing else, it is too sensitive.
 
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