RPM Flucuation

While at crusing speed, 2,00o-2,500 RPM, the RPM's will at times increase by about 250 for a second or so and then return to the previous level. Could this be a throttle position sensor problem, or is there another more likely problem?
Ed Wulf #67

well... it can be couple other things as well: how clean is your TB? There is sensors there that affect your RPM's as well...

Kejtar

Ed Wulfe+,
Here's my thought -- you have an automatic transmission...

While cruising at 2K to 2.5K rpm you'd be rolling comfortably locked in 4th gear in the AW4... the key word is "locked" as in with the torque converter locked.

With 31's (and no regear), you're throttle pedal requires some pressure to maintain that RPM (as comfortable cruising)

A slight MAP or TPS change will cause your torque converter to unlock... the added slip in that system accounts for the RPM increase that you're seeing ...

... Y'wanna prove it?
Drive comfortably cruising at/near that RPM range and watch your tach as you gently apply the brake pedal with your left foot (you should only need enough brake to light the lights, not so much as to slow) while maintaining speed with your throttle-foot...

... If you were "comfortably cruising" with the TC locked, the slight aplpication of brake should've unlocked the converter and you'll see the "slip" as increased engine RPM for the speed you're travelling...



(In the unlikely event that you have a manual tranny, and your engine speed increases for a given road-speed, suspect clutch slippage)

I never thought about the torque converter. I have regeared to 4:10's, but that could still be it. It does have 187K on it. Does anyone know how to check the TPS sensor? I know that on a GM product it can be done with a voltmeter. How about the MAP sensor? Can that be checked? Thanks to all.
Ed Wulfe
#67

Been wondering about that too.... I've seen mine do the exact same thing. I have a 5 speed, new clutch. Engine speed doesn't increase, tach needle just takes a quick 250rpm blip. I've been waiting 3 years for a sensor to fail so I can find the culprit.

I am running the AW4 and when the flucuation takes place the engine speed does increase. I pulled out my Hanes manual and they describe how to check the TPS. I'll do the test and post the results. Thanks

Some TPS data - http://www.naxja.org/forum/showthread.php?s=&threadid=2171

Some MAP data - http://www.naxja.org/forum/showthread.php?s=&threadid=1497

MAP should not be providing shift-inputs (mostly derrived from the TPS and speed sensor data)

How about the idle step motor. If malfunctioning, could slightly increase the rpm past idle. I would check everything previously listed first, use this as a last resort. I just got tired of guessing and replaced all of the sensors. Runs smoother now too. Good luck!

a 250RPM hop is a big step for the Idle Stepper (full in it'll support like 2200 rpm on a warmed motor with a base idle bleed of 475rpm -- so the total trave on the stepper is like 1750 rpm or so... a 250rpm "Blip" would be roughly a 0.10-inch motion - even with an accelerated clock that'd take some doing to get in a short time (OEM travers speed for that would be nearly a second).

HOWEVER, on that though - icing in the idle bleed and/or the stepper flow area could get you alittle pop, but the vehicle's ground-speed would always show the change...

if the tach changes and the ground-speed doesn't - the tach is wrong (suspect loose grounds!) or there's some slipping in the driveline...