rapid rpm change

[tmaxxexpress]

New to the forum. I have a 1998 Yukon 1500SLT. V8 5.7. No codes thrown so far. So, here is what is going on. Idle is perfect. over 40 for oil pressure. temp is 185. from a dead stop the machine accelerates as it should through all gears. the problem is within the rpm range of 1200-1600 rpms. If im going over 60 miles an hour and rpm is 1600 or higher drives perfect. When driving 40-58 miles an hour and just cruise a constant speed i will be at say 1200 rpm. the rpm will go click up to 1500 rpm then click back to 1200. You can feel the motor change. it is sudden. Like I say it will do this in overdrive and also in drive. So, is the a throttle position switch bad? ideas? I appreciate your input.

 

[Doubeleive]

sounds like transmission to me, but could maybe be fuel pressure (surging). But the suddeness would point me towards the transmission, a more suttle feeling would maybe be fuel or something intake related.
you might want to find a good transmission shop and ask them to check it out which should involve a technician/mechanic test driving it.
you could try a couple things yourself, such as driving it in a lower gear on purpose up to and above the prescibed RPM's and see if the issue is only present in drive.

 

[Marky Dissod]

Can you check the function of the torque converter clutch?

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

I am in agreement with @Doubeleive that your transmission may be the source of your issue. @NickTransmissions is our resident expert on transmissions, and he will chime in with his thoughts.

 

[NickTransmissions]

New to the forum. I have a 1998 Yukon 1500SLT. V8 5.7. No codes thrown so far. So, here is what is going on. Idle is perfect. over 40 for oil pressure. temp is 185. from a dead stop the machine accelerates as it should through all gears. the problem is within the rpm range of 1200-1600 rpms. If im going over 60 miles an hour and rpm is 1600 or higher drives perfect. When driving 40-58 miles an hour and just cruise a constant speed i will be at say 1200 rpm. the rpm will go click up to 1500 rpm then click back to 1200. You can feel the motor change. it is sudden. Like I say it will do this in overdrive and also in drive. So, is the a throttle position switch bad? ideas? I appreciate your input.

Is the RPM fluctuation always happening at the same road speed at the same gear change intervals (i.e. always at the 1-2 or always at the 2-3 or always at the 3-2 transition(s) or does it vary?

Put a scan tool on it that's capable of rendering live transmission and engine data then take for a test drive...First scan for codes then look at transmission shift speeds while the vehicle is moving. Speeds greater than 1 sec indicate applied element slip. Since you don't have an 07+, which as a turbine speed sensor in addition to the vehicle speed sensor, you won't throw any gear ratio error codes. You may have P1870 but that's just a guess.

Anytime you have unusual drivability symptoms, regardless of what the direct causes may be, you always use a scan tool to see what's actually taking place. From there, proceed with further diagnosis and repairs, as necessary.

 

[Eman85]

Your Tahoe has torque converter clutch that is pulse width modulated controlled from the factory. It all sounds good and works great on paper until the vehicles get miles on them and things wear. The valve body in the transmission is aluminum, and modulating that steel valve back and forth wears the valve body which causes problems. With PWM TCC you don't see or feel the TCC apply. All of that said if the transmission had any problems or was overhauled there's a good chance that valve has been modified and or updated to simple direct apply TCC.
Drive on a straight wide road and get it up into 4th gear, watch the tach and feel every shift. When you stop accelerating and cruise you should feel TCC apply and see the tach drop. Maintain that speed and just rest your foot on the brake pedal enough to turn the brake lights on and you will feel TCC release and watch the tach increase.

 

[B-train]

Agreed with all above. I was going to say similar in that it sounds like he's cruising at the TCC lock/unlock load and/or speed.

Try running in 3rd at the same speed and see what it feels like.

 

[NickTransmissions]

Your Tahoe has torque converter clutch that is pulse width modulated controlled from the factory. It all sounds good and works great on paper until the vehicles get miles on them and things wear. The valve body in the transmission is aluminum, and modulating that steel valve back and forth wears the valve body which causes problems. With PWM TCC you don't see or feel the TCC apply. All of that said if the transmission had any problems or was overhauled there's a good chance that valve has been modified and or updated to simple direct apply TCC.
Drive on a straight wide road and get it up into 4th gear, watch the tach and feel every shift. When you stop accelerating and cruise you should feel TCC apply and see the tach drop. Maintain that speed and just rest your foot on the brake pedal enough to turn the brake lights on and you will feel TCC release and watch the tach increase.

The 96-2000 had lots of issues with the TCC isolator and regulator valve however starting in 96 those valves were anodized aluminum, not steel and the castings themselves were prone to wear, not necessarily the valve. The casting was revised in 01 and again in 07-08 and the occurance rate of p1870/p0894 dropped sizably compared to 96-00.

Fitzall's TCC regulator valve kit is the best kit available as it:
- seals off the bore via o-rings on the valve
- converts the PWM to a true "on-off" mechanical regulation strategy.
- costs less than anything else out there for that same problem.

@tmaxxexpress needs to tell us exactly what is happening and when it's happening in terms of vehicle speed and gear(s) upshift or downshift transitions assuming he logs back in and decides to reply.