Fluctuating RPM's in 98 GMC K-1500

[Sutty]

Not sure if it is related, I recently changed my 02 sensors, ran sea foam in the oil for a while and reset the computer by disconnecting the battery for a couple hours. Its been three weeks since I did this and have no check engine codes and have taken it on a 8 hour road trip with no issues.

Anywho, now when I am going 40- 50 mph my rpms fluctuate by a 2-300 rpm. I was told to try tapping the break on the highway to see if the rpms go up to check my torque converter and they did

Tranny going south? has 2 hundo on it.

[Shack]

Is the eng. actually revving up and down?

Is the trans slipping as the r.p.m.'s are revving up?

Are you holding gas peddle steady and this is occurring?

[Sutty]

I can't feel the Tranny slipping, I am holding the gas steady and on level pavement. I can't detect the engine revving but it only goes up a little bit and than back down a few seconds later it will do it again.

[Shack]

Is it the tach needle you are basing this on?

When holding the gas peddle down steady, do you hear a fluctuation in the eng., noise wise?

I am just trying to rule out an elec. issues with the readings the tach is getting (or the tach its self), or a drive train issue.

[Sutty]

Yep basing it on the tach, No it sounds real good if I step on it both at idle or on the road.

[Sutty]

I didn't mention it because I didn't think it is related but my alternator went out about two weeks ago I replaced it and the battery since it was pretty old as well.

[Shack]

Well, good thing is we ruled out a major trans issue pretty much, could still be a T-converter issue though.

I would have a battery test done A.S.A.P...

Most shops will do it for free.

Good luck!

[Sutty]

ok thanks

[Jeremy airjer W]

One more thing Sutty, What was your heater control set to? Is it possible that you are feeling the a/c compressor kicking in and out like it might be low on freon? Just a thought.

[john.wells]

I was going to mention the A/C compressor. My 97 blazer does this when the freon gets low, and anytime it kicks in. Just more frequent cycling and more noticeable when low.

[Shack]

Ya good points guys! I have also seen this be an issue in the past. Worth checking out.

[Sutty]

It has had a slow leak in the past, last fall I put a can of 134a in that included stop leak but it sure might not have sealed it up. Thanks for the tip I'll check that out.

[fishinJohn]

My old car used to have the rpm's go up unusually and it was the clutch slipping and needed to be replaced. Doubtful but hey...