Tach fluctuates 200 RPM at highway speed

[jtb74168621]

I recently bought a 2002 ZR-2 with 98,800 miles on a 4.3L with an automatic transmission. The truck is in pretty good condition for its age - no rust or major dings.

I noticed something odd while driving on the interstate this last Sunday - the tach will raise by 200 RPM (one mark) and then drop back - somewhat rhythmically. I believe the engine RPM is advancing rather than just the tach moving by itself. The tach is steady at 600 RPM at idle - no fluctuation. But driving in town and on the interstate it fluctuates. I tried it with the cruise control on and also tried it in 3rd gear - still fluctuates 200 RPM.

There are no lights on in the dash that would indicate a problem.

The previous owner did tell me he had a problem with the service engine light (sometime) before and replaced the sensors in the air cleaner and when that didn't solve the problem he took it to a mechanic and found the catalytic converter needed replacing which he did. I have not seen the service engine light on myself other than when the key is first turned on.

I haven't checked the gas mileage on it yet but it seems to be consuming more fuel than other S-10s I have owned (needle on 1/2 tank after 170 miles). But then, I am recently use to driving a Subaru Forester and the difference may just be more noticeable. But, if while driving the engine speed is constantly varying by 200 RPM then it would stand to reason that the fuel consumption would be higher.

Any suggestions or ideas?

[jtb74168621]

Just an update:

Took the truck to Autozone to get the codes read. Unfortunately their hand-held device will only read codes that cause the service engine light to remain on. Mine hasn't been on.

Before I bought the truck it had the catalytic converter replaced but there were a couple exhaust leaks around the welds. I had those repaired at my local garage and it seems to have made a difference. I haven't been on the interstate yet to see if the tach fluctuates but in a short stretch of 4-lane in town I checked it with the cruise control on. I noticed a slight fluctuation on the tach but can't tell if that was normal due to a slight incline of the road.

[jtb74168621]

Drove on the interstate this weekend with the cruise control on. The tach was steady!

So the problem was caused by two exhaust leaks around each end of a recently installed catalytic converter.

[jtb74168621]

After a few miles (basically an oil change) the tach is fluctuating again but not when using the cruise control. Strange...

[old_master]

Possibly the torque converter clutch solenoid in the transmission is failing or has a poor connection to it. A scan tool that is capable of monitoring the transmission, (not a code reader), will be able to spot it.