Surges or bucks at steady speed, 40 to 50 mph

Maybe a contaminated throttle body.

If you had a emissions problem and it has been corrected and the check light is out, then it should be cleared after 200 miles.Or will just show is a inactive fault

Honestly sounds like the throttle position sensor, if the car is cold it will be choked and thus the throttle body flap almost closed easy to crank, more fuel less air.... when the air warms it moves the throttles actual flap opening wider, thus more air less fuel and harder starting, not as much fuel to combust and if the computer cannot sense the correct position, it will cause acceleration hesitating, poor idle, and what is called stumbling, where you press the gas and it kind of sounds like the motor sputters for a second and then goes or the occasional surging if at highway speed of say 60 mph and you feel the car kinda speed up mile or 2 and then back to the original speed.

My 93 nissan p/u had that issue, but the tps was replaced and it takes 2-3 revolutions on the motor and it's running smooth, hot or cold.

Hope this helps.

You may check the Air sensor on the engine.

Hello! The Drive-Cycle is as follows...Check engine light off, codes cleared; cold start...Warm up engine 5 minutes...Drive (w/ steady throttle) at a steady speed between 40 and 60 mph for 8 min...Stop and idle 3 min...Drive (w/ steady throttle) at a speed above 20 mph for 2 minutes...Turn key off, leave off 10 min...

Guru.............Saailer


I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

An additional clarification to terian_5's suggestion of checking the vacuum lines, usually the rubber elbow on the back of the throttle body gets cracks in it.
Remove the plastic throttle body cover, and check the rubber elbow, and the vacuum line for this. You might have to look close, sometime they are hard to see.

Ignore the IAC notation in this picture, it is the best one I have that shows the rubber elbow, it is the elbow right below the text


This is ~ 20.00 ( + tax/ship ) from Ford to get a new one. Replace this, clear the codes and do 2 driver cycles ( steady throttle up to 47 mph min, back to a stop and back up to 47 mph again ).
If you do not have a code scanner to clear the codes, disconnect the battery fro ~ 10 min, reconnect battery, start let idle to warm up, and do 2 drive cycles.

Performing a GM OBDII Driving cycle:

1. Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F (50°C) and within 11°F (6°C) of the ambient air temperature at startup.
   Do not leave the key in prior to the cold start or the heated oxygen sensor diagnostic may not run.
   
2. Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire, and - if closed loop is achieved- Fuel Trim.
   
3. Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph (88km/hr) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.
   
4. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.
   
5. Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 20 mph (32km/hr). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.
   
6. Accelerate. Accelerate at 3/4 throttle until 55-60mph (88-96 km/hr). This will perform the same diagnostics as in step 3.
   
7. Hold Steady Speed. Hold a steady speed of 55mph (88km/hr) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.
   
8. Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

you are to replace thermostat

Sounds like one of your chambers isn't firing. Run your car for a few minutes, shut it off then pull your plugs one at a time and see if one of them is wet. Might just need a new plug.

The issue may be the EGR valve. Sometimes the valve can test out OK with a vacuum pump but can still cause issues with surging. Is this happening between a quarter and half throttle? The reason you wont notice it before it warms up is because the ECM doesnt activate the EGR valve when it is cold. One temporary test you could do is unplug the EGR valve and try and drive the car once it is warmed up. If this stops the surging this is the issue. One other issue could be your Fuel pressure regulator. Your fuel pressure could be low from this and it will cause your fuel injectors not to be able to vaporizefuel as much causing the bucking feeling. Keep me posted. If you found this solution helpful please rate it when you get a chance.

Thanks - Tech101