My malibu has 210k miles on it. I've noticed that when an internal fan (don't know which one) kicks on under the hood, my temp gauge drops from operating temp (200) to flatline (100). When it does this, my RPMs jump up from around 500 to b/w 1100 and 1500. The car will surge forward, and if I'm at a light/stop sign/whatever, I have to press very hard on the brake to keep from moving forward. If I go to accelerate during the surge, my transmission shifts hard going into 2nd gear, and if I'm backing up during the surge, the transmission "yanks" for lack of a better word as I shift into drive. I do alot of interstate driving and even at a constant speed I notice the temp gauge floating up and down and the car surges then as well. It didn't have this problem when we bought it in February, but now that the summer temps are here, it does it constantly. We read on another forum last night that it may be the throttle position sensor, so we changed it about an hour ago, and we are still having the same problems. The check engine light is on too. Sry for such a long-winded thread..help me please!

If you go to AutoZone they will use their scan tool for free and pull the codes. You can use them to guide you toward a solution. They were instrumental in fixing 2 issues that happened simultaneously on my old truck so I bought the scanner after buying the 2 parts and fixing it. The 2 parts and the scanner cost me less than getting it diagnosed at the dealership!

Sounds like a sensor and/or detection circuit is the biggest culprit. It's good to be able to use the scanner as a guide, so long as it actually helps.

Are you sure its not a Charging System problem? The extra current draw from the engine fan causes Voltage fluctuations. Which would make the engine run crazy.

Once I saw an bad Ignition Module in a Cadillac Deville. Ran fine until the owner turned on the headlamps. Then started bucking. New module cleared it up.

Jeez, we seem to be getting a lot of these questions lately. Search engines must love us. Given the age of the car, it could be sensors, wiring, control module, anything. It sometimes is cheaper to have a shop properly diagnose the problem than just throw parts at it.

Okay, don't get me wrong. I'd like you help but without a good set of scanner readout, it could be anything. The relevant facts are:

1. Surges after the fans kick in and (either before or after, which?) the coolant temperature drops (keep surging after the fans cut out?).
2. Surges in idle or while cruising (do the fans go on while cruising?), eliminates idle air control valve.
3. It didn't do it when cold.
4. It apparently wasn't the throttle position sensor.

It's obviously having a hard time maintaining the proper air-fuel ratio. There may be pending codes that have not be promoted yet. Like I stated, it'd be so simple to hook up a good scanner and monitor what's going on.

Ok, so like I said, the TPS didn't work. On my way home today, the Check Engine light came on, so I ran it up to Advance Auto. They put the scanner to it and it pulled the code P0118: "Engine Coolant Temperature Circuit, high imput". So, I decided to go ahead and buy a replacement ECT Sensor. After I give the engine a little while to cool down, I'm gonna go ahead and replace the old with the new and hope that fixes the problem. It has been brought to my attention though that this particular sensor might be the one that simply operates the temp gauge in the instrument panel, and may cause the temp gauge to read properly while the surging continues to take place.

STARSHIP: once the fan has kicked on, and the temp gauge falls is when the surging starts. It DOES surge slightly while cruising occassionally..usually as a slight rocking sensation. I can't actually hear the fan kick on while cruising due to road noise, but I assumed that it is b/c it's causing the surging at idle.

I'm glad that you're still with us. The additional infomation helps a lot. I think you have a stuck-open thermostat. I'm either assuming or emphasizing the relevant bits:

1. The coolant temperature gauge on the instrument cluster is calibrated okay, i.e., the needle is where it should be when cold and sweeps the whole range.
2. The needle is near the top 3/4 when the fans kick in.
3. Before the fans cut out, the said needle is at or near the bottom.
4. The problem started when the fans started kick in as weather warmed up.

Since the fans kick in and out, the engine coolant temperature (ECT) sensor is probably okay. It may have shifted out of calibration but it doesn't need to be that accurate. It's the thermostat's responsibility to restrict or cut off the cooland flow to the radiator to maintain a minimum operating temperature. When the coolant temperature dropped below the threshold, the ECM then got out of the closed fuel loop and kick up the idle speed to keep the motor from stalling (what happens in the morning). Thus, if I were you, I'd start with the thermostat.

Are you aware there is 2 Coolant Sensors on the engine? One is for the Gauge, The other is for the ECM.

Yes, I am and OP hasn't presented any evidence that the ECT sensor and sender (the one for the instrument cluster) disagree wildly.

I am glad you brought that up. I failed to establish that I am assuming that fact also. I'm also not saying there are other things that could be not quite okay with the car but the thermostat is a logical starting point.

Thanks for the replies guys!

As it stands right now, the ECT sensor that I bought and replaced last night seems to have fixed the problem. I was aware that there are 2 different sensors, so I was just hoping that I got the right one. (the young kid at Advance didn't seem to know how to tell the difference b/w the two) I'm hoping that this is permanent fix. I had absolutely no problems during my 100 mile round trip commute to and from work today. No fluctuations in the temp gauge needle, no surging, no transmission slipage. I've got my fingers crossed!

Okay, thanks for the update. So, what did you replace, sensor (2-wire connector) or sender (1-wire)?

Its the 2 wire sensor.

I believe I replaced the sensor. That's what the part that I bought was called. (Engine Coolant Temperaure Sensor) http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Coolant-Temperature-Sensor-BWD_5012767-P_290_R%7CGRPSENSAMS_575863004___

Let me kno if that link didn't work...

of course it didn't....you can find what I bought at advanceauto.com

Congrats.:D I just wanted to file away the right information. A ECT sensor having shifted out of calibration would certainly be consistent with the symptom.