Car Acts Weird When Idling

1998 Saturn SL2, automatic, 104K miles.

Since last Saturday, my car has been acting weird whenever it idles.
[li]It sputters, and the RPM’s go up and down dramatically.[/li][li]The A/C stops blowing cold air and instead blows lukewarm air.[/li][li](Most alarmingly) The temperature gauge starts to climb, until the car is moving again.[/li][/ul]

My ideas:

  1. The air conditioner is almost out of coolant: Good theory, except that it blows cold air when the car is moving.
  2. It’s just really really hot. OK, but it’s been hot all summer. Why is it acting this way now?
  3. Low engine coolant - Nope, just checked. The reservoir is full.
  4. Spark plugs need replaced - I just replaced them three or so weeks ago.
  5. Something else? Perhaps a belt issue?

A combination of the above?
A/C blows warm except when moving + engine starts to overheat while idling says look to ensure that the electric fan(s) at the radiator is operating. If this fan quits, no air over the rad= o/heat, + no air over A/C condenser = lousy A/C performance. Start moving again, and everything gets better until you stop. The crappy idle could be a part of this or separate.
You changed the plugs three weeks ago? How about you damaged one of the plug wires, and it is not firing properly.
Also to check the coolant, you need to have the car cool and remove the cap on the top of the radiator, not just look at the overflow tank. (I remembered this time Gary T)
Fix the overheating first, then if the idle is not corrected, work on that. Lastly go after the A/C (but I suspect you will find it is a fan issue)

I am not a mechanic, but our old Ford Escort was doing the identical thing, and it turned out that when the sparks had been replaced a few weeks back, the wiring harness got damaged - I guess it is easy to do. It going wonky while idling was because of the different pattern of vibration while idling.

Luckily for us, the mechanics skipped ordering us a new harness ($400) and were able to hold the thing together with zip-ties.

(Unluckily for us, the car was totalled a few weeks later…)

Check for a vacuum leak.


We’ve somewhat isolated the problem: the cooling fan is not kicking on.

My stepdad and I looked at it, and we’ve narrowed it down to one of two things. The fuse and relay seem to be in good repair, although I’m going to confirm this by taking them to Auto Zone and getting them tested. Assuming they work, he thinks it’s going to be either an in-line fuse (or possible a short in the wire) or the fan assembly itself is fried.

If it’s the fan assembly itself, I found one for $118, and my stepdad thinks he and I can install it.

If it’s an in-line fuse or a short, I’m screwed, since he and I can’t work on this.

Can anyone provide any more insight?

Maybe carburetor icing?

Yeah, I know you have fuel injection and not a carburetor but the phenomena goes like this:

Atmospheric air goes passed the throttle valve and expands to fill the vaccuum inside the intake manifold. As a gas expands, it cools…perhaps below freezing.

Atmospheric moisture condenses and then freezed inside the intake manifold, and as the IM heats up with the engine, the ice melts and liquid water trickles into the cylinders.

The roughness, hesitation, and stalling is due to the spark plugs getting intemittently wet.

Now, there is supposed to be a line of antifreeze connected to the throttle body to prevent icing…maybe yours is glogged or otherwise malfunctioning?

Before you start yanking out innocent parts, you need a bit more troubleshooting.

Start at the fan itself. Is it getting power? If it is, but it’s not running, then odds are you need a new fan.

If it’s not getting power, work your way back through the circuit. Check at any fuses or plugs. Inspect the wiring end to end, looking for chafes or pinches. When you arrive at the relay, check that it’s getting a signal to turn on. If power is on its input terminal, and it’s getting the “on” signal, but no power gets through it, you probably have a bad relay. Or maybe a bad relay socket or associated wiring.

If the relay is not getting an “on” signal, your world got more complicated. You’ll probably need a wiring diagram to trace out what’s between the coolant temperature sensor and the fan. In older cars, it was simple - coolant temp sensor switch -> relay -> fan. Now, it’s more likely:

sensor -> computer--> relay -> fan.  
                |---> dashboard gauge
                |---> emissions controls

Damn I’m good. :smiley:

Most if not all fuses can be “tested” by looking at them. Look at these two pictures. See the sideways U in the first one, and the Z shaped piece in the second picture? Those are the fuseable link. Those will melt when the fuse blows. Like this.
I was unaware of AZ tested relays. I kinda doubt they have anyone smart enough. However if you go to AZ but a 12V test light and you can test the circuit yourself. The test light will have a ice pick looking thing that has a wire coming out of the top. On the other end of the wire is an alligator clip. You clip the alligator clip to a good ground, and test on a known good voltage supply (positive cable of the battery is always a good candidate) When touched to a power supply the light inside the test light will light up.
Start the car, warm up the engine and turn on the A/C. Push the probe into the power wire at the cooling fan. If the light lights, stop, you have a bad cooling fan. If the light does not light, unplug the fan and go check the fuse for the circuit (there may be more than one) many of the fuses of the type I linked to have two little tiny holes the top where you can stick a test light in. If the fuse has voltage on both little holes it is good. If one hole has power and the other does not, the fuse is bad. If the fuse does not have holes, pull it out and eyeball it. Replace any and all bad fuses with the correct rated fuse, and then go back and test for voltage at the connector to the cooling fan (connector still unplugged) If you have voltage now, that would indicate that the cooling fan is probably drawing too much current causing the fuse to blow. There is a slight possibility that the fuse blew for no good reason, but don’t bet on it. If the fuse blows without you plugging the fan back in, you have a short somewhere. If you still have no voltage at the fan connector either you have a bad relay, or the relay is not getting activated, or there is an inline fuse. To check for an inline fuse, follow the wires back from the fan to where they disappear in the harness. If there is an inline fuse or fuseable link it will be here. If you don’t find one you can probably rule that out. Look at the fan relay Does it have two or three electrical connection on the bottom that are way bigger than the others, with big fat wires going to them? If so this is the power in and out to the fan. With the key on, there should be power to one of those wires. With the relay activated (warm car running +A/C) the other (or one of the other two) should also have power. if you don’t get power anywhere with the key on, check your fuses again, the relay isn’t getting power. If when activated, no power is coming out, or if the light is dim, you have a bad relay, or possibility what is supposed to activate the relay isn’t. You can try buying a new relay, but if you guess wrong, they won’t take it back. If the relay is not getting an activation signal, then the repair is beyond the scope of a message board reply.
Good luck

Ah, no. This would not explain the lack of A/C performance at idle and having it work fine at cruise. The fan not coming on fits the symptoms to a T

I own a 97 SL2 Saturn (238,000 miles); the electric radiator fan is the culprit. It died (@150k ) and the same symptoms occured. I replaced it my self. I (the electrical engineer) opened it up to find the brushes were worn out and the inside pretty corroded. It’s a cheaply made unit.

Of course, nothing would be complete without the fans thermostat starting to go. Nowadays, I need to leave the A/C on. Otherwise, on a warm day, the radiator starts to boil over. (The fan kicks on automatically when the AC is turned on). I’ll get around to replacing someday

Be careful not to get into the path of the fan blades, when doing anything near it. It could power on at any time.