220K miles. Recent cylinder head gasket replacement. No codes.

Occasionally stopping at red lights the idle in N goes to 1,800 RPM. Turn it off, turn it on, the restart may momentarily go high, but it will always settle down to the typical, expected, 650 RPM. Then for days in a row it'll stay constantly in its low range, but then it'll get a bug and start staying in its high ranges again.

From what I've read, it's most likely a bad throttle position sensor. If that's so, are there preferred brands to buy or from which to avoid? Some have written that "ALL AFTERMARKET" TPSs are poor quality. I'm thinking rockauto as my source.

If it's sounding like a vacuum leak—which was another possibility based on what I've read online on the forums—that's a strange one to me. I'm not sure how I'd test for that. Thanks.

 

Third generation SOHC s cars had an intermittent issue during factory engine assembly where the intake manifold gasket was getting folded by a robot during the install. Over time, part of that gasket would get pulled into the intake, creating a leak, a vacuum leak, at the intake manifold. Usually when that happens, you get a corresponding PO30x with x telling you which cylinder has a misfire. But you said you have no codes.

Could be some position sensor, could be throttle body needs a good cleaning. Every now and then my 95 SC2 will behave that way for about 10 minutes after it warms up with no drop to normal idle speed. It goes away on its own or I clean the throttle body with throttle body cleaner.

Checking for a vacuum leak can be done multiple ways. The easiest is to bring some throttle body cleaner or choke cleaner around all of the vacuum connections and around the runners of the intake manifold to each cylinder. Be sure to spray the entire circumference where it meets the head, not just the top. If you hear a change in idle speed after spraying, it means you have a leak.

Things like this, I always start with a good throttle body cleaning.

How much oil does it burn per thousand miles?

 

Thanks Fred, uh, derf! I hadn't noted it, but its intake manifold gasket was replaced two summers ago.

Oil consumption? Is "lots" accurate enough? ;-) I keep a 5 gallon container in the trunk, check the level every other fill up or so, topping off regularly. I don't have a solid number for it. I'll estimate I go through a 5 gallon container per quarter, but then I can't estimate my mileage accurately. I've always read that they go through it quickly, though.

Throttle body cleaning is all about removing the carbon buildup, correct? After watching a few YouTube videos, carbon buildup at the butterfly plate might be holding it open.

Is there a reset procedure that needs to be run on a cleaned SL1 throttle body?

 

Cleaning the throttle body is always a good idea. Do you have a code scanner? I had a situation in late 2023 where idle speed would not drop to idle until 8 seconds after I stopped at the stop sign (and there was other idle speed weirdness too). I bought it used, so I didn't know the car's total history. After a few weeks of this nonsense, I used my code scanner. My coolant temperature was never getting over 168F (2002 DOHC SL2 5-speed). As soon as I replaced the thermostat (November 2023 at 164,681 miles) the problem was fixed. I suspect it had the original thermostat, since it looked exactly like the thermostat I took off my other Saturn (2001 SL2 5-speed) that I have owned since new.

 

I have a scanner and a bluetooth OBD transmitter attached to the connector under the dash. My temperature indicator never goes above 3/8 position, never making it to half way, which I've always believed to be normal operating temperature.

I drove home tonight and put the car in park. RPMs went up to 1,800. I left it on for twelve seconds but the RMPs stayed at 1,800. I shut if off, restarted it, and it settled at 750 rpm.

My temperature sensor was replace last summer with the newer, brass tipped one most Saturn gurus on the forums recommend.

 

(Reprise of the same, old thread.)

The throttle position sensor is now new.

I now get intermittent high idles at red lights, whether AC is on or off. A day or two it happens, then it goes away for a week. High idles sometimes go to 1,500, sometimes to 1,700. I’ll wait eight to ten seconds to see if it settles down, but it never does.

I used to get temperature sensor low voltage values codes. I checked the sensor connections for corrosion, and I looked inside its connector, but there isn’t any corrosion. The TPS was replaced with the brass tipped one six years ago. Dang—maybe the sensor’s going bad. I hadn’t even thought about that.

When I shut off the engine, it shuts itself off in a rather rough way, like it’s gasping, “protesting,” (as an engineer, that looks so funny…) or popping… or rough. Restarts return to 650 for no AC, or 750 for AC running.

I’m beginning to think I need to do Derf’s engine cleaner or carb cleaner spray test to look for a vacuum leak. The intake manifold gasket—with its robot deformation on cylinder #1 on manufacture—that was replaced 5 years ago.

When spraying around the vacuum hoses… how do I know what’s a vacuum hose or not? How do I know where any vacuum is? The question is intimidating, honestly.