Saturn '97 SL2 5 speed - Fast Idle

Starts and runs well. Push in clutch while driving (approaching stop sign for instance) and tach will drop to 1600rpm and stay there until FULLY stopped at stop sign for 3 seconds, then drops to 900rpm. Idles smoothly. Why will idle not drop when coasting very slowly?

Have cleaned throttle plate and throat. Have taken out spark plugs… clean, only small white deposit on back of ground wire. Gas mileage seems ok but have not checked.

If there is no check engine light (is there), then the next thing I would clean would be the idle air control (IAC) valve and the mass airflow (MAF) sensor. Clean the IAC with whatever you used on the throttle body. For the MAF use MAF cleaner or electronics cleaner (essentially the same things).

Check for a vacuum leak of the intake manifold gasket around the #1 cylinder.

Throttle Position Sensor (TPS) and Idle Air Control (IAC) are against firewall on backside of Throttle Body. Removed Throttle Body to remove IAC. Cleaned IAC and both throats within Throttle Body. Reattached and sealed Throttle Body. No Effect, although now, when coasting, it idles at 1900 rpm. Check Engine light has always been on, and will read codes today. I don’t have much faith in the codes, but will try anyway. Should the plunger on the IAC be able to move when out of throttle body by depressing spring? I expected it to depress, but could not move the plunger.
There isn’t a MAF but a Manifold Air Preasure (MAP), which I assume is the same thing. Will clean it next and report.

Sprayed ether around gasket when engine at idle, no increase in RPM as expected if leaking. Is there a better way to check for leaks?

Get the codes. I have no idea why you “don’t have much faith” in them. The whole thing really isn’t some kind of scam or something - the computer controls the engine and monitors the major systems. When something is out of spec it stores the error code - they’re called “diagnostic trouble codes” (DTCs) to help you diagnose trouble.

When you have them read get the exact code(s) - as in P1234

You sprayed ether around the engine compartment of a running engine looking for vacuum leaks? Well, there’s probably no better way to set your car on fire.

If you’d like to look for vacuum leaks without blowing yourself up or setting anything on fire, you might try spraying something like WD40 LIGHTLY onto suspected leaks.

Thanks for all your help !!
Read codes (same 2 codes as below were stored)… cleared codes… drove around city in a square, 8 blocks on a side… each coasting caused 1600-1800 RPM idle… no check engine light appeared… read codes - no codes registered… drove another 4 blocks, check engine light came back on… read codes… P0300 (multiple misfire) and P0301 (misfire #1 cylinder) were the only 2 codes generated… I was hopeing for something about idle speed but nothing.

As I was driving after this code reading process, the idle characteristic changed. It now reduced RPM to 1100 when coasting and would drop to 600 RPM and then recover to 1100 RPM… back and forth… makes me think IAC again. This characteristic has been there before the high idle (1600 RPM) occured. Then later as I was driving the idle would stay steady at 1100 RPM when coasting, and return to 900 at stop sign. I suspect this may be what should be happening… correct?

After this I cleaned MAP with electrical component cleaner… no change.

Looks like I have an issue with misfire as well… will check ohms on plug wires… anything else I should check… like I said earlier, plugs are clean with 20,000 miles, gaped at .040 … so misfire does not seem to be a major problem or contributor to the idle issue.

Mom never said I was the sharpest knife in the drawer. I do take precautions… only on a cold engine, only on intake side of engine, spray frugally, outside in a breeze, with extinguisher handy. So far (sound of knocking on wood) I’ve been lucky.

When spraying WD40, will this increase idle speed if there’s a leak?
Seems like there should be a technique using dye and pressurize intake manifold to find leak by dye leaking out. Similar to pressurize crankcase and then look for bubbles in antifreeze… which I’ve heard is possible, but have never tried.

Actually spraying ether or pressurized propane around suspected vacuum leak areas is a common practice and has been done for years by many mechanics with no negative results. If there is a vacuum leak, it will show up with increased engine speed. Spraying a non flamable liquid such as WD-40 will sometimes detect a leak, but since it isn’t air borne, it has to be exactly sprayed into the leaking area. Where airbore gasses can be sucked into the vacuum leak if it’s even close to the leak.

You need to check when the engine is hot, not cold. Either or propane will increase the idle. If you have a high idle that is a little rough and you spray WD40 over the leak, the idle will actually smooth out and go down to normal.

BTW, it is normal for a Saturn to hold a high idle for a couple of seconds after taking your foot off the gas, but then it usually drops to 750. It will set a P0507 if the idle is 950 or higher for one minute on a warm engine. That is a sure fire indication that the intake manifold gasket is leaking. The idle will be just slightly rougher and gas mileage drops about 2 mpg.

I found problem of error codes… P0300 (multiple misfire) and P0301 (misfire #1 cylinder).
2 coil packs at the front of the engine compartment… the one controlling 1 and 4 cylindar had corroded contacts… cleaned contacts and replaced plug wires. Used dielectric grease on all connections. 13 Michigan winters with salt spray are hard on contacts. Will have to wait and see if codes re-appear.

Thank you for the detail… no idle speed approaches 950 when at a stop sign (after 3 second wait it drops to 800-900)… so it’s ok that I’m not seeing P0507 - I understand. The characteristic of not dropping below 1000 rpm when coasting is the symptom I’m trying to resolve. I have some time later today to change IAC and think things may settle out after this.
BTW, I found misfire error code problem, see above entry in this string, titled “Misfire codes”. At this point, I do not think gasket leak is a cause.
This has been a great experience and learning process for me.

Thank you, propane is a good idea… hadn’t thought of this alternative. I can use the canister I sweat the copper pipes with…

propane does work…only just be sure there are no problems with your plug wires…if you have any arcing you can ignite the propane in your face.

Although your issue sounds complex…I would consider replacing your wires. These cars use waste spark ignition (2 coils, 1 coil for 2 cylinders) and are grounded on the secondary side between both spark plugs and wires. If there is any chance that the plug wire ends don’t snugly on the plug end then 2 cylinders will intermittently misfire. New plug wires are cheap, but are also easily damaged when removing them to check plugs. (I would just replace them each time you remove them)

The guys have a good idea, an leaking intake manifold gasket can play with the MAP sensor and affect your idle speed. It does sound like the problem is only while the engine is decelerating and the throttle plate is closed. The IAC stepper motor can be checked for correct operation (movement), and correct position feedback by a service center. It’s also possible the problem is erroneous sensor information from the throttle position sensor, since this is also checked along with the MAP to indicate desired engine speed. It could have a flat area which gives the computer erroneous info.

You could have this checked along with the IAC, and the MAP at a service center OR on your own with a DVOM if you have a correct specs. The IAC is more easily checked with a scan tool which can command the stepper to move.