erratic idle in traffic

[madpogue]

Originally Posted by DIYguy

Discovered that the throttle butterfly plate was more or less 100% closed!! Perhaps it is just the result of 113,000 miles of wear and tear.

Turned the idle stop screw 1 turn CW so that light was barely visible at the edges of the butterfly plate. This will force the IAC motor to a new setting.

The throttle plate was 100% closed when the TB was bolted to the intake manifold in Spring Hill. It's been 100% closed every time you release the pedal, for the last 113,000 miles. Forcing the throttle open with the stop screw just does what the IAC is supposed to do on its own. If you have to fake out the IAC by forcing the throttle open, it's probably either bad, or still dirty (or bad connector, or bad wiring to it, or.....).

[DIYguy]

I cannot say for sure one way or the other, as I've only owned the car 4 months and am just becoming familiar with it (perhaps a bit more familiar than I would prefer, quite honestly).

However, presumably there is a reason that both the service manual sets I have cite matching procedures for adjusting the manual idle screw to a position other than 100% closed.

Perhaps it was 100% closed (or very close to it) at the factory but requires adjustment as a PM service item some time later when things break in? I really don't know.

If adjusted to 100% closed, the car would not idle at all when the IAC is fully seated. The procedures say to adjust it to within an RPM range (450-650 or something like that) with the IAC fully seated and electrically unplugged. Same basic procedure for most GM vehicles I've owned.

Anyway, thanks, I ALWAYS appreciate the feedback. It is yet another clue to the puzzle.

If the IAC does turn out to be bad, it's off to the happy hunting grounds with it where it can keep the old grungy plastic IAT sensor company.

Thanks again!

[madpogue]

Originally Posted by DIYguy

However, presumably there is a reason that both the service manual sets I have cite matching procedures for adjusting the manual idle screw to a position other than 100% closed.

Is either of these a Saturn FSM? If so, I stand corrected. Conventional wisdom here has been that the throttle stop screw is factory set, and should never need resetting. But of course, this applies to my '96, and specs/procedures change over the years.

[DIYguy]

No, one is a printed Haynes, the other is a Mitchell CDROM. The procedures are slightly different from one another, but both basically say the following-

- "The TB idle setting is factory preset and should never need adjustment."
- Tampering, wear or replacement of components may require readjustment.
- The coil resistances of the IAC should be about 5 ohms each.
- Remove the adjustment cap and air cleaner.
- With the engine warm, either plug the IAC air passage or use a special IAC tool to fully seat the IAC closed (the two manuals differ slightly on this).
- Disconnect the IAC cable if not done already, and start engine.
- Adjust to 600 rpm (+/-) and replace the adjuster cap. Engine off.
- Remove the IAC passage plug (as applicable) and reconnect the IAC cable.
- Put anything else that was removed back (air cleaner, etc) and that's it.
- The car should now idle at 800-900rpm warm.

The Haynes is awfully generic. It says "dealer service item" a lot.
The Mitchell is very specific to the model and year, and is supposedly more of a "factory" manual. It mentions a lot of special tools.

I'll check for your 96 and see if it is different.
Is yours a TBI engine by any chance? Mine is MFI (quad injectors).

I have my eyes open for a factory manual set for the SC2, and will buy one pretty soon. Interestingly the procedure described above is also exactly the same for both of our GM trucks, for which we do have factory manual sets. One of them is TBI, one is CPI (a real oddball). Curious that procedures for all of them would be about the same.

Again, I admit I am not that familiar with Saturns (yet). They do seem to do things differently than the rest of GM, so your original statement about it being 100% closed from the factory could very well be true. I woudn't dismiss it just yet.

Perhaps someone else on the forum can provide an excerpt from a genuine factory manual on this?

Thanks again.

[DIYguy]

Ooooh, check this out-

http://www1.autozone.com/servlet/UiB...3d8019a00e.jsp

[DIYguy]

A brief update-

I suspected a sticky/sluggish IAC for various reasons, so I replaced it a week ago (Neihoff). The car ran SO MUCH better and had done so all week (~325 miles). However yesterday it started to have the weird idle again occasionally under certain very specific circumstances. So much for optimism.......

It apprears to be related to a rapid throttle snap (sharp release of the pedal) in heavy traffic situations. The idle motor does not seem to be able to react fast enough, which is why I replaced it. It then appears to overreact and the idle surges until I put it under some light load. Then it smooths out and stays smooth even after releasing the load. Again though, this happens ONLY when the vehicle is at full operating temperature.

I did another leak check during the week (sprayed carb cleaner at everything I could think of) with none found. Read the plugs- wearing in nicely, no signs of oil or rich operation. I just did another compression check tonight- 197,198,197,196. Hardly anything to get upset about, I'd think.

I will return the idle screw to exactly where I found it originally (I kept careful note of the amount of change), pull the battery cable for a few minutes and try another week.

I will also do a fuel pressure check some time this weekend. Haven't done an in-depth one such as descrubed elsewhere ob the forum so far. I may even devise a way of setting up a gauge to monitor it during driving conditions.

Anyone have any suggestions....please? I am running out of ideas fast. Still getting 35-36mpg in commute traffic though. I am just annoyed at the occasional problem and it always keeps me on edge.

Thanks in advance.

[fdryer]

Nice post. You may not know that the response of the IACV isn't supposed to be rapid as you've found out from the Autozone library. It doesn't specifically state how fast it is supposed to move the valve as it is used for idling purposes. Another piece of info you'll need to be aware of, if you haven't researched it already, is that emissions control requires the idle speed to be higher than idle during closed throttle deceleration. The idle is raised deliberately to approximately 1200 rpm when decelerating from speed until a certain lower speed is reached where the higher idle speed is returned back to normal idle.

You may have to look back to the EGR valve operation again if it interferes with part throttle operation between 1200-1500 rpm (I think). Luke pointed this out in another post as to what rpm the EGR valve opens and closes.

[DIYguy]

Thanks very much for the reply. I hope the new IAC was not a waste of money, but it really did work beautifully for a few days. I was actually beginning to fantasize about fixing the bodywork paint......

I drove it all day today (about 75 miles), and it is getting to be positively aggravating again. Literally getting worse by the mile. Now it pulls the idle trick almost every time I get to idle. When I briefly pull a slight load (clutch in gear) it smooths out and is OK, until the next stoplight. I wish something would just die so I can isolate it.

I checked the fuel pressure this morning, it is 36psi rock solid with the vacuum hose on. This is at the high limit of the range per both my manuals. The pressure took over 30 minutes to drop by 5psi after shutting off the engine (spec is 3psi max after 5 minutes). So no smoking gun there, but no bad check valve, leaking regulator or injectors either.

I messed up on the compression test late last night, by the way, I forgot to hold the throttle wide open, so the pressures are certainly higher, but at least it shows uniformity between cylinders. When it is not going nuts, it idles smooth as silk.

I will try to block off the EGR again, but point out that the bad idle occurs at 800-900 rpm in neutral or clutched in gear sitting in traffic or at a stop light. It also does not usually come on instantly, it takes a moment or two.

I am discouraged to say the least. I have worked on a lot of cars (some rather exotic and complex) but this one is driving me crazy.

Thanks again. I'll try look at the EGR again.

[DIYguy]

I finally adjusted the throttle body idle setting properly yesterday.

There is now no doubt that someone, somewhere, during the life of the vehicle messed around with it and screwed it up big time.

I had to add almost another complete turn (plus the turn I did before) just to get it up to 600 rpm with the IAC passage plugged. After letting it recalibrate itself, it runs very nicely again. We'll see what happens after 300-400 miles of traffic this week.

By the way, if anyone needs or wants to do the procedure (Autozone has it online), a small rubber stopper from a hobby or hardware store works perfectly to plug the IAC bypass passage. Don't trust the IAC to be fully closed.

[DIYguy]

Update- (97 SC2, 115k miles, 5 speed)

It's somewhat better but not gone. Still acts up now and then. Slight clutch load clears it up.

I added another engine ground strap on a hunch. The problem seems to be somewhat related to changes in electrical loads (lights on or off, etc). It helped my truck a few years ago. No long term effect good or bad for the ground strap on the Saturn.

EGR blockoff has no effect. PCV blockoff has no effect. MAP tested good. Spraying carb cleaner everywhere has no effect. Regapping the plugs has no effect. Plugs run clean. Ignition coils are clean, control module and bolts are clean. Wires are new. ECTS and connector are new. IAC is new. Fuel pressure is good. Every engine sensor and control connector I can locate is clean. Compression is prefect. Idles perfectly except when it goes nuts once in a while in traffic. 36mpg+.

I think that the TPS may be the next item on the change list even though I tested it carefully.

Help?????

[tomneri]

I am having the same problems. I have changed everything but the O2 sensor after the catalytic converter. Most people are un aware that there is one there.n I am hoping this will solve the problem.

Tom

[DIYguy]

My sympathies, believe me. It is way too annoying a problem for anyone to have.

However, I may have found something else that could help you too.

As I posted a few days ago, the TPS seemed like the next likely culprit. I am really tired of spending money on things that don't work though.

It seems to me that if the TPS were getting flaky it would be at or near the idle position because that is where it spends much of its life. I have a sensor tester (not a great one mind you, but it sort of works). I tested the TPS very carefully and it seemed OK. I also tapped on it, and have checked the connector and wires to it, ad nauseum.

It dawned on me a few days ago that it could be slightly out of tolerance or could have an erratic connection near idle position. This might not have showed upon the tester because it wasn't a bizzilion degrees hot when it was being tested. Also, there is a maximum "idle" position voltage for the engine computer and it could be on the edge of that limit. If so, sometimes it might be telling the computer that it was "not at idle". This could be a big problem because guess what happens then? The IAC changes position !!!!

So I added a 24K ohm 1/4 watt resistor at the connector between the ground and output terminals (I just pushed the resistor wires into the connector along side of the wires and ohmed it out to be sure it connected).

This would have the effect of pulling the TPS output to a solid ground/idle condition if the sensor's resistor element is worn out or disconnected at that point.

Sure enough, with NO other modifications or changes of any kind to anything else, it seems to work , at least so far. However I've been down this road already. Give it another week and another 300-400 miles in traffic and I'll know for sure, but so far it is 250 miles and counting. The idle is a normal emissions idle (bureaucratically acceptable), but NOT erratic like before.

If this works out and remains stable for another week, I will replace the TPS, but I am tempted to leave the resistor in place or to move it to the computer connector permanently.

I'll let you know in a few days.

By the way, the second O2 sensor after the CAT is a monitor only. It does not (so far as I am aware) control the engine in any way. It is just Big Brother watching your motor. It can definitely turn on the SES light and can set codes, but your engine should still run even if you take it out and smash it with a hammer (not a bad idea.... hmmmm).