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Can anyone help me interpret scan tool data for rough idle?

This is in regard to a 2002 Daewoo Lanos 1.6L Automatic.

I have owned this car for approximately two months. It ran great for the first month, I even had a professional mechanic change the timing belt on it. Now, for the past few weeks, it doesn’t run properly. It has a very rough idle, feels like it’s misfiring and going to stall, but it never does. If I put my foot on the accelerator to raise the RPMs, it smooths out. When the car is moving, it has decent performance and acceleration. The only trouble code stored in the computer is P0442–evaporative emissions system small leak detected.

I took the car back to the mechanic who did the timing belt, they double-checked everything, confirmed that the mechanical timing is correct, the engine has proper compression on all cylinders, proper fuel pressure at the fuel rail test port, measured resistance of the fuel injectors, etc. They claimed that their scan tool provides only basic information for this model, and cannot access the diagnostic functions provided in the real factory scan tool, but based on all their tests they don’t see a problem. They even did a smoke test, but the only thing that found was a hairline crack in an emissions check valve near the fuel tank.

I looked into getting a real Daewoo scan tool, but at approximately $3000 for a used one, that would be insane. So I bought a decent low-cost OBDII scan tool online, which provides live data and sensor values if supported by the particular vehicle.

So with my scan tool, I see that the short-term fuel trim constantly fluctuates from -0.8 to 3.6, as the engine warmed up at idle the long-term fuel trim went from -6.3% to -11.7% to -14.1%, the engine RPM constantly fluctuates from 857 to 948 RPM, the ignition timing advance for #1 cylinder constantly fluctuates from 2.5° to 13.0°, oxygen sensor B1S1 constantly fluctuates from 0.160V to 0.825V, and oxygen sensor B1S2 only fluctuates from 0.845V to 0.865V.

The fact that the fuel trim trends more and more negative the longer the engine runs would tend to suggest a fuel pressure problem or fuel injector problem. Is there any way for me to test the fuel injectors other than measuring their resistance, which was already done? Also, I have heard that this model is known for a defective camshaft position sensor (unless already replaced) and this can cause serious drivability problems without setting a misfire code.

Injector balance test, but you’ll need a fuel pressure test kit and some way to “buzz” the injectors, such as this


This is where the pro-level scan tool would come in, because it very well might have the capability to run that bi-directional test, thus negating the need for the separate tool

It sounds like your idle is too high. Does it ever drop down to, say, 750rpm? By chance, does the underhood emissions sticker say what the idle speed should be?

Your oxygen sensors are apparently performing their duties correctly

Has anybody ever bothered cleaning the throttle body?

Does this thing have an idle air control valve?


My first thought was the IAC (if equipped) could be bad or the port plugged with carbon, etc and not set a check engine light.

I believe if it were a fuel delivery problem, you’d probably notice a power loss / drive ability issues other than only a shaky idle.

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Yes, throttle body was cleaned. And it uses an IAC, but apparently only the real factory scan tool can operate the IAC and view its current position. Also, I replaced the spark plugs and ignition wires myself, NGK brand wires and plugs, but it made no difference.

I just checked the underhood emissions sticker–it says “Idle speed, idle air/fuel mixture, and ignition timing have been pre-set at the factory and are not adjustable”.

I have a spare throttle body, complete with the IAC and TPS, which I purchased from the local “you pull it” junkyard, together with a bunch of other parts to keep as spares for this car. I have not tried swapping the throttle body, IAC, or TPS.

One other thing is that this car is difficult to start if it has sat for more than an hour. In that case, it requires 3 or 4 tries to start, and when it does start, it misfires for several seconds, then keeps running. If it has sat for less than an hour, it restarts on the first try.

I’d suggest swapping the iac valve. Won’t harm anything and you can always swap back. If it doesn’t help, at least you’ve ruled that out.

I don’t mind “throwing parts at it” if the parts are free!

I will say your last post makes me think it might be a fuel issue of some sort. But, the free iac valve is worth a shot.


I’d hardly say you’ve “ruled that out”

Let’s imagine this scenario . . .

op isn’t sure his iac valve is good

he swaps in a used iac valve from the junkyard

nothing changes

what does that tell us . . . not much, since we don’t know if the junkyard iac was any good

There have so many times when I’ve seen guys swap in a used part, only to discover the “good used part” was a piece of garbage, after all. Waste of time, resources, etc. :-1:


I would prefer trying the free (used) iac valve that he has on hand rather than buying a new one if it were my money. If absolutely nothing changes, I agree, it doesn’t totally rule the iac valve out as the problem. But 2 failed iac valves are less likely than one. Installing a brand new iac valve wouldn’t rule it out as the problem 100% either, as it’s not unheard of to get bad new parts, albeit far less likely. If you’d like to tell him how to test the iac valve, by all means, go ahead. I don’t know how to do that myself, other than watch the pintle move (or fail to move).

I will also add that the EGR valve that’s currently on my car came off a scrap car and everything’s hunky dory, no more cel. Sometimes you win, sometimes you lose. But if the part is held on by 2 screws and has one electrical connector, it’s worth a shot. :+1:

This is starting to get slightly amusing. I would have to agree with my man @db4690. You rule nothing out till you rule it out right n proper.

Salvage yard parts are handy and helpful if you know how to wield them. They do not prove anything out until that part is actually proven out…until that happens, all you do by utilizing them is inject more mystery and suspicion into your current conundrum. Things spiral out of control Quick…when you start “assuming”. Ask me how I know.

The crazy thing these days is new parts aren’t as robust and trustworthy as they should or could be…another discussion entirely. How many times have I told someone to grab an old “Actron sensor tester”…worth its weight in gold I tell ya. It proves out what you cannot prove out easily sometimes, it’s an old school item, as I am and someday you too shall be. Nothing wrong with Old School.

Use the junkyard wisely…and if you know what you are doing you will be OK… I cannot possibly teach the salvage yard parts usage lesson however.

Mind Mr DB46 he is definitely passing on some hard won knowledge here.


Long term fuel trim seems to tell me you have a leaking injector. Too much fuel is getting in and the computer is trying to rectify it. The hard start sure sounds like a leaking injector bleeding off pressure. Doesn’t explain why iit doesn’t happen later. Maybe heat is binding up an injector internally only when hot.

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The IAC is easy to remove–two phillips screws and one electrical connector. I will try the used one when I get home from work today. My intuition is that all of the parts on that car were good, other than the engine, because it had very low mileage, and went to the junkyard due to a failed timing belt. I will post back with the outcome…

Here’s another vote for a leaky injector(s).
The symptoms fit.
The IAC won’t throw off fuel trims or cause rough idle at 800-950 rpm.
Hook up a fuel pressure gauge and watch the pressure when the engine is turned off.
It should fall very slowly or not at all.

I agree with @Mustangman and @circuitsmith. Long term is off. Checking how fast fuel pressure drops is key.

I absolutely don’t disagree with either you or db. Don’t want to imply that. I’m a shade tree mechanic at best, and I freely admit it. Like I said, if you guys know how to test the iac valve function to rule it out (you probably do, I do not), then that’s absolutely preferable to parts swapping. No, I wouldn’t source parts from a scrap vehicle (unless you work at a scrap yard daily :grin:). But if the parts are there already, easy to replace, and free…:metal:t5:

I doubt the iac valve will fix it. But bad idle, it has an idle air control valve, I’d try it considering the effort is so minimal.

If an injector leaked down… it would be cylinder specific and we would see hard misfires on that cyl. It should stick out like a sore thumb in this scenario. No?

If this vehicle is known for cam sensor issues… then start there, test it, replace it… Because if the cam pulses are incorrectly correlated to the ecu so are the injector pulses… most likely early (possibly during exhaust event) which would produce a rich exhaust… which would then try to be leaned out… Or something along this line of thought. Or is it just late and I’m completely exhausted again?

Good point.

One other thing is that this car is difficult to start if it has sat for more than an hour.

Delayed starting could come from a failed cam sensor also because the ECU had to guess compression tdc.

Other thoughts:
Another reason for delayed starting might be fuel pump check valve. A slightly leaking fuel pressure regulator can cause richness. Could be multiple problems?

Alright, so I swapped the IAC and re-did the idle learn procedure. It made no difference. The junkyard here still has this car, so I’m going to get the set of fuel injectors. I picked up a new set of O-rings, as I’m not going to re-use the used ones.

Fix the hairline crack in the emissions system. Get the valve off the junkyard car while you are at it. It could just be a classic vacuum leak.

Good job on changing the timing belt. This was the end of many older GM Daewoo products.

My GF used to own a 2006 Suzuki Reno which is a newer version but another GM Daewoo product. This is the hatchback version of the more common Suzuki Forenza sedan but otherwise they were the same car. Get ready to track down this and other gremlins. It was always some $50-100 sensor each month but I figured it was cheaper to keep this on the road than get into a new car payment.

Finally the car started to get a pretty good backlog of more expensive repairs. A rock hit the windshield and it was an oddball so nearly $400. The struts were finally just about worn out. It was going to need a new battery and tires very soon. We started to make a list to prioritize the replacements and then the head gasket or something similar let go. I told her it was time to move this car along when I saw and could SMELL exhaust gasses coming up through the coolant. That is the smell of death right there. Even my GF who knows nothing about cars was commenting on that smell.

I hope you have better luck with this car and hope it is a fun/project/spare car and not your primary daily driver.

Instead of spending time and money on more junkyard parts . . .

Why don’t you buy that fuel injection pulse tool and a fuel pressure test kit . . . ?

Then at least you’ll know what’s going on, as regards the fuel system

Please don’t take this the wrong way, but here goes . . .

You’re taking the shotgun approach . . .

Throw many parts at the car, hoping one of them fixes the problem

And even worse, you’re literally throwing junk parts at the car

Without the Daewoo scan tool I would guess that an oscilloscope would be the best method to determine the operation of individual injectors. A digital scope would give a great deal of insight into the problem by viewing all the inputs and outputs involved with engine management. And a digital scope is useful for all makes and models.