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

Do you have a tach in your dashboard? If so, what is the idle speed? It should be around 750 rpm.

If it is idling much below 700, then the rough idle could be due to the throttle position sensor (TPS). The TPS should have three pins on the connector and is easy to check with an ohm meter or VOM meter. Everyone should have one of these if you pan on repairing your own vehicles.

Simply unplug and measure the resistance between the center pin and one of the outer pins. It should sweep smoothly from around 0 to max or max to 0 as you or someone presses the gas pedal. If you find it hitting max at both ends and varying in the middle, it is bad and needs to be replaced. If this isn’t working properly, the computer can’t control the idle because it won’t know that it is supposed to.

Do also fix the P0442 issue because that could be allowing unmetered air into the intake manifold.

If I’m testing a tps, I’ll be backprobing and measuring voltage as I go from closed throttle to wot, and back again

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Potato Patato

Does anyone actually pronounce it “patato”? :thinking:

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If you solve the -10% and worse long term fuel trim problem you’ll likely also solve the poor engine performance.

  • Too much gas getting into the engine: faulty injector, too high of fuel pressure, incontinent fuel pressure regulator diaphragm, faulty purge function in the evap system

  • Inaccurate metering: faulty O2 sensor, faulty MAF or MAP sensor, faulty coolant temp sensor

My guess is either a faulty purge function or MAF sensor.

The engine is running at the correct “warm” temperature right?

No, but then no one tests a TPS by disconnecting it from the vehicle and testing resistance either. Sensors and solenoids are properly tested when they are “loaded”, that means connected to the voltage and ground source they are used in and operating with the systems carrying a load. Ohms testing circuits is not productive.

To quote an old Star Trek saying “Resistance is futile.”


You’re obviously some sort of a trekkie . . .

Did you ever watch the Star Trek Voyager series . . . ?

If so, remember the episode where some of the crew went back in time, and the pilot . . . Paris? . . . had an opportunity to drive an automobile and took it, thinking he’d be good at it, because he was good driver on the holodeck. Turns out he wasn’t a very good driver in real life, and it was pointed out to him

I think the automobile in question was a VW Bus, one of the earlier ones

That is a valid test for a TPS. A load test is for the entire circuit, a resistance check is a confirmation check for an individual component.

Edit: Personally I would test under load first for troubleshooting purposes, but if the load test fails, I would confirm the TPS with a resistance test to make sure the load test failure wasn’t due to a bad connection or other fault.

Nope. Ohms testing can tell you if a component is bad, but it can’t tell you if it’s good. Unless you operate an item under the voltage provided by the control unit you haven’t confirmed or denied anything.

EDIT: yes, all testing is done by backprobing to ensure you don’t disturb any connections. Only when you are confident in your direction do you actually disconnect anything.

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That is one of the absurd things I have ever heard.

Resistance simply tells you what the continuity of a circuit is while it is static. That’s useless information. What matters is the ability of the circuit to carry an electrical load, whether it be a headlamp or the 5 volt load to the potentiometer in a TPS. You can ohms sweep a TPS and find an open circuit along the way and find it’s bad. But sweeping it with an ohm meter will not find a spot where it’s failing under load, since your meter does not supply the same electrical load that the car does.

I’d like you to take 3 feet of 14ga stranded automotive wire and strip all the insulation off. Now untwist the wire and take one single strand out and measure the resistance. Should be darn close to 0. That’s a good circuit with no resistance, right?

Now remove the positive battery cable from your car and replace it with that one strand. The resistance from the battery to the starter will still be 0 ohms, that’s a good circuit with no resistance, right? Now start the car. Of course it won’t work, the circuit, although good, will not carry the load.

That’s an extreme example but similar to what we find under the hood on a regular basis where things get very hot, very cold, very wet, very dry, and very oily.

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We are not talking about a battery and starter motor here, its a TPS. It doesn’t get loaded that much. BTW, all my ohm meters use a 9V battery so it does provide a pretty close load to a 12v Battery.

I fully understand your thinking, I used to test commercial 3 phase transformers up to 2750 kVA, that 2.75 megawatts to those uninitiated in power systems. I actually worked with Electrical Engineers that didn’t fully understand that concept, but you have to consider the context here.

It is a valid test and probably safer for those who are DIYers.

Not to disrespect the DIYers, but you do have to be careful that you don’t give them advice where they might accidentally let the magic smoke out of the box.

So now, I am even more stumped. We had to go to a city that is approximately 75 miles from where we live for family matters today. Normally, we would take the wife’s car–the 2004 Toyota Corolla–but unfortunately it sprung a coolant leak at the thermostat body, so we drove the Daewoo instead.

Once the speed got over 70 MPH (about 20 miles from our destination) the check engine light began to blink, and I wasn’t sure if I was feeling excessive misfires or just the bad road surface. On the drive back, again taking the speed above 70 MPH made the check engine light blink, and it kept on blinking until the engine was turned off.

When I got home, I kept the engine running and retrieved my scan tool, and connected it. The only code stored in the computer now is P0300–Random/Multiple Cylinder Misfire Detected. With the engine still idling in park after a drive of several miles, the long-term fuel trim was -11.7%, the engine RPMs kept fluctuating from about 830 to 921, and the ignition timing advance for #1 cylinder kept fluctuating from 2.0° to 11.5°. There is also a freeze frame data stored, due to the trouble code.

I am beginning to question whether this is even a fuel system problem at all, or the camshaft position sensor, which was known to fail. The problem is that this sensor is expensive and difficult to install.

No, I followed The Next Generation from beginning to end, and some of DS9, but never got into the Voyager.

I do remember being 20 years old and wondering how long it would be until I could get a holodek of my own!

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No. “So let’s call the whole thing off,” or so goes the tune as I recall…

You should get a code for the camshaft position sensor or the crankshaft position sensor, but I have seen a crankshaft position sensor fail without giving a code other than the P0300. You could replace both, they aren’t as expensive as paying for a troubleshooting and they really aren’t that hard to replace. You can find You Tube videos.

If they don’t fix the problem you are not out that much. However, I’d still fix any air leaks that might allow unmetered air into the manifold first.

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You (or a mechanic who knows how) can back probe the cam and crank sensors with a scope to see if their signals are kosher.
There are still the fuel injector tests to be done, yes?

Can this be done even without the Daewoo Scan-100 tool that no one seems to have anymore? I have the printed factory service manual, and the troubleshooting flowchart for P0300 would have me view the camshaft revolution counter and crankshaft revolution counter to see if the readings increase at the proper rate for the commanded RPMs. There are also cumulative misfire per-cylinder counters that would be really useful if I could access them.

Well, the only fuel injector test I can do is to try another set of injectors. With the Daewoo scan tool, the recommended test procedure would be to install a pressure gauge on the fuel rail Schrader port, with the engine off, command the fuel pump to come on just long enough to pressurize the fuel rail, pulse one injector a few times, and observe the pressure drop. This test is to be repeated with all four injectors, and the pressure drop from pulsing each is to be compared to see if one sprays more or less fuel than the others.

I went to the “you pull it” yard today and got the complete fuel rail with injectors, MAP sensor (factory original, date code 2001), and camshaft position sensor (updated/revised part number, date code 2006). I previously purchased, but have not yet installed the fuel pump (aftermarket brand, date code 2015). The only reason I want to replace the fuel pump is because the gauge will not go above 75-80%, and replacing the instrument cluster (due to a grossly inaccurate speedometer) did not help with the fuel gauge, though it does show the correct speed now.

I assume the first thing I should do is try replacing the fuel pump, which I was planning to do anyways, and install a new fuel filter. If that makes no difference, what would you suggest next? Replace camshaft sensor, or try the replacement fuel rail (with all new O-rings on the injectors of course)?

Camshaft sensor problems aren’t a common cause of misfires. While it’s used for other stuff, the camshaft sensor’s principle purpose is to tell the engine computer whether the crank sensor’s pulse is for an exhaust stroke or an intake stroke. The computer won’t fire the plug on exhaust strokes. When the crank sensor fails the computer may fires the spark on both intake and exhaust strokes. This keeps the engine running, but can cause the coil to overheat & eventually fail if the problem remains unresolved. If the crank sensor failed in a manner that made the computer think the exhaust stroke was the intake stroke, that could cause misfires. But the most common failure mode is probably that it just stops working altogether.

I’d bet you lunch that the various Snap-on Solus scanners or Modis scopes would do all of the factory bi-directional tests

This is good information, because those models I mentioned are quite common, as far as pros go

You don’t even NEED the daewoo scan tool to do that

If you buy a fuel pressure test kit and that otc fuel injector tester tool which I pictured, it’ll do exactly what you just described, provided you can actually get to the injector connections

A decent fuel pressure test kit isn’t that expensive. Neither is the other tool I pictured, and there are a few different brands out there, if you want to “window shop” . . .