Trivia of the day (Can you solve this?)

[Tofuboy]

It has come to my mind that there are hundreds and thousands of people asking questions everyday in this forum. I figure there must be some people out there has learned something instead of just thinking it's another place to chat. Feel free to post your answer with explanation (it doesn't matter if it's right or wrong) to see if you can solve this problem, also to see if there are any intelligent minds out there.

The situation is like this, my car's RPM is fluctuating while idling from approx. 900RPM to 1500 RPM. Check engine light is not on and there is no previous problem exists in the engine. What could this problem be and how to fix it?

I am pretty sure what the problem is already, I will double check and post up the answer tomorrow.

Hint: I have taken out the injectors and ran them through the injector cleaning machine and tested, then replace them back.

For those who are interested in the result from the machine, the details are below:

Flow % - Cyl #1=100 #2=100 #3=100 #4=100

Pintle % (24MS @ 7200RPM) - Cyl#1=99 #2=98 #3=100 #4=98

Leak Down Test- 60 PSI and above (didn't want to turn the PSI too high since the spec is 40-47 PSI)

[ivymike1031]

is the rpm "bubbling" up and down, or is it smooth at 900, then smooth at 1500, then back to 900, etc.?

[Tofuboy]

is the rpm "bubbling" up and down, or is it smooth at 900, then smooth at 1500, then back to 900, etc.?

RPM is smoothly fluctuating from approx. 900 to 1500 RPM

ex. smoothly from 900 rise to 1500 RPM and smoothly from 1500 drops back to 900 RPM and repeats over and over.

[Frostbyte]

The problem is that you keep hitting the gas pedal with your foot.
You can fix the issue if you stop hitting the gas pedal with your foot lol.

[ivymike1031]

son of a biscotti... I typed this long-ass response and it didn't post. I hate that.

To me it sounds like a controls problem rather than a problem with hardware (excluding wire connections and sensors).

You didn't say whether or not the car is warmed up, or if that matters, so I'll assume that the behavior is the same hot or cold.

You've recently cleaned the injectors, so potentially they're flowing better than they were before. If they were flowing especially poorly before, then the long-term injector trim would likely be a fairly high value. I would expect that the short-term injector trim would be adjusted to compensate (assuming closed-loop operation) and you'd see no problem.

I'd start by looking at the IAC valve and the circuit that controls it, to make sure that the inputs and responses are correct.

dunno... post the solution when you find it, I'm interested to hear what it is.

[kane2g]

if it doesnt go away after warmed up, i would suspect fast idle valve. I just replaced it on a 92 lude and it was doing exactly what you said.

[Tofuboy]

If they were flowing especially poorly before, then the long-term injector trim would likely be a fairly high value. I would expect that the short-term injector trim would be adjusted to compensate (assuming closed-loop operation) and you'd see no problem.

if it doesnt go away after warmed up, i would suspect fast idle valve. I just replaced it on a 92 lude and it was doing exactly what you said.

Nice approches. This is happening during warm engine idle. Note that there were no previous problems at all and there is no check engine light. Engine was running like a champ.

The injectors were working fine I just felt like giving them a bath.

[b16a3sol]

check your air filter and PCV valve.

[crxlvr]

id check the throttle position sensor, as well as your idle control arm and your idle sensors.

[Tofuboy]

Ok, here is what I did.

At first I was suspecting a vacuum leak around the injectors from removing and replacing them as I might have damage the O ring(s).

The reason why I would think it's a vacuum leak is because:
There is no check engine light, which it would be impossible that the IACV is not working plus that I have taken out the injectors.

So I started the engine, let it warm up, idle and try to hear for the leak as the RPM speed going up and down, but I couldn't hear any, at this point I have change my mind about the vacuum leak as the problem (I will explain why I change my mind later). But I used the propane method to try to find the leak anyway, thinking to myself that I probably couldn't hear it as the engine noise is covering it.

After using the propane method and no leak was found, so I have proven my point on changing my mind on the problem was right. The reason that I changed my mind was because I was thinking that if a vacuum leak that can make such a noticeable difference on idle, there is no way that I can't hear it.

So I have move on to my second though as the IACV is probably not working properly. After removing two bolts and two hoses that attach to it, I have taken out the IACV. Inspecting and found that the IACV, the air passage and the screen that filters the air are pretty dirty, so I used some crab cleaner to spray them clean and replace the IACV back and WOW LA the engine is idling properly again.

The reason why this would have the fluctuating RPM symptom is because the idle circuit has some dirt buildup causing the idle to drop and the ECU is detecting that and attemps to raise the rpm before the engine die, as the rpm being raise to a certain point the ECU is detecting that the rpm is too high and try to lower it back resulting the fluctuating idle speed.

Ivymike was pretty close to finding the problem, great thinking!!