Poor idle

[Spetz]

Hi guys,

I have a 2003 Mitsubishi Diamante 6G74 that has a poor idle.

It feels like it misses a bit but not too horribly.
Idle quality is inconsistent. It ranges from smooth (rarely) to quite poor (also rarely) and usually is somewhere in the middle - Not horrible but not really acceptable either.

I have changed:
Injectors (these were cleaned and flow tested)
Distributor cap
Distributor rotor
Spark plugs
Plug leads
Fuel filter
Throttle body & ISC clean performed

Vacuum leak was performed - No leaks
Connected onto onboard diagnostic - No codes and all sensors seem to be reading within spec
Idle rpm are stable and where they should be

What would be the next thing for me to test/check?

As on rare occasion the car idles smoothly it seems that it is possible for it idle smoothly all the time.

Help would be appreciated!

 

[Silk]

Try the most neglected diagnostic tool, a vacuum gauge. I bet it will have low vacuum at idle. Of course finding out why is the hard part. Apart from the obvious, check the EGR, PVC.

 

[NavyVet88]

Originally Posted By: Silk
Try the most neglected diagnostic tool, a vacuum gauge. I bet it will have low vacuum at idle. Of course finding out why is the hard part. Apart from the obvious, check the EGR, PVC.


+1

My Buick in the Sig had the same exact problem so I bought a bottle of carb cleaner and grabbed an old tooth brush and cleaned the gunk out of the butterfly and it's been idling fine for the last 1500 miles.

 

[North to Alaska]

Try lazy o2 sensor
I believe it should cycle at leadt 2 times per second?

https://www.google.com/url?sa=t&source=web&rct=j&url=http://akinsautorepair.com/oxygen-sensors.html&ved=0ahUKEwi0mrTxp73UAhUBWj4KHaoiAP0QFgheMA0&usg=AFQjCNExibG6iyYWD3bCLfCeNEtFxl76-A&sig2=BlfgLWKmUqFDtlgeJsQvLg

 

[Spetz]

The EGR valve you mean?

I cut a piece of coke can material and blocked the EGR passage into the intake manifold but that didn't make any difference.
I'll try again but presumably this should be a good enough way to test it? While perhaps checking for vacuum leaks around the EGR passage?

 

[Spetz]

Also with the vacuum gauge, did you guys mean install a vacuum gauge inside the car or use a vacuum gauge tester?

With the O2 sensor I was told that it is ignored at idle and the ECU uses the last known value to make assumptions - Does this sound right?
The car runs well besides the idle and fuel efficiency is good. During the diagnostic test (some 30,000 miles ago) the voltage readings from the O2 sensor were fine

 

[North to Alaska]

It only ignores it till it warms up enough.
Lazy means it is not jumping back and forth fast enough.
If this is slow, the computer will fast enough information to adjust the air fuel mixture. This can happen without throwing a code

 

[Silk]

Just plug a vacuum gauge into a port on the manifold...use the brake booster hose if you can't find another. You should see at least 18 in/mg.

 

[eljefino]

Originally Posted By: Silk
Try the most neglected diagnostic tool, a vacuum gauge. I bet it will have low vacuum at idle. Of course finding out why is the hard part. Apart from the obvious, check the EGR, PVC.


This x2.

It is also possible your TPS is flaky in such a way that there aren't codes but that it changes between startup and bad-idle-time such that the computer feels like it doesn't have an obligation to control the idle... or pops between two different programs.

If you idle poorly, turn the car off, and restart, is it cured?

 

[Silk]

Mitsubishi always had an issue with the idle control motor. I used to pull out the pintle and clean that too. If it has an idle airbleed screw, clean that too...not on that model though I think.

 

[Spetz]

Thanks guys,

Is there a way to test for a lazy O2 sensor?
Would I be able to check the A/F ratio at idle and come to any conclusion?

I haven't tried restarting the car if it idles poorly but there are times that idle quality changes between traffic lights.
It idles better when not fully warm but I think this might be due to higher idle rpm?

With the TPS (and all the sensors for that matter) they were checked not with a diagnostic tool but rather a laptop with the Misubishi software that allowed to reflash the ECU etc and all sensors (including TPS) read within normal ranges

 

[Spetz]

Hi guys

So I performed a vacuum test.
The needle was steady and at idle was showing 20inHg.
If I blipped the throttled it would go to zero, then drop quite a bit, then settle at 22inHg and slowly move back to 20inHg.

What should I be testing next?
I will likely be replacing the O2 sensor as it is 144,000 miles or so currently and due for replacement anyway.

 

[mk378]

I assume when you say "idles poorly" you mean that it misfires at idle. Does the engine have adjustable valves? A valve that is too tight will lose compression especially at idle. Also you could unplug the fuel injectors one at a time to see if it is always one cylinder that misfires or it is generalized.

 

[Spetz]

It does feel like a slight miss, but it is not consistent or rhythmic as such.
The valves are hydraulic but either way seeing as I did a vacuum gauge test on the car it does not seem to be a compression issue

 

[Spetz]

Bump... Anyone?

 

[mightymousetech]

Do a compression test, if a cylinder is low, do a leak down test.

 

[Lubener]

Look at fuel trims with a scanner.

 

[Merkava_4]

Originally Posted By: Spetz
Is there a way to test for a lazy O2 sensor?
Would I be able to check the A/F ratio at idle and come to any conclusion?


Fuel trim numbers will tell you. But even if the O2 sensor wasn't reporting fast enough, it would still only be a possible cause for a rough idle issue; nothing definitive unless the numbers were way off.

 

[Spetz]

How would I check fuel trims?
Do you mean take it to a tuner or something?
Or get an A/F ratio gauge?

 

[Merkava_4]

You only need a scanner that can read live data, which is what the computer is seeing.

If the O2 sensor is not reporting fast enough, then the computer won't adjust fast enough.