I noticed my idle gets rough once the car has been running for a while.
What seems to happen is the rpm drop to just over 500 in D.

I've heard the idle speed can be adjusted, but it is only a matter of time before the ECU compensates and brings the idle back down.

Is it possible to reflash the ECU such that the base idle is kept a bit higher when fully warm, in effect getting rid of the rough idle?

The ecu cant compensate for the idle control acrew afaik. It liturally stops the throttle from closing. The morevits open the higher the revs at idle ahould be.

If you adjust the air screw without the MUTT plugged in and setting the stepper motor correctly the ECU will compensate and drop the idle again

You need to properly clean the TB and then adjust everything correctly as a base start, rough idle can be caused by many factors and even new some of these cars were not the best when it came to idle unfortunately.

I've noticed though that my idle only gets bad once the revs drop close to 500rpm.
When the car is fully warm but hasn't been driven long and the idle is at 700rpm or so it is smooth.

I plan on cleaning the TB fairly soon as I know it really needs it:
http://i26.photobucket.com/albums/c104/spetz83/IMG_0657_zps4656f6ac.jpg

Reason being is that the air control valve has not fully closed yet, before you chase a low idle clean that TB

Yes cleaning the TB is next thing to do on the list, I am just coinciding it with a spark plug change in 2,000km (service time).

Is it possible that once the TB is cleaned out my low/rough idle will disappear for good?
From what I've read it seems no one has successfully managed to get rid of the dreaded rough idle so common to our cars

I was always under the impression that the rough idle was caused by the transmission? Since they idle rough in drive and ok in neutral.

My solution was to buy a newer car lol

In my car in D the revs drop to just over 500 yet in N they are stable at around 700 giving a smooth idle

In my car in D the revs drop to just over 500 yet in N they are stable at around 700 giving a smooth idle

Spetz - higher idle rev speed will always be smoother - i've had a rough idle a few time and for me it's usually a reminder to run some inject cleaner through (I've only ever used RON 95 or 98 in the can in 12 years as well), but what I define as rough is slight minor 'miss' now and then in drive.

My TJ gets a rough idle when hot too. Painful at traffic lights. Just nudge it into neutral if stopped for any length of time, or put your left foot on the brake pedal and raise the rpm by 100 or so with the right foot. Works for me.

Thanks MadMax but neither one of those seems like a "solution" :)

Thanks MadMax but neither one of those seems like a "solution" :)

I said it works for me. I didn't say it would work for you.

Clean out the throttle body, check your ISC is working, get someone with a MUT tool to check and up your car's idle speed. No ECU flashing needed.

But this is just temporary and the ECU will adjust the idle speed down again?
I will of course do that but I was hoping for a more permanent solution

AFAIK the MUT instrument can be used to tell the ECU that the engine idle should be higher, and will be a permanent adjustment.

I'm looking at getting one of these ..

http://www.ebay.co.uk/itm/Car-fault-reader-code-scanner-diagnostic-tool-OBD-2-CAN-/230644533100?pt=UK_Diagnostic_Tools_Equipment&hash=item35b37c476c#ht_3219wt_1143

I'm looking at getting one of these ..

http://www.ebay.co.uk/itm/Car-fault-reader-code-scanner-diagnostic-tool-OBD-2-CAN-/230644533100?pt=UK_Diagnostic_Tools_Equipment&hash=item35b37c476c#ht_3219wt_1143

That looks near identical to the JarCar one I bought a few years ago, except a helluva lot cheaper.

If it is the same as the JarCar one then it won't work on a 3rd gen, I tested mine on two 3rd gens & no go.

Shouldn't work because the magna is MUT, not OBDII.

AFAIK the MUT instrument can be used to tell the ECU that the engine idle should be higher, and will be a permanent adjustment.

Sorry no. The MUTT will tell the mechanic how many steps the stepper motor is sitting at so the air screw can be correctly adjusted but will not allow the user to up the idle

Bother!
Can the ECU be recoded to up the idle?

(Shouldn't be this hard for a simple adjustment - well, on a carby car it is anyway. lol)

MadMax this is what my question was, whether it would be possible to raise the idle with a reflash of the ECU

MadMax this is what my question was, whether it would be possible to raise the idle with a reflash of the ECU

You can definitely. You need an openport cable and the defs.

Wether you should actually bump the idle up is another thing...

This is often done with motors with modded cams fitted as an example, I have done it a few times. Some resort to also bumping up the idle stop screw but this is a very bad idea and will make baby Jesus cry

You need to find the reason your idle is low and looking at the pics of the TB I think I know why?

So, I cleaned the TB, took off the ISC and all.
As well as new fuel, air and oil filter, new oil, 6X new spark plugs, but the engine still idles rough.
I think it is better and more consistent in the rough idle than before.

What could this be?
Does it in R and D, so I assume not engine mounts?
First 15 mins of driving the idle is smooth, after that it gets rough.
It can idle rough at one traffic light, and then 5 mins later at another the idle is a bit better.
RPM seem consistent at all times

i hope you took off that TB to clean it, that way you can get in to every little air passage, also i have found that on the air mixture screw if you remove it entirely and clean out the passage way through there, and replace the O ring as this can be hard and brittle and not seal properly

No I left the TB on, but I took out the ISC to give it a clean as well as the passages there.
I read the TB does not need to come off?

Are there any vacuum passages that could be blocked anywhere that may cause a rough idle?

it is easier to clean the TB with it out, and to replace that O ring on the mixture screw i think you will need to have the TB off. im not talking about the isc. its that plastic mixture screw on the top of the TB, O ring is about 20c from any bearing place

I replaced the ancillary drive belt idlers on my TJ a while ago and didn't bother to tension the belts correctly. They vibrated badly at idle and were noisy.

The engine felt rough at idle like that, but when I finally got around to tensioning them correctly, the idle became a lot smoother and quieter.

Could just be coincidence, but worth a check anyway. Start it up, and let it idle. Check with a torch for vibrating belts. They don't jump around if tensioned correctly.

Belts are new and tensioned correctly (considering no special tools).
I had a vibration + noise before around 1,100rpm but after the new belts it was gone.

I will however have a look at them with a torch as recommended.

Any other possible culprits?

You can most definitely re flash ECU to adjust idle RPMs, my car was converted to manual, so i had a tune done which also reduced idle value to where manual needs to be be (auto idles higher).