Jump to content

Slightly Rough Idle

Recommended Posts

Hey. I've noticed that my car can idle a *tiny* bit rough when it's at normal drivng temp.

I did have it checked out by Lexus before I purchased it, on the little ticky sheet, it passed the "idle too low/too high" component.

They did say the engine mounts are sagging a bit, but didn't require any attention for around 30,000 k's.

The guy I bought it off wasn't sure when the Timing belt was done last. He think's recently though.

Does anyone think either of these could cause my problem?

I know it's bad, but what happens if I take too long to change the timing belt? And could lexus give me an indication of when I may need to change it?

Cheers again.

- Gav

P.s. Sorry guys, I feel like a real dog just asking questions and not being able to give anything back.

Link to comment
Share on other sites

depending on the year, timing belts usually should be changed at 90-120k. My 02 states 90k.

depending on the year of your lex, if the timing belt breaks the engine will just die and not cause problems (non-interference type engine) or it will die and could cause problems (interference type engine). The 1MZ-FE VVT-i engines (like mine) are of the interference type.

Rough idles can be caused by several things,

poor fuel pressure.......faulty fuel pressure regulator

poor fuel delivery......clogged or dirty fuel injectors or fuel filter

poor air flow at idle......idle air control motor, clogged/dirty air filter, MAF sensor, PCV valve, air temp sensor (is it ever "rough" on cold idle).

improper air/fuel mixture......faulty O2 sensors, failing EGR valve, faulty air/fuel sensor (aka, pre-cat O2 sensor) these will usually set off a check engine light

Easy/cheap fix for now:

1. replace air filter with new one if not done within the last 15k miles or 12 months.

2. run a couple of tankfuls of gas with a bottle of Cheveron w/Techron fuel injector cleaner (read instructions on bottle) with each tankful.

3. run 91-93 octane gas.

4. if the fuel filter wasn't replaced by 60k miles, then think about this.

If maintanence was done by Lexus, all record should be available as to what was done when.

don't feel like a dog, this is how we learn. For all you know, someone else may have wanted to ask the same thing but didn't. This is what the forum is for, so ask anything you want.


Link to comment
Share on other sites

Ok, I'll have a look at doing that.

I'm sure the t-belt would have been done already, it's up to just over 200,000 K's I'll prob just get it done for the hell of it as I cant afford to make a serious mistake.

I actually use petrol with a 98 octane rating. Am I right in thinking this is just as good (if not better) as the 91 -93?

And alas, the owner before me owns a few service/petrol stations, so the on site mechanic took car of his car for him, which pretty means I've got bugger all service history after the first 80 k(ish) done by Lexus.

- Gav

Link to comment
Share on other sites

gav, where are you located (nothing listed in you screen id)?

just so we are clear, the "k" in my post meant thousand not kilometers.

I am curious if the US octane rating is the same in other countries, or is there a conversion factor here, too.

good luck with your car.


Link to comment
Share on other sites

I had a rough idle problem which also cause the car to stall on steep hills and so took it to the dealer to get it looked at. They diagonosed my air idle control valve to be failing. Which would have cost about $700 to fix with them. I declined the repairs that time, but after a few months I decided to get it fixed. But when they diagnosed it again to be sure it was the valve. They found that it was actually the hose that connects to the valve that was cracked. They replaced it for $195 parts and labor. Seems expensive for just a damn hose huh? Car idles fine after that. Anyway, they said the hose cracks because my engine mounts were worn and the engine movement tugging on the hose caused it to crack. I had the dogbone engine mount replaced elsewhere for cheaper.

Link to comment
Share on other sites

Ahh, thanks lex :) That makes a bit of sense, I know that my mounts are sagging, so I guess it's possibly that...

Hey steviej, I'm from Australia, so yeah, kilometers... And as far as the octane rating goes. it's only our "high end" premium unleaded that has that. I'm pretty sure most premiums are 96 (ish) and reg is 91.

I know the T-belt should be done between a soft 150 and a hard 90 k, but since I don't have history for the last few years, is there a way lexus can tell?

- Gav

Link to comment
Share on other sites

in the US (I speak for New England) I think it goes something like this:

87: regular unleaded

89: special unleaded

93: premium unleaded

94: ultra unleaded (Sunoco only)

I forgot to mention another possbility and lex jogged my memory. It ain't alzheimers, it just the beginning stages of CRS (can't remember sh#@)

Toyota has been plaqued with a simple problem. The plastic duct (accordian style) that connects the air box to the throttle body is prone to getting pin prick holes in the folds of the plastic.

The tiniest hole can screw up the ECM signal from the MAF sensor. It is tiny so that under accleration or heavy air flow, you don't feel anything. But, under idle, enought 'extra" air gets in after the MAF sensor to cause a slight roughness.

Take the hose off, put a small MAGLITE (high intensity flashlight) in it, cover both ends with something (I used two styrofoam plates). Sit in a totally dark room and see if you see any pin prick holes. They will light up.

I have seen this be the cause on more than several occasions.


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...