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#4754849 - 05/12/18 08:39 AM Rough idle - Running lean?
Spetz Offline


Registered: 01/23/14
Posts: 400
Loc: Australia
Hi everyone,

Is there a product in a spray can that you can inject into the intake of a car to check whether a rough idle is causing by a lean condition?

I am not sure if butane gas in a can would work?
_________________________
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 150,000 miles. Shell Helix Ultra 5W40 at 6k OCI

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#4754865 - 05/12/18 08:52 AM Re: Rough idle - Running lean? [Re: Spetz]
supton Offline


Registered: 11/09/08
Posts: 11705
Loc: NH
Always thought anything flammable would work. WD40 on up. Usually one fogs in an area to see if recovers--pointing to a vacuum leak. Don't see why butane wouldn't work.
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2011 Toyota Camry, base, 2.5L/6MT, 159k, hers
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#4754868 - 05/12/18 09:00 AM Re: Rough idle - Running lean? [Re: Spetz]
Snagglefoot Offline


Registered: 12/31/17
Posts: 1530
Loc: Alberta
Carb cleaner, but it will increase your revs and possibly mask the issue.

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#4754871 - 05/12/18 09:02 AM Re: Rough idle - Running lean? [Re: Spetz]
mk378 Offline


Registered: 09/27/15
Posts: 1390
Loc: USA
Check the fuel trims. Do a cylinder drop test to see if it is one cylinder misfiring or generalized.

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#4754922 - 05/12/18 10:04 AM Re: Rough idle - Running lean? [Re: Spetz]
Trav Offline


Registered: 11/20/06
Posts: 18536
Loc: MA, Mittelfranken.de
I use a propane torch with hose among other things when looking for a vacuum leak, just turn the gas on and move it around the suspected area the idle will smooth out when you find one (not it isn't OSHA approved so do it outside). It does not work well with fans that run all the time.

Like this one.
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#4754953 - 05/12/18 10:42 AM Re: Rough idle - Running lean? [Re: Spetz]
Dave1027 Offline


Registered: 05/02/18
Posts: 20
Loc: California
Google "smoke test". +1 to the fuel trims idea.

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#4755045 - 05/12/18 12:42 PM Re: Rough idle - Running lean? [Re: Dave1027]
Trav Offline


Registered: 11/20/06
Posts: 18536
Loc: MA, Mittelfranken.de
Smoke testing is all well and good if you have one but most DIY usually don't happen to have one handy. Checking trim while looking for the leak is helpful when looking for a leak.
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#4755536 - 05/13/18 03:31 AM Re: Rough idle - Running lean? [Re: Spetz]
Spetz Offline


Registered: 01/23/14
Posts: 400
Loc: Australia
Hi everyone,

Thanks for that.

I've had a smoke test done before and it showed no vacuum leaks.

I was thinking perhaps the AFM or something was faulty causing a rough idle.
The car idles worse with colder ambient temperatures.

The butane/propane test was purely to isolate or exclude any fueling issue.
At least in my mind that's what it'll show as if idle gets better I know it's a fueling issue and if it doesn't I know it's not - Would that be right?
_________________________
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 150,000 miles. Shell Helix Ultra 5W40 at 6k OCI

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#4755554 - 05/13/18 05:10 AM Re: Rough idle - Running lean? [Re: Spetz]
Trav Offline


Registered: 11/20/06
Posts: 18536
Loc: MA, Mittelfranken.de
Originally Posted By: Spetz


The butane/propane test was purely to isolate or exclude any fueling issue.
At least in my mind that's what it'll show as if idle gets better I know it's a fueling issue and if it doesn't I know it's not - Would that be right?


Correct, it is only to isolate the leak. If the idle gets better you have a vacuum leak in that area.
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ASE L1, Master. Deutsch Meisterbrief.

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#4755557 - 05/13/18 05:22 AM Re: Rough idle - Running lean? [Re: Spetz]
Spetz Offline


Registered: 01/23/14
Posts: 400
Loc: Australia
Trav,
I was actually planning on feeding the butane gas directly into the intake just before the throttle body to isolate or exclude a lean running condition that may be causing rough idle.

Would my theory be sound?
My tests so far have shown no vacuum leak however perhaps an injector is faulty or something causing a lean condition in one cylinder?

As the car does periodically idle smoothly I believe whatever is causing it must be able to be corrected
_________________________
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 150,000 miles. Shell Helix Ultra 5W40 at 6k OCI

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#4755562 - 05/13/18 05:31 AM Re: Rough idle - Running lean? [Re: Spetz]
Lubener Offline


Registered: 01/22/11
Posts: 2535
Loc: N.Ohio
I wouldn't do a thing till I checked the fuel trims first.
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#4755573 - 05/13/18 05:43 AM Re: Rough idle - Running lean? [Re: Lubener]
das_peikko Offline


Registered: 01/01/18
Posts: 458
Loc: San Antonio, TX
Originally Posted By: Lubener
I wouldn't do a thing till I checked the fuel trims first.


What are we looking for when we check fuel trims?

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#4755627 - 05/13/18 07:53 AM Re: Rough idle - Running lean? [Re: das_peikko]
Trav Offline


Registered: 11/20/06
Posts: 18536
Loc: MA, Mittelfranken.de
Most engines have one O2/AFM (aka wide band O2) per bank, some inline have 2 and some modified engines use one per cylinder which is much better but more expensive and complex.
Given this V6 has one per bank fuel trims will tell which bank or both are adding fuel (eg +10) for a lean condition or taking fuel away (eg-10)because its rich.

If one bank is hovering around 0 give or take a couple of points and the other is +10 for example then that bank is running lean.
The problem is that bank has multiple cylinders making it difficult to pinpoint the exact one if the misfire count is the same for all.

Enriching the whole system through the TB wont help pinpoint the issue, checking the fuel trims to find out what is happening is the only real way to get a good starting point.
Back in the day when engines has secondary air system ports for each cylinder and no OBD 2 we removed them one at a time and put a gas analyzer probe down it to identify the offending cyl.
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ASE L1, Master. Deutsch Meisterbrief.

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#4755750 - 05/13/18 10:11 AM Re: Rough idle - Running lean? [Re: Spetz]
Spetz Offline


Registered: 01/23/14
Posts: 400
Loc: Australia
The car only has a single O2 sensor.

I know the test I am proposing won't pinpoint the offending cylinder but all I am trying to achieve here is to establish whether or not a lean condition (on a single, multiple, all etc cylinders) is the cause of the rough idle.

If the car idles better once I inject butane into the intake then I know I should be looking at a lean idle condition.

How do I check fuel trims?
_________________________
2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 150,000 miles. Shell Helix Ultra 5W40 at 6k OCI

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#4755847 - 05/13/18 11:59 AM Re: Rough idle - Running lean? [Re: Spetz]
Trav Offline


Registered: 11/20/06
Posts: 18536
Loc: MA, Mittelfranken.de
With a scan tool. This engine has both banks going into a single pipe with one O2? GM V6 engines were like like.
Yes enriching the mixture to see if it smooths out will tell you if you have a lean condition but you need some numbers.
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ASE L1, Master. Deutsch Meisterbrief.

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