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Rough idle only at idle. Consistant, 2007 Eclypse 3.8

Customer Question
Rough idle only at...

Rough idle only at idle.

Mechanic's Assistant: Is the rough idle intermittent or consistent?

Consistant

Mechanic's Assistant: What is the model/year of your Mitsubishi?

2007 Eclypse 3.8 Spyder

Mechanic's Assistant: What year is your Spyder?

2007

Mechanic's Assistant: Are you fixing your Spyder yourself? What have you tried so far?

New timing belt. New plugs. New air filter. New MAP sensor. New O2 sensor , Bank 1 sensor 2. Cleaned with spray thottle intake.

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Mileage is 145000. a few mechanics are stumped. Other than the rough idle car runs great.

Submitted: 9 months ago.Category: Mitsubishi
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Answered in 1 hour by:
9/14/2017
Mitsubishi Mechanic: 350matt, Engineer replied 9 months ago
350matt
350matt, Engineer
Category: Mitsubishi
Satisfied Customers: 23,197
Experience: Mechanical Engineer BEng Hons22 years experience in car engine industry, 10 years in formula 1
Verified

Hello
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth cleaning out the throttle body as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

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Mitsubishi Mechanic: 350matt, Engineer replied 9 months ago

Hi
do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

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Customer reply replied 9 months ago
I have not had time to get my mechanic to perform the search.
Mitsubishi Mechanic: 350matt, Engineer replied 9 months ago

understood

just get back to me on this post

the post will remain open after rating so if my information is helpful please consider rating my answer

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Customer reply replied 8 months ago
Check vacuum lines with butane torch
No change in engine rpm. Can you tell me specific location of pc valve on 3.8 eclipse engine?
Customer reply replied 8 months ago
3.8 not 3.0 are they the same?
Customer reply replied 8 months ago
Are 3.0 and 3.8 the same?
Mitsubishi Mechanic: 350matt, Engineer replied 8 months ago

yes they are essentially the same engine just the 3.8 is a bored and stroked version

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Customer reply replied 8 months ago
Hey Matt, just installed new pcv. No change in idle.
Mitsubishi Mechanic: 350matt, Engineer replied 8 months ago

Hello

OK running out of options here.

one other thing to try is clamping shut the brake booster vacuum line

as I've known them to spring a leak internally and then provide a leak path / vacuum leak into the manifold

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Customer reply replied 8 months ago
Nothing involving evap purge valve? My gut feeling is just a stupid small vacuum leak that I'm not finding. I used butane to check around the small lines. Would you try starter fluid on a cold engine (fire hazard) and just be careful about where to spray?
Mitsubishi Mechanic: 350matt, Engineer replied 8 months ago

Hello

the flash point (temperature required to ignite) for most combustible fluids is around 750 Degrees F so you need a hot glowing exhaust or a spark to set them off

so yes using starter fluid or brake cleaner can be used to detect a leak

Ask Your Own Mitsubishi Question
Mitsubishi Mechanic: 350matt, Engineer replied 8 months ago

Hi
do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! Thank you

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