Idle in Issue at Traffic Lights

[fazilsingh]

Hi folks,

I've been pulling out my hair trying to solve an issue with no avail and decided to ask for some help as i'm currently stuck.
VEHICLE: 2010 CY3A JDM Lancer, Galant Fortis, with a 1.8 [4B10] engine with CVT transmission with all stock parts and tune.

Basically the car idles fine all the time without exception [hot/cold/1st start/restart] when the vehicle is in Park or Neutral, observing diagnostic output via DLC i see target idle is 703rpm and vehicle idles at same, which is all good, no vibrations anything.
The problem occurs solely when the vehicle is in Park or Reverse and the brakes are applied, for example when at a traffic light or backing out of driveway. When this occurs i see the target idle and actual rpm of the engine is 500rpm which is way too low and thus vibrates alot and feels like the vehicle will stall but it doesn't. It actually has stalled once or twice when driving quickly and had to come to complete stop very fast, upon the vehicle stopping the rpm will drop too quickly causing the engine to stall and die, necessitating a manual restart.
I have scanned the vehicle with multiple diagnostic scanners and no DTCs are shown on any system even after stall. In my testing the only way the condition doesn't occur is if i unplug the IAT sensor at which point the idle stabilizes at 700rpm when in drive/reverse with brake applied, but i don't think this is a solution and most likely works properly due to a fail safe in ECU.

So far i have done the following to no avail:
- change air filter
- change air intake temp sensor
- change plugs
- service injectors
- bleed brake lines
- service throttle body
- service intake manifold

Unfortunately here in Trinidad [in the Caribbean] our local Mitsubishi dealership refuses to service vehicles from Japan and i'm at my wits end with this problem, can anyone point me in the right direction or have experience with a similar issue? Are there any additional diagnostics or manual tests i can conduct to hopefully pinpoint the issue?

Please help.

[bumblebee]

Is the inside of the Throttle body absolutely spotless?
If the butterfly sticks it can cause rpm drop similar to what you described.
It wouldn't be a bad idea to get a tin of something like "start ya bastard" and with the car at idle and warm up have a quick squirt where the inlet manifold meets the head.
If rpm goes up you have a gasket issue.

[lancerdarryl]

Agee with bumblebee it may be a vacuum leak. You said it occurs when you apply the brakes,disconnect the brake booster hose from the intake manifold and plug the fitting in the manifold. Now try the car and see if it still has the same fault. If the fault doesn’t occur you will need to check the booster doesn’t have a hole on the diaphragm. Try not to drive the car while the booster is disconnected as the brakes will be very hard to apply

[bumblebee]

Awesome call from Lancerdarryl

A few things to try

Let us know how it goes

[fazilsingh]

lancerdarryl will try that and get back to you. So if the booster is the issue, a replacement should resolve it?
an bumblebee, yea i cleaned it twice, its absolutely spotless. Will try to spray around the manifold and see if the rpm goes up as well.

Is there any chance that engine is damaged? Hence the reason for all this? Other than this issue the car runs fine.

Anyway thx for the help will test and get back to you.

[fazilsingh]

Agee with bumblebee it may be a vacuum leak. You said it occurs when you apply the brakes,disconnect the brake booster hose from the intake manifold and plug the fitting in the manifold. Now try the car and see if it still has the same fault. If the fault doesn’t occur you will need to check the booster doesn’t have a hole on the diaphragm. Try not to drive the car while the booster is disconnected as the brakes will be very hard to apply

hey this seems to be the problem rite there, tell me if i remember correctly this problem started not too long after installing aftermarket rotors and pads on the front wheel.
Could those rotors and pads be causing an issue like this? Booster is pretty expensive so i want to make sure first.

[lancerdarryl]

Pads and rotors are not your problem. You may need to seek a second opinion from someone who can see the problem

[lancer2014]

check the EGR valve that has no carbon deposit this causes problems with the rpm

[bumblebee]

Good call
Use some throttle body cleaner on it after you remove it