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Discussion Starter · #1 · (Edited)
From the beginning.
Car sounds and feels fine, but fails emissions, take it to the mechanic, he does stuff and gives it back. Immediately I notice a Rough Idle and take it back, he says it's normal. I accept and continue. Next day I get a P0400 trouble code, call mechanic, he says "Disconnect battery" and it'll reset. It always comes back on with the same code. I ignored it for about a year and a half.

Recently I get tired of the rough idle and presumably bad gas mileage so I get a manual and start the Troubleshooting.

EGR System
Step 1 - Check vacuum lines (Inspect for cracks, clogs, or disconnections. All vacuum lines checked out to be in perfect condition)

Step 2 - Check PCV Valve (Inspect for negative pressure and operation of the valve itself)

PCV Valve checks out to be in perfect working condition, including negative pressure.

Step 3 - Evaporative Emissions Purge Solenoid - (Remove solenoid and connect directly to the battery, with voltage, solenoid will not hold a vacuum. without voltage, solenoid holds vacuum) Resistance should be 30 - 34 ohms
Solenoid works properly and shows 33 ohms

Step 4 - EGR System Check - Connect vacuum pump to EGR Valve with a T-Fitting and run the engine (Revving while cold = no vacuum.
revving while hot = a rise in vacuum
I did not get a rise in vacuum while hot, instructed to to perform the vacuum control valve check and EGR port vacuum check.

Vacuum Control Valve Check - Connect vacuum pump directly to the Vacuum Control Valve and idle the engine. Correct results would read 6.3 - 7.1 Hg
My results were about 4 - 5 Hg. Replaced part, got the same results

EGR Port Vacuum Check - Connect vacuum pump directly to throttle body and idle engine. Correct results will be about 15 Hg.
My results were exactly 15 Hg.

EGR Valve Check - Remove EGR Valve, connect vacuum directly to the valve and apply 20 Hg of vacuum to open the valve, air should flow through the ports. Drop to 0 Hg and the valve should close, blocking the flow of air through the ports.
My results show the EGR valve works properly

Step 5 - EGR Valve - Connect the vacuum pump directly to the EGR Valve. Idle engine, apply vacuum of 8.7 Hg (Opens EGR) and engine should run rough. With no vacuum applied, engine should run smooth.
My results showed it was working properly. Open = Rough Idle/Closed = Smooth Idle

Step 6 - EGR Solenoid - Remove solenoid and connect directly to battery. with voltage, solenoid should hold vacuum. without voltage, solenoid should not hold vacuum. Resistance should read 29 - 35 ohms
My results - Solenoid is working properly and at 33 ohms

What I understand:

When revved, the EGR System is supposed to drop vacuum when the engine is cold, but keep vacuum when the engine is hot. (Step 4)
Mine drops vacuum no matter what when revved.

The vacuum control valve is supposed to show a vacuum of 6.3 - 7.1 Hg at all times. (Between Step 4 and 5)
My results show 4 - 5 Hg with my old part and new part.

The only thing that I can think of is something in the system is keeping my EGR Valve open during idle. The only thing that makes me think this is because when I test the system with the vacuum pump connected directly to the EGR valve, I can apply a vacuum to open the valve, which makes the engine idle roughly. When I release the vacuum to close the valve, the engine runs smoothly. But all the parts check out to be functioning properly when tested away from the system except for the Vacuum Control Valve, but I've even replaced it and receive the same results as the old one.

Why is it that my car has a rough idle and produces a P0400 trouble code?
Why does that Vacuum Control Valve show that it's not producing the proper amount of vacuum even after replacement?
Why does my EGR Valve stay open during idle?

I put together a little video and thought at first my rough idle was a separate problem from the P0400 trouble code because my spark plugs were a little gooped up with oil, but I think it was from the Valve Cover leaking down into the spark plug hole.

Info on the car:
2000 Mitsubishi Eclipse GS, Standard, 180k Miles

In 2006 the timing belt slipped and bent all the intake valves.
Rebuilt the head assembly myself, had the car towed to a shop to have the valves lapped and the head shaved to prep it for a new head gasket.

But this rough idle all started from the mechanic that I took it to for passing emissions.

Thanks in advance!

3 Posts
I am having a very similar issue.. I am getting a P0400 code but after doing the diagnostic testing, after testing my vacuum control valve, I get 0 vacuum.. I am hoping that after I buy a new one, it will clear the code. Any updates? Did you fix it? Any info would help thanks.
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