Rough idle


#1

i have a 95 mitsubishi diamante es 3.0 sohc auto trans. it idels rough i have replaced plugs wires cap rotor button air filter and fuel filter. checked the timming belt ok checked the fuel pressure it is 40 psi the book calls for 38 psi. if i rev it up to 2000 rpm’s it smoothes out.what else could i check? thank you


#2

Check it for vacuum leaks, including around the intake manifold. My own preferred DIY method for this is to feed unlit propane around all of the vacuum lines/fittings & intake manifold. If you hit a leak the engine will respond.

You should also have a look at the EGR system on the car. I don’t know how it works on that car so I can’t give any specific guidance on that. But a poorly seated pintle on the valve will act like a vacuum leak. Its pretty easy for minor EGR issues to escape the computer’s notice in case you’re wondering about codes.

Speaking of codes - do you have any? (Check engine light?)


#3

Cigroller knows his stuff, I was going to say the same exact thing. Vacume leaks are the first suspect to idle issues.


#4

their are no codes on the car no check engine light


#5

did a cylinder cut out test their was no difference when i cut out numbers 2 and 4 cylinders.


#6

checked the codes again got code 21 coolent temp out of range has a new sensor wires at plug look good where else might i check?


#7

Describe your cylinder cut out test.

You have a code 21 for the coolant temp sensor. Did you actually test the sensor? This would make sense if the rough idle appears or is worse when the car is warmed up.


#8

Make sure all the injector wires are well attached to thier connectors. I had a similar episode with my 3.0 Eclipse. The was one injector where the wire had come out of the crimped socket. I ended up soldering it…fine thereafter.


#9

checked the sensor it is good. the car is hard to start and runs better after it has warmed up. the cylinder cut out test was done using a solus snap on scanner.


#10

The first step when it comes to engine performance problems is to run a compression test. A cylinder, or plural of that, that has lower than normal compression can cause a rough idle that will go away when the engine is revved but a vacuum leak, EGR stuck open, etc. can do the same thing.

If compression is down you can throw parts at it forever with no improvement so any possibility of mechanical faults needs to be eliminated at the start.
This engine should have about 180-190 PSI on all cylinders. A slight variation is acceptable; within reason of course.