What internal engine issues cause poor idle?

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Spetz

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Hi everyone,

I have a poor idle issue with my Mitsubishi Diamante.
It feels choppy, but the rpm are steady.
Sometimes it idles smooth, sometimes really bad, but most of the time just bad.

I've had all the usual culprits checked/replaced.

Is it possible something internal in the engine is causing a poor idle?
And if so what are these possible causes and how would I test for them?
 
Low compression from worn rings or valves out of adjustment due to worn HLA's will cause that on a Mitsu.

Things to check first:

Actual Idle RPM should be between 650-700?
Dirty Throttle Body?
Dirty Injectors?
Compression on each cylinder?
 
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I have no way of confirming this now, but in college I had a friend with a Diamante and it was idling poorly occasionally. Not all the time but just once in a while. Over time it got more frequent. Eventually his tranny gave out (known issue on Diamante) and after replacing a tranny with a junkyard unit over the weekend the poor idle never came back again for couple years until he finished college. Don't know the history of the car after that point.
 
Originally Posted By: Vladiator
I have no way of confirming this now, but in college I had a friend with a Diamante and it was idling poorly occasionally. Not all the time but just once in a while. Over time it got more frequent. Eventually his tranny gave out (known issue on Diamante) and after replacing a tranny with a junkyard unit over the weekend the poor idle never came back again for couple years until he finished college. Don't know the history of the car after that point.


I've read reports of people using Lubegard Red in their transmission which helped solve poor idling issues. Worth a try in the OP's car. They tried a bunch of different products and were just using the Lubegard which unexpectedly solved the idling problems. Cleaning the t-body should be a cheap, potential solution.

Have the injectors removed and professionally cleaned by a specialist. Someone on here does them but they would have to be sent to the U.S. Hopefully you have someone closer to you.
 
What has been checked or performed.
Most mechanics are fair at BEST so they could have missed something.

Make sure PCV and all Vacuum lines are not leaking (including any to a vacuum modulator on the trans).

Crusty Throttle body and MAF and MAP are always suspect on old vehicles.


Read plugs and do a warm dry and wet compression check while the plugs are out.

What does that tell you


Next the possibilities are endless from failing cam/ crank sensors, to burnt valves to failing fuell pump with low pressure bad fuel filter to clogged injectors with poor spray patterns.

You have OBDII so whats that say?

good luck.
 
I had a buddy with one of those old Mitsubishi "MCA Jet" engines that idled like a coffee can full of rocks. It had some type of "third valve" that was supposed to boost performance, or some such nonsense. He never could get it fixed. He finally dumped it.
 
Originally Posted By: billt460
I had a buddy with one of those old Mitsubishi "MCA Jet" engines that idled like a coffee can full of rocks. It had some type of "third valve" that was supposed to boost performance, or some such nonsense. He never could get it fixed. He finally dumped it.
Interesting, I wonder if that was like Honda's CVCC system from the mid-70s?
 
Look for vacuum leaks . Faulty sensors . Faulty coils , wires ( if it has wires ) and spark plugs .
 
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Originally Posted By: billt460
I had a buddy with one of those old Mitsubishi "MCA Jet" engines that idled like a coffee can full of rocks. It had some type of "third valve" that was supposed to boost performance, or some such nonsense. He never could get it fixed. He finally dumped it.


Those were gone by the end of the 80s or the early 90s. Not because of anything inherent with the jet valve, but just the fact that the head castings would crack no matter how often Mitsu re-designed it. Mitsubishi apparently cannot build a cylinder head. Bottom ends are great, heads either crack, drop valve guides, or flush oil past the valve guides at a high rate. Remember all the blue-fogging Chrysler minivans of the 90s... almost all of the ones that smoked had a Mitsubishi 3.0.
 
It was bad valve seals most of the time in those Mitsu's we rebuilt a ton of heads because of that and it also affected the 2.6L 4 cylinder as well.
 
So many things could cause this. I’ll add dirty fuel injectors. Maybe run some Redline S1 or Techron fuel treatment and see what happens?
 
The 1.5(?) in my wife 85 Mitsu ( Dodge) Colt was fine and it had a pre chamber on AVERAGED 43 MPG.

Checked valves at 120K and they were all well within .001" of Nominal. Even the VC gasket was pliable and reusable.
 
It has the 6G74? I remember a while back someone complained about the same issue on their 6G72 (which is pretty much in close relation to the 6g74) and it ended up being a bad rear motor mount causing the bad idle. They only found out since they were in the process of replacing it anyways. In some cases I've heard of the PCV inlet on the cam rocker getting coked up over time.

Edit: When I replaced the spark plugs last year, I installed the throttle body gasket incorrectly which gave me a bad idle, flipped it around and it was back to normal.
 
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Originally Posted By: StevieC
Low compression from worn rings or valves out of adjustment due to worn HLA's will cause that on a Mitsu.

Things to check first:

Actual Idle RPM should be between 650-700?
Dirty Throttle Body?
Dirty Injectors?
Compression on each cylinder?



Those things are not going to give you an intermittent rough idle as the OP said.
 
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Torn intake manifold gaskets will do this. They either plug with dust and grime (sometimes) or only show when the car's really lean and under little accessory load.

Similar to degraded injectors that do all right under load or richer cold starts.
 
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