Gen 3 poor idle

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egoxrt
New Member
Posts: 2
Joined: 26 Aug 2018, 20:26
State: Victoria
Model: Mitsubishi Magna TJ
Engine: 3.5-Litre 6G74 V6 24V DOHC
Transmission: 4 Speed Straight-Shift Automatic

Gen 3 poor idle

Post by egoxrt » 26 Aug 2018, 20:33

Has anyone worked the cause of the poor idle attribute on the 3.5l gen 3's. Is it ecu related?

WytWun
Valued Contributor
Posts: 535
Joined: 09 Apr 2011, 23:22
State: Australian Capital Territory
Model: Mitsubishi Magna TJ AWD
Engine: 3.5-Litre 6G74 V6 24V SOHC

Re: Gen 3 poor idle

Post by WytWun » 28 Aug 2018, 00:29

egoxrt wrote:
26 Aug 2018, 20:33
Has anyone worked the cause of the poor idle attribute on the 3.5l gen 3's. Is it ecu related?
I guess it depends what you describe as a "poor idle"?

egoxrt
New Member
Posts: 2
Joined: 26 Aug 2018, 20:26
State: Victoria
Model: Mitsubishi Magna TJ
Engine: 3.5-Litre 6G74 V6 24V DOHC
Transmission: 4 Speed Straight-Shift Automatic

Re: Gen 3 poor idle

Post by egoxrt » 31 Aug 2018, 00:47

The inconsistent, rough idle that plagues most TH and TJ 3.5l's which i'm sure your familiar with. I'm sure it's either a electrical issue or ECU related, or perhaps a combination of both. I have never heard of any type of resolution after countless years of complaints.

Also, you seem to have a very good grasp of the ECU circuit on these vehicles, so you might be able to shed some light on a issue that has driven me insane for months now. I have this weird electrical noise emanating near the relay box in the engine compartment. It's a high pitched noise that makes it way through all high current components in the car, the alternator and my amps being the worst effected and happens both with the car running and key on, not accessories. I have tracked it down to the ECU circuit. When you pull the 20 amp fuse in the relay box it disappears.

I'm not saying it's the ECU itself, because the noise is not coming from the unit itself, but perhaps something it is powering.
I've pulled all the connectors in the engine bay one at a time with no luck. I might try and upload a vid on youtube.

Have you ever come across anything like this?

WytWun
Valued Contributor
Posts: 535
Joined: 09 Apr 2011, 23:22
State: Australian Capital Territory
Model: Mitsubishi Magna TJ AWD
Engine: 3.5-Litre 6G74 V6 24V SOHC

Re: Gen 3 poor idle

Post by WytWun » 01 Sep 2018, 01:26

egoxrt wrote:
31 Aug 2018, 00:47
The inconsistent, rough idle that plagues most TH and TJ 3.5l's which i'm sure your familiar with. I'm sure it's either a electrical issue or ECU related, or perhaps a combination of both. I have never heard of any type of resolution after countless years of complaints.
Well, I can't say that I have an inconsistent or rough idle in my car. I have been in a KJ Verada which had what I would describe as a random "miss". The owner of that car thought my car's idle was quite good, though neither of us would claim it to be perfectly smooth.

Idle is actually one of the most difficult parts of a petrol engine cycle, as the intake vacuum at it's most extreme so cylinder charge density is very poor, charge ignition can be difficult and flamefront propagation can be inconsistent - all contributing to a fluctuating power output from each power stroke at low revs where such variation actively fights against the flywheel. Once the throttle starts to open and intake vacuum decreases all these things usually behave more consistently. Plenty of opportunity for poor "idle quality" in other words...

There are a number of possible causes I've been given for what might be a variable idle:
- air leak between MAF and intake valves, more usually between throttle body and intake valves
- one cylinder is running much leaner than the others (causing random or persistent "miss" depending on severity)
- faulty oxygen sensor causing over (or rarely, under) fueling
- ignition issue (i.e. coil/rotor/cap/leads/plugs)
- sticky/faulty ISC stepper/valve
- manufacturing issue with one or more components in the rotating assembly that causes a static or dynamic imbalance (i.e. similar effect to having a poorly balanced tyre which is quite often more noticeable at some speeds than others)

The cylinder leanness issue could have a number of causes:
- pinhole leak in a gasket or casting in the upper or lower manifold runners or the flanges
- a casting fault in the manifolds or heads affecting air flow
- lazy/faulty or fouled injector
- a sticky intake or exhaust valve or poor valve seal
- damaged injector connector or wiring

Small intake leaks will have most effect at maximum intake vacuum - i.e. at idle.

The rotating assembly balance issue is problematic particularly for V6s as I've been told by a very experienced engine builder that getting the dynamic balance perfectly right under a wide variety of operating conditions in a V6 is a very non-trivial engineering/manufacturing problem and it doesn't take much manufacturing variation to resulting in a deteriorating dynamic balance. Static balance is easier to achieve, but of limited value if the dynamic balance is poor.

Air leaks can be temperature sensitive, especially with metal components.

Injector performance at idle can be problematic, specifically the way the injector responds to the initial "open" signal (often referred to as the "latency"), as this is usually the part of the injector's performance envelope where manufacturing variation can be significant and most after-market testing cursory at best (often just check flow rate). An injector or two that are much slower to open than the others will produce lean mixtures at idle in the respective cylinder(s) that the closed loop oxygen sensor feedback won't entirely compensate for.

Because of the way the ECU juggles the ISC and ignition timing to try and maintain a target idle speed, any problems with components involved with those functions show up in the idle. The only check you can do is to put the ECU into BSS mode which is used to properly set the idle speed screw on the throttle body. I believe there's a connector on the firewall that can be used to do this in the E/F/H models, but a MUT system or something like Evoscan is required for the J/L/W models :roll:. If the idle quality improves in BSS mode, there's an issue with something the ECU is trying to vary to control the idle speed otherwise it is something else that the ECU is fighting to try and get a controlled idle speed.

egoxrt wrote:
31 Aug 2018, 00:47
Also, you seem to have a very good grasp of the ECU circuit on these vehicles, so you might be able to shed some light on a issue that has driven me insane for months now. I have this weird electrical noise emanating near the relay box in the engine compartment. It's a high pitched noise that makes it way through all high current components in the car, the alternator and my amps being the worst effected and happens both with the car running and key on, not accessories. I have tracked it down to the ECU circuit. When you pull the 20 amp fuse in the relay box it disappears.

I'm not saying it's the ECU itself, because the noise is not coming from the unit itself, but perhaps something it is powering.
I've pulled all the connectors in the engine bay one at a time with no luck. I might try and upload a vid on youtube.

Have you ever come across anything like this?
I think I've seen one other posting about a similar issue some years ago. I can't remember whether I saw a resolution of it :(. If it is related to the ECU circuit then there are a number of relays directly driven by the ECU:
- "engine" relay (actually it controls power to the injectors)
- fuel pump relay
- in automatics, there's a relay to control power to the shift solenoids
- fan relays (main hi/lo, ac)

Some of these (esp fans) I think are in the engine bay relay box, but I seem to recall the engine relay is on the firewall inside the cabin? Not sure where the fuel pump or AT relays are.

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