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1990 2.4 rough ideling in gear. Constinat. Mighty max. Plugs…

Customer Question
1990 2.4 rough ideling...

1990 2.4 rough ideling in gear

Mechanic's Assistant: Is the rough idle intermittent or consistent?

Constinat

Mechanic's Assistant: What is the model of your '90 Mitsubishi?

Mighty max

Mechanic's Assistant: Are you fixing your Mighty Max yourself? What have you tried so far?

Plugs wires rotor cap

Mechanic's Assistant: Anything else you want the mechanic to know before I connect you?

Checked timing belt nd timing thats all good

Submitted: 7 months ago.Category: Mitsubishi
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Answered in 4 hours by:
9/27/2017
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago
350matt
350matt, Engineer
Category: Mitsubishi
Satisfied Customers: 23,076
Experience: Mechanical Engineer BEng Hons22 years experience in car engine industry, 10 years in formula 1
Verified

Hello
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth cleaning out the idle speed control motor / valve as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

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Customer reply replied 7 months ago
Have already checked most of that vehicle only idles rough when in gear park and neutral is fine
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Hello

when its in gear the engine will be under greater load so if you haven't checked fuel pressure then that would be wise to do so.

if that's OK then I'd say try clamping off the brake booster hose to see if this helps at all

if it does then possibly the brake booster is leaking and so allowing air into the engine as a vacuum leak

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Customer reply replied 7 months ago
Will check tonight and advise
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

understood

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Customer reply replied 7 months ago
No vacuum leaks found booster isn’t leaking and fuel pressure is good cleaned map and idle valve still idles very rough in gear and fine in park
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Ok

if there's a lack of power as well as the rough idle then it would also be worthwhile doing a compression test across all 4 cylinders

but if there's no power loss then check the condition of the air filter and it would also be worthwhile cleaning out the throttle body with a lint free rag and some brake cleaner

I'd also check that the main engine ground connection is tight and clean, do the same for the main battery connections and also there should be a smaller ground wire that runs out of the engine bay wiring loom and is secured to the engine with a ring terminal

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Customer reply replied 7 months ago
No loss of power at all I’m lost
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

hello

can you check the ground connections and the air filter as suggested?

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Customer reply replied 7 months ago
Runs rough with air filter removed and ground wire running for cab to block is disconnected I will reconnect and try
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Ok

just get back to me on this post

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Customer reply replied 7 months ago
Connected ground wire didn’t help the situation
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Ok

does the truck drive ok? plenty of power and pulls as it should?

if it does then the fuel pressure is OK , if not then the fuel pressure needs to be checked

if this is OK then

This could be a wiring issue as things tend to expand when hot and its possible a poor or partial electrical connection is being pushed apart on one of the major sensors so check the connection plug and wiring to the following;
Airflow meter
Crank position sensor
Cam position sensor
O2 sensor
Throttle position sensor

unplug each in turn and use some switch / contact cleaner spray on the pins to ensure its all grease / dirt free

Also worth checking the connections to the fuel pump for the same reason - unplug each in turn and give them a good squirt of switch / contact cleaner - Not WD40

and also unplug and inspect the main power relays and fuses for signs of overheating or corroded terminals - so the ones for the ECU and the fuel pump

And also unplug the wiring loom connections in the engine bay one at time and inspect for corroded or recessed pins.

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Customer reply replied 7 months ago
Truck runs great doesn’t even miss when idling in park just shakes so bad when you put it in gear
Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Hello

OK this could be worn mountings - check the tightness of the engine and gearbox mounts and their condition - this is best done by using a jack to take the weight of the engine off the mounts and seeing if any cracks or splits open up in the rubber.Also check the tightness of the mounts bolts

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Mitsubishi Mechanic: 350matt, Engineer replied 7 months ago

Hi
do you still need help?
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350matt
350matt
350matt, Engineer
Category: Mitsubishi
Satisfied Customers: 23,076
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Experience: Mechanical Engineer BEng Hons22 years experience in car engine industry, 10 years in formula 1

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