Question about 1998 Mitsubishi Galant

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Running rich fouls plugs and starts missing seems to run better on higher octane fuel been told could be o2 censor how do i test this or bypass it

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Well you can't bypass the sensor. If the sensor was faulty the check engine light would be on.
If you have replaced the plugs you may have to get a mechanic to check out the computer and other sensors.

Posted on Dec 11, 2012

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Posted on Jan 02, 2017

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SOURCE: running rich and missfiring

could be the fuel injector (spark plug connect too) or could be the wires

Posted on Apr 18, 2009

Mustgo
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SOURCE: On a cold start my 2002 estate 2.4 gdi missfires.

Since you vehicle is 7 years old. The temperature is due for a change.

the temp sensor give the exact engine block temperature information to the computer (ECU).

An old sensor or sensor giving incorrect temp to the ECU will cause this problem.

The correct term is call close loop. When the engine at start up the computer will only count on the temp sensor to report temp.

When temp is reaching 195F,the mode change to open loop. All sensors comes into play and at the point the fuel inject will scale down for the warm up state.

Running rich condition can be cause by an old temp. sensor.

Change the sensor and the RICH condition after warm up will go away.



Posted on Aug 18, 2009

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1 Answer

Is Premium and higher octane better than regular gas in Mazda 3, 2012 car?


Octane ratings have nothing to do with fuel economy or power. The octane rating only indicates how much the fuel air mixture can be compressed before it ignites without the spark plug firing in a hot running engine. Some really high compression engines require higher octane fuels so they don't knock when running. If you don't have a sticker under the filler lid that says you have to use one of higher octane fuels, then don't use high-test.
It used to be that using a low octane fuel in an engine designed to use high octane fuels could damage the engine.
Nowadays, engines are all computer controlled and most high octane engines will re-tune themselves on the fly so that they don't knock if the wrong grade of fuel is used.

Feb 20, 2015 | Mazda MAZDA3 Cars & Trucks

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Octane Myth


What is octane? Octane cannot be seen, but it is of utmost importance when it comes to gasoline. One thing for sure, higher octane fuel costs more. Allot more! Simply put, octane is a measure of gasoline's ability to resist detonation, which you hear as pinging and knocking in your engine. The higher the octane the more the fuel can be compressed without detonating before you want it to. Detonation, ping, knock, whatever you want to call it, occurs when the air/fuel mixture ignites before the spark plug fires. The mixture ignites from compression and not from the flame of the spark plug. In other words, the higher the octane the less likely it is to ignite prematurely. The only benefit to a high octane fuel is that it allows an engine to run at a higher temperature and with a higher compression ratio without pinging. Higher octane fuel does not provide more energy, more power, better mileage, more torque, burn cleaner, clean your engine, and is not better for the environment. If the engine is pinging when using the correct octane fuel, then it may be necessary to move to the next higher octane to prevent pinging, and damage to your engine, unless there is another problem. If you are using higher octane fuel for any of these reasons, STOP, you are throwing your money away. Also, never use a lower octane fuel than is recommended by the manufacture. If the manufacture recommends 89 octane then use 89. If they recommend 87 then use 87. The key is what was the engine designed to run at to achieve optimum performance and mileage? One exception is when you are towing a heavy load with a vehicle designed to run on 87 and you experience pinging. In that case it may become necessary to switch to 89 while towing. In conclusion, race car engines are designed to run on high octane fuels due to their high compression engines. You cannot make your engine a race engine just by upping the octane. Save your hard earned money, and use exactly the octane you need.

on Jun 19, 2010 | Honda Accord Cars & Trucks

1 Answer

Miss fires at ideal smells like to much fuel out of exhaust. Runs better at higher rpm. 2000 Pontiac Grand am 2.4L


If it improves with rpm, I would suspect a stuck injector (not closing) causing your engine to run rich, or a bad temperature sending unit. You should check for fault codes (they will do this for free at Autozone). You will most likely have a code for a bad oxygen sensor. Don't replace it until you correct the rich fuel mixture with the bad one unplugged. The rich mixture is the cause of the bad O2 sensor, not the symptom.
I you unplug the O2 sensor the ecm will substitute a generic value, allowing you to correct the real issue without destroying another O2 sensor. They are not cheap.
You can verify a stuck injector cyl by cyl by inspecting the plugs. If they all look the same, you may want to unplug the cold start injector and see how it runs.

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1 Answer

After i start my truck black smoke comes out of the tail pipe


sounds like you are running rich try and change the plugs to better quality to burn excess fuel off and higher octane gas

Sep 11, 2010 | 1994 Chevrolet C1500

1 Answer

93 jeep grand cherokee. Starts and idles ok.


Being injected, the control system varies fuel quantity and does not directly regulate air flow. Therefore, having black soot on plugs tells you that there is too much fuel going into the engine. The O2 sensor tends to react to, more than control that. Missing can be the result of plug fouling. Backfire may be the result of a too rich fuel mix permitting unburned fuel to enter the hot exhaust where it re-ignites.
Where I would look first in your case is to the manifold air temp sending unit and the coolant temp sender as well. If either or both are out of range the computer may "think" it is running a cold engine and therefore adding additional fuel for cold start up,and not switching to normal run mode when the engine reaches operating temp. Once the plugs foul the engine will continue to run poorly regardless of other conditions. You can test each component individually using either chilton or haynes manual as a guide. You will need a decent digital multi-meter to do the tests.
You can also have a code test done at most major parts stores, but often if sensors are not bad enough to set a code but together are causing a problem, you may not see anything except the O2 sensor.
There are other possible causes such as worn out injectors, but most often that won't happen in all cylinders at the same time. Do testing first, correct whatever you find and then make sure plugs are clean. If after running, the O2 sensor continues to register a fault, then replace it.
Good luck...any further questions are welcome and don't forget to rate answers!!!

Feb 22, 2010 | 1993 Jeep Grand Cherokee

1 Answer

Hello. my 2000 4runner v6 has been running rough. when i start it, it runs fine untill it warms up. then it begins to miss. have replaced spark plugs, and battery but didn`t make a difference. diagnostic...


Change your oxygen sensors - they're responsible for feedback that the computer uses to determine the air/fuel mixture. They usually are good for about 60k miles and then start to go downhill. When they get to a point that the computer won't trust them, it ignores them and runs on preprogrammed fuel maps that run excessively rich (too much fuel) to safeguard the engine. You've probably seen a decrease in fuel mileage as well. And the O2 sensors have to be up around 900 degrees before they'll work - the car ignores them until then. That's why you run fine after a cold start but run rough once the car is warmed up.

The O2 sensors you want to change are in the exhaust system between the engine and catalytic converters (the "pre-cat" sensors). There are O2 sensors after the cats as well, but those are used comparatively to determine how well the cats are functioning. The primaries are the ones used for fuel mixture. If it's been this way for a few weeks or so, I'd put new spark plugs in as well - rich fuel conditions can foul them and contribute to the rough running.

Dec 09, 2009 | 2000 Toyota 4Runner

1 Answer

Starts dicent when cold but when it warms up it starts running so b ad it wont accelerate and will finally die


Do you see black smoke coming out the back at all, once it warms up? To me, it sounds like your oxygen sensors are going bad. If you have a 1996 or newer, change the oxygen sensors between the engine and the catalytic converters. If it's a 95 or older, you'll only have one set. Change them out.

The O2 sensors determine what the fuel mixture for the engine will be. They are ignored when the engine is cold, as they have to be up around 900 degrees to work. Once it warms up, feedback from the O2 sensors is used to determine how much fuel is needed. When they fail, the car defaults to a rich-running condition, with excess fuel being dumped in. This is because the computer doesn't trust the sensors, so it adds extra fuel. Running rich fouls spark plugs over time, and causes hesitation and bogging and bad gas mileage, but it's better than running lean (too little fuel) - that blows engines. So the car is programmed to err on the side of caution and richen the mixture. Change the O2 sensors and the spark plugs, and you should be good to go.

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I use 87 octane with Lucas Oil fuel additive. Car runs noticeably better, gets better mileage and solved problem with engine fouling in cold weather (which caused check engine light to trip). Audi dealer service rep told me that premium cold weather fouling has been traced to additives commonly used in premium fuel. That service rep "unofficially" recommends using 87 octane to customers experiencing problems with engine fouling in cold weather starts.

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1 Answer

Fuel /emmision


If it ran extremely rich for months and now it won't start, the spark plugs are fouled out. Replacing them will fix your problem -- temporarily.

You'll probably want to check the O2 sensor(s) and replace them as they are probably the cause of the car running extremely rich.

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