94 B3000 rough idle

[94MazdaB3000]

Hey guys, I have a 94 Mazda B3000, lately it has begun to idle extremely rough. It doesn't matter if it's just parked in the driveway or stopped at a red light, the longer it sits, the worse it gets until it is seriously shaking you inside the cab. It doesn't do this once the truck starts moving. I am far from an expert so I don't know where to start. Any thoughts or opinions would be greatly appreciated.

 

[RonD]

1994 3.0l still used a distributor and TFI ignition system.

When did you do the last tune up?

Distributor cap, rotor and spark plugs, and spark plug wires if funds permit.

These parts wear out and you notice it first at idle.
If you change the spark plugs keep track of where each old one came from, spark plug tip can tell you alot about how the engine has been running.
Very Light brown is good, dark brown or blackish is rich burn, maybe some oil, black is way rich and could be oil.


Also worth while putting a can of Seafoam(or other injector cleaner) into the gas tank once a year.
Dirty tips on the injectors can cause rough idle, because they drip fuel vs spraying it.

O2 sensors are good for 100k miles, after that the engine will tend to run rich(see spark plugs) at that time it is worth while changing the O2 sensors, they will pay for themselves in fuel savings over the next 100k


Once engine is warmed up and idling unplug the IAC(idle air control) valve, idle should drop down to 500rpm or engine may even stall, either is good, it means no vacuum leak.
If idle doesn't drop down then you have a vacuum leak that is causing to rough idle.

 

[94MazdaB3000]

Ok, I bought the truck off of my dad and I am pretty sure he replaced the spark plugs within the last few years so I may start with the o2 sensors and work from there. Thank you so much for all the advice and tips, I got plenty to go on from it!

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[RonD]

Pull out a few spark plugs, if they are darker brown then yes, replace O2, if they are light brown then new O2 won't help, won't hurt, but won't help for current issue.

Computer controlled systems rely on sensors and controls to run the engine correctly, computers and sensors rarely fail, DIYers are quick to think any problems are sensor related, it can be, but it just isn't very likely, they are way down on the list.
And most can be tested with Volt/OHM meter($25) vs replacing($100+).

Computer "thinks" it is running engine correctly, thats all it can do, if you pull out the spark plugs you can "see" if what it "thinks" is correct.

Besides the volt meter, a vacuum gauge($25) is good tool to ID a problem.
Vacuum gauge tests here: http://www.gregsengine.com/using-a-vacuum-gauge.html

 

[monstermazda]

Look at egr valve hanging open when egr valve is one will cause you motor to run rough at idle. Then when you accelerate goes away if it's barely open. If not I can't remember if that year model had a DPFE (differential pressure feedback sensor) in some cases it could be going the wrong signal to the pcm.

If egr is 100% open will cause it to run extremely rough and die all the time.

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