Engine vibrates in idle (I know, another idle tread, I've tried most things though)

EddyMerckx

Member
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2003 Mazda Protoge5
At engine gradually started vibrating pretty badly, mainly when in Drive and braked, but slightly in Park too. Read the threads and got it checked out, the front and side mounts were bad. Replaced, $$.

Vibrated a lot less but still had bad rpms. Got is checked out, bad idle control motor, replaced, $$.

Also had bad coils last fall with misfires, replaced those and plugs then.

However, right now, after warming up, the engine has a constant 400 RPM, but the engine is vibrating a fair bit, causing the hood to vibrate too. Probably what wore out the mounts in the first place. Also have replaced a few other random things, and ran seafoam in the oil and gas for about 40 miles.

TLDR
Engine vibrates when idling in Drive.

New:
plugs, coils, idle control motor, EGR, 4x engine mounts, PCV valve, air filter.

Possible causes:

Vacuum leak? I actually can hear a slight hiss from the cabin but the mechanic said if I could hear it it was almost certainly heat/AC related not a vacuum leak. Checked the big hose from the air filter and there are no visible cracks.

Timing Belt? I bought this used last year, it has 135k on it, and I dont know if the timing belt was replaced at 100k. But got two quotes for $1k to replace it, and since I've already spent $1.5k getting it fixed so far, not really ready for that just yet.


Idle adjust? I don't know, maybe I should adjust the idle? Should I reset the idle?
Any tips? Any are appreciated!!
 
If your idle is constant at 400RPM it seems pretty low to me. Mine idles around 700-800RPM

If idle is that low i would check the simple things first. Make sure the throttle body is clean, when they get goobered up with gunk it can cause a low idle and a vibration. Possibly vacuum leak, though you won't hear it inside the cabin. If you can hear a vacuum leak inside the cabin then you've got major issues! Restricted exhaust (clogged cat), timing issues (stretched timing belt) or other causes could be at fault here as well... though many times would also cause a check engine light.

I would start simple. Clean the throttle body so it's nice and shiny and clean. Make sure the ports to the IAC are clean as well. Then disconnect the battery for 5 minutes, let the computer reset itself. Many times (at least in newer cars) this is all it takes to clean up idle issues. If this doesn't work then it may be an endless path of checks until you find it, and may be cost prohibitive unless you REALLY love your car.

As for the timing belt, if you have any mechanical aptitude and can follow a repair manual, it's not very difficult to replace. For most people you could do it in a day, or a weekend worst case. For a mechanic with some general know-how it shouldn't take more than a couple hours. You can replace the valve cover gasket, timing belt and water pump for around a hundred bucks or less. And trust me, it's better to replace it now if it's never been replaced than to be stranded somewhere because it decides to break... this generally happens at the most inconvenient time/place, usually when you're 100 miles away from home.
 
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