Replaced spark plugs. Engine jumps between 500 and 1000 rpm when i let go of gas?

[JEDI]

2005 mazda3i, 100k miles

OEM spark plugs: NGk itr5f-13
New spark plugs: NGK IX
(both itrium plugs)

test drive since installing the NGK IX:
there's a small hill near where i live (25mph speed limit). i usually just let go of the gas and let gravity do the work. i never noticed the tachometer fluctuating w/OEM plugs.

with the new NGK IX, the tach goes between 500 and 1000 rpm constantly. I'm at like 20mph at the top of the hill when i let go of the gas.

Why is the tach fluctuating? And what do i need to do to the new spark plugs to fix it?

 

[bruceb]

It is probably coincidental that the problem occurred after the plugs change.
I would check and clean the inside of the throttle body. Carefully remove carbon and varnish buildup. Also, check for vacuum leaks and maybe a bad Throttle Position Sensor.

 

[dawp]

Might also make sure you didn't knock something loose.

I did the plugs, wires and valve cover gaskets on my daughter's car not too long ago and it was doing the same thing. turned out I had missed reconnecting an electrical plug to some sensor in the bay. once that was hooked up, every thing was fine.

Is the check engine light lit?

 

[JEDI]

Might also make sure you didn't knock something loose.

I did the plugs, wires and valve cover gaskets on my daughter's car not too long ago and it was doing the same thing. turned out I had missed reconnecting an electrical plug to some sensor in the bay. once that was hooked up, every thing was fine.

Is the check engine light lit?

no engine lights, but I've only driven the car 20miles since putting in the new plugs.

power and acceleration seem the same. rpm have hit 6k a couple of times w/o a problem.

One thing i did notice was that at 50mph i let go of the gas, and rpm stayed/dropped to only ~2k.
Is that normal?
Why didnt it drop to ~750rpm since no gas is going thru the engine?

 

[dawp]

did you disconnect the battery when you did the plugs? maybe the ecu reseting itself if you did.

 

[JEDI]

did you disconnect the battery when you did the plugs? maybe the ecu reseting itself if you did.

i disconnected the negative terminal.
how long does it take for the ecu to relearn everything?

interesting note:
after i hooked back the negative terminal, my clock stayed at the time when i disconnected the negative terminal and my saved radio stations didnt get erased.
Why not?

 

[alphatarget1]

sounds like a vacuum leak...

 

[Gibson486]

Throttle body....

did you ever get yours cleaned? They become VERY dirty after 50K.

 

[CraigRT]

sounds like a vacuum leak...

It does.

The battery does not have to be disconnected to change spark plugs, as there is nothing to re-learn or learn in the first place.

Snoop around under the hood and see if there is anything obvious, vacuum hoses broken through or disconnected..

 

[JEDI]

It does.

The battery does not have to be disconnected to change spark plugs, as there is nothing to re-learn or learn in the first place.

Snoop around under the hood and see if there is anything obvious, vacuum hoses broken through or disconnected..

will take a look under the hood, but i dont think i touched any wires except those connected to the spark plugs.

and if a vacuum hose is broken, why is it only affecting 500-1000 rpm range?

 

[AnnonUSA]

Check for a loose plug.... Are you sure they are all tight?

if there is a compression leak, this will happen as the engine computer tries to compensate.

 

[CraigRT]

Check for a loose plug.... Are you sure they are all tight?

if there is a compression leak, this will happen as the engine computer tries to compensate.

This is also a good suggestion, although I would imagine if it was loose enough to cause problems, you would also notice power loss.

Nonetheless, a good thing to double check all plugs.

The problem is not specific to the 500-1K RPM range, its because when you are off the gas your car is producing vacuum.... (at idle or any time off throttle)

 

[JEDI]

Check for a loose plug.... Are you sure they are all tight?

if there is a compression leak, this will happen as the engine computer tries to compensate.

1) hm.. when i removed spark plug #3, there was oil on it from the hex nut down to the threads. all other spark plugs were clean. compression problem in cylinder #3?

2) also, i hand tight all the new spark plugs, then used a wrench for 1/8th turn more to approximate the recommended torque.

For plugs #3+4, i could have easily tightened the spark plug more after the 1/8th turn.
For plugs #1+2, i could only tighten it maybe 1/32 of a turn past hand tight.

so could plugs #3+4 not be tight and cause this compression leak?

 

[AnnonUSA]

I recently Had a plug work loose and Blow out of the cylinder head, taking the threads with it and launching the plug and coil into parts unknown....

While the engine did run, at idle with a plug missing, it did exactly what you are describing here.

 

[slag]

I recently Had a plug work loose and Blow out of the cylinder head, taking the threads with it and launching the plug and coil into parts unknown....

While the engine did run, at idle with a plug missing, it did exactly what you are describing here.

Was it in a 97+ F150?

 

[CraigRT]

Was it in a 97+ F150?

Wasn't that problem only in the 4.6? (just wondering)

 

[CraigRT]

1) hm.. when i removed spark plug #3, there was oil on it from the hex nut down to the threads. all other spark plugs were clean. compression problem in cylinder #3?

2) also, i hand tight all the new spark plugs, then used a wrench for 1/8th turn more to approximate the recommended torque.

For plugs #3+4, i could have easily tightened the spark plug more after the 1/8th turn.
For plugs #1+2, i could only tighten it maybe 1/32 of a turn past hand tight.

so could plugs #3+4 not be tight and cause this compression leak?

I think it's possible you may have a small compression leak if the plugs are not tight enough, but your jumpy idle problem still sounds like a vacuum leak or something along those lines.

Have you checked the Mazda forums?

 

[JEDI]

I think it's possible you may have a small compression leak if the plugs are not tight enough, but your jumpy idle problem still sounds like a vacuum leak or something along those lines.

Have you checked the Mazda forums?

my idle is fine, both while stopped at red light and in park. (rpm=750)

it's when i let go of the gas while traveling under 20mph that the rpm fluctuate rapidly between 500 and 1000 rpm.

will re-tighten spark plug #3+4 after the engine cools down a bit

 

[soydios]

i disconnected the negative terminal.
how long does it take for the ecu to relearn everything?

interesting note:
after i hooked back the negative terminal, my clock stayed at the time when i disconnected the negative terminal and my saved radio stations didnt get erased.
Why not?

clock/radio had a built-in battery to save them.

 

[LTC8K6]

clock/radio had a built-in battery to save them.

More likely nvram.