Rough Idling?

[PepeEAS]

Well today i bought my first Mazda lol off of a buddy of mines and he told me that it idles rough and one of the pistons isnt working. We pulled the plugs its getting spark but #3 piston is bad, also, he said he had it checked out and said that the head gasket, head or bent valves could be the problem. Any ideas?

 

[jdwk]

How do you know #3 is bad? One piston can't just stop working unless you break a rod in which case it usually takes out just about everything else with it, and not simply be a rough idle.

The springs keep the valves closed, and they can break, but is rare on these cars. The cam pushes them open, and they can break, but nothing will work if that happens.

Did you pull the plug, put it in the wire, and just look at it while you turned it over with the starter?

To check the other items, just pull the valve cover, raise the front of the car, remove the front passenger wheel, and turn the crank with a 21mm socket while watching the valve train for anything off like a valve not closing. Pull the spark plugs and it will make turning it much easier, and you can feel the air being pushed out the spark plug hole. You could also pay for a compression check or buy a tester from harbor freight (cheaper option).

The simplest solution is usually the right one.

 

[PepeEAS]

do you know the compression #'s for each cylinder? i bought my compression tester today

 

[hornsfan10609]

I believe you want at least 175 compression, 180-200 is a good, healthy motor.

 

[dandres54]

This fixed my rough idle. http://www.mazdas247.com/forum/showthread.php?94788-How-To-EGR-Removal-and-Cleaning-Rough-Idle-Fix

 

[PepeEAS]

well today i changed the spark plugs and coil pack, the idle went down dramatically but now its missfiring at cylinder #1 and there is a slightly rough idle? any ideas?

 

[TurboGixxer928]

If he said he had it checked and it was the head, gasket, or valves, then they probably did a compression check and it failed, and changing spark plugs and coils isnt going to do any good. I'd get a compression check on it asap, I think you can even get them from the loan a tool from places like autozone. If there's a problem, one cylinder will be way lower than the rest, and if it's bad, you'll have to pull the head and start troubleshooting from there. Do you know if it's losing water or if there's any water in the oil? if so, you're looking at a head gasket

 

[PepeEAS]

we did a compression test on it yesterday it has good compression on each cylinder and with a change of coil and spark plugs the idling changed dramatically however it still idles a little rough and doesnt shake as much.

 

[sde780]

How do you know #3 is bad? One piston can't just stop working unless you break a rod in which case it usually takes out just about everything else with it, and not simply be a rough idle.

A piston ringland can break without any interference from the rod.

I believe you want at least 175 compression, 180-200 is a good, healthy motor.

I don't want to sound like an asshole, but you're incorrect. Different compression testers will give different readings. The MOST IMPORTANT thing is that there is no more than 10% variance between the cylinder readings. Actual PSI numbers are arbitrary and vary from gauge to gauge.

we did a compression test on it yesterday it has good compression on each cylinder and with a change of coil and spark plugs the idling changed dramatically however it still idles a little rough and doesnt shake as much.

How did you do the compression test? Car must be warmed up.
1. Pull the ECU fuse
2. Remove spark plug, thread in tester
3. At WOT (wide open throttle), crank the car a few times until the gauge stops jumping up dramatically. Generally 3-5 seconds.
4. Jot down reading, remove tester.

You have finished a "dry" compression test. A "wet" test is optional for your purposes.

5. Add a few drops of engine oil into the cylinder
6. Repeat steps 2-4.

If you notice a significantly higher reading from the "wet" test, your piston rings are most likely shot. But again, that doesn't sound to be the case here.

 

[PepeEAS]

So what would your guess be here?

 

[sde780]

So far you've changed the coil packs and plugs; is it still misfiring?

Are all of the injectors firing? Any CELs?

Has the fuel filter been changed?

 

[hornsfan10609]

I have never used a compression tester that didn't read within those ranges but it's not like I do comp. tests every day. Our cars don't have a fuel filter just a sock on the pump and they are usually fine for well over 100k miles.

 

[sde780]

Cool, scratch the fuel filter idea then. The more important part is the consistency between the cylinders, not the specific reading.

 

[PepeEAS]

havent checked the injectors yet, but my guess would be between that, timing, egr or just bent valves, if i did bend a valve (which i hope i didnt) im just gnna take everything apart and turbo it