So I have read these forums for about 1.5 years trying to figure out what is causing the rough idle on my 99 626 V6 auto, so i decided to create my own thread on the issue. When the car is in drive, and warmed up, the car idles very roughly at a stoplight. I have found that it doesn't appear to idle as rough when the engine is still cold. In neutral the car doesn't vibrate. Specifically the car idles very roughly and the occassional dip in RPMs occurs. At idle in neutral, the gauge reads 740-750rpms, while in drive it reads about 690-700. Most of the time it's just a very rough idle without a huge noticible misfire and not too much RPM fluctuation, but when it's in drive and the RPMs do dip, its usually a powerful rumble you can feel in the engine with a fluctionation from about 650-700. The dip in RPMs is intermittent. It usually starts after about 3 minutes of driving and i come to my first or second stoplight, then it occurs many times throughout my trip after that.
To add to that issue, here is something else i recently noticed as a symptom. When I turn on my inside air when i'm sitting at a stoplight, the car vibrates real bad for about 2 seconds then returns to a somewhat normal RPM level, about a hair under what it was before i turned the fan nob on. You REALLY notice it when you quick turn the nob from 0 setting to 4, but you can still feel it cause the entire engine to rumble greatly when you turn that nob from 0 to 1, 2, 3 or whatever. With that said, does that imply a grounding issue? Grounding and/or coil pack issues? Something else? I tried turning my lights on real fast and head unit, but those didn't seam to cause the engine rumble like the fan did (maybe they are just not noticeble enough to the driver?)
Next issue, possibly related. I recently had some work performed by a friend mechanic, and i also did some tune up procedures over the last 1.5 years that i have detailed below to help with the idle. My friend had replaced the valve cover gaskets on both valve covers because the car leaked oil from both covers ever since i bought the car 9 years ago. So that fixed the leaks, not a drop under the car for about 4-5 months, then slowly a drop, then multiple as time went on. Now, about 11 months after the front gasket was replaced, the gasket has splits on the left and right of the front valve cover, i don't think the rear has any leaks (yet). I don't know if this implies bad mechanic installation or pcv issue, but keep reading on about what i have also done!
So here is what has been performed on the car since i started with all this.
-Checked for vacuum leaks with starter fluid, multiple times. Found nothing.
-Replaced PCV valve and gromet with oem mazda (i tested the new pcv valve before installation, it worked good.
-Cleaned PCV hose to intake manifold, and there was no residue of any kind built up. Was pretty clean really. I also cleaned the port the hose attaches to at the intake manifold, also not too bad.
-Cleaned throttle body (did not remove IAC from TB) after i removed from the manifold, and I also cleaned the EGR ports right inside the intake manifold.
-Replaced throttle body gasket when reinstalled TB
-Replaced air filter (mazda oem, i used an 01 filter since that's what the dealership ended up giving me, i assume it's the same filter for my 99)
-Replaced fuel filter
-Cleaned MAF wire (sensor was replaced about 8 years ago, but seems to function fine from what i can tell)
-Replaced Spark plugs and wires with NGK recommended ones.
-New Battery about a month ago (old one went bad anyways, lasted 4 years).
-I have used techron concentrate 3 times in the last year, and i have used fuel injector cleaner (lucas) for many tanks as well.
What my mechanic friend did
-Replaced water pump (didn't have to remove timing belt)
-Replaced valve cover gaskets
-Replaced intake manifold gasket
-Replaced the passenger side motor mount, driver side transmission mount, and the "dampener" underneath that bolts to the crossmember with beck/arnley mounts (seemed well made for aftermarket). However, we could NOT find factory OEM mounts for the front and rear, or even beck/arnley mounts, which is what we used for the 3 we replaced. I found 2 DEA months that were the right design, but the througbolt hole ont hose DEA mounts DID NOT MATCH the bolt that comes with the car, so i refused to put them on and put the old ones back in. I would not allow rattling aftermarket mounts to be installed. By the way i watched as he did all these repairs at his shop. The rear mount is in very bad shape, and the front is very worn as well. I know this is not helping the vibration issue, but the motor isn't running right at idle. So i can replace the mounts, but i feel all i'll be doing is masking the issue, that's if i could ever find the dang things. (anyone know where i can still get OEM mounts for my exact car, 99 626 V6 Auto? because 5 dealerships i've called all say i'm SOL in finding one from a mazda dealer)
This rough idle started and was about as bad as it is today before I attempted everything i listed above and well before my friend mechanic performed his work. I am at a loss on what next to do. I have not tried the following,
-Have not tested fuel pressure (because i didn't think this mattered if it accelerates and operates well under high throttle, am i wrong?)
-Have not pulled the 6 injectors out to inspect for dirtiness (because i didn't think this mattered if it accelerates and operates well under high throttle, am i wrong?)
So with all that said, should i give up, or do any of you have advice as to what next i should do or even redo or test further? I hope i have including everything that will help the experts figure out my neverending rough idle. Thanks in advance just for taking the time to read my issue!