UPDATE
A lot of mistakes were made, but I've fixed them all now. The culprit was the valve cover gasket leak. Originally I only changed the front as that was the only side leaking, but the rear valve cover gasket also started leaking.
It takes a long time when doing this stuff for the first time. No garage and rainy weather makes it tough as well, so I wanted to avoid doing more stuff with limited sunny weather.

Originally installed NGK Iridium spark plugs, but they were not pre-gapped like some of the copper and platinum plugs for other vehicles. So I had to re-gap them because the RPM gauge was fluctuating incorrectly. They were gapped too narrow: 0.051" (I know, even .001" makes a difference). I gapped them all to .054", in the middle between the 0.052-56" range.
I used the coin-style spark plug gauge because I couldn't find anyone selling a wire gapping gauge higher than .05". I know it's not recommended to use the coin, but I was far more gentle than the wire gapping gauge I saw people use, and I didn't have much of a choice. I then double checked it with a feeler gauge.

When I changed the coil packs and replace the valve cover gasket, the engine ran louder; not uncomfortably loud, but it's not as whisper quiet as it used to be. Not sure if it ever was supposed to be that quiet, but the properly gapped spark plugs made it a bit quieter, but not by much.

I found something called self-fusing tape or fuse tape. Apparently this is a more permanently solution for repairing hoses, so I fixed the minor vacuum leak with that.

The random shaking / jitter is gone now, and it no longer hesitates when going uphill at ~80km/h when it tries to switch gears.


Replace valve cover gaskets, ignition coils, spark plugs (double check gap before installing), get some self fusing tape if you've got any damaged hoses / tubes.

So far, so good. Now I just got some random electrical connectors that are deteriorating from age.

-----------------------------------------

I've been trying to diagnose this for a while now and just can't seem to figure it out. I'm hoping someone here has the same issue or knows what's up.

When I'm driving, there's a noticeable shake, like hitting a small bump in the road. Jitter / shake, not sure what to call it, but it's very noticeable. It's not a consistent vibration either, it's more like 1 really strong vibrate, but happens somewhat randomly. Like shake, then 3 seconds pass, shake, then 1 second, shake, then 2 seconds, shake, etc. It's more noticeable when idling as the engine at a high rpm to mask it.

I notice it happens more when the car is fully warmed up, after driving for like ~10-15 minutes.

I first noticed it after changing all the spark plugs and boots (not ignition coils though) using this guide: I'm wondering if maybe I put too much dielectric grease on the connections? I tried cleaning the iac valve, changed pcv valve, cleaned things around the area, checked hoses for any holes, used some fuel injector cleaner in the gas tank.

I had to leave for a few months and the car was sitting in a garage for months. I got someone to help maintain it by starting it and taking it for a short spin every now and then. The check engine light came on and it was a cylinder 4 misfire.
There was a bit of oil in the spark plug socket, so I cleaned it out, changed the valve cover gasket, and re-seated the ignition coils, spark plugs, boots in the front. That fixed the check engine light, but that shake / jitter is still happening.

If an ignition coil is bad, are the symptoms consistent or intermittent? I don't want to throw a bunch of money at a new set of ignition coils just to have it not do anything, but I don't know what else to do. How long do ignition coils usually last?

Appreciate any help.

 

I'm just boiling down [and bolding] what I see as the heavy hitters of the issue:

O2 sensors are known to 'get tired' with age and perform poorly in monitoring fuel management (but without always throwing a code). There's 4, $29 ea.

 

Hey, thanks for the help. It's got 185k km / 115k miles.

Yeah, I also thought it was something I did when I first changed the spark plugs, but I re-did the whole thing twice (removing the upper intake manifold), making sure all the way I put everything back on properly, torqued everything to spec using Chilton's service manual. I cross-referenced with the manual and a few Youtube tutorials to make sure I'm doing everything properly and they all match up.

Can you elaborate a bit on the vacuum leaks? Can I use carburetor cleaner instead of gumout? I thought the gumout was some fuel stabilizer.
And blue spider legs in pitch black? I'm assuming there's some blue light coming out somewhere when you spray while idling?

I came across this video by Scotty Kilmer with the car hesitating. The symptoms seem similar to mine, but it doesn't need to be moving, can be stopped at a light and it'll shake and the RPMs fluctuate a bit too.

I went ahead and bought a 6 pack of ignition coils, gonna try and see if this fixes it. Hoping I don't need to get o2 sensors.
Wish this stuff would just throw me some codes so I know what to do, oh well.

 

113k with that old is not much, so your ride may be in real good condition and the $$$ you're putting in it is investing, not wasting. And good you're tearing into the issues yourself - both learning AND not chewing up expensive mech. hours.

Chilton's service manual + Youtube = a good approach, but getting a real Factory Service Manual (eBay), or even a .pdf version with a subscription from alldatadiy.com is better
Can you elaborate a bit on the vacuum leaks? You're already doing it by inspecting all the vacuum hoses. Also wiggle connections while idling to find leaks. I didn't notice where you did the 'clean the MAF sensor screen' procedure.
Can I use carburetor cleaner instead of gumout? Yes, anything that ignites and doesn't hurt rubber or plastic basically. You're looking for a change in idle either quicker or stall.
I thought the gumout was some fuel stabilizer. Gumout is a solvent. Techron and such is the fuel treatment that goes in the tank. Throttle body cleaner for the TB, and MAF Cleaner for the MAF. Certain components are touchy about what gets on them.
And blue spider legs in pitch black? I'm assuming there's some blue light coming out somewhere when you spray while idling? Dryed rotted insulation on spark wires arching which weakens current getting to the plugs. And you don't have to spray anything, they'll arch if they're bad. You can do the same in daylight by running your hands down the wires for a 'shocking' experience.

I went ahead and bought a 6 pack of ignition coils, gonna try and see if this fixes it. Yah, not throwing codes makes you not sure if doing that is the do-all be-all solution. But replacing them (for just the parts cost involved) is a good bet to guarantee they're not (or no longer) part of the problem.
Hoping I don't need to get o2 sensors. Get a reader app on your phone (from 'Free' to $15-25) and an OBD II dongle ($20-120) and plug it into the OBD port under the dash. You can read codes and clear them, and watch the performance of at least the two upstream O2es.Plus monitor numerous other operating parameters.

Wish this stuff would just throw me some codes so I know what to do, oh well.
Someone will eventually happen by here a lot more versed in these cars than me.
One thing you'd get good insight from is 'borrowing' a fuel pressure tester kit from a parts house to weed out a failing fuel pump as the problem. It's a straightforward procedure to check its initial priming pressure (needs to be 60#), whether it holds or bleeds down too quickly, and whether keeping a steady running pressure.

Keep at it and beat it into submission.

 

Alright, so my crankcase vent hose / tube had a hole in it; just in the rubber elbow hose / tube that connects at the very front. I taped it up with electrical tape for now since I can't seem to find anywhere that sells the replacement part in Canada. That, or I don't know what the proper name is; it's just a small rubber elbow that connects the plastic vent tube to the valve cover.

Replaced all the ignition coils.

So far, so good. But we'll see how things go once I take it out for a long drive; only drove it for 30 minutes so far after re-learning the idle trim fuel management thingy from disconnecting the battery. I will update the main post once everything is good.

 

That, or I don't know what the proper name is; it's just a small rubber elbow that connects the plastic vent tube to the valve cover.

You simply need to run a basic search [pcv tubing 2003 mazda tribute] and either match up the illustrations or find a parts exploded view:

This may OR MAY NOT BE what you're referring to:

https://www.ebay.com/p/1725424548

When you get to installing that 6-pak of coils it's just about mandatory to also replace the PCV - it's the only time you'll have access.

FTR and BTW that tape can be relied upon to last at least 4 minutes.

 

It's this thing here:

Don't know what else to do in the meantime, so I just covered the hole with some tape.

I've already changed the PCV valve and the hose looked fine. Even the old one wasn't that bad when I changed it.

Yeah I didn't mention the other minor things I did like cleaning the MAF sensor, as that was fairly cheap and easy to do, but didn't really accomplish anything.