A tune up today is not the same as it was 20 years ago. Follow your manufacturer schedule for what is needed when. Todays vehicle need much less then they used too.
If you have a problem you are self diagnosing as needing a tune up. Before you throw money at it (unless you just know you are way past due), consider having the problem diagnosed before you tell a shop you just want a "tune-up"
To tell a quick story, I did my own "tuneup" last summer with about 125K on my car.
It would frequently run rough and randomly reduce engine power-sometimes with near accidental consequences.
A code reader reported multiple misfires(I think on three separate cylinders), as well as throttle body issues.
The COPs are a known problem source on my particular car-the covers that protect them didn't have the best insulation, and they are known to get water and/or oil(from using the fill cap, not leaking) down around them. It was a common enough problem that Ford/Lincoln quietly extended the warranty just on the COPs to 100k/10 years. I knew that at 125K, I was on borrowed time, and when I started having the rough running problems knew that was likely the issue. Some folks on the Lincoln LS Owners Club told me that the throttle body code was actually a common symptom of the #5 coil(the one right next to the throttle body) going bad, as it would occasionally thrown off random RF signals that the car's computer would mistake as being a throttle body problem.
In any case, I broke down and bought 8 coils along with 8 NGK Platinum-Iridium spark plugs(this is the OEM plug, although the ones I bought were marked NGK and not Motorcraft). I was advised to just go ahead and do all 8 coils, as getting to them is 3/4 of the battle and that I was basically on borrowed time any way even with the still working ones. The coils take about 5 minutes each to replace once the cover is off(which is the hard part, thanks to the poor bolt clearance on several of the bolts), and changing a plug takes another couple of minutes once the coil is out. Thus, as advised, I went ahead with all 8. The original plugs were
probably still serviceable, but did show a fair bit of erosion, and, again I was deep enough into the engine already that for the $5 each they cost I couldn't justify
not replacing them.
Once everything was back together-a job that took me two days working on and off-it was quite literally like driving a new car. You don't notice the gradually diminishing performance, but when doing something that effectively brings part of the engine back to new condition it's amazing how much of a difference it makes.
By the way, when things were messing up the check engine light came on(the first time in my nearly 8 years owning the car). I didn't bother to reset it, and it cleared itself within two or three days of my changing the plugs and coils. That was all the confirmation I needed that I'd fixed the right problem.
