How do you know it's time for a tuneup - 1990 Toyota Cressida

first thing you need to know is to have the fault codes read
if nothing comes up don't bother with a tune up as the ECM is doing that every 60 seconds

Typically, these days, a basic tuneup consists of replacing the spark plugs. They will inspect plug wires (if your vehicle has them). They will remove and replace the distributor cap and rotor if the car has one and you ask for it and pay for them. There is no timing to be set on newer cars, so they don't do that. They will usually test drive the vehicle first, and if there is any performance issues, they will note them before work starts so you cannot blame them for inducing problems that were already there. They do absolutely NO troubleshooting during a routine tuneup. If you are having performance problems (Check Engine light is a good indicator of them), then they will expect you to request and pay for an engine diagnostic. That will let them hook it up to the scanner and engine diagnostic machine(s) to try to determine the problem. Then they will come to you with a laundry list of things you need and things you should have. It's all rather pricey, but that"t they way auto mechanics are. Ask for a tuneup, get a tuneup. If it fixes your problem, ok. If it don't, don't expect them to offer repair suggestion for free. That's not how they make money.

its TUNEUP TIME. 99% of time.
ever do the 60,000 mile tuneup , listed in the lost operators guide.
the guides are easy to find on all hondas, made, (unlike nasty toyota's)

tuneup time skipped? the 60k miles tuneup?
scan it? if there are DTC errors, it might in LIMPHOME mode, with RPM limits dropped for good reasons.
or is misfiring. that too can be felt or seen with the scan tool.
the poor idle proves its running bad, no other symptom better prove this. its misfiring, only queston is , why.
50 reasons
or 1 to 10 simple tests to find true cause, ask.
after a full 60k tuneup,(repeats)

that is called misfire, not a work argument, but is misfire
and knowing that word, helps solve the riddle.
misfire has about 4 causes?
1: bad compression on 1 or more cylinders. (burn valve?)
2: bad spark (ever the a tuneup at 60k miles or this decade?
3: bad fueling (too much or too little.
4: leaks. say coolant is leaking into a combustion chamber.

EGR stuck open, is common, if it even has one , some engine dont.

all it takes to find this is tests. and tools
got tools>? (gauges too, compression gauge, fue pressure, vacuum , timing light)
if not? the get er fixed
that is why we have mechanics, they have skills, the book and the tools.
DIY takes the same thing, or its

there are tests for every failure group, it take me about 1 hr to do it.
(after the skipped tuneup, and cures most)
In the USA, our cars are OBD2 that year, and in Canada.
So we scan all cars, first. (after the tuneup)
a scanner is like $9 off fleabay
i use an Innova 3160 , it gets the newer codes on newer cars.

if you do your own tuneup the money you saved buys the good tool.

Check your coil pack ,injector,don't know what kind tuneup ,.some people think tuneup is oil change,but it's not ,good luck

hmm, Let me guess, Sideways V-6, can't reach rear plugs? In most cases on these you have to remove the air intake system to get to the plugs. Everything else is basically the same as any other vehicle. There is no timing adjustment because that is electronically controlled although it can still be checked. If it isn't a sideways V-6 it's the same as any other tune up except for setting the timing part. I will try to answer any questions you have about the process however, if it is something specific to the Concorde I recommend you get the Haynes Repair Manual on the vehicle for step by step instructions. If the vehicle does have a Distributor cap and Rotor instead of a coil pack then everything is the same as a regular engine to include setting the timing.

the timing gears have a notch or line on the them that you align pointing togather slip belt on and it is timed

Fuel filter, air filter , timing, faulty wires, plugs, vacuum leaks, egr, low compression, brakes binding,, tranny fluid low,... start checking!

Plugs, distributor cap, Plug wires, rotor, and it wouldn't hurt to replace the air filter and fuel filter too.