I forgot to mention (and it seems to always happen in posts where I
don't mention this) that this site does not want people to open new
questions in response to a question that haa already been posted. The proper
way to post new information and feedback about an existing topic
is to go to the original question, and click the "answer this question"
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In response to the two new questions you have opened; 3 cylinder Metros are probably more sensitive to spark plug choices than any other vehicle on the planet. And the choice of plug that works best will change, in response to any other changes you make to the engine. I have owned a 3 cylinder Metro for the last 19 years; and have serviced and advised the owners of many others. This is a most amazing vehicle; but it is also extremely vulnerable; if abused or not maintained properly.
The loss of mileage, the shaking at idle, and the exhaust smell are all signs that the motor is out of adjustment. But the adjustments which address these issues are so critical that I hesitate to recommend them to the average owner. An impatient, distrustful, or headstrong, or fearful owner will inevitably get into a huge mess if they go into this process. But if you are willing to follow instructions, and realize that the results will depend on how thoroughly and carefully you apply these recommendations; as well as on the mechanical condition of the vehicle; then you will be likely to end up pleased with the results.
If you want to get into this (which, by the way, is my specialty) I will work with you; but it will require some tools which you'll probably have to buy, plus modest amounts of money spent on parts which may not end up being permanently used; along with doing some experimentation; which sometimes makes things temporarily worse, before they get better.
A few notes on wires,rotor and cap: most replacement distributor caps for Metros are not adequate for service on this motor. The best cap to use is either a Beck Arnley # 1746959, or a Standard Motor Products # JH165T. Please do not use any other brands or part numbers. The JH165T must have the "T" suffix in the number. I am well aware that the Standard Motor Products "T" series is usually considered as a lower quality line; but this particular cap is made in Italy, and uses a brown color material that is a better electrical insulator than the higher priced caps. The Beck Arnley cap also uses a similar material. Also buy an Airtex # 4R1044A or a Wells # DR970G rotor. These parts may be available locally; and are also available from (www.rockauto.com)
Please do not buy a wire set at this time. It will require some custom work, and there are other things which need to be done before that.
I am concerned about the frequent use of fuel injector cleaner you have been adding. This type of product is basically a solvent; and its excessive use tends to dilute the motor oil which is in the upper cylinder area. This can damage piston rings and cylinder walls. So I would like to see the results of a compression test on this motor before we go further. Buy an accurate compression tester which screws in to the spark plug threads; not the type with a tapered rubber cone that presses in. NAPA parts stores would have such a tool.
Before running a compression test; drive the car for at least 3 miles, and then let it cool off for 45 minutes. Disconnect the small diameter wires from the ignition coil before running the test, and tape the metal ends or secure them in a location where they cannot touch each other or touch any metal parts. Take off the cover of the fuse box that is just in front of the strut tower, next to the battery, on the driver's side of the engine compartment. Locate the fuel pump fuse in the diagram on top of the cover, and remove that fuse.
Remove all the spark plugs, screw the compression gauge into one spark plug opening, and have someone in the car hold their foot about halfway down on the accelerator while turning the ignition key to the "start" position. Have them crank the starter until the cylinder has gone through four compression strokes (you'll see the gauge jump four times). Stop the cranking after the fourth compression stroke and read the gauge. Write down the pressure reading. release the pressure in the gauge, and move on to the next cylinder. Do this on all 3 cylinders. The cylinders are numbered 1-2-3 running from the fan belt end to the transmission end of the engine.
The compression on a new motor is 195psi. The minimum allowable pressure is 165 psi. There also cannot be more than 15psi difference between any two cylinder pressures.
Post the compression readings you get; and we'll go from there.