Every engine has basically the same parts that wear out and are critical to the operation of the engine. These include spark plugs, spark plug wires, and a distributor or distributor rotors and caps. Also included on this list are sensors. Depending on your car, one single sensor can make a huge difference. On an LS400, a Coolant Temperature Sensor that is bad will cause a loss of throttle response and a good 20-30hp! That's pretty extreme but other important sensors are the Throttle Position Sensor (TPS), Idle Air Control Valve (IACV), Crankshaft and Camshaft Position Sensors, and Oxygen Sensors (you should get a code if one of these goes bad). All of these send information to the ECU and so fixing bad ones will typically restore lost power, throttle response, fuel economy, and even shifting if your transmission is automatic.
I want to explain how to do these things aswell so you can actually do it on your car. If you use a mix of junkyard and store bought parts, you should be done for under $200.
Spark plugs should be changed every 100,000 miles or so. They develop carbon deposits, causing a bad spark. You will see this on old spark plugs as a layer of black carbon on the metal parts that create the spark.
Whatever your engine is, you will have to get to the point where you can unplug the spark plug wires so you can access the spark plugs. I would change them one at a time. This way, you can unplug one wire, change that spark plug out, plug the wire back in, and move on to the next. You won't lose track of which wire goes where this way. You should also lay them out in order so if you see something like oil on one, you know which cylinder it is (one of mine had oil on it but it didn't mean anything except that the old spark plug probably wasn't firing correctly).
There is a specific way to tighten the new spark plugs. You untighten them like anything else using a spark plug tool or a socket with a long extension. When you put the new one in, however, you want to tighten it to the following specifications:
Tightening Spark Plugs: Turn it until it's tight enough to where it first rests near the bottom of the thread. This is called "hand-tight" but that term can be misleading. Basically, you should feel it hit bottom - you will suddenly need significantly more force to turn it. It is at this point that you turn it HALF of a turn more. That is perfect tightness.
I do my wires separate mainly because it's easier to keep track of which is which. You should typically buy new ones but you can also get good ones from the junkyard. What I did was test the resistance of mine (5-12 ohms) and of the sets of junkyard wires I was looking at (1.1 ohms, all of them). That test at least let me know there is proper resistance and with the fact that they looked new, it was a good indication they were new. And they worked very well. This can save you $60-80.
When you take the wires, lay them out in order. Decide which cylinder is 1, 2, 3, etc. For my V8, passenger side bank was 1-4 starting from the firewall and the driver side bank was 5-8, again from the firewall. At the junkyard, I marked them with dots (4 dots for 4 or 8) and laid them out in two piles corresponding to the two banks. I used the clips that were there to clip them together aswell.
When you start changing them, just do it one at a time. Sometimes pulling the plugs that attach to the spark plugs can be a pain if they've been sitting for a long time. Just go side to side, don't pull TOO hard (although you are changing them...), and use a screwdriver to push the rubber boot off the cylinder while you pull if you have to. Doing the wires one at a time this way prevents you from mistaking wires. Take off one old wire, put on the corresponding new wire, go to the next.
One other thing you need to change while you're doing your spark plug wires is the coil pack. This is what feeds the appropriate amount of electricity to each wire. A bad coil pack will certainly make your motor run rough. The timing may be off along with power and throttle response. I would recommend going to the junkyard for one and trying to find the newest looking one you can unless the coil pack is only $20-30 more new, in which case just get that. I believe I saved almost $100 by using one from the junkyard.
Just be careful not to get a really old one. Look for what effect the elements have had on it. If it looks new, it's probably relatively new. I know that comparing my old one to the one I got is a big difference. My old one was faded. Of course, this isn't a surefire way of knowing that the coil pack is good but it's a safe bet and it's still cheaper even to get a second one from the junkyard. I said this before but get a new one if it's only a little more.
Dirty EGR Valve.
One of the things you NEED to change if your temp gauge isn't working in your dashboard is either the Coolant Temperature Sensor or Engine Temperature Sender depending on your car. Some cars use the info from the former for the gauge while others (like my Lexus LS400) have a separate sender. They're right next to each other. I mention this because it is extremely important to be able to see your engine temperature because you can easily blow a head gasket if it overheats. Also, like I mentioned, the Coolant Temperature Sensor will bring back some power in most cars because it sends information to the ECU so if you have a bad sender aswell, change them both.
If you have weak throttle response, meaning your engine is slow to respond when you push the gas pedal, you might want to look at the Throttle Position Sensor (TPS). If yours is old, replacing this part should make your car more responsive.
The EGR Pipe recycles exhaust gas and sends it back to the engine. If yours is cracked or the valve is stuck, your car will get bad fuel economy and probably run bad aswell. If it's just the valve (pray), you can take it off the car and clean all the deposits with a carb/throttle body cleaner. If your car is old, it's good to clean this anyway so that you don't have problems in the future.
Another important component that you can clean is the Idle Air Control Valve or IACV. When this one goes bad or doesn't function properly, your car will have a rough idle and maybe even stall out.
You need to check the slack on your throttle cable. One reason a car develops bad throttle response is a sagging throttle cable. It has to be taut but not slack and not tight either. If you can move it up and down significantly with your finger, it needs to be tightened. You should be able to press it down a little. I think all cars actually have specifications on how tight the cable should really be but if you tightened it like I said, it will be fine. Push on it gently.
To tighten the cable, most cars have a setup where you can adjust it by turning two nuts. The nuts sit next to each other, tightened against one another, usually with some sort of thin metal in between. What you do to adjust the cable is loosen the nuts by using two wrenches; you will be holding one in place while you turn the other one to loosen it with your other hand. Usually as soon as you loosen it, you should be able to twist both nuts either way with your fingers. If the throttle cable is on the left, you need to turn the nuts towards the left, moving the contraption to the right and tighening the cable as it moves. It is actually quite self-explanatory when you look at it. The contraption has a thread that these nuts sit on and allow you to tighten it at any point. When you get it where you want it, bring both nuts to the metal part that sits between them and tighten them together with your fingers first and finish with the wrenches.
I shouldn't have to mention this but you should check your fluids. Change your oil if it's extremely dark and make sure you have proper coolant. There's something in particular that reminded me of this - my roommate's Mustang. The coolant in that thing, if you can call it that, is orange because it's just water with a whole lot of rust. Kids, get the coolant specified for your car by its maker. It's important not to let anything that has to do with your engine rust and your radiator will be able to cool the engine better.
If your transmission fluid doesn't smell burnt, you should be okay. Maybe think about changing it at 200-250k miles as it's supposed to be a lifetime thing but it does go bad eventually.
Dirty engine bay.
I recommend you take the time to clean your engine if you're taking off a lot of covers or parts and can get to the hard-to-reach places. It just looks better and might help keeping it cool, even, although that's if your engine is covered in oil... Also, if your engine is completely clean and you get a leak, then it is much easier to find because you should be able to see whatever it is that's leaking without confusing it for all the other liquids that have accumulated.
Clean engine bay. (Mine will be like this next year...)
One of the last things I can think of is obvious: your brakes. Check that the car doesn't pull to one side when you brake hard (that's a sticking caliper) and make sure your brakes and tires are good.
If I missed something, please leave a comment and get to wrenching!
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