And I've got something to tell ya
You won't believe how many people straight doubted the flow
Most said that I was a failure…
(Nelly)
Not necessarily everybody doesn't know what i'm about to say but i'm convinced it'a a majority.
Have you ever noticed an oil smell when turning the heat on? That is from oily dust from engine compartment that made its way past the hood's seal and into the ventilation system and on the heater core.
Even on a new car. One single drop of oil left from an oil change on a rough surface composite material of a valve cover will spread due to fast and unpredictable flow of air under the hood everywhere.
Which means. Attracting dust. Oily dust will settle and glue on any surface, incoulding connectors. Tiny amount of oil will get inside the connectors. Engine oil has very harsh detergents that will corrode the surface of the contacts. If contacts are gold coated like most are they may be covered with a non-conductive layer that in time will cook and harden.
Battery connectors which take hundreds of amps when charging or starting, are secured with bolts and nuts on the terminals. But if oil gets on those, that will allow loosening of the nuts. Though it will charge and start, the battery won't be able to fulfill other functions, like smoothening of the rectified voltage from alternator. Voltage going to the sensors and actuators will fluctuate unpredictably. Together with problems at the connectors of the sensors and actuators themselves, the car computers will get erroneous readings and the actuators will actuate unpredictably.
All these problems won't be signalled by the car's computers because the software is not smart enough yet and probably will never be until the full advent of electric to detect small, abnormal fluctuations coming from sensors.
The good news are
First make sure you have towing on your insurance so you can tow your car to your mechanic if you screw anything. Just kidding.
First you should inspect all the painted surfaces under the hood. If you see dust, then it's time to perform the following procedures:
Wipe clean any oil residue left from last oil change around the oil cap or dipstick.
Use a large soft brush and dislodge dust from any visible surface. The dust will fly away at the first drive. Finish with a wet cloth. Do not forget the hood itself, under.
Disconnect both battery connectors starting with negative. If the connector's screws are parallel with battery's terminals, use a tool to hold the connector in place to prevent rotating and twisting the terminal which may damage the battery inside.
Disconnect all connectors that you may see. Start with easiest. Most have a small pad that when pressed with a finger will "unclick" or unlock the connector. Use caution and be gentle.
Use some contact cleaner spray that is plastic and rubber safe, ideally with a straw which will allow "focusing" the flow of the cleaner on the desired area. In a ventilated area (ideally outside far from other people) use a rag under as you spray cleaner outside and inside the connector a few seconds or until no more dirty cleaner comes out, including on any piece of wire that come out of a harness. Shake if possible and let dry. Some have soft rubber seals that may expand because of the solvents in the cleaner. Be patient and let those dry to get back to their original shape or you won't be able to plug the connectors back. If cleaner is too harsh and melts the plastic or rubber, it means you have the wrong cleaner. Test first. Don't forget most important, battery's connectors and terminals. If terminals have been damaged by acid, or have been on a shelf for a long time clean first with a wire brush or sand paper. Wipe clean with a paper towel wet with contact cleaner all contact surfaces.
Plug the connectors back one by one. Finish with battery connectors. If you can't use a torque wrench use the following rule: Tight the nut first by hand and then by wrench to the point of resistance, and then while holding the connector with another tool, like an adjustable, to prevent from rotating and twisting the terminal, in one move gently tight until it stops firm, usually less than one more quart of a turn.
If you didn't damage anything (which could only happened by gross negligence or possible by bad mechanic's work done previously), the car will then start and run as new again, without vibrations, full power and mpg.
Use a 15 dollars LED voltage indicator that doubles as a phone charger plugged in the cigarettes lighter plug. It will tell you in real time many interesting things about battery and car's electrical system.
Invest in a torque wrench from NAPA, Autozone, etc.. Every once in while torque the nuts on the wheels to specs or right under 90. Loosen and torque them one by one (not more than one at the time with the car on wheels) to make sure they weren't overtorqued previously, using a by two sequence or "star pattern" Do not torque the nuts one after another cause this may warp the wheel or rotor.
This may increase driveability at high speed. Also check periodically tire inflation, again with important effects on driveability at high speed and mpg. Tire pressure varies with temperature so it should be done on a cloudy day with a temperature that is average for the season, after the car and tires have cooled from the most recent drive. Sun on one side of the car will heat the tires and will give you false (higher) reading on that side.
Did you know they have serious jump starters at Amazon, under 40 dollars which is way less than a jump start by towing and buying one is way less embarrassing than searching for a jump start at 2 AM at the casino parking lot? Got one that when fully charged (on a phone charger) could start the car several times, even with battery completely drained or old or even damaged (like it was in my case).