Many a time, I see people complaining about their perfectly fine cars giving horrible fuel mileage even though “nothing is wrong”. There are actually a large number of possibilities in your case. This article will be discussing one of the most important one out of those, which is hardly ever looked at in general. This particular activity is absolutely vital to perform on each and every vehicle that you own if you want to really enjoy the ride and fuel economy for a long time to come.
Most of the modern day cars are electronic i.e., the critical vital signs of the engine and atmospheric environment such as fuel pressure, temperature of the Air being sucked into the engine, the crank and cam shaft rotation information, the engine timing information, the pressure inside your engine’s air intake manifold, the amount of unburned fuel being released into the exhaust (the after-burn mixture information); are different kinds of information that are communicated to the Engine Control Module (ECM) of your car by various sensors that are in place.
Most of these sensors are multi-wire sensors which are taking their VCC and Ground reference voltage inputs from the wiring harness which brings it directly from the ECM pins and from the chassis grounds. Here’s the trick! If these sensors are having unstable VCC or Ground reference voltages, their output voltage signals are going to be highly unstable as well. This basically means, your car’s sensors will start sending incorrect feedback data to the computer which will paint a wrong engine and environment picture for the ECM. This will happen because output signals of the sensors are directly dependent upon their input and ground reference voltages for stability and accuracy.
When either of these connections is bad, a vicious circle of bad feedback information being sent to the ECM, resulting in bad control decisions being made by the ECM at every level begins, which will eventually destroy your car’s mileage. In some cases, it can destroy your car’s sensors such as a car running rich for prolonged periods will damage the O2 sensor which will become foiled and “laggy”. This will in turn blind your car’s ECM from properly seeing the amount of unburned fuel going into the exhaust manifold and as a result in this case alone, your car’s petrol consumption will go through the roof. At this point, your mechanic/electrician might tell you everything seems good and you can try replacing this part or that part. Watch out for those greedy money makers as they will try to sell you unnecessary solutions that your car does not need without performing this absolutely important DIY job.
The simple solution:
Clean your car’s chassis and engine block ground connections. You might be wondering why only Chassis and Engine Block Ground connections? Well here is the answer. The VCC input reference voltage wires of sensors are getting voltage directly from ECM pins which is taking 12 volts directly from the positive terminal of the battery. This makes corrosion of this part of the sensor input signal pathway comparatively less likely. However in case of ground, the ECM takes ground voltage signal from the chassis ground and passes it on to some sensors while other sensors get ground reference voltage directly from the engine block. The engine vibrates the entire time making engine block ground connections loose overtime while the chassis ground wires attached directly to the frame or other parts of the chassis can get corroded overtime due to rust. It is highly advised to not rely on measuring resistance between ground terminal of the battery and the chassis ground connections at this point to determine whether those connections are good or not. Why? Because if a ground connection looks good, that does not mean it is really good. Let me explain:
Due to constant vibrations and corrosion, ground connections have increasingly lesser area of contact which makes their connection pathway to the negative terminal of the battery more and more restricted. Upon resistance check with a multi-meter, it might tell you that there is good ground. But when the car starts up, many sensors start drawing a huge amount of current through those already choked connections. For instance, most oxygen sensors used nowadays in modern cars are heated oxygen sensors which can draw up to 2 Amperes of current alone! Then you have all the lights and other accessories relying on the same ground connections. As the total current drawn increases, the actual active resistance at these connection points dramatically rises. Sometimes it can get so bad that your car might start throwing random error codes due to excessive distortion in sensor data while running because all kinds of signal noise is introduced into the wiring as the engine is running. Minimal vibrations by engine as well as your car’s sound system add to the instability of these already unstable ground connections.
It’s important to note here that in most cases there is actually nothing wrong with any of the sensors or the wiring itself. I saw the exact same thing happen to multiple cars that were brought to me for check-up over the last 2 years by friends so consider this a very real scenario. Sometimes, the owner of these cars told me that they had already purchased multiple new replacement sensors on advice of their mechanics only to find out later that they had wasted several thousand rupees unnecessarily. Sometimes those error codes were random while at other times the behavior was predictable. For instance, a friend’s car was towed to my garage as it was shutting off at random while driving. I scanned the car and it gave crankshaft position sensor error code. After inspection, it turned out to be a bad ground connection to the sensor only at run-time that was causing it to give up at random intervals, which would instantly stall the engine. A simple overhaul of chassis and engine block ground connections with some good old carb cleaner fixed that problem and it was reported back to me later on that the car’s mileage increased considerably and that same car is still running strong after 150k miles on it.
Here’s what you can do to remedy this situation:
- If you have Japanese car, all you need is a 10mm socket/wrench/T-handle/L-socket and some carb cleaner. Look around under your car’s battery box or somewhere around the sides of the frame in your car’s engine bay and you will see many black wires tightened with 10mm bolts to the chassis. Refer to your car’s service manual to find the exact locations of chassis ground connections as they can sometimes be hidden pretty well under car’s plastic interior trims and other tricky locations due to close proximity of ground connections required by some sensitive electronic components. Those are basically ground wires. Usually there is only one engine block ground wire and one chassis ground wire that is connecting ground to the ECM and through it, to all the sensors.
- Take the negative terminal of the battery off first. Then all you have to do is to take those 10mm bolts off one by one, clean the connecting surface of those ground connectors with a sand paper and wipe out the connection surface on the chassis with a piece of cloth and some carb cleaner. Clean them nice and dirt-free. Also, clean the threads of the bolts that come out with some wd40 and a tooth brush in order to get rid of any rust fragments stuck in between the threads. This will ensure maximum good connection surface as you put things back together. After everything is clean, start putting these connections back one by one. Avoid over tightening of these bolts into the frame/chassis for obvious reasons. You don’t want to strip the threads so make them nice and snug; no need to crank them down hard; and you are all done.
- The last step: Reconnect the negative terminal of the battery to give your car a fresh start. By the time you are done with cleaning all the connections, the ECM’s memory containing your stale driving patterns which were built using noisy signal data input will also be gone. This will ensure that your car will now learn your driving pattern based on fresh clean noise-free signal data from the sensors instead of relying upon bad past data patterns. Not only will you feel a noticeable increase in the car’s overall power, you will also observe noticeably better car mileage overtime. If your car’s O2 sensor is foiled due to prolonged rich running, chances are that it will gradually return back to normal as your engine progressively runs better making the carbon deposits to burn off from the sensor. However in some cases, O2 sensor needs to be replaced or cleaned. How to clean it will be covered in another article though.
Some other factors which can yield the above mentioned problem:
There can be other reasons of bad fuel mileage too; for example, running your car without a thermostat valve forcing the engine to run cold for a prolonged duration, coolant mixed with engine oil causing the engine to run rough (usually due to blown head gasket), bad or excessively slipping clutch, bad axles overloading the transmission due to excessive friction, non-stock spark plugs; bad MAP sensor, bad MAF/IAT sensor, bad or laggy O2 sensor, a clogged CAT converter, worn out piston rings and/or valves etc. For those, you will have to run the necessary tests in order to pin-point the exact problem in your car but only approach those grounds after you have done this simple
It is usually a good idea to repeat this fix every now and then. I personally clean my car’s ground connections at least once a year religiously and have never faced any issues other than usual wear and tear of running components on my cars.