Catalytic Converters and Gas Mileage: What’s the True Connection?

At the other end of the phone was one of my friends who lives miles away. Like most car owners, when he noticed that his car was gulping more gas than was reasonable, he approached his ‘mechanic’, and this was their verdict.

I wondered for a second, the connection between catalytic converters and gas mileage, and how someone who calls themselves ‘mechanic’ had managed to reach such a hard-to-understand conclusion.

“If you can, install the absent catalytic converters,” I answered sincerely. “But, it won’t do anything for your gas mileage.”

There was a long, awkward pause , one that made me certain that a lot of thoughts were going on in his mind. At long last, he spoke.

“Why so? The mechanic is so certain.”

“Is that so?” I half asked and half stated. Without giving him the chance to respond, I continued. “Then, go with him. After that, be sure to report back if things have improved.”

Inwardly, I was happy that the hitherto catless car would now produce less harmful gases (which is great for the environment). Deep down, though, I knew that my friend’s concern was MPGs and not the environment. I also knew that disappointment was looming steadily in the background, once the moment of truth became imminent.

We caught up and eventually hung up the phone. He also promised to call me when the new catalytic converters had been installed.

The Call I Had Been Expecting…

Finally, the call I had been expecting came through — and from the first second, I knew it was exactly as I had predicted.

“What’s up?” I enquired.

“The car is still consuming gasoline like an annoyed fish takes water!” He almost yelled.

I was taken aback, unsure if I had mistakenly told him that new catalytic converters would solve his issue.

“What’s your mechanic saying this time?” I asked.

“He’s telling me there’s nothing wrong with the MPGs — and it’s really frustrating. He wants me to believe that everything is in my head.” He wrapped up, clearly frustrated.

I knew better…I also knew MPGs don’t lie — and there was no way they could be ‘in one’s head’. Importantly, my friend also knew better than to believe his mechanic blindly (this time around, at least). 

We were back to square two (not one, since the car now had functional catalytic converters, which is very good in its own right).

I decided to do what no mechanic likes doing:  run a remote diagnosis.

Catalytic Converters and Gas Mileage: What Actually Helps?

I made a mental list quickly and asked my friend how his car compared to the list. For what he wasn’t sure, I simply asked him contextual questions about the subject matter to deduce what could be wrong, if any.

Here’s what I came up with.

1. Binding Brakes

The brake system on a traditional Internal Combustion Engine(d) (ICE) vehicle appears to be a complex thing until you understand its working principle.

Explained simply, when you depress the brake pedal, the hydraulic fluid (brake fluid) in the system, by means of the master cylinder (or brake booster, if you will), travels along the hydraulic lines and creates pressure behind the brake caliper piston(s).

The brake caliper piston(s), in turn, travel out of their caliper housing and compress on the back of the brake pad, which in turn compresses on the brake rotor, which, depending on the level of pressure exerted, slows down or completely stops the vehicle.

However, in practice, many things can go wrong, things that negatively impact MPGs. Chief amongst these things is binding calipers.

The caliper guide pins can get stuck in their brackets, failing to exert pressure and release as needed. The brake pad guide plates and shims can also rust out, causing stickiness and all manner of challenges relating to pressure exertion and release.

When these happen (sticky guide pins, messed-up brackets, rusted shims, and guide plates), one thing is consistent: the brakes are ‘applied’ even when you have no such intentions.

The results?

Binding brakes, burning smell, premature wear of brake pads (and associated components), and, importantly, because you now need to accelerate harder against brakes that have some degree of hold, terrible MPGs.

2. Wrong Engine Oil Weight

The function of engine oil in an ICE, traditionally, has been to mitigate heat and manage metal-to-metal contact by acting as a thin layer between the various moving parts.

To efficiently achieve this, engine oil needs not only to maintain overall viscosity, but it must also be of the right viscosity to start with.

This is where figures like 5W3o, oW20, 0W8, etc come in: the number before the ‘W’ serving as viscosity when cold and the number after, the viscosity at operating temperature.

These may all appear like some unnecessary jargon at first. However, messed up, this logically forms the basis of outrageous MPGs.

But how does this happen in the first place?

Modern internal combustion engines, to meet the strict emissions (and fuel efficiency) standards governments around the world are imposing on them, are increasingly making do with thinner (and lighter-weight) engine oil.

Take Toyota, for example: from 5W30 to 0W20, 0W16, and now 0W8, the goal has always been to go lighter as far as lubricants are concerned.

The engines themselves are designed with tighter clearances around them to achieve both efficiency at the pump and generally less emissions, using lighter-weight engine oil.

The irony?

Most ‘mechanics’ working on cars, especially in the developing world, make the huge mistake of recommending 20W50 engine oil for vehicle owners, irrespective of express recommendations from the manufacturer, forgetting that modern engine oil isn’t only designed to keep lubrication, but it also serves hydraulic purposes for such systems as the lifters.

The summary?

Vehicles made to work with 5W30 (or lower weight engine oil), for instance, will struggle to push heavier engine oil weights through their systems. Not only that, lubrication will be minimal, wear and tear accelerated, and overall, very poor MPGs will logically follow.

Sad truth to type, but an honest reality, nonetheless.

3. Vehicle Running Without a Functional Thermostat

The engine thermostat, regulating an ICE, is an amazing but simple and clever device. Usually mounted between the radiator and the engine block, the thermostat closes shut when an engine is started and remains shut until that engine reaches a preset temperature (its operating temperature).

At an engine’s operating temperature, the thermostat opens – and allows coolant to flow into the block, keeping the engine running within ‘spec’, avoiding overheating.

Essentially, thus, a thermostat’s sole function is to allow an engine to heat up quickly to operating temperature and, having heated to such a temperature, remain so.

As straightforward as this appears, many mechanics (again, in the developing world) get it wrong: they remove the thermostats from vehicles, either immediately they are bought or once they begin overheating, in the name of ‘hot weather’.

This, without question, is usually the beginning of higher-than-normal gas consumption.

But why is this so?

An internal combustion engine isn’t meant to run cold. As a matter of fact, an engine running cold is as bad as one overheating – you don’t want either.

The trick is to stick within the range the engine thermostat calls for – and get there as fast as possible. This is why, on cold starts, your vehicle’s transmission hardly shifts beyond the third gear. The logic is to get your engine and tranny heated quickly up to operating temperature.

Unfortunately, when you remove the engine thermostat, it becomes impossible to have your engine run at this all-important operating temperature. And, when your engine runs cold, the oxygen sensors don’t get heated enough to send valid air-fuel ratio data to the ECU, regarding driving conditions.

Without this much-needed information, the engine runs at what is termed the ‘open loop’, which is a fancy word for preset values, as against real-world realities. The preset values here? Dumping fuel in cylinders to forestall stalling, just like what a choke does on a bike, on cold starts (remember, the engine believes it has a cold start situation).

The irony? The ‘cold start’ situation runs as long as the engine remains on, and the dumping of fuel remains a perpetual issue, ensuring very poor MPGs, both at cold start and throughout the day.

Talk of a classical lose-lose case!

4. Running Cooling Fans 24/7

The cooling fans of most vehicles with an ICE today are electric. Being electric (as opposed to mechanical), they aren’t driven by the crankshaft, so they don’t turn with the engine (or immediately you power on the ignition) – at least, under ideal working conditions.

Unfortunately, as was discussed immediately above, most mechanics in the developing world aren’t only wrongly obsessed with thermostats; they also complement their wrongful thermostat removal with running the fans directly to the battery so that they run all the time, immediately the ignition key is turned right up to the moment the vehicle is switched off.

But why is this act dangerous to MPGs?

Simple: minus when the air conditioner in a vehicle is in use, the cooling fans only come on periodically, cool the engine for about 30 seconds, and go off again.

The theory behind their operation is simple: they only come on to keep the engine from overheating when it begins to head beyond acceptable thresholds. It achieves this by means of the engine coolant temperature sensor, which dips into engine coolant by design and reports to the ECU directly.

With a compromised cooling fan that runs all the time, only two things are certain: overcooling of the engine, making it harder (or impossible) for the engine to reach (and maintain) operating temperature, and at the same time, increased cooling fan wear as a result of working around the clock.

None of these is what any vehicle owner wants – or will appreciate: they tax you immediately at the pump and indirectly in fixing the cooling fans that will need replacement sooner (rather than later).

5. Dirty Engine Air Filter and MAF Sensor

Many functional parts make an ICE work in the manner it’s intended. These parts, while many and seemingly overwhelming, especially for the newbie, are all interconnected and work in perfect harmony. Screwing up one usually means screwing up the next, and bad values got from one are likely to also affect the values of others, in that loop.

Two such parts that when messed with will create a series of MPG problems are the engine air filter and the MAF sensor.

So, how do these two innocent-looking parts affect MPGs, especially the outrageous variants that everyone is literally scared of having?

It’s simple. Allow me to explain.

For combustion to take place and a car to move, three (3) things must be present: spark, fuel, and air (oxygen, basically). These ‘constituents’ usually need to be present in a manner dictated by the stoichiometric ratio for that engine (which is essentially the sum of the parts that need to come together for complete combustion to happen).

Unfortunately, we don’t live in a perfect world, and sometimes, that ratio is badly screwed up. Two such instances where this can happen are when the air filter is dirty, affecting airflow to the engine, or an equally dirty MAF, throwing incorrect readings to the ECU.

In both cases (unless remedial actions are taken immediately), more fuel than is needed winds up in the combustion chamber, with the vehicle not only consuming more fuel than needed but also, painfully, not burning up everything in the rich ensuing mixture, usually leaving a dirty tailpipe as clear evidence of what is happening.

6. Bad Oxygen Sensors

The MAF and engine air filters aren’t the only things that have the power to critically mess up your MPGs, especially to the point where you actively believe that you have a leaking tank or something along those lines.

A faulty oxygen sensor, also called O2 sensor, can equally mess things up greatly for you, and if proper care isn’t taken, you’re likely to believe that your lack of a catalytic converter (or catalytic converters) is responsible for the mess.

This is how it all plays out.

The precat oxygen sensor(s) monitor exhaust gases for the presence of oxygen (the postcat oxygen sensor(s) are concerned solely with the functionality or otherwise of the catalytic converters themselves). If and when found, this confirms that incomplete combustion is taking place, with unburnt elements.

With this realization, it signals the ECU to adjust the air-fuel ratio to ensure a better mixture (more air for the same fuel or less fuel for the current air pact) and, accordingly, achieve better MPGs.

However, it always isn’t this easy nor is it always very straightforward: a bad oxygen sensor will give a false reading (or no reading at all), keeping the ECU in the dark (or feeding it with wrong information) and ultimately, giving it no option but to revert to default values which don’t serve you both in the long and short term (at least fuel economy wise).

7. Poorly Inflated Tires

Black by default, the tires of a vehicle powered by an internal combustion engine have the basic duty of moving the vehicle forward and providing some level of comfort from road irregularities.

However, this isn’t all: tires achieve much more than propelling a vehicle forward. They also serve much more than making sure that you don’t get stuck when the going gets tough.

However, that’s just the tip of the iceberg: tires also contribute extensively to fuel economy – or the lack thereof.

Here’s how.

A vehicle’s tire bears its weight as the vehicle travels from point ‘A’ to ‘B’. To achieve this, a vehicle relies on the proper inflation of these tires to cancel inertia and brilliantly ensure smooth movement.

This is why on the door jamb of modern vehicles, there’s usually an advisory indicating what this inflation point, usually in PSI, should be.

Unfortunately, a vehicle rated 33 PSI, for instance, will have poorly inflated tires, reading about 15 PSI, and will still appear perfectly normal to the visible, naked eye. However, such readings will offer intense resistance to motion, and the net result would be an increased, unhealthy MPGs, caused by increased throttle efforts to achieve basic movement.

8. Running AC in the City

On sunny days, especially during the hot summer months, the AC of a vehicle is a lifesaver. Drive with it, and everything is fun. Switch it off or have a bad unit, and all hell is let loose.

If you won’t be bothered with details, this is about all you need to know – and enjoy your ride.

However, if you’re pro details, it’s much more than switches, cold air, and ultimate comfort.

The air conditioning system of a vehicle powered by an internal combustion engine runs primarily off engine power. This means the engine is robbed of some oomph to run the compressor, which makes sure that refrigerant flows through the system and cold air is supplied.

This, by default, means higher than usual MPGs, since the vehicle’s engine works harder than normal with the air conditioning on.

On the freeway, this extra burden (and fuel burnt) is lower than the drag that happens when the windows are rolled down. So, it makes sense to roll up and instead use the AC, even if it will tax the engine and consume (slightly) more fuel.

In the city, however, the game is completely different: the wind resistance isn’t usually more than a passing nuisance and often, is nothing serious. Accordingly, using the AC in the city (or stop-and-go traffic) generally consumes more fuel than if it were used on the freeway.

Things take a headway in chaotic traffic if you insist on using the air conditioner and have to commute in a city mad with traffic all the time.

9. Bad Driving Habits

When ‘bad’ driving is mentioned, images of accidents and a vehicle filled with body filler, such as the photo below, fill the human brain.

Granted, this is one of several challenges of driving in a bad, rough manner. However, that’s just the tip of the iceberg – and the very obvious.

Away from such challenges, there are subtle signs of bad driving, signs that don’t immediately scream to your face or make their presence clear, at least, from the fore. One such reality is consistent bad MPGs.

This is how it all pans out.

Driving looks simple enough on face value. However, it’s one of the most complicated acts ever, with various laws of physics playing out at any given time.

For instance, it takes a lot less power to keep a vehicle moving than to start it from a standstill. This is generally (and summarily) why freeway driving takes less gas than traditional city stop-and-go traffic.

Now, if you’re a lunatic who loves to drive like a Fast and Furious actor/actress/fan, every time you brake and pick up speed again, you burn more fuel than is necessary, and this ‘off and on’ action, though insignificant initially, quickly adds up, and winds up as poor MPGs and frequent trips to the gas station.

10. Running Regular Gas in a Vehicle Requiring Premium

Most performance vehicles are rated to use premium gasoline only. This warning is usually contained on the gasoline filler cover, in the user manual, and everywhere else you’re likely not going to miss it, especially when you decide to visit the gas station.

Unfortunately, many folks who drive vehicles that call for premium gasoline comfortably use regular gas, thinking that ‘gas is gas’ and as far as the pistons move and the vehicle takes them to where they’re going, they have succeeded in saving money.

It turns out, gasoline grades aren’t only advisories in most cases, nor are they mere, simple recommendations. They are important considerations that must be religiously followed if the vehicle is to run at optimal capacity and efficiently.

This is where the story gets interesting, and importantly, this is where things take another dimension, beyond the pings and knocking lower octane gas is known to have in vehicles that call for premium gasoline.

How does this work in reality?

Premium gasoline burns evenly, without forming any air pockets. This reality maximizes power while minimizing energy losses. With lower octane gas, the opposite is true: air pockets form randomly, knocking occurs, and peak power isn’t delivered through the combustion process.

To overcome this challenge, the accelerator works harder (but hardly more efficiently). This results in one thing: poor MPGs.

So, Was the Issue Sorted?

Yes, I discovered that he was driving his car without a functional thermostat — and that was the biggest contributor to the MPGs nosedive the car was experiencing. 

He took the vehicle back to his ‘mechanic’, who swore that it wasn’t the issue. Getting to the end of my patience, I advised him to ignore his original mechanic and approach another.

He did, and that was when the ‘drunk fish’ finally became sober (and changed from being ‘a fish’, in the first place)!

So, What Does a Catalytic Converter Actually Do?

Mounted on the exhaust system of vehicles, a catalytic converter (or catalytic converters) serves the singular purpose of converting the harmful gases a vehicle produces to non-harmful end products like steam and water.

Unless it’s blocked, its absence doesn’t have any bearing on a vehicle’s fuel efficiency. That said, for vehicles that have their cats removed completely and the oxygen sensors damaged (or also removed) in the process, the rules are different; not because of the absence of the cat but because of how its absence also affects the oxygen sensors, sensors very crucial for the right MPG settings (air-fuel mixtures).

Wrap Up

The thermostat in a car plays a vital role in fuel efficiency (as do several other options mentioned above). Unfortunately, many have no idea that this little unit plays a super important role in keeping the engine hot and economical.

For my friend, a unit costing just a few bucks was responsible for his woeful MPGs, and replacing it sorted the issue most classically.

For you, your challenge could be any of the other nine issues mentioned and treated above. Or, if you’re cursed enough, the lack of a functional thermostat could also be the reason why you’re on a first-name basis with all the gas station workers along your route!

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