Yes, your car is thirstier thanks to the muffler you installed

Hi Baraza,

First, let me first say  I appreciate your vast knowledge of machines. I have only two questions.

1 I drive a Subaru Legacy BP5 year 2003 make. It works superbly. I had an average consumption of 11-12 km/l, until I fitted a sound muffler on my double exhaust and now I am getting an average of 8.2 km/l. Can a muffler make it behave this rich?

2 When driving downhill in cold weather, my engine temperature drops considerably, thereby increasing fuel consumption. I had the Thermostat removed sometime back. What could be the reason for the fall in temperature and what is the remedy? Eric W



1 Yes, tinkering with the exhaust system can affect fuel consumption, and in more ways than one. The first effect is psychological. I assume the “muffler” does the exact opposite of what its name implies;

it must be making your exhaust louder and more

rorty to enhance that Subaru rumble and create more excitement when driving. I don’t think a stock BP5 needs any more “muffling”, given how quiet it usually is.

The effect of that “muffler” noise on your mind is that it makes you step on the accelerator just a little bit harder and more often and hold on to the revs until a little higher in the range. That is my prime

suspect for the increased consumption.

If you manage to limit yourself to your previous throttle openings, then the muffler could be having an effect on exhaust back pressure. Back pressure is the resistance or opposition to desired flow of a

fluid within a confined space, in this case the confined

space being the exhaust pipe. Too much back pressure can strangle the engine, leading to air flow problems into the engine, thus reducing power.

Increased load (which the loss of power could be misinterpreted as by the ECU) will cause the engine to burn more fuel in compensation. Too little back pressure is also not very healthy: part of the intake charge could easily escape into the exhaust manifold

during valve overlap (period when both intake and exhaust valves are open), and this “rush” of air straight through the engine will misdirect the MAF sensor: the sensor will “see” that a lot of air is going into the engine and thus deliver more fuel to keep the air:

fuel ratios correct, only for part of this mix to bypass the combustion process entirely and escape with the outgoing exhaust gases from the previous combustion cycle.

That is wastage, which can also be manifest as increased consumption. Typically, too little back pressure is accompanied by a misfire/backfire and could damage the exhaust valves or poke a hole in the pipe, which lowers the back pressure even further.

2 The truncated Merriam-Webster Dictionary defines thermostat as “an automatic device for regulating temperature”. Let me explain: the thermostat controls the engine temperature and maintains it (or tries to) within the optimal range for operation.

In cold conditions, the thermostat shuts down the water pump and the fans to prevent coolant circulation, thus allowing the engine to warm up faster.

Once the engine is hot enough or the temperature goes beyond the optimum range, the thermostat activates the cooling system to bring the heat back down to desirable levels. The thermostat controls the fan and pump speeds: hotter operations call for higher

system component speeds while cold operations call for the opposite. That is why for a car with a thermostat, on a cold start the fan and water pump don’t “work”, and most cars will have the fan(s) suddenly and audibly start operating just after stop

You had your thermostat removed. To prevent overheating, the palliative measure for this is to hook up the cooling system directly to the electrical system, so that it is working full time, even when it’s cold outside.

There is no upside to this, apart from the fact that the thermostat will never bother you again. This is your situation: you are driving in a place with very low ambient temperature. The cold wind is also blowing into your radiator and chilling the coolant in it as

you drive along. And your cooling system is working at full power all the time.

The temperatures are so low it’s a wonder you don’t find ice cubes floating around in the radiator expansion tank. Now the engine knows this (via the temperature gauge and the ECU), and the engine wants to get warm for maximum operational efficiency.

To do this,the engine  burns more fuel and creates as much heat as is necessary to get to its desired temperature range.

This is against a cooling system operating on overdrive and nature’s cold breath pouring ruthlessly and relentlessly through your grille. As a result, consumption suffers.

There is another downside to operating a cold engine, and that is the oil. Cold oil is very viscous and flows slowly, so it takes longer to reach where it is needed.

This leads to damage of those parts (if you have a turbo then you really need to watch out, especially in a diesel engine). The oil itself gets degraded very fast because the engine running rich in its quest for heat means a lot of (cold and probably condensed)

fuel is dumped into the cylinders, where it 1. Washes oil off the cylinder walls (more damage) and 2.  Mixes with the oil, which tends to dissolve the oil. If you are not careful to dramatically shorten your service intervals when running such a powerplant, a

replacement will be on the books soon.

Remedy? Reinstall the thermostat.


Dear Baraza,

Thank you for making reading about mechanics so much fun. I have two questions.

1 Can I get past issues of your column online?

2 I have  had a 2007 VW Touran TSI BLG engine 1.4 for a year now. Lately, it seems to lose power when I accelerate suddenly, for example when overtaking.

3It has a very hard start, usually after I stop, and sometimes just shuts down and stops whenI am driving.

4I have been advised to take down the engine and check the  pistons.

What would you advise?




1 Some, not all the past issues of this column, are online.

Now, 2, 3 and 4: Before tearing the engine apart,  do a compression test. That should tell you whether or not the pistons are the culprits here. The loss of power, hard start and stalling could be symptoms of failing plugs  (or high tension leads), so have those

checked too. Eliminate all possible theories that do not involve disassembling the engine before actually disassembling it.


Hi Baraza, After reading your column on January 27, I realised that you answered what I had enquired about sometime back as narrated in the next paragraph, which is still not clear to me.

“And now the malfunctioning diverter valve and “fake-sounding’’ ‘air injection system’. How can they be fixed and is it a complicated affair because I think I can do with my Mark II the way it is as I have no other issue with it, or what are the consequences of not fixing the valve?”

I have noted that when I start the engine in the morning,  the revs are usually high (1000-1100 rpm) and in an earlier review you rightly explained that the engine revs up fast to attain the operating temperature in a shorter time.

However, I have also noted that the exhaust produces a “popping” sound  that is louder than for same range of rpm when the engine is hot. Is this normal?  

I have also noted that when I  brake (a bit hard) while going downhill at 80-90kph, the transmission engages a lower gear and the revs go up. Is this normal and does it happen in all auto transmission systems(cars)?

Do the higher revs mean more fuel consumption although the foot is off the gas pedal?  The revs remain high for a while even when I release the brake pedal.

Previously, I had a manual transmission car (Toyota Corolla G-Touring) and I have noted that for the same speed, the auto gear revs seem to be higher. Is there an explanation for this or is it because my current car seems has four forward gears while the

manual one had five? What are the signs that the ATF requires change if one does not have the service history? Motor enthusiast,  L. Magambo


The diverter valve can be handled only by a mechanic if you do not know your way around the engine. Replacement is the best option here.

The high revs at idle in the morning are, as I said earlier: the engine is warming up quicker. The louder decibel range is something I have noticed in almost all cars, and is the result of the high idling speed. If, you get a chance to adjust the idling of a vehicle,

try and note: adjusting the idle speed to 1500rpm creates a much louder engine than simply revving the engine to 1500rpm. I don’t know why.

The downshift on braking does not seem all that strange, especially if your transmission is in Sport mode or the equivalent of Sport mode. It provides better engine braking as well as keeping the vehicle “primed” for a more aggressive acceleration should

your right foot change pedals. Provided you are off the throttle (foot off the accelerator), do not worry about fuel consumption. With no load on the engine, the injectors are either delivering very little fuel into the engine or they are shut off (depending on how advanced your engine management system is).

There is an explanation for the difference in engine speeds between the two cars: first, the cars have different engines. That means the engines develop different peak power/torque figures; and at different levels.

That in turn means the transmission ratios have to be engineered in such a way as to take advantage of the different power/torque curves.

Second: one is a 5-speed manual, the other is a 4-speed automatic. The gear ratios are different: hypothetically, both cars cut off at 180km/h. To achieve 180km/h, one transmission has 5 gears while the other has 4. There will be points where there will be

overlap (same speed, same gear), and points where there will be two distinct and discrete gears (say 3rd in the automatic and fourth in the manual for a given speed). If we were to force both transmissions into the same gear “number”, this is what would

happen: shifting the auto to fourth (if possible) causes a dramatic drop in engine speed. Alternatively, shifting to third in the manual causes a dramatic flare in the revs. Same road speed, same gear but very different engine speeds.

Third: the automatic gearbox uses a torque converter, which is a wet clutch with plenty of slip engineered into it. That slip, before the transmission locks up, also causes high engine revs for relatively lower road speeds in an automatic.

When you check the ATF and discover it has changed colour (it should look like honey), it has black or dark flecks in it, or it smells like burnt bread, know it is time to change it, and quickly.

But I would advise that you rely on service histories, if available, and manufacturers’ recommendations.


Hi Baraza,

I have been using a Honda Airwave 2006 for just over two years now and it has served me well. I service and change CVT on time. 1. However, I have noted recently that when I stop and am shifting the lever from drive to neutral, it shudders.

  1. I also find the lever very tight when shifting from P to D when I am starting on an uneven surface or on a slope. This has been an issue since I bought it.
  2. There is also the issue of plugs. I am still using the ones it came with, having driven 46,000 kilometres. Whenever I take it for service and I ask about them, I am told they are fine. I am getting nervous.

Is this right of should I be looking at the mileage covered to change them. I bought some at the local dealers and despite having to wait and pay through the nose for them, they don’t look exactly the same so I kept them.3. There is also a slight jerking that

only a keen driver will note when accelerating. What’s the problem?

Would appreciate your input.

Thubuku W



  1. Check the transmission fluid level. It might be too high or too low. If the level is fine, another diagnosis could be either bad engine/transmission mounts or bad driveline (constant-velocity) joints. In both cases, replacement is the only option.
  2. This does sound like an issue with either the CV joints or transmission mounts. The uneven surface/slope means that the engine/transmission combo is not exactly in a straight line and has, therefore, been “torqued” (external stress applied has resulted in a

bit of “twist”). With poor mounts, the twist might be more apparent than in a car with good mounts, and with the driveline slightly crooked, getting into gear is not easy.

  1. If the plugs are fine, then just keep driving. When the car starts developing a slightly hard start and/or losing power no matter how subtly, you will know your plugs are due for change. Most people will not agree with this (preventive maintenance is

always better than curative maintenance), but some plugs are meant to go for high mileages and changing them early might be a waste.

Check your vehicle handbook and/or the information detail on the plug to know exactly how long they should go before replacement.



Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.