Direct or indirect: Which tire

Jun 21, 2023

I rendezvoused with a friend this past weekend at a farm that currently has a barn under construction. On the way home, both of our vehicles developed tire leaks, resulting in our low-tire-pressure indicators coming on. I’m going to assume we both picked up some construction debris while on site.

My 2018 Volkswagen has an indirect tire-pressure monitoring system (TPMS), which only tells me which tire is low on air but not the exact tire pressure. My friend Stephen's 2017 Dodge Caravan has a direct TPMS, which displays the exact tire pressure for each tire. Amazingly, his Caravan does not come equipped with a spare tire, which in my opinion is a significant oversight on the part of the manufacturer, but that's a topic for another day.

Direct versus indirect TPMS: What's the difference? Which system is better? My Volkswagen uses the existing wheel-speed sensors. A leaking tire will become smaller as it deflates and therefore spin faster when compared to the other three tires. The onboard computer sees that one tire has an elevated wheel speed and deduces that a low-pressure situation is occurring and warns the driver accordingly. Indirect systems are cheaper for manufacturers because the wheel-speed sensors are already installed in most vehicles for anti-lock-brake use, making the addition of TPMS a software-only addition.

Alternatively, the Caravan comes equipped with a tire-pressure sensor located in each wheel that transmits data via radio frequencies to the onboard system. Live tire-pressure data is then directly displayed for the driver to monitor. The problem is that every time the tires get rotated on his van, the sensors move with the wheels to a new location. Unless the TPMS is relearned at every tire rotation, the tire-pressure information is now displayed out-of-location. These sensors also require replacement all too often, meaning most people just ignore them once their light comes on.

I managed to easily get my spare tire on, but Stephen, knowing his vehicle wasn't equipped with one, decided to push his luck. Even though his vehicle comes with a small air compressor, he didn't use it. In hindsight, I’m sure he wish he did, as his leak turned into a full blow-out requiring the vehicle to be towed off the side of the road. Ultimately, it was not the manufacturer's fault that he had a tire blow-out, but it really shouldn't be this difficult.

So, which one is better? Personally, I have no need to know my exact tire pressure while I am driving. I only need to know when there is a problem and where that problem is. While the addition of the exact tire pressure offers a slight convenience, it is not worth the additional service required to continually relearn the TPMS systems, troubleshoot and replace failed sensors. Plus, winter tires wouldn't require the added expense of a second set of sensors. My vote is to get rid of TPMS sensors as all manufacturers are now capable of providing just the right amount of tire-pressure information without them.

Hi Lou,

My daily driver is a MK VII Volks GTI, and like many newer cars, it has a direct fuel-injection engine. From what I understand, carbon buildup on the intake valves is more of a problem with this design, since the fuel no longer "washes" the valves when it enters the combustion chamber.

However, there is no mention of decarbonization in the maintenance schedule, and I am wondering what type of service you recommend, how frequently it should be done and about how much I can expect to pay.

Thanks,

Tom B

Decarbonization is a relatively new service procedure. While all dealers provide this service for their customers now, it almost feels to me like manufacturers have their collective heads in the sand and won't admit their engines actually require this service. Hence the reason why you won't see this service in your factory maintenance schedule.

Some direct-injected vehicles will see a minor decrease in fuel as early as 40,000 kilometers due to carbon build-up on the intake valves. At 80,000 kilometres, those same vehicles will have noticeable cold-morning start misfires, eventually turning on the malfunction indicator lamp (MIL). I advise my customers in two different fashions; those, like you, who are looking for a preventative maintenance decarbonization and those who are looking for a solution to a drivability problem. For customers that opt for a preventative maintenance decarbonization in the 80,000-100,000 kilometer range, they usually report a noticeable improvement in fuel economy, more power and better idle.

There are differing procedures from chemical decarbonization to manual decarbonization, which is where the technician removes the intake manifold and literally chips away manually at the carbon, to a walnut-shell blasting procedure. Prices will vary from $350 to $800 depending on procedures and severity.

Why is my hybrid using a lot of gas? I bought it in April 2019 at 80,000 miles, and it now has 110,511 miles. I just filled it up, and I’m on a half-tank already and only drove 115 miles. I just had the oil, air filter and cabin filter changed. I work out of my car and have the AC on all day, but it just started getting worse all of a sudden. I put a can of SEAFOAM in it, but I don't know if that is bad for hybrids. The car runs fine; it just eats gas. I was using low-grade gas, but the last two weeks I’ve been using the premium or 91 grades to see if that makes a difference. I thought hybrids got great gas mileage? Please help.

Michelle M

Yes Michelle, hybrids are designed to be more fuel-efficient than their internal combustion engine (ICE)-only counterparts. Keep in mind, however, that all hybrids still do have an ICE and are still susceptible to all the malfunctions they can encounter. You haven't told me what kind of vehicle you have, so I can't provide any specific help. General tips include maintaining tire pressure, replacing filters as you have already done and making sure other tune-up items, such as spark plugs, are up-to-date.

That your vehicle had a sudden change in fuel economy suggests something else has gone wrong. Be aware that changing the grade of gas will not improve your fuel economy, so don't waste your money. Take the car in to your local dealer or an independent shop with hybrid experience and explain the recent change in fuel economy. Have them perform a full inspection, including looking at everything from dragging/seizing brakes to possible failing engine air/fuel ratio sensors.

Lou Trottier is owner-operator of All About Imports in Mississauga. Have a question about maintenance and repair? E-mail [email protected], placing "Lou's Garage" in the subject line.

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