Engine Diagnostics: Mazda – UnderhoodService

Engine Diagnostics: Mazda

We’re going to take a look at the Mazda line of cars and trucks with an eye on driveability service and diagnosis. In particular, we’ll look at some driveability problems that may result in an illuminated check engine light, as well as some problems that will illuminate the light but may not result in a driveability complaint.

Mazda continues to offer a diverse line of vehicles that deliver not only good, reliable service and value, but a strong performance image as well. Although it’s up to us keep the “zoom-zoom” under the hood, there’s no magic involved — just a good maintenance program, smart diagnostics when required and knowing where to look for help when needed. The most common failure we see on Mazda models is a torn air intake hose between the airflow sensor and the throttle body. The symptoms that result from this failure are hard to miss. The usual complaint is a big stumble or stall on initial acceleration. As the engine torques on the mounts, the crack in the hose will open, allowing unmetered air into the manifold while, at the same time, reducing airflow through the sensor. Of course, when the engine stumbles, the crack closes, allowing the engine to accelerate, starting the entire process over. Many times the customer will mistakenly think the transmission is the problem.

Although it’s simple to diagnose with a visual inspection, I’ve seen some good technicians tricked by this one. These hoses become hard and brittle over time, and you may not find the fault simply by squeezing the hose. Be sure to inspect the hose closely for cracks that won’t be obvious but will become evident when the engine is loaded against the mounts.

Another common driveability/ check engine light problem on Mazdas is a dirty hot wire mass air flow sensor (MAF). The first indication will be a malfunction indicator light (MIL) with the system lean code but no driveability complaint. More severe cases will cause hesitation, stalling and poor power complaints. If there is a check engine light, start your diagnosis by retrieving codes. Be sure to check the freeze-frame data, making note of the long-term fuel trim. If you have a lean code, you should find a long-term number in the 20s, indicating that the engine is getting all the fuel that the ECU is willing to deliver, yet the oxygen sensor is still reporting lean.

Of course, there are a lot of reasons for an engine to run lean, and if there are multiple codes that could contribute to a lean condition, they should be checked first. Take advantage of your scanner’s capabilities; look at the data that would affect mixture. Look at temp., throttle and 02 sensors and make sure the thermostat’s working and that the ECU knows it. Many times things like poor grounds and wiring problems show up as bad sensor readings first.

Short-term trim is your window into how the fuel’s being adjusted; it’s valuable information that shouldn’t be overlooked. At a steady throttle opening, look for a close-to-zero number with a slight variation as the ECU works to keep it there. We know that a negative number means fuel is being removed, so since we’re looking for a lean condition, we expect to see a positive number as fuel is being added to compensate for the lean condition. If you see a positive trim number at idle, check closely for vacuum leaks; don’t overlook the crankshaft ventilation and EVAP systems as sources for unmetered air. We’ve seen intake manifold leaks on four-cylinder Mazdas as well, although they’re usually accompanied by a rough idle complaint. Don’t overlook the basics.

If you have the desired near zero reading at idle, take the car on the road with an assistant, if needed, and make note of the trim number. Again, at steady throttle you’re looking for a close-to-zero number. At the same time, keep a close eye on the MAF sensor signal voltage.

With the MAF signal, we’re looking for an increase in voltage as the airflow increases; max airflow results in a signal of approximately 4 volts. During your road test, make a few full throttle accelerations, taking note of how the car responds as well as the sensor voltage. Keep in mind that a plugged catalytic converter or air filter can contribute to a low signal output, but if the air isn’t there you probably won’t have the lean code as much as a poor power complaint. While we’re talking about the exhaust system, it’s also important that there’s no exhaust leak before the cat that would trick the oxygen sensor into thinking the engine is lean.

If there’s still some question, this is a good time to grab your graphing meter or labscope and take a closer look at the MAF sensor. This also gives you the opportunity to check signal voltage into the sensor as well as confirm the integrity of the ground circuit. Watching the graph for a nice clean signal, do a couple of snap throttle accelerations. Again, you’re looking for a clean linear signal with that 4-volt peak. In the example, the signal never got to 3 volts. As you can see in the photo on page 33, this was an extreme case that resulted in both a poor performance and MIL complaint.

The sensors can be carefully cleaned, but be gentle as they are fragile. We were able to clean the one in the photo and put it back in service on this high-mileage, older car, but a safer route is to clean the unit to confirm the diagnosis and recommend a new sensor to ensure a successful repair. Once the problem is diagnosed and repaired, make sure that the bolts holding the sensor to the air filter housing are tight, that the air filter box is not allowing unfiltered air into the sensor, and that the air filter element is of high quality and in good condition. There have also been reports of the sensor housing bolts coming loose, letting dirt into the air stream. Be sure all the air is being filtered.

Another check engine lamp issue that can give you fits is P0400, excessive EGR flow. The first problem is the fact that an excessive flow code will be thrown when the actual problem is insufficient flow. The excessive report is the result of the ECU commanding more EGR when the engine doesn’t respond, as expected, to the EGR valve being opened. The most common cause for this lack of response is plugged EGR passages in the manifold. By removing the throttle body and EGR valve, the passages can be cleaned using a liberal amount of carbon cleaner and a piece of speedometer or hand brake cable in a drill. On some models, it will be necessary to remove the upper plenum. Be sure the vacuum line and manifold nipple are clear going into the MAP-type sensor that serves as the EGR boost sensor.

While we’re working in the area, it’s a good time to service the throttle body. It doesn’t take much of a buildup on the throttle plate and housing to affect performance. Everything from a hard start to poor idle quality can be traced to a dirty air intake. In extreme cases, the buildup will cause the throttle plate to stick in the closed position, with the customer complaining of having to snap the throttle open.

Another problem that’s been reported on the air intake system is the idle control valve sticking in the closed position. Although the valve is not available without buying a throttle body, many techs have had good luck cleaning the unit. Get a gasket, remove the valve blocking off the cooling lines and clean the passages. It may take a couple of soak and flush deals to get it clean. While you’re at it, clean the passages and the idle adjustment port. When you’re done, check your service information for any special procedures required to set base idle speed.

Another common code, the P0300 series indicating misfires, moves us to the ignition system. Many times a misfire code will have a driveability complaint associated with it, but either way the diagnostic strategy is similar. If the miss is always evident, it shouldn’t take you long to figure it out. Be sure the engine is mechanically sound with good compression and spark plugs that are in good condition.

While you’re there, take a good look at the wires for any cracking or indication of carbon tracking; look closely for pinholes burnt through the plug boots that would let the spark get to ground. If any oil is evident, replace the valve cover gasket with spark plug tube seals. If the oil is deep enough, that could very well be the cause of your problem. Grab your stethoscope and be sure the injectors are opening and closing by listening for that distinctive click with the tool in the same location on each injector; they should all sound the same. I will say that we see very few problems with Mazda injectors.

In recent years, Mazda has made the move to a distributorless ignition (DIS). Using waste spark, coil pack and coil-on-plug systems, they all have the same job of providing spark at the correct time. None of the systems have been immune to problems. Plug wires have been the most common failures, but we’ve seen some coils fail as well. In the best case, the OBD II system is reporting the offending cylinder, sending you in the right direction. If not, it can be tough to pinpoint the offending cylinder, especially if it’s an intermittent problem.

While you can use a low-amp probe to look at the coils, it can be difficult to catch the problem. On a higher mileage, four-cylinder car that uses two coils and two wires, I would tend to replace all the components. On coil-pack cars, history tells us wires are the more common failure. With six-cylinder, coil-on-plug cars where the cylinder can’t being pinpointed, you have to decide if you should change them all or wait for the failure to become more evident. That decision would be between you and your customer.

If you’re diagnosing a driveability complaint on older cars with a distributor, check the plug wires first. Even if there is no apparent misfire, an open plug wire will send electrical forces bouncing around under the hood, causing driveability problems you wouldn’t expect from a bad wire.

The other weak link in the ignition system is the distributor itself. You may have a complaint that the car cut out but when it was started back up, it seemed fine. Or maybe it just cut out and wouldn’t start. There are no serviceable parts in the Mazda distributor, so if you have a no-spark condition that is traced to one of the components housed in the distributor, the unit will have to be replaced.

Actually, on some models, the igniter is available, but it costs as much as a rebuilt distributor. The aftermarket has high-quality rebuilt units available, but they all don’t come with the O-ring. You may be able to carefully switch the old one, but it’s much safer to have a new one on hand. When the time comes to put the timing light on, be sure to check your service information for any special procedure required to set the base timing.

One thing holds true no matter what ignition system is used: If you ignore a misfire code for any length of time, you will be faced with a P0420 catalytic converter efficiency code. Again, using the data side of the scanner, we want to take a look at the front and rear O2 sensor to see if the cat is doing its job. Unfortunately for the customer, we usually see the sensors mirroring each other, telling us the misfire has damaged the cat converter. If the front sensor was switching nicely while the rear stayed steady, we’d know the cat’s doing its job. Keep in mind that the cats are covered with an eight-year, 80,000-mile warranty.

Mazdas also have their share of EVAP codes. A P0455 indicates a large EVAP leak. By catching some purge valve vacuum in the tank (by closing the vent valve), the ECU looks at the tank pressure while the vent valve is then opened. If it doesn’t see a pressure change, it assumes there is a large leak and the code is set. You have to be careful here since the ECU doesn’t know if the vacuum wasn’t held because of a leak, or if the vent lines are plugged not allowing the pressure to change as expected. If your software supports it, monitoring the fuel tank pressure will let you know what problem you’re facing. One tip that can save you some time is that the vent valve will click when the ignition key is turned on to at least telling you it’s working, but won’t if it’s sealing. Other EVAP issues include the gas cap and the usual leak suspects of filler necks, canisters and lines.

With Mazda’s reliability, there aren’t many silver bullets to share, but there a couple of things worth mentioning. The early OBD II Millenias will throw O2 heater codes if the battery voltage dips below 9 volts while cranking. The car will crank and start fine but, being a two-trip code, it will be set on the second start after being cleared. Another issue on the older cars is an airflow failure that will result in a no-spark, no-injector pulse condition. A shorted airflow meter will pull all the 5-volt reference signals to ground, shutting down the system. Simply unplugging the airflow meter will have the car running in limp.

I hope this article gives you the confidence to handle any Mazda that may show up in your bay with a check engine lamp or driveability problem. I also hope it will motivate you to take full advantage of your scanner’s capabilities and make use of the valuable fuel trim information that’s at your fingertips.

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