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Diagnosing a newer car is easier than you think. Since 1996, all new cars like the Volvo S60 and BMW 325i have been fitted with the OBD-II system, which monitors the engine very closely, specifically the emissions systems. If anything looks wrong, and the computer cannot regulate it, it will set a fault. By deciphering these faults, you can determine what component has failed, and make repairs accordingly. Using an OBD-II scanner, you can diagnose most problems at home in no more than afternoon.

Get what your budget allows

obd-scanner-pce-obd-2

When you're shopping for an OBD-II scanner, make sure you get one that can read live data. It will be a little more expensive than one that just reads codes, but you will need it to do any real diagnosis. Get what your budget allows – these tools have a wide range of prices.

Now, an important thing to realize when dealing with OBD faults is to realize that the engine computer is not as smart as you think it is. If it was really smart, it would just tell you what the cause of the problem is. Since it's stupid, it can only tell you what the symptoms are, and you, the smart one, have to figure out the cause. Dead fuel pumps cause stalling, but the ECU doesn't always know the fuel pump is dead. The ECU sees that the engine stopped for no reason, and that air has stopped coming in, and sets faults for engine speed sensor and airflow sensor, because that's where it sees the problems.

Check the freeze frame data

Lean faults are fortunately very easy to diagnose with an OBD scanner and a can of parts wash.

There's three main things that can cause a lean fault:

  1. Bad oxygen sensor
  2. Bad mass airflow sensor
  3. Vacuum leaks (Most common)

If you have a lean fault, check the freeze frame data in the OBD scanner. This will tell you everything you need to know – coolant temperature, engine load, vehicle speed, fuel trims, throttle position – at the moment the fault was set.

The first step of diagnosis is to reproduce the concern. By reading the freeze frame data, you can figure out what temperature the engine should be at when you start looking in to it. With the engine at the correct temperature, look at the live data. Read the short term oxygen sensor regulation, or fuel trims – they should be bouncing around, positive, negative, all over the place. If the values are mostly positive, that means lean (think: positive means adding fuel), and negative means rich (think: negative means taking away fuel). Check your long term fuel trims, at idle, and see how lean they are. They don't have to be perfectly zero, but should be within +/-5%.

Next, perform an extended visual inspection. Be unscrupulous; if you don't like how that vacuum hose is looking at you, put it on the list of parts you'll replace. Check for fine cracks on booster hoses, brittle rubber, broken dipstick tubes, and collapsed vacuum hoses. Unless you see a vacuum hose actually physically hanging off, don't replace anything until you've completed the next step.

Now, the fun part. If your readings are lean, then it's time to find your vacuum leak using aerosol parts wash. (Carb cleaner, brake cleaner, etc.) Make sure you're in a well ventilated area, and have at least a full can of parts wash. Watching your scanner, start spraying the engine down in small areas, but don't spray too much. You're not trying to clean the engine!

Popular MechanicsDon't spray like you're cleaning it

The idea here is to find the small leak in the air intake. By using parts wash, extra “fuel” is added to the air intake through the small leak, and the oxygen sensors read it, thinking the engine is too rich. When you do find your leak, the readings will go full negative, and you know where to focus your search, and eventual repair. Go slow, and don't spray it down like you're cleaning it. If you can, it works well to just dribble on the vacuum hoses. Spraying too much will cover too much area, and won't tell you too much.

Focus on the vacuum hoses, air intake boot, check valves, and breather hoses. On a lot of Volkswagen engines, the suction jet valve can crack at the seam, causing lean faults. If you're not getting any results, don't be afraid to tug around on hoses gently while applying parts wash. If the leak is there, it will show itself.

Doing your own diagnosis is a crucial part of doing it yourself. Without proper diagnosis, replacing parts is a gamble. And with European cars, it becomes a very expensive gamble very quickly. So check it right, replace it once, and get on with your life!


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Written by :
Chris Stovall

Chris is a journeyman mechanic from Berkeley, California, specializing in late model Volkswagens and Audis. A glutton for punishment, his spare time is spent rebuilding every component of his ’83 Rabbit GTI.


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