Two Guys Rally

Hunting for Performance Loss Causes
[ May 20th, 2008 ] By: Mark Ozimek Posted in » Tips and Tricks

Recently I have noticed a lack of power in the upper RPM range of my S70. Considering that it is now 10 years old and has just over 146k miles, I’m sure there are a whole bunch of things that are causing problems in one way or another, from dirty fuel injectors to leaks in the air hoses. Part of being a good driver or co-driver on a rally team is being able to quickly and accurately diagnose faults with your car that impact the performance. Also, being the engineering oriented car enthusiast that I am, I like knowing what is going on under the hood of my car.

So immediately, there is a list of probable causes to this performance reduction.

1. A hole or tear in an air hose.
2. Compressor bypass valve (CBV) failure.
3. Turbo control valve (TCV), AKA boost control solenoid (BCS) failure.
4. Significant restriction in the exhaust system.

A hole or tear in an air hose can cause a vacuum or boost leak, depending on where it is. This will cause the reading from the MAF sensor to be incorrect, and will throw off the base pressure upon which some of the turbocharger’s boost control devices operate on. A significant leak will cause the ECU to throw an error and turn on the Check Engine light, as well as significantly decrease fuel economy, neither of which have happened, so it’s actually rather likely that this isn’t the cause.

The compressor bypass valve (CBV) is a known weak spot, being integrated into the compressor housing, it has to endure a lot of heat and vibration. Tears in the valve diaphragm develop over time and allow air to circulate back to the compressor inlet when the engine is under load, which we definitely do not want.

Another part that is known to fail on this car is the turbo control valve (TCV), or boost control solenoid (BCS). Either name works, they’re the same thing. It operates based on a duty cycle from the ECU and the pressure difference before and after the compressor, and sends the ‘resultant’ pressure to the wastegate actuator to control the boost pressure. If this solenoid is stuck open, the wastegate will open before it should, and the turbo will never develop significant amounts of boost.

A restriction behind the turbocharger can also cause a significant performance decrease, for example, the catalytic converter failing and clogging up. Any backpressure on the turbine wheel increases how hard the turbine must work to generate a certain boost level by a lot, so this is also a sensitive spot. It is also the easiest to check for, and is what I am doing today.

As you can see, it is most likely related to the turbocharger. Unfortunately Volvo did not see the need to include a factory boost gauge in the instrumentation cluster, nor do I have a gauge on hand to test to make sure the pressure is at 10 psig like it should be.

So, with limited diagnostic resources at hand, there are really two choices I have; Check for leaks, or see if there is a restriction in the exhaust. I decided to check for the latter, since checking for vacuum leaks would entail replacing hoses to see if anything changes, and I do not have appropriate hose with me.

Starting off, there is a nice heat shield around the exhaust manifold and turbocharger to keep the engine bay temps down a bit, so I’ll remove that to get to the downpipe which I will unbolt to allow for gas to flow directly out of the turbine housing, bypassing any restriction. Be sure to do this work when the car is cold, as the exhaust system can get VERY hot.

Now with the heat shield out of the way, I have access to the bolts holding the downpipe on. A 13mm socket fit perfectly. After loosening the bolts, I pulled the downpipe off the turbo outlet flange:

After pulling it back enough to create a significant gap, I started the car, and went for a short test drive. I could tell that a significant amount of air was bypassing the exhaust system through that gap from the sound of the air blowing through, and the exhaust smell in the cabin. I ended up opening the windows to get some fresh air, since carbon monoxide poisoning sucks.

After doing some hard acceleration after the car had warmed up, I found that there was very little difference in the performance of the car. It had become significantly louder though, quite similar to the sound of a diesel truck, and surely in violation of noise ordinances. Fortunately, that rules out restriction in the exhaust causing the performance issues, since it would have been an expensive fix, working against the money Charles and I are trying to save up for the rally car. So I put everything back together. Always make sure to use proper torque for tightening things up in the exhaust to prevent leaks or cracking. Volvo specifies 30 Nm of torque for the downpipe bolts.

Before putting the heat shield back on, I decided to investigate two of my other leads: the CBV and the TCV. The CBV is totally caked in dirt and oil, which makes me highly suspicious of a leak there. The only way to find out if it has failed would be to replace it, which I plan on doing since it is an inexpensive part.

The TCV was harder to look at, since it was hiding underneath the air intake hose. It did appear normal aside from the electrical tape, but looks can be deceiving, so this is next in line if replacing the CBV diaphragm doesn’t do anything.

It looks like I need to brush up on my diagnostic skills a bit, since I was not able to find the fault in an afternoon’s work. When I find out what is wrong, I’ll post a guide on what happened and how to fix it, since boost related performance issues appear all the time on turbocharged cars as they age.