Breathing free

A closed valve
If the exhaust valve did not open in an OHC engine, would this create a breathing issue for the engine? Yes, it would. 

Let’s think about the engine’s four strokes for a moment:

• During the intake stroke, the piston would draw the air/fuel charge into the cylinder through the open intake valve. This is the result of negative pressure created by the piston being pulled downward towards BDC. With the intake valve open, the piston is actually pulling against the throttle plate, creating a negative pressure within the intake manifold that will draw fresh (atmospheric) air in.
• Next, the power stroke would normally compress the air/fuel charge.
• Then the energy being created would push the piston downward.
• Before BDC, the exhaust valve would begin to open, and as the piston was traveling upward to TDC it would push the spent gases out of the cylinder and into the exhaust system.

But if the exhaust valve didn’t open as the piston was traveling to TDC, wouldn’t the gases in the cylinder start to compress again? What normally occurs at 360 degrees of crankshaft rotation? Valve overlap occurs. This is when the intake valve opens before TDC and the exhaust valve closes after TDC.

So back to our mechanical issue. If the gases build up again in the cylinder because the exhaust valve did not open, wouldn’t this pressure buildup enter the intake manifold and displace the negative pressure when the intake valve opened just before the piston reached TDC?

This mechanical issue will cause multiple cylinders to misfire. Very common are occurrences of cam followers becoming dislodged due to high mileage lifter wear, over-revving the engine while driving and low oil levels while operating at higher RPM and load conditions as well as lack of maintenance issues that allowed sludge to form in the lifter galleys and lifter oiling ports.

Carbon buildup
We all have had experiences with driveability complaints created by excessive carbon buildup on the backside of the valves, intake runners and ports. Yes, it does impede the airflow into the cylinder creating excessive turbulence. This can lead to rough/unstable idle conditions as well as misfires. Not to mention the carbon absorbing some of the fuel charge as well as creating lean conditions within the cylinder.

Excessive exhaust back pressure
Checking for excessive exhaust back pressure should be part of your baseline test. When a vehicle has drivability issues, it is part of mine. We listed all the possible drivability symptoms the vehicle could have when the engine is not breathing properly. I see too often that a technician will first inspect the air filter housing and fresh air intake ducts, then literally stop at the timing belt when he observes the crankshaft (CKP) and camshaft (CMP) alignment marks are correct, even though he/she may suspect an engine breathing issue is creating the driveability issues.

 I believe there could be several reasons for this:

• Not having a full understanding of how an engine breathes and the variables involved;
• The difficulty of accessing an HO2S for removal for testing purposes or not wanting to deal with damage threads in the bunt or with the sensor itself, which could lead to a high cost for parts and labor to repair just to perform an exhaust back-pressure test; and
• Not knowing what amount of exhaust pressure is acceptable and what is considered excessive.

It has been my experience with diagnosing vehicles that have large volume intake (vacuum reservoir) manifolds (especially the late-model vehicles with movable intake runners) that the use of a mechanical vacuum gauge doesn’t reveal the information I am looking for in most cases, when compared to older vehicles with low plane intake manifold designs where the mechanical gauge reveals more information.

For example, I was diagnosing a misfire that was created by a burnt valve in a 2000 Dodge Caravan FWD with a 3.3 liter engine. The vehicle had more than 150,000 on the odometer and was equipped with an automatic transmission. The use of a pressure transducer confirmed the valve was not sealing, and the cylinder leak down tester verified my diagnosis. Even though the misfire was severe, the vacuum remained steady at 19inHg. I like to see 21inHg from a good breathing/mechanically sound engine. I would have expected to see a much lower value or at least some movement with the mechanical gauge, but that was not the case. I find the use of a vacuum transducer is much more informative with the late model vehicles, or vehicles with large volume intake manifolds.

Case Study:
2009 Honda Civic
60,000 miles
1.8 liter engine
Lacks power when accelerating

Lack of power
The first step is to verify customer’s complaint; this may require a test drive to duplicate the concern. But this is not the time to be attaching any diagnostic equipment.

The engine had a normal idle speed and snapping the throttle repeatedly in the bay did not reveal any issues. The road test was different and revealed a lot of need-to-know information.

• Moments before the lack of power became evident, the engine started to have a steady misfire. Not enough to prevent high RPM operation at first, but the more aggressive the acceleration was, the sooner the lack of power issue would occur.
• The MIL lamp was blinking during the misfire events, and when the load and RPM was decreased, the MIL stayed on steady. After cycling the key off then restarting the vehicle, the lack of power was no longer present and the vehicle could be driven under light load conditions.

The vehicle was then brought into the service bay where a visual inspection was performed. There were numerous new body and mechanical components replaced on the front of vehicle because of a recent collision.

Is the lack of power complaint a result of the misfires at higher RPMs and loads? The engine was equipped with an (ETC) Electronic Throttle Control. And the answer to the previous question is yes — the misfires were putting the engine management system into a Limited Power Mode. This mode is activated when the driver intent cannot accurately be determined or when the output of the engine’s power is impaired, during a mechanical issue, for example.  

One of the quickest tests I can perform is to place a pressure transducer in place of a spark plug and start the engine. The pressure transducer waveform (once learned) can reveal a lot of information about the mechanical condition of the engine, such as cylinder compression levels, valve timing, valve opening and closing events and cylinder sealing. This information can help reveal the root cause of the misfires or eliminate these possible mechanical issues as being the root cause.

Pressure transducers (positive/negative) will save a lot of shop diagnostic time and money. For instance, if you want to check the engine valve timing to see if it is correct all you need to do is observe the details of the pressure transducer waveform. The only time you would be removing front engine covers is to verify your diagnosis or perform actual repairs, if needed. You can observe valves not opening or closing without removing engine components first by observing the waveform. There are other benefits as well.

The pressure transducer waveform in most cases can also identify restricted exhaust systems. Using the pressure transducer for this diagnostic step can save time and money because you do not have to remove an oxygen sensor, and it removes the risk of the threads being damaged during removal or reinstallation while performing an exhaust back pressure test. 

Now take a look at the pattern it provided on the troubled Honda (Figure 4).

The Honda’s waveform pattern (Figure 4) is of one cylinder with engine idling. It reveals normal compression levels (all four cylinders showed the exact same details), valve timing, valve open and closing events and no cylinder sealing issues. But remember, this engine had no symptoms when idling; this is a baseline test.

Next, I performed a “snap throttle” test (Figure 5).

This in-cylinder pressure that remained through the entire Snap throttle is the result of a restriction in the exhaust system not allowing the gases to properly exit the cylinder during the exhaust stroke. Since my baseline pattern showed that the cam/crank correlation was correct, the only culprit left is the exhaust itself.

Common Causes Of Restricted Exhausts

• Broken up substrate.
• Collapsed inner wall pipe in exhaust.
• Broken off baffle in the muffler.
• Severe bend/kink in an exhaust system.

Exhaust back pressure - quick explanation.

Exhaust back pressure will vary from vehicle to vehicle, due to the different configurations (length of piping, numbers and placements of the catalysts and the mufflers themselves) and the inside area available for the expanding exhaust gases.

I usually see 0-1.5 psi at idle, and 3.0 -4.5 psi on a hard snap on a naturally aspirated engine. I would strongly suggest making your own library of exhaust pressures. Anytime you have a vehicle equipped with an EGR pressure valve/transducer (Chrysler, Toyota, Nissan, etc.), tee into the exhaust side and take a quick reading using a good quality exhaust pressure gauge.

Exhaust pressure has to do with the volume. As with any gas, an increase in volume results in an increase in pressure. With a low throttle opening, the exhaust gas base will be low. As the throttle opening increases so will the exhaust gas base. The exhaust gas base has thermal energy that is trying to expand the gas base as it is forced out of the exhaust chamber into the exhaust port, by the upward movement of the piston. This means the volume is increased by the temptature of the gas as well as the (exhaust) gas base. There are two forces at work as the combustion chamber is being scavenged. First the upward movement of the piston forcing the volume out of the cylinder and secondly is the force of the exhaust moving through the inside area of the exhaust system. The exhaust gas base contains mass. As the exhaust (mass) is accelerated through the inside area of the exhaust system it creates a siphon effect that pulls the exhaust gas out of the combustion chamber. In order to have this siphon effect, the velocity of the exhaust flow must be as high as possible. This means during the design of the engine the exhaust port area will be calculated for the range in which the power will be produced. Most vehicles operate below 50% throttle opening a majority of the lifespan of the vehicle. Which means the velocity of the exhaust will be set at a lower throttle opening were the power is needed. This however will create resistance (exhaust back pressure) at higher exhaust gas volumes (greater throttle openings).