Exhaust Manifold Leaks - Oxygen Sensors - Engine Management

Exhaust Manifold Leaks Cause Issues For The Oxygen Sensors

The engine management system does not know there is a crack in the manifold.

When the exhaust valves open, the exhaust gases are pushed out into the exhaust manifold by the upward movement of the piston. The escaping gases form a pulse in the exhaust manifold. This pulse has high pressure at the front and negative pressure behind it as it travels in the exhaust runner.

Imagine a small crack in the runner of the exhaust manifold. As the front of the pulse encounters the opening, some of the exhaust gas escapes. As the pulse passes, the low-pressure can suck in gases outside the manifold.

To your ears, it may sound like a pop or a puff. To the oxygen sensor, the change to the exhaust gases in the manifold can cause inaccurate readings. The escaping gases and the outside gases cause the sensor to read too lean. 

The engine management system does not know there is a crack in the manifold. All it knows is that there is an incorrect  ratio of oxygen in the exhaust gases. Consequently, the engine management system will add fuel to burn the oxygen. Depending on the leak’s size, the fuel trim will eventually reach a point where it can no longer compensate. At this point, a code will be set like P0170 or P0171 for a lean condition, and the long-term fuel trim will be maxed out. 

Repairing the leak in the exhaust manifold is critical to restoring the engine management system’s ability to control what is happening inside the combustion chamber. 

Repair Options

Most exhaust manifolds fail in the high-stress areas where the flange meets the head or where the runners converge into the collector. On some exhaust manifolds that have a connected catalytic converter, cracks can occur at the flange that might connect to the downstream catalytic converter.

Some DIY solutions are epoxy or putty to seal the leak. These never last due to the constant expansion and contraction of the manifold. 

The other option is to weld the exhaust manifold. The problem with this approach is the hole or crack might not be accessible to the welder. Most manifold assemblies are either cast iron or stainless steel, two of the most challenging metals to weld. Also, to properly weld cast pieces or stainless steel requires a MIG or TIG welder and a skilled welder.

The other option is to replace the exhaust manifold. This means you have to deal with the fasteners that secure the manifold to the head. These fasteners are subjected to extreme heat, vibration and water. Always plan for broken studs and seized fasteners.

There are methods for removing these stuck or broken exhaust manifold studs. Some methods include penetrant chemicals applied to a hot or cold manifold. Others involve welding a bolt to a stud or tools to grip the stud. No matter how careful you are with the stud or your level of experience, you will eventually have a stud break off in an aluminum head.

The measure of a professional technician is not if they can avoid snapping off a stud, but how quickly they can extract and repair the threads. This typically involves tooling up for the task. There are kits available to help with the removal and repair of exhaust fasteners. These tools can be engine specific, like for the exhaust manifolds on a 4.6L Ford V8. These guides can align drill bits, extractors and thread repair tools.

You May Also Like

GM Catalytic Converter Break-In Information

A service engine soon light may be experienced on low-mileage vehicles.

2002-2018 Cadilac Escalade2002-2013 Chevrolet Avalanche2002-2018 Chevrolet Express2002-2018 Chevrolet Silverado


A service engine soon light may be experienced on low-mileage vehicles, or vehicles that have recently had the catalytic converter replaced and DTCs P0420 and/or P0430 may be stored.

Diagnosing P0420 Or P0430 DTCs with Catalytic Converters

It’s important to understand the basics of catalytic converter chemistry and how catalytic converter failures are diagnosed.

Exhaust Repair Options

If a replacement does not meet or exceed the flow characteristics of the original, it could mean more than a loss of power.

Catalytic Converters: Efficiency Code Diagnostics

Understanding catalytic converter efficiency codes can be a long road to a happy customer or an expensive comeback.

Catalytic Converter Theft Crisis – Due to Covid-19?

Catalytic converters have become a hot product among thieves. Has the global pandemic played a part?

Other Posts

Lightweight EM 210, EMP 210 Welders Deliver Big Power

Both welders weigh just 29 lbs. and deliver up to 200 amps of MIG welding power.

How a Manual Impact Driver Can Make Your Day

We’ve all had screws and other fasteners that just refuse to budge. Years of corrosion, heat cycling, and gunk buildup due to weather exposure and normal vehicle operation can literally freeze a screw in place. And we can guarantee the screw head will strip if you use too much force trying to remove it. But

Tech Tip: Close-Coupled Converter And Engine Replacement

If an engine breakdown were to occur, debris may be deposited in the converter through engine exhaust ports.

Catalytic Converters: Regulation, Replacement and Theft

Catalytic converter replacement is easy – it is the ordering and associated paperwork that can be difficult.