Exhaust Manifold Problems

It used to be that exhaust manifolds were arguably a rudimentary part. Boy, has that changed. The modern manifold, while still devoid of moving parts, is equipped with temperature, pressure, and oxygen sensors, along with branch connections to feed emissions control devices such as an exhaust gas recirculation valve (EGR). Due to its complexity, higher exhaust gas and under-hood temperatures, and the use of weight-saving lighter materials, today's exhaust manifold often needs attention.

Common maladies include cracks between the cylinder branches, studs, and bolts that fail from thermal cycling; face warping that facilitates gasket failure; and damage from servicing one of the sensors housed in it. If you keep any engine a few years beyond the warranty, there is a good chance you will be working on the manifold.

bowed manifold

A very common issue with modern exhaust manifolds when they are removed from the cylinder head is that they bow slightly (the holes no longer line up with those in the cylinder head). If you have never experienced this before, you will be dumbfounded, since the manifold is not visibly shorter. There is a simple workaround for this problem, but it requires a special tool called an exhaust manifold spreader. There are many different styles, but the least expensive spreader resembles a turnbuckle. It is placed between the manifold branches and is expanded with an open-end wrench until the fastener holes are aligned. You insert and loosely snug the fastener and then move the tool to the next branch, if need be.

Being unaware of this tool and the need for it has many mechanics using a pry bar – in frustration – to attempt to reinstall the manifold. With rare exception, this is met with either a broken cylinder head or other components being pried against. If you plan on doing any exhaust manifold work, buy this tool first.

The use of lightweight materials along with the high rate of thermal cycling both in frequency and magnitude produces a transverse load on the exhaust manifold fasteners (side to side). The action can also cause manifold castings to twist. The load eventually snaps the fastener, requiring it to be drilled out in the cylinder head. The goal would be to accomplish the repair without removing the cylinder head from the engine block.

There are many model-specific tools made to allow the fasteners to be drilled out that do not require disassembly. They usually resemble a jig and come with the required drill bit. It is not uncommon to find an engine today with four or five broken fasteners to the cylinder head.

I do not want to mislead you: Even with the tool, this job is no picnic. But it is better than taking the engine apart.

With luck and patience, the job can often be done in a few hours. If there are multiple broken studs, then I usually replace the exhaust manifold with a new casting since it, too, is near the end of its service life.

cracked castings

You may also run into cracked castings, which are tricky since you need to determine if the casting failed due to fatigue or from an engine running problem such as timing or fuel delivery (diesel) or lean mixture (gas engine). Anything that will increase the exhaust gas temperature will impact the manifold casting's integrity. If the manifold is being replaced, always use a new gasket(s) and fasteners.

Apply antiseize compound to all threads so that they do not freeze in the head. Take special care when removing the old fasteners to not pull the threads out of the cylinder head casting. Some controlled heat with a torch (if possible) and soaking with a good penetrating fluid go a long way in saving you from a major headache.

Keep in mind that any exhaust leak also introduces oxygen into the manifold, allowing the fuel delivery to skew since the engine sensors will see the false air.

If a sensor is being replaced, then your mind-set is to save the threads in the sensor bung and not the sensor. Using a combination of sound mechanical procedure, heat, and a rust release agent will allow the task to remain true to its intent and not turn into a major headache.