Like the sludge that collects in our arteries, the carbon deposit that builds up inside a later-model diesel pickup’s intake manifold over a 100,000 miles or so hurts the engine’s performance.
Maybe not a lot. But a soot-laden EGR manifold does have an effect on idle quality and fuel economy. It can also throw a P0404 code, indicating there’s an issue with the EGR system. Mainly the build up of the carbon deposit is not allowing the proper exhaust gas flow past the EGR valve.
According to OBD-Code.com, EGR systems are inherently problematic due to carbon buildup in the intake manifold. This normal buildup can lodge in an EGR valve, holding it open when it should be closed. If this is the case, the engine may idle rough, or not at all.
If the valve has failed and is NOT opening, then symptoms would be higher combustion temps and as a result, higher Nox emissions.
So if you are doing any sort work that requires removal of, or easy access to, the intake manifold, it pays to give it a thorough cleaning. In fact, many shop mechanics and dealers will do this at 100,000 miles as part of preventive maintenance.
The process is not that difficult. It’s just a dirty 10 minute job. Use a screwdriver or narrow gasket scraper to dislodge the oil carbon buildup from the manifold. It removes easily.
After the deposits are removed, give the manifold a good dousing with carb cleaner or other aggressive solvent. Then flush out the manifold in a good parts washer and use an air gun to dry it before re-installing.
The result will be smoother idle and possibly better fuel economy as the manifold now recirculates those hot gasses (unburned fuel) back into the system.