Excessive black smoke
I have a problem with my 2008 Renault Grand Espace 3.0 dci, which has done 125,000 miles and runs well. However, it emits black smoke when accelerated hard, which can obscure any following cars from view; when driving normally, I don’t notice any smoke in the rearview mirror.
The problem is much worse when the engine is cold. This extra smoke when cold seems to be associated with a noisier engine. It takes several miles for the rattling to subside, after which the smoke problem is much reduced, but still present to a degree. I also notice the automatic gearchange down seems a bit jerkier when cold, particularly from 2nd to 1st. When the engine is warm, the gearchanges are almost unnoticeable. I’ve heard it said that jerky auto gearchanges can be a symptom of injector problems. There are no indications of faults on the electronic dash.
I took it to my local garage, but they weren’t sure what the problem could be. They believe the turbo is working fine and speculated that it might be the EGR valve, but it’s difficult to access on this engine so they have not removed it. They did notice that the intercooler seems to have a leak on its lower corner. The bodywork in this area also shows evidence of a minor shunt – could this have damaged the intercooler? I thought the intercooler was filled with charge air, so why is the leaking area black and oily?
Black smoke is usually due to excess fuelling or insufficient boost on a diesel engine. A leaking intercooler might be a possible cause, but why would excess fuelling be so much worse when cold? Hugh O’neill
The charge-air side of the induction system carries tiny droplets of oil that have escaped from the engine breather system or have bypassed the turbo seals. This small amount of oil is normal, and when removing any intercooler hose a light film of oil will be found in the system. This oil has a very useful secondary purpose in that it will reveal any leaks in the system by depositing oil around the escaping air. With such a tell-tale sign on your intercooler, this is confirmation that the intercooler is leaking and requires replacement. The problem will appear worse when cold because the cold running mixture adjustment is more crucial and any inconsistencies will be far more noticeable.
The black smoke is, as you say, due to excess fuelling or insufficient boost. The black smoke will be worse with a cold engine because, when the engine is warm, the carbon particles that make up the dense smoke have a higher chance of burning before being emitted through the exhaust.