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300tdi very sluggish.


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A friend of mine has a 96 300tdi. It has covered 130k miles. The car is generally very sluggish, does not seem to have a willingness to rev. When started cold it struggles to run, catching on 2 or 3 cylinders before all 4 start to fire, with a lot of black smoke.

Once running and warmed up it sounds normal, with no excessive rattles or noises. It uses very little oil, and uses no water. On acceleration it smokes quite a lot (black smoke). The head gasket was replaced 6 months ago, the problem existed before this job was done, the head was crack tested and found to be ok. The valves were good.

So far, we have found the egr valve to be already blanked. The intercooler has been cleaned externally, and flushed with solvent. The air filter has been replaced.

What should we be checking next? I am not a diesel expert.

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You could check to see if your glow plugs are working and not burnt out. Best way is to remove them, put one in a vice and put jump leads on the input and the earth. Be careful, they should glow red hot at the tip within a few seconds. That could be your starting problem.

For the lack of power and black smoke it may pay you to get the injectors tested and adjusted at a Bosch agent.

Otherwise there is a very interesting post on tweaking your injector pump in the Tech Archive.

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There's an easier way to test your glow plugs than taking them out!

Check the resistance from the power connector to the engine block. The resistance should be very low (a few Ohms at most). If the resistance is very high (k Ohms) then the glow plug is probably shot.

Note that the engine will start with just one plug working but will then be rough until it warms up. I had a similar problem with a VW Golf (pre-Tdi). When the final plug failed it was an absolute pig to start. With just one plug working it would start but run rough for a couple of minutes until the combustion chambers warmed up.


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I wouldn't trust a resistance check to confirm glow plug operation.

I replaced all four of mine recently. I bench tested them with a multimeter and a set of jump leads.

1 worked out of the 4

3 had low resistance values, with no discernable difference.

1 had high resistance - yes, it was one of the 3 that had failed

So, in short, I wouldn't say that the test results were in any way dependable.

In any other 'sluggish to rev' situation, i would suggest the lift pump failing, but since there's black smoke there's too much fuel in the cylinders anyway. I'd fully check the air inlet route from filter box to manifold, as well as querying the health of the injectors.

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These comments "Once running and warmed up .... On acceleration it smokes quite a lot (black smoke)" make me agree with Raceface120.

You are getting the smoke and the sluggishness because you are getting the fuel but not enough air. Has anyone checked the actual boost pressure at the inlet manifold?

I agree the poor starting could be duff glow plugs, the blocked hoses / lack of boost wouldn't affect the starting.


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