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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by ALLDATAdiy.com users over the course of many years.
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  Jim Grant's Tech Tips

'99 VW Jetta TDI, Performance Let Down

Q: My ’99 VW Jetta TDI has just turned 100k miles. I have serviced the engine regularly with oil and filter changes, air and fuel filter changes. At 70k miles, the timing belt was changed by VW service. Recently, two glow plugs were malfunctioning and all four were replaced. The engine does not have the regular pep and performance. It’s hard to start when hot and I have to press the accelerator pedal to the floor. It slows down dramatically on hills with no load. This is very unusual for this car since it has performed as gas engines previously. No "Check Engine" light has come on. I use good diesel fuel. I am now adding Marvel Mystery oil to every tankful. What could the problem be?

A: Having to press on the accelerator and hard starting when hot is a good indication that your injection system has a problem. The injection system on your Jetta is computerized, but the computer cannot compensate for base systems that are not adjusted or operating correctly. I would suggest starting with the basic checks, like injection pump timing and injector inspection. It does not take much of a change in injection pump timing to affect overall performance of a diesel engine. Leaking and/or worn injectors can cause starting and performance problems as well. To correctly inspect injector performance on a diesel requires special equipment to monitor the opening pressure, spray quality and to check for leakage. It is not uncommon, with age, for the opening pressure of injectors to drop. This drop in pressure affects fuel delivery in many ways that just robs the diesel engine of power. Leaking injectors will cause hard starting problems and if they’re leaking it is likely that the spray pattern of the injector is bad as well. A Jetta with a TDI diesel engine is just getting broken in with 100k miles, you have many greats miles left in that engine. All it needs now is some special attention.

 
     
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