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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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