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98 Passat- Mysterious problem stumps my mechanic! What do you think?

This is just starting to get annoying. I just heard back from my mechanic and he has no clue. Really, what’s the deal?

Car Info:

98 Volks Passat; 140K miles


Sometimes it doesn’t want to turn over.

It happens almost every other day or so. Never in the morning, usually after I’ve been at the doctor, grocery, or whatever. I turn the key; electronics come on; I hear the crank sound; but no ignition. After a couple of time of holding the key i’ll finally hear the engine start to putter on. Of coarse when I take it to my mechanic it works FINE. He keeps it overnight and drives it around, let’s it sit for a couple hours and it fires up beautifully he says. He also does the electronic reading and no codes were found.

One issue is said it COULD be was a temperature sensor, but he isn’t too confident that is the issue. I live in the bay area so it is constantly between 45F-65F. Never too hot or cold, just right.

Maybe you have an idea??

You have a wire,connection or sensor problem.
The ONLY way to find and fix is to have someone who knows how and has tools to track down the problem WHEN it happens.

It sounds to me like you may have an ignition component becoming heat soaked.

When you’ve driven the car and shut it down for a short spell, the cooling system stops drawing the heat away from the cylinders to dissipate with the radiator and the air stops flowing by the engine’s exterior taking heat away with it. The heat around the cylinders has no place to go and begins to dissipate out the exterior of the engine as radiant heat. Everything in the engine compartment initially gets heated up.

Some components, especially those with windings like coils (coil packs) and relays can get heat sensitive over time. The varnish like coating that insulates the coil wires (polyimide-amide) has a different thermal expansion coefficient and begins to develop microfractures. When the wire expands, the fractures open, the windings short, and the induced voltage drops.

This condition can be intermittant, but is also diagnosable. Perhaps if you tried another shop…

Or perhaps if you made arrangements for you yourself to replicate the conditions rather than relying on him to. Drive it to his shop, let it sit for a bit, then have him come out and show him exactly what you’re experiencing. Even if it takes a few visits to get it to act up.

First things first !
Get your codes scanned ! Autozone does it free and some other stores might also .
Temp sensors on these cars are notorious but are very easy to replace . If I had real strong fingernails I could replace mine without tools but a screwdriver is really all you need !

My guess is the #109 relay, but it is not a sure thing. This relay seems to be a weak point on Some VW models. Mountainbike as a good list of things to check.

I like your mechanic’s temperature sensor theory. A failing temperature sensor may not generate an error code because the problem happens on startup before the engine goes ‘closed loop’ on the computer. The computer may not see any error because on startup it does not know what temperature reading to expect.

If it is a temperature sensor problem, it may be either very rich or very lean and therefore fail to fire when you try to start it. If it is very rich, you will probably see black smoke from the exhaust when the car finally starts.