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2010 VW Jetta Sportwagen TDI jump/hiccup inconsistently at ~30mph

2010 VW Jetta Sportwagen TDI – manual
One owner.
No repairs in history of car. Regularly maintained at dealer.

From beginning about 1/month will be driving ~30 mph in 3rd gear when car seems to hiccup once before returning to normal operation. Does not seem to have any repeatable attributes. Only seems to happen when about constant velocity with very slight acceleration.

Dealer says computer doesn’t show anything and of course can’t replicate.

Looking for any advice.

Update: As the weather cools down and the car is colder when first driven this occurs more often. It is happening about once each morning and does not occur once the engine warms up.

There a bunch of TDI specific forums. Hopefully common enough a known fix is out there. Good TDI hesitation maybe?

Yeah, and I’ve searched those and tried some of the fixes (emptying possible oil near the end of the intercooler, soot buildup, etc) but nothing has worked so far.

The diesel fuel is more viscous when it is cold. My first guess is there is some obstruction in the fuel line and the colder it is, the more it blocks fuel flow. When was the fuel filter last changed? Is there a chance you got some water in the fuel tank? Have you used a different gas station recently, just before this symptom started, one you never or seldom use?

Interesting thought in terms of an obstuction. I guess I don’t see why that would only happen at around 32mph and not earlier in the drive.

The fuel filter was changed about 10,000 miles ago. I don’t see anyway that water is in the tank, and given that this has been happening infrequently over the past couple years I don’t think that’s the cause. Same with the station I use (switches frequently).

And I should note that this was occurring before the fuel filter was changed.

hmmm … well, it could occur at 32 mph b/c at speeds lower than that, the engine needs less fuel flow to run, and the obstruction isn’t blocking the fuel flow enough to prevent enough gas to meet the engine’s needs, but above 32 mph the engine needs more gas, fuel and at that point the engine becomes fuel starved. From what you say though, after the hiccup occurs at 32 mph, then you can go faster w/no other symptoms. I suppose some type of fuel starvation blockage could still be the explanation, but I admit it’s a reach. If you couldn’t go faster than 32 mph, then the blockage idea would make more sense I guess.

10K on a fuel filter isn’t much. It’s probably ok as is. So what else? Maybe the clutch is slipping a tad right at that speed? Possible if some grease or oil has somehow contaminated it. You could ask your mechanic to do the standard test for slipping clutches. Since it happens w/slight accel, that would be consistent w/a slipping clutch. But it would also be consistent with loose/worn engine or transmission mounts, so good idea to ask your mechanic to check those too. Have the mounts every been undone or removed? If so, maybe they were not torqued properly when re-tightened.

The good thing w/a TDI is you know it isn’t the ignition system. But it could still be some intermittent electrical problem of one sort or another. Ignition key, fuel pump relay, etc. And with diesels I’m always suspicious of the injector pump with drivability problems.

Besides the above, make sure your shop checks all the TSB’s and recalls. Other than that, you may have to live w/it until it becomes frequent enough that it will reliably do it for the shop to observe.

I’m also leaning away from it being a fuel line issue. However, I haven’t looked into the slipping clutch but that may be a good idea.

As for the mounting being undone or removed, that has not happened unless it wasn’t torqued properly in the factory.

Shop still says they haven’t heard of anything like this occurring with other vehicles, so I’m still waiting for the frequency to increase or more reports about it.

Thanks for your ideas.