glcowner

My 2-year GLC just went through service B (costing over $1000) and shortly after that the engine literally became twitchy and when I opened the hood I would literally see the passenger side of the engine jumping a little.
This has been happening for over 2 weeks. Dealer's rep asked to take the car in when it starts happening "more frequently". And yesterday it did, and things got serious, the car died on a freeway with the yellow check engine showing up.
Up until then we haven't had the check engine light go up at all.
The car got fixed very quickly, but something doesn't make sense in service's explanation.

They said that a lower octane fuel may have been used, and that led to the engine trying to compensate and in result "jumping". They did also say that usually this would have resulted in check engine light going off - but why hasn't it gone for the whole 2 weeks and only just now ?
As a fix it seems that they replaced spark plugs (can spark plugs go bad only after 2 years - especially right after service B ??), and also reprogrammed some sensor unit (likely related to engine timings etc).

Does that even make sense ?

Everyone I asked says that there's no way spark plugs could have gone bad that soon - this must have been long-time coming, but then why wasn't it caught during service B ?
We've ALWAYS used 91 Octane gas, not even once anything lower than that.


Thanks

ItalianJoe1

Spark plugs do fail from time to time on the M274 engines, we usually see the insulator loose, when it falls down it causes an intermittent misfire. The part # for the plugs has been updated so the replacement ones shouldn't ever do that. Not the end of the world, sounds like they fixed it, you should be good to go.

peet

First off - A vs B is just that B adds a brake flush. So that shouldn't make a difference. Second, did you ever get ripped off on the price! $1000? High price on these is 500-600 with most around $450. Check that they didn't charge you for extras and ask for a refund.

Regarding plugs - they theoretically could go bad, but unlikely. These days plugs are 50-60k mile parts.

What you described, sounds timing or injector more likely. If they changed a sensor and reprogrammed timing then it was likely a failed sensor. For an engine to jump, a misfire wouldn't necessarily do it, not an inline 4. If a V and half of the V did, maybe.

glcowner

Thank you both.

From what I recall, during B they realigned the suspension, did the brakes. I'm not sure whether that qualifies as extras, I'll definitely complain about the cost.
It is really strange that this happened right after the service and they're implying it's due to bad gas octane rating...
The car literally has 12000 miles on it (we don't use it that much), so spark plugs going bad is really hard to believe unless they developed mechanical fault (or during service B inspection they didn't reinstall them properly).
Does B even include/require spark plug check ? - when I took my porsche to a nearby indie guy (great mechanic) for a basic oil replacement - checking the plugs was nearly the first thing he did...

Thanks.

peet

No - plugs are not touched at A or B at all! They could have bumped a plug cable, so you ran on 3 cylinders for a while which could have caused more damage later. That's a stretch though.

Plugs in modern cars are rarely touched - the first time I have heard of plugs being needed early is on new Alpha Romeo Stelvios (I was checking into those) but even there it's at 30k miles.