Eternal Flame: The Check Engine Light that Never Dies (Part 2)

Part Two: The Gas Cap

In the first post of this series, I talked about how the thermostat’s gasket goes bad and causes a check engine light (CEL). In this post, I’ll be talking about another common cause – a leak in the emissions system.

The vast majority of CELs I’ve seen in Astons are caused by the emissions system. The problem with it is that it’s hyper-sensitive and any number of things can cause a fault. If a bad thermostat gasket can trigger the fault, just imagine what else can! The faults are concerning not because of potential damage to the car, but rather because they can lead to the automatic failing of an inspection.

One of the many components of the emissions system is the Diagnostic Module Tank Leakage (DMTL) pump. The DMTL pump checks to see if there are fuel vapors escaping the fuel tank to atmosphere. Fuel vapor leak is a no-no so it gives you the emissions system-based CEL. The most common spot that leaks is the gas cap.

The gas caps that came with early V8 Vantages were often faulty – even when properly secured, they’d leak just enough to trigger a fault. They’re easy to replace, taking only a minute, but they’re unnecessarily expensive (around $250!!). Maybe if they had the Aston Martin wings logo engraved in them they’d be worth so much. But they don’t. So they aren’t. But they still cost an absurd amount of money. Anyway, many were replaced under warranty. Some weren’t and it will come as a shock to the new owner of a used Vantage that something like the gas cap could be so costly to replace.

Owners of newer Astons will sometimes have this same issue. If you’re one of those people, don’t worry just yet! Before you get pissed about needing to pony up the cash for an overpriced gas cap, try reclosing your current one. It’s possible that it didn’t seal all the way when you closed it up after your last stop at the gas station. Give it another try, then drive around. After a few drives the CEL might go away on its own.

If your gas cap isn't the problem, your best bet is to have a smoke test done. This basically entails smoke being pumped into intake system (and all the associated plumbing) to see where there might be a leak - the smoke makes the leak visible.

I was asked at one point why I go through all the effort of diagnosing problems, figuring them out, putting in the innumerable hours that I do to understand what causes these issues and how to fix them, and then documenting it all for other people. Well, because I'm not just doing it for the sake of it. I don't go looking for problems. The problems... they find me.  The gas cap you see in the pic above - that's mine. Yep, it's a nice new, inexplicably expensive one. The emissions fault and CEL in the pic below? Yep, that's mine, too.

I've been dealing with the CEL problem for a while in my own car, so I've got through these diagnostic steps myself. In the next post in this series, I'll be discussing my next hunch: A bad MAF sensor. Sounds odd, but I think my reasoning is sound. Let's find out...