QOTW #69 – What are the real side effects of thermostat failure in the V8 Vantage 4.3 or 4.7?

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Welcome to Bamford Rose and another question of the week. Before we get started on this question, it really helps us if you can subscribe to our channel. Click us a like and comment about what we feature today. The content that we put out here everything we do on social media is absolutely free of charge. We will never monetize the channel or have adverts. All we ask is, you give us a subscription for our efforts. Without further ado here’s this week’s question of the week. Which is, ‘What are the real side effects of the thermostat failure in the Aston Martin V8 Vantage 4.3 or 4.7?’

Here’s a picture of the thermostat. Now what happens is the rubber seal around the outside neck of the thermostat. Where the thermostat seals in the housing that starts to break up, and then two things happen. One is the debris from that rubber gets jammed in the spring and wedges the thermostat open, so it never properly closes. The other worst case is where that rubber has degraded completely. So, now the thermostat is not sealed in the housing. And in both cases whether the thermostat was jacked open a little bit by some rubber debris or the seal is completely gone, the end result is still the same. A significant part of water volume goes past the thermostat.

So, the thermostat isn’t properly doing its job of closing and limiting water flow through the system, so the engine gets hot. And then when it reaches the predetermined temperature typically 90 degrees, the thermostat opens allowing circulation. With the rubber seal degraded and there being significant water circulation, the engine never really gets up to full operating temperature. This is really common on V8 Vantage and it’s quite wise on your own car to start from cold and then certainly go a few miles. Certainly within it depends how far you’ve driven. But two or three minutes should see that gauge start to rise quite quickly and get to halfway on the coolant gauge on the dashboard.

When the seal starts to degrade allowing water flow through the thermostat before the engine is reached its normal operating temperature. What you’ll do is probably just see the gauge start to hang up about one bar below the fully warm or the middle set point. In worst cases, you could go for a 30-minute drive and it literally is a quarter of the way between cold and fully warm. However, this should really be picked up by whoever is doing your servicing. Because on a road test, the time to reach fully operating temperature normal operating temperature should form part of a road test. Now as development engineers here you know road test to us is second nature. I can quite empathize with your normal garage technician taking a car out on a road test.

You know they’re road testing it from more or less a layman’s perspective. It’s much different than a design engineers development engineers road test. Perhaps they’ve got a novelty factor as well of driving cars, where you know most of us here have been driving around in Aston’s myself literally for 18 years. Well over any novelty value of driving in Aston. So, whilst they’re swept away by that novelty of driving your Aston on a test drive, it’s probably quite easy to miss that the coolant gauge is like one bar below normal operating temperature. On the particular car that I’m going to show you the side effects of running prolonged with thermostat failure, probably that was the case.

The car came to us with a fault that the prolific Aston brand seller couldn’t fix. So, it had to come here for our attempt to fix it. Which was another one of those classics where the other outlet had had the car for about two or three weeks and quite literally two or three minutes in the car park. Well less than that actually I’d say 20 or 30 seconds in the car park would fix. The problem was easily fixed; it was just an earth strap. So, we can’t claim that we’re rocket scientists or anything for fixing that car’s problem. But the other garage’s failure to diagnose that really quickly, meant them stripping significant parts of the car unnecessarily.

During that strip the oil circuit came out and the water circuit too. And when they put it back together again the road test should have checked that both those circuits were functioning properly and obviously getting up to normal operating temperature, should have formed part of that check. So, not only did they not solve the root cause of the original problem, they stripped the car unnecessarily. And then they failed to identify that the coolant circuit was unhappy while the thermostat was unhappy on the road test. So, with the original problem fixed and the car out on road test to clarify that, then we just happened to notice that it wasn’t getting up to normal operating temperature.

It turns out this car has been like that for a significant period of time and no one in a garage has identified it. What happens when the thermostat is failed and the engine fails to get up to normal operating temperature is that, the engine is running a bit colder than it normally does. What that causes is, the engine to never come off its cold start enrichment phase. So, the car is running rich it’s running over field and it’s running that permanently. That causes an excess of hydrocarbons and unburned fuel in the exhaust gases. Which travel downstream into the catalyst which is very hot typically 900 degrees. And then that fuel vapor is going to combust on the face of the catalyst.

Fortunately, a V8 the catalyst is quite a way downstream and the likelihood of ingestion of the cat debris is very low. But allowing it to be run over a ridge for that significant period of time, the fuel vapor being burned on face of the catalyst has completely failed the catalyst. As you can see on this catalyst, the matrix, the honeycomb matrix is completely molten. It’s broken up and the molten segments are obviously reducing gas flow. Now the next one, this is from the other side and as you can see the catalyst is completely missing. The fuel vapor has exploded on the catalyst, broken up the ceramic matrix. And instead of being inside the catalyst can, the ceramic catalyst matrix is now probably residing either in the silencer or on some pathway somewhere.

The thermostat seal kit is approximately 65 pounds, plus V80. Probably, half an hour’s labor is fair to install that. In this car’s case, if that had been detected a lot earlier it would have prevented the need to replace the catalyst which as you can see, I completely failed. I wouldn’t be overly paranoid about this issue every once in a while, start the car up, take it for a drive just to check it gets up to full operating temperature within a reasonable time frame. Unfortunately, this is one of those faults where the on-board diagnostic system isn’t going to ping a light on the dash to make you aware of it. Hope you like that question of the week, and we’ll see you in the next one.

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