Chapter 7- The Valve That Sticks

The fault that had been logged fourteen times concerned exhaust gas recirculation, and the code plainly said insufficient flow, which sounds like a diagnosis and is not.

Insufficient flow means the car asked for a certain amount of exhaust to be routed back into the engine and did not get it. The reasons that can happen include a valve that is stuck, a valve that is working while a passage is blocked, a cooler that is clogged, a sensor reporting incorrectly, or a vacuum or electrical supply problem at the valve.

Which of those it is determines whether the repair is a cleaning job or a several hundred pound component, and no code distinguishes them.

Commanded Against Actual

What does distinguish them, on a car that reports it, is a pair of live values that Ffion had to hunt for and that she now checks every month.

The engine management system commands a position and it measures the position the valve has actually reached. On a healthy system those two figures track each other closely.

On Ffion's car they tracked closely for the first twenty seconds of a pull and then diverged, with the actual position lagging further and further behind the commanded one.

What the car asked the valve to do, against what the valve actually did.

That is not a sensor reporting nonsense and it is not an electrical fault. It is a valve that is physically moving less than it is being told to, which points at something mechanical restricting it.

She photographed the screen and did not conclude anything out loud, which is a lesson she got from a friend in Minnesota and which appears in Chapter thirteen.

Why It Sticks

The explanation is the same one as Chapter five and it is the reason the two faults on that screen were related rather than coincidental.

The system deliberately routes a proportion of exhaust back into the intake, which reduces combustion temperature and therefore the formation of certain emissions. That exhaust carries soot, and the intake carries oil vapour, and the two combine into a wet, tarry deposit.

On a car that regularly gets hot and works hard, some of that deposit is managed. On a car doing four mile school runs it is not. It builds on the valve seat and in the passage, and it packs rather than blowing through.

Why the valve on a short journey diesel ends up doing this.

Eventually the valve cannot open as far as it is told to, and the car reports insufficient flow.

So Ffion's two symptoms had one underlying cause, which is the pattern of driving, and the same fortnightly longer journey that addressed the filter also slows this down.

What It Cost

She took it to Rhodinerrs Motors Ltd, the independent garage in the village, with photographs of the commanded and actual traces and the count of fourteen occurrences.

The valve came off, was cleaned, the passage was cleared, and it went back on. One hundred and forty pounds including labour.

She is careful to say that this was the good outcome and not the guaranteed one. The same code on a valve that had actually failed would have been several hundred pounds, and the same code on a car with a blocked cooler would have been more again. Nothing she did determined which of those it was.

What the readings did do, according to the man who did the work, was tell him where to start. A count of fourteen occurrences said this was chronic rather than a one off. The commanded and actual trace said the valve was moving but restricted, which is what a cleaning job looks like from the outside. He said that without those he would have started by testing the valve electrically, which is the correct first step from cold and which would have taken the better part of an hour.

What She Nearly Bought Instead

Before Rhodinerrs saw the car, Ffion had found the part online for eighty one pounds and had it in a basket.

Her reasoning was that the code named the valve, the valve was cheap, and fitting one is within the range of things her brother in law would do on a Saturday.

Three things stopped her and all three are worth stating because this is the commonest expensive mistake made by people who have just bought a scanner.

The code did not name the valve. It named a symptom, which is insufficient flow, and the valve is one of five candidates.

An eighty one pound part fitted to solve a problem caused by a blocked passage leaves the problem in place, with the money spent, and produces the conclusion that cheap parts are rubbish.

And the actual repair was a cleaning job. A new valve would have been fitted into the same blocked passage and would have stuck in exactly the same way, on the same driving pattern, within a year.

Her summary is that the scanner had made her feel qualified to buy a part, and that feeling qualified to buy a part is precisely the state in which people waste money. What it had actually qualified her to do was describe the problem to somebody who could tell her which of the five it was.

What Ffion Learned

• Insufficient flow means the car asked for something and did not get it. It does not say why.

• The same code covers a stuck valve, a blocked passage, a clogged cooler, a sensor fault and a supply problem, which range from a cleaning job to several hundred pounds.

• Commanded against actual position is the pair of live values that separates a valve moving badly from a valve not moving at all.

• A valve that tracks at first and then lags under load is being physically restricted rather than failing electrically.

• The deposit that restricts it is soot and oil vapour combined, and short journeys never get hot enough to manage it.

• The filter problem and the valve problem on a short journey diesel usually have the same underlying cause.

• Readings do not choose the repair. They tell a mechanic where to start, which is what a diagnostic hour is actually spent on.

This curated preview is provided for personal reading. Copyright remains with Nobo Astoria.