MoT test instruments: how accurate are they?
- Thread starter Alec_t
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I think the phrase is "breaking the law in a very specific and limited way"? (Should be fine for his MOT if he's a senior Tory....)
I had a quick look online, but the specifications for the analysers seems remarkably hard to find. DVSA publish a list of accredited ones, but I can't find the limits for accuracy and resolution. This sort of thing often crops up though. The line has to be drawn somewhere and a fail that's just on the wrong side of that limit always causes consternation - but it cuts both ways though. If it had passed with a lambda of 1.029 you wouldn't have thought "gosh! That's way too close for comfort!" and insisted that they have a look at it!
A couple of years ago, I was donated my sister's 2001 Freelander diesel, as she was going to scrap it. Like you, she hated revving it for fear of "hurting" it. The car had only ever been driven on light throttle openings and at low revs. Among the things wrong with it, the thermostat was stuck open. In the course of fixing that, I took the inlet manifold off and found it absolutely chock-full of filthy, sooty gunge. I'm surprised any air managed to get down it at all! Sadly, I didn't take a piccy, but proceeded to scrape as much of it as I could out of the manifold with an old knife and lollipop stick.
Last MOT, it only just passed the smoke emissions which is fair enough, it used to smoke like a dog when I booted it after I first got it). Its next MOT is booked for Wednesday next week! In preparation, I thought I'd clean out the Exhaust Gas Recirculation valve (which was actually fine, by the way)! I also took the manifold off again, as it's easy to do, and was pleased to see no further accumulation of gunge in the areas that I had previously scraped. However, I did have a bit of a dig in some other parts of it that I hadn't previously scraped and took a piccy this time...
What I think happens, is this...
50 years ago, cars just vented their crankcase fumes to atmosphere. There's air inside the engine, above the oil, which expands as the engine warms up, so it needs somewhere to go. It used to just go to atmosphere, but of course, it is mixed with oil mist and any unburned fuel that gets past the piston rings. Regulations then required it to be routed to the air intake so that the engine burned it. However, that's not really what happens because it's cold in the air intake, so it just condenses back into dirty oil and sits there in the pipework. This is especially true of modern turbo diesels where the pipework is very long because it has to go through the turbo and then through the intercooler, right at the front of the car, before going all the way back to the engine.
On yet more modern cars (to help with emissions of oxides of nitrogen), the problem is even worse because they often have EGR valves which force the engine to recirculate some of its exhaust gas back into the inlet tract so that it burns a bit of its own exhaust gas a second time, completing the combustion process and lowering the combustion chamber temperature so that NOx is less likely to form. This adds soot to the already filthy oil in the inlet tract.
Finally, there will be whatever oil seeps past the turbo oil seals, also added to the mix.
When demand for air is high (which really means at maximum revs in the case of a diesel), a lot of air will be coursing through all that pipework, "scouring" any liquid oil with it and dragging it into the cylinders, where it will get burned. On cars that don't often see maximum power demand, the rate of air flow never gets high enough to do this, so the stuff just stays there. When you switch the engine off, it cools. After some months, the more volatile fractions start to evaporate and you're left with a much thicker deposit that can't easily be "scoured" from the pipes by the passing airflow. This just sits there and then each time the engine is used, more of the stuff sticks to the first layer (and so on) until your inlet tract is full of gunge.
The moral of the story, (I believe!), is therefore to "drive it like you stole it" once a month or so. High revs, big throttle openings. Obviously you don't need to hit the rev limiter, but you do need to work it hard. This Freelander (it's a TD4) redlines at about 4500 revs, so I generally give it 4000 on a wide open throttle (usually accelerating up a hill or towing the trailer so I don't lose my licence!) for 30 seconds or more. I think it's well worth doing, but I'll find out on Wednesday!
EGR has been the death of many an engine, if you think of all the particulates in Diesel exhaust gas no wonder it clogs the manifold!.
I ran water injection years ago on a car (for anti detonation), the manifold was spotless. Almost worth running it (or a Methanol mix) just for cleaning purposes..
I ran water injection years ago on a car (for anti detonation), the manifold was spotless. Almost worth running it (or a Methanol mix) just for cleaning purposes..
Yes, but generally only on lightly-driven ones, I find? It would be interesting to go on a model-specific forum and do a poll to see if there was any correlation between those who got the best MPG and those who had EGR and manifold sooting-up problems! Necessary to try and keep NOx down though. Forums are full of advice to blank them off, but I now see we're getting 60 MPH limits on some stretches of motorway to try and improve air quality. To some extent, at least, we reap what we sow!
On a diesel???!!!
I'm not a petrol-head, but wouldn't gunge build-up in the inlet manifold restrict air-flow and result in an excessively rich mixture? It's my understanding that a lamba reading above the top limit indicates an excessively lean mixture.
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Admittedly, I was talking about an old diesel there, which wouldn't get checked for lambda anyway, and the problem shouldn't be that common in petrol engines! To answer your question though, I think most cars these days have some way of metering air going into the engine and the airflow meter (or more recently "MAF") tells the ECU what's going in there, so the EU gives an appropriate amount of fuel. I therefore think (but am open to correction!) that the mixture would stay pretty much as it should be, but maximum power would be reduced. Also (it varies from engine to engine) but for most of the bottom half of the engine's power and, the lambda sensor will tell the ECU to make small corrections so is it's running a bit rich or lean, it will tell the ECU to give slightly more or less fuel. It's certainly true that a high lambda means a weak mixture. Sometimes the "Italian tune-up" gets exhaust temperatures high enough to burn contaminants off the lambda sensor wire, which might be affecting its reading. Hard to say what the probe was, but glad it's sorted for another year!
Thanks for checking. I couldn't find anything relevant either.
Yes, but generally only on lightly-driven ones, I find? It would be interesting to go on a model-specific forum and do a poll to see if there was any correlation between those who got the best MPG and those who had EGR and manifold sooting-up problems! Necessary to try and keep NOx down though. Forums are full of advice to blank them off, but I now see we're getting 60 MPH limits on some stretches of motorway to try and improve air quality. To some extent, at least, we reap what we sow!
On a diesel???!!!
They do use it on Diesel engines but direct injection, a small nozzle before inlet manifold would do no harm (AFTER you've cleaned the gunge out).
"Abstract: Addition of water to the diesel process decreases combustion temperatures and lowers NOx emissions. The most common methods of introducing water are direct injection into the cylinder, a process commercialized in certain marine and stationary diesel engines, and water-in-fuel emulsions."
The issue is oil from the turbo, being recirculated back to the engine mixed with the hot recirculated gasses from the exhaust - which is a really bad idea, at best a botch to improve NoX emissions when a vehicle leaves the factory. The problem it causes is the oil is burnt by the hot exhaust gases, choking up the EGR, the intake and the valve ports. The parts are not on any service schedule for cleaning, so soon the engine is so choked up the emissions become much worse than they would have been without the EGR system at all. There are EGR bypasses, which can be bought for many diesels, including the BMW engined Freelander. The MOT cannot presently check for Nox emissions and a bypass fitted is not easily spotted.
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