Sorry this is off topic, but before trawling the net, I wonder if anyone on
this site can educate me about Lambda sensors and EGR valves.
This is due to the wife's car now being on its third EGR valve at only 47000
miles and I cant get to grips with the relative associations of the valves
to each other and why cleaning an EGR valve makes it work? What's on it and
Are you diagnosing the EGR as at fault, or is some lazy sod plugging
in his reader, pulling an EGR system fault code and replacing the valve?
Vaccuum or electric valve?
There may be other things at play than just the EGR valve being
faulty, though it is possible that the car just eats them. :-/
The purpose of the EGR valve is to recirculate a set amount of exhaust gases
depending on engine load, speed and temperature. And it's all for emission
purposes, as it helps to lower exhaust temperature (the same could be done
by running lean, but then you stray into lean burn territory), which in turn
affects the exhaust gas composition (IIRC it reduces NoX, but increases
other gases which the catalytic converter can then deal with, by converting
them into water).
Older vehicles tended to use an open loop system for the EGR valve, so that
if it stopped working, chances are you'd be none the wiser, unless it
happened to stick open.
Newer vehicels tend to all be closed loop systems, so that the ECU knows
exactly how much the valve is open, and by monitoring the MAF (Mass Air
Flow) sensor, it knows that exhaust gases are getting through (so blanking
it off will be detected, and the Emissions warning lamp turned on).
The usual failure for EGR valves is for them to get gunked up, and start
sticking. This can happen one of three ways. They stick shut, they stick
open, or they become jerky in operation. Stuck shut is not a major problem.
Stuck open means too much gas will get recirculated, and you'll get
constantly rough running (if bad enough, the engine will strangle itself at
low revs). If they become jerky, can also lead to several problems, but the
usual being hunting, more so when under partial load.
Lamdba sensors, or Oxygen sensors, are used to detect how rich/lean the
exhaust gases are.
In the ideal world, for emission purposes, petrol engines will always run at
the Stoichiometric ratio (14.7:1 air to fuel ratio).
Lamdba sensors detect when the exhaust gas is rich, and generate a small
voltage (Usually somewhere from 0.4V to 1.2V depending on sensor). This then
signals to the ECU, which in turn reduces fuelling slightly until the Lamdba
sensor voltage drops to zero, at which point it starts to increase fuelling
until it sees a voltage from the sensor again, and it coninually goes
through this process, and good sensor should switch at least once a second.
Since 2000, cars have had to meet the EOBD (European OnBoard Diagnostics),
off which one spec is the fitment of a post cat sensor. This second sensor,
is used for the sole purpose of monitoring the performance of the emissions
control. It's signal should vary only occasionally, and is only monitored by
the ECU, and not used to control the running of the engine.
If you want the full info on Lambda sensors, check the wikipedia -
PS. How come Emimec's (Bobs) posts never appear on some news servers?
I only see them if someone happens to reply to them.
I am sure your comprehensive description is correct, but what a stupid
system. Just how many complicated and expensive things to go wrong do
they want under a bonnet? A valve handling exhaust gases is sure to
clag up. Besides these mods seem pointless as any improvements in the
cars around here have been more than offset by all the A-road speed
limits being lowered, which puts up journey times, so increases the
number of cars on the road at any given time, and so causes endless
Enough to meet the european emissions standards!
Modern petrols are actually less complicated than a modern diesel, and it's
only going to get worse.
On the plus side, it does mean I get lots of nice courses, to keep me upto
date with technology.
Modern valves are far more robust, but longetivity pretty much depends on
how the car is driven. A car that gets long runs, and is driven hard will
have a far cleaner engine, than one that only ever gets used to potter about
town on short journeys.
There is no denying that modern car emissions are pretty low, and that can
only be a good thing, but it's coming to point where vehicles are no longer
the main source for pollution, yet they're the easy target. It's not quite
so simple cutting the emissions from other common sources.
Many thanks to all who replied. I feel a lot more knowledgeable now. Seems
the trick is to make sure the car does a bit of distance work, which is not
the case with the wife, 4 trips a day of only about 2 miles each way.
Another good bit of advice is to get some carbon cleaner spray to keep the
valve free, and also buy a hand held code reader and reset the thing myself
instead of paying the stealership loads of money.
The main dealers theory of there being remnants of the old "Cat" in the
system was hard to accept.
I must say, it is in my opinion a ludicrus idea/system of how this EGR valve
works, anything in the flow of an exhaust is surely asking for trouble.
Thank heaven my car is pre all this twaddle !!!!
When the cat destroys its self ..usually happens after failure of one
or two gizmo's that end up making the fuel too rich.....in most cases
there will be chunks of it in the exhaust system rattling away for
months unless you buy a complete new exhaust.
they cant be emptied by dismantling either ..
I know because i have that exactly.
new fangled crap.
all the best.mark
Ditto, Side effect of an engine bay that requires the entire three part inlet
manifold to be taken off (10 O clips) In order to change the spark plugs, plus
a design that causes the rocker covers to weep oil into the spark plug wells.
We had several on the fleet, and the spark plugs was the least of the
The EGR pipe down the back used to crack (rough idling when cold, and a
slight exhaust rasp), and that was always fun to do. Crow's foot spanner was
the answer, aswell as being double jointed.
Thankfully we never kept them long enough to have to timing belts on them!
Yeah it's coming to the point alright ...that, there will be that many
gizmo's on them.
That after five years, you add up the total for repairs and
replacement and diagnostics of the gizmos.......after it fails its MOT
on a simple emissions fault ....that the car will be
scrapped ...thereby necessitating that Joe public go's out and
purchase another new car ......
So in the end, more cars have to be made ....cars will have a five
year life span instead of 10..the manufacturing process uses more
resources ....and puts out more pollution ...so it don't make any
sense......as far as the environment is concerned.
and more cars scrapped means more scrap and other non biodegradable
materials to deal with
well the only people it does make sense to... is the manufacturers,
because they make more profit from selling more consumable cars.
....and that's the future of the car industry according to mark ...LOL
Absolutely, plus the equipment the garages have to keep buying puts up
their costs, so the labour rate rockets too (plus the training
courses). I think the law makers realise that this will put up the
cost of cars and hence tend to reduce the number, but its all going to
end in some sort of 'orrible crisis if they are not careful.
The only vehicle I had with EGR was a V8 LandRover and I was told the
EGR was nothing but trouble, and to throw it all away. Which I did. I
don't remember any valves, just some corroded leaky pipes connecting
the exhaust to the inlet. It struck me as a silly thing to do in the
first place, and the MOT man says it has very low emissions.
So from the little I know the EGR allows some exhaust gas to be
recirculated through the inlet manifold. The EGR valve presumably
prevents any chance of the flow going in the other direction, which if
it did would cause the Lambda sensor to give the wrong reading (as it
detects oxygen). It would draw air into the exhaust which will be
detected by the Lambda sensor, so the computer will think the mixture
is lean and richen it up.
The action of the valve is guesswork on my part. Does this tally with
the symptoms ? I suppose it would go rich on the overrun.
If lambda sensor is the same as Oxygen sensor...
The O2 sensor runs the fine tune on the mixture, with the computer
reading the voltage generated and richening or leaning out the mix on
The EGR valve draws dirty old exhaust back into the nice clean engine
in an effort to help balance out the emissions on the over-run. The
system on my diesel truck is vaccuum operated. The one on my Mazda car
has a 6 or 8 wire plug on it and runs off a stepper motor ($$$).
The one on my truck is slowly being made redundant. I ignore the check
engine light on the car that says that there is an EGR fault. I suspect
actually that the EGR is fine, the wiring however is suspect. I set the
valve in the closed position to avoid continuously sucking burnt gasses
back into the engine (who thought that was a good idea anyway?)
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