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k_d_ch

Co Emissions High: Overfuelling?

15 posts in this topic

I have just had a new cat fitted to my Ford Escort (1995, 1.4 petrol) and the CO emissions reading is still high. I wonder what the chances are that the engine coolant temperature sensor is faulty. I recently
replaced the lambda sensor, and lambda readings are OK.
Many thanks!

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I would start by having the Lambda sensors checked, if they are not correctly calibrated or working then this is likely the cause of the issue. The engine temperature sensors will have little to do with overfuelling as this is purely used for controlling engine cooling behavior.

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I would start by having the Lambda sensors checked, if they are not correctly calibrated or working then this is likely the cause of the issue. The engine temperature sensors will have little to do with overfuelling as this is purely used for controlling engine cooling behavior.

Many thanks for your comment.

Today a guy at a garage sprayed some stuff (they said "brake cleaner") around the inlet manifold and the emissions on the computer shot up. The guy suggests there is an air leak in the inlet manifold. Just to check: my understanding is that an air leak like this would make the oxygen sensor send a signal to the ECU to inject more fuel, which would be the way it overfuels. Would this logic be correct, were there to be an air leak?

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Not quite. What actually happens is that the air goes through the inlet manifold and then escapes.

To keep the emissions good it expects (for descriptions sake) 1 part fuel to 3 parts air. The air goes through the airbox and the car knows at this point, there is three lots of air in the system, but as it passes through the manifold, half the air escapes. By the time that the engine gets the air and fuel, its now at a 1:1.5 not 1:3 ration. in essence, to keep it balanced the car should inject half the fuel (0.5 parts for 1.5 of air) but its not, its actually injecting twice that, hence the overfuelling.

If it is a leaky inlet, then it would be best taking it off rubbing it down and finding the leak. It might be something cheaply repairable, or may just need a new one.

How confident where they that this was the case?

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Not quite. What actually happens is that the air goes through the inlet manifold and then escapes.

To keep the emissions good it expects (for descriptions sake) 1 part fuel to 3 parts air. The air goes through the airbox and the car knows at this point, there is three lots of air in the system, but as it passes through the manifold, half the air escapes. By the time that the engine gets the air and fuel, its now at a 1:1.5 not 1:3 ration. in essence, to keep it balanced the car should inject half the fuel (0.5 parts for 1.5 of air) but its not, its actually injecting twice that, hence the overfuelling.

If it is a leaky inlet, then it would be best taking it off rubbing it down and finding the leak. It might be something cheaply repairable, or may just need a new one.

How confident where they that this was the case?

I can see the logic in your explanation. I saw on a website (I think it was about Polos though) that the pressure inside the manifold can be lower than outside (is this what people mean when they use the word "vacuum" in this context?), hence the sucking in when an "air leak" occurs. It may well be that on Escorts it is the other way round, or that they were ill informed...

Regarding the garage guys, I would not put too much money on their claim. They were in a rush, as many of them usually are. At the time another problem occured as we spoke: the fan connector on the fan switch stopped working. It had not looked very healthy for years and now finally gave way. Following which I got a new connector from a scrapyard, which was lucky, and the fan is cutting in fine.

I then went back to the emissions guys to ask them to give the emissions test another go, which they offered to do free of charge in the first place. (This was kind of them, but also explains why they would not want to spend too much time on my problem.) At that point they said that there was no point in retesting since the first time round it was an air leak and the fan connector and switch could not have been a reason for high emissions, which seems fair enough to me.

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to take this back to the beginning...few corrections

coolant temp sensor plays a vital role in emissions, its this sensor that informs the ecu of engine temperature...ie, cold or hot.

think of it as a modern choke system...a faulty sensor is a common cause of high CO.

an air leak anywhere in the system will also play havoc with CO, at idle/low revs, the only place air can escape is through the exhaust, as, as you correctly stated the manifold pressure is lower than outside pressure, so rather than air escaping, its actually sucking extra air in...this extra air is unmonitored as its after the air flow sensor, so the air flow sensor is telling the ecu one thing but the extra air is detected by the lambda sensors in the exhaust system, which therefore tells the ecu to increase the fuel to counter it as it believes the engine is running too weak.

so....if spraying around the manifold changes the CO, then yes, its an air leak problem...check all vacuum hoses/gaskets/breathers and the manifold itself...you could double check this yourself if you either didnt witness the test or just doubt the diagnosis...run the engine till warm, then with everything connected, spray wd40 all round the manifolds/pipes/breathers etc..if air leak exists, you should hear a difference in the running as you spray.

matter of interest, is this a single point or multipoint injection engine (either has round air filter on top of engine, or square filter to the side of engine)

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NA (naturally asperated/ non turbo) sucks, turbo blows

On an NA, (4 stroke) during the induction stroke the inlet valve opens and as the piston moves down it forms a vaccum, normal air pressure is 14.5PSI so this pressure differential (the vaccum in the cylinder vs the normal air pressure outside) is what draws air into the cylinder

this means if there is a leak in the inlet manifold, it will draw air in, not blow it out on a NA (NA sucks, turbo blows)

On a turbo engine, however, the air is compressed by the turbo (compressor) so air is forced in to the inlet, above atmospheric pressure, it may act like an NA at low revs/ low load, but at higher revs/ load if there is a leak in the inlet manifold it will blow air out (NA sucks - turbo blows)

So on an Na, if there is a leak in the inlet manifold, it draws air in the leak, on a turbo, it blows air out

So the turbo may run rich and the NA run lean, on a modern engine the fuel is directed directly into each cylinder,(multi- point fuel injection) older engines may have single point fuel injection or even carbs, this may change whether an engine runs rich or lean if there is a leak, but in general it mucks up the fuel-air mixture as the leak bypasses/ is not measured by the MAF or boost is lost

On a modern engine the ECU uses the engine temperature to set the fuel/air ratio (richer, when it is cold, leaner when hot) - like how an old school choke works - thats why if the engine temp sensor (now normally directly on the head, not the coolant temp) is faulty it may put the engine into limp (home) mode as it relies on this sensor for this

EDIT - just noticed Wase16iis post - snap!

PS - also check the crankcase breather hoses for spits or leaks and the PCV (positive crankcase ventilation) valve is ok as they can also draw air not measured by the MAF (inlet airflow sensor)

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one other thing...the lambda sensors are the main sensors for setting/adjusting the fueling, so just because your lambda readings are fine, doesnt necessarily mean the lambda sensors must be ok.

the machine is measuring lambda from the exhaust gasses/tailpipe, not what the lambda sensors are doing...

a high lambda suggests a leak in the exhaust system, low lambda suggests an air leak before the lambda sensors...thats a very basic explanation, can get a bit more complicated than that but suffice to say, a correct lambda reading doesnt necessarily mean everything is hunky dory.

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Problem sorted. It was an ECU glitch, apparently. It was not supplying the required voltage (0.5V) to the lambda sensor.

The garage guy who fixed it said everything was in order once he probed (i.e. gave an extra ground) the signal wire that goes from the ECU to the lambda. Many thanks to everyone!

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By the way, can anyone suggest where I can get a fault code reader for my Escort (1995)? Apparently, OBD2 readers work for

cars from 1996 onwards. It seems a bit bizzare, but my Escort's ECU has got a socket that OBD2 plugs into (and even tries to start when plugged) but then gives an error message and is of no further use.

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cannot suggest a code reader as its a long time since i plugged into an old ford..but from memory..

95 was, i think, the year a 16pin socket was fitted, but you often find there is the older type 3 pin socket under the bonnet too..it was the crossover point.

there are code readers that connected and read fault codes on these 16 pins, but they were also very limited beyond that, i.e, live data, system readiness etc...for that you need a genuine ford tester which can be very rare and pricey

if your looking for something purely to read codes, then cheapest route is to check if you have the 3 pin socket too, if so, Gunson do a connector for around £15, gives you the codes via a flashing LED, could probably pick one up on the internet for pennies, there was also people that manufactured their own readers that converted the flashing LED to text, do a search and you may find some up for sale...

for 16 pin readers..be careful, as the early Ford dont always work on the standard obd 1 and 11 readers...need to confirm if your ecu is EEC IV or EEC V, and ask if the reader is compatible for this system

if you have the 3 pin socket, i have a genuine Ford tester i can sell you cheaply, did advertise it here couple of months ago so should be somewhere on the for sale section, never got round to putting it up for offer anywhere else

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Many thanks for your comment.

Today a guy at a garage sprayed some stuff (they said "brake cleaner") around the inlet manifold and the emissions on the computer shot up. The guy suggests there is an air leak in the inlet manifold. Just to check: my understanding is that an air leak like this would make the oxygen sensor send a signal to the ECU to inject more fuel, which would be the way it overfuels. Would this logic be correct, were there to be an air leak?

FOCA - the above post gave us reason to believe there was an air leak around the inlet manifold - we have to rely on the information you give us and draw our conclusions/ make our suggestions from that

Problem sorted. It was an ECU glitch, apparently. It was not supplying the required voltage (0.5V) to the lambda sensor.

The garage guy who fixed it said everything was in order once he probed (i.e. gave an extra ground) the signal wire that goes from the ECU to the lambda. Many thanks to everyone!

FOCA - So the air leak in the inlet/ manifold was a "blind alley" - the garage has direct access to the car - we have to rely on the info you post

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At least it's sorted now, perhaps the inlet is less an issue though lol

Sent from my MY SAGA C2 using Ford OC mobile app

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FOCA - the above post gave us reason to believe there was an air leak around the inlet manifold - we have to rely on the information you give us and draw our conclusions/ make our suggestions from that

FOCA - So the air leak in the inlet/ manifold was a "blind alley" - the garage has direct access to the car - we have to rely on the info you post

I am sorry if you feel that it was a waste of your time. I am grateful for your air leak comments: I personally have learnt from them and shall take them on board in the future. The garage that said it was an air leak was different to the garage where I took the car in the end (I was wating for my appontment with them at the time). There are garages to just do an emissions test and there are garages to solve a challenging problem! By the way, the guy who did the job said that there is no air leak, he checked it first.

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I learned on this one as well, so I wouldnt worry about it! we all come here to learn and share, on this occassion, my offering was incorrect, and proven otherwise and I learned from it too, so I wouldnt worry about it. But as FOCA said, we help based on what we are told, and if the garage tell you something that you pass onto us, then we can all only work with what we are given, so dont take it personally :)

On the plus side though, its fixed, and no longer being a total pain in your posterior.

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