78 replies to this topic

[pete l]

Thanks bat. Would either of these problems cause the mixture to be over rich ?

[Wiggy]

If an exhaust valve isn't sealing, unburnt fuel will go straight down the exhaust.

You seem to have covered pretty much everything electrically.

Have you checked the valve clearances? If they're too tight the valves won't be closing properly.

[tmsmini]

Although this picture is from an SPi, the MPi version is in the same place. This picture is on this site in the SPi troubleshooting threads.

 

The white pointer in the picture shows it. There is a description of its function in the Rover service manual:

Purge valve

The purge valve receives battery voltage on an NK wire from link 4 in the engine compartment fuse box on an N wire from the closed contacts of the main relay (providing it is energised) and a control signal from the ECM on a BW wire. The valve remains closed when the engine is cold and at idling speed to protect engine tune and catalyst performance. When the purge valve is open, fuel vapour from the charcoal canister is drawn into the throttle housing for combustion.

[Bat]

Hi,

The canister under the wing its connected to stores fuel vapour, so it's not impossible for it to richen up the mixture, let's say if the valve is stuck open...

Cheers 

[tmsmini]

My thought was that a leaking or stuck open purge valve would actually cause a lean condition and closed loop mode would compensate for this. The port though which the vapor is introduced into the manifold is pretty small, so the impact would be minor most likely.

[pete l]

I will check this out. Thanks.

[Sprocket]

The purge valve can provide a fair chunk of fuel if it is stuck open. My fuel map has a huge dip in the VE table on overrun where the valve is active because of it.

 

First off, the mechanical aspect still applies. Many mechanical faults can cause high emission readings on any engine. General maintenance items still apply, and a good seal on the exhaust from the head to the tailpipe is imperative for accurate tailpipe readings.

 

Lets understand a little of the theory behind the exhaust gas readings.

 

What goes in must come out. The process of burning (decomposition) of the chemical mixture in the cylinder breaks down molecular bonds and creates new ones. so here are some of the basic bits that we hear about the most. The first two here are the ones that garages seem to get fixated on these days, as they seem to have lost the skills they once had when they had the basic stand alone exhaust analysers, which had 4 or 5 different gas readings. The new machines still have this facility, but often, the monkey operating the machine doesn't know how, or, as i've had in the past, they simply reserve the machine for MOT tests and not diagnostics, which makes a mockery of the diagnostics in its self.

 

So, the common two.

 

Carbon Monoxide (CO) is partially burnt fuel. It is fuel that has decomposed as a result of the high temperature and pressure in the cylinder, where there is not enough oxygen to completely burn the fuel (a rich mixture). The high carbon and low oxygen produce more CO rather than CO2 that ideally we want. Normally a lambda 1 (AFR 14.7:1) stoichiometric burn will produce around 14% CO2 and minimal CO. A rich burn (less than Lambda 1 or 14.7:1AFR) will produce less CO2 but more CO (what goes in must come out, maybe not as the same thing). so, with a high CO reading, it is an indicator of a rich mixture in the cylinder. If you could also read the CO2, it would be low, maybe 12% or less depending on how rich the mixture is.

 

Hydrocarbons (HC) is raw fuel. It is fuel that has passed through the cylinder without decomposing (burning). The nature of a piston engine means there will always be HC in the exhaust as fuel gets stuck in areas in the cylinder where it just wont burn, the gap around the top of the piston above the rings for example. An excessively rich mixture will also allow more HC to pass through the cylinder as it just cannot sustain any efficient burn, but you will know when this is the case as you'll smell it, if your eyes don't burn anyway. A leaking exhaust valve seat, damaged valve / bent valve (yes, it does happen), large overlap (sporty) camshafts will allow HC to pass through the cylinder and always with some oxygen too. A simple missfire (doesn't have to be regular) will pass all that fuel straight through the cylinder and with it a lot of oxygen too.

 

The other three gasses that seem to get miss placed when it comes to engine diagnostics are CO2, O2 and NOX.

 

Always remember the readings are an average, that means there are both high and low readings, since the ECU is causing both a momentarily rich and then momentarily lean conditions when you see the lambda sensor volts on the car fluctuate. You also have to consider where the reading is taken..... at the tailpipe. And we arrive at 'Lambda'. I'm not talking about the lambda sensor on the car here, i'm talking about the calculated lambda value the emissions analyser spits out. Lambda is simply put, a factor of actual air fuel ratio against the ideal air fuel ratio (stochiometric ratio), for example, 14.7 / 14.7 is Lambda 1.00. Lambda 1.09 is equivalent to  16 /14.7 ( a lean mixture). Lambda sensors actually read oxygen content in the gas, so if there is oxygen present, its lean, but hold on a minute, between the exhaust valve and the tail pipe, there are several mechanical joints, some welded joints, and a large moisture trap called the back box that rot from the inside out. A small leak, barely noticeable otherwise on any one or a combination of them all can lead to an artificially high Lambda reading at the tailpipe.

 

Lets consider your test results. CO say's its burning rich, black plugs also suggests its rich. HC say's there is un burnt fuel in the exhaust but the lambda reading say's it lean. Somewhere you have to consider that this cannot be without there being a leak in the exhaust system somewhere, it might be that there is a more of a regular miss fire than just the odd one, this is both HC and oxygen, but why the high CO (rich) if the lambda sensor on the car is seen to fluctuate on its own, which would suggest the ECU is targeting lambda 1. It could be the exhaust leak is before the lambda sensor on the car, which can introduce oxygen and the ecu is trying to over compensate for something thats not passed through the cylinder.

What about the Cat Converter? if there is a good burn before the cat, any HC that might be in the exhaust will be reduced down to almost zero and CO will be negligible. A high lambda might be a lean mixture, but the exhaust leak might still introduce excess oxygen at the tailpipe too. Maybe the cat is working hard as the CO and HC are much higher leaving the engine and the cat is in fact doing its job, but it is simply overwhelmed. You should be able to determine if the cat is working hard as its temperature will be much higher than the exhaust before it. I've seen cats that have been so hot they have melted the ceramic core, usually glow red hot and accompanied with a burning smell, but that really is an extreme case.

 

Ok so i've gone on a bit, and i've really only scratched the surface on what is a complex subject, but I can only suggest you make sure the ignition system is in good condition, the correct plugs, and leads, all wiring and connectors are in good condition the engine has had a good service in general, have the cylinder head valve seal tested (a leak down test might help indicate this), smoke test the entire exhaust system for leaks (plenty good demonstrations of this on youtube), check there is in fact a core inside the catalytic converter (before you do the smoke test obviously).

 

I'm not concerned about the lambda sensor, it works, what more do you want it to do? its telling the ECU what its suposed to do, and the ECU is responding to it. I would be chasing why you have a high lambda reading at the tailpipe.

 

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Edited by Sprocket, 18 July 2018 - 02:03 AM.

[pete l]

Wow, thanks for that sprocket.

I did see the co2 reading when it was re tested with the new lambda and it was around 13/14 %
The lambda was at 1.02
The co was lower, 0.99%
And HC around 470 if I remember correctly.

So, what actually do I do now ? Smoke test the exhaust ?
Do a leakdown test,
I'm up for changing the cat which might allow it to pass the MOT but its not going to stop it running rich.
Do I loosen off the tapets ?
It has new sparkplugs, leads, sensors, air filter. I've sprayed easy start all around the inlet area and got no engine note change.
I will check out the purge valve area.
Thanks
Pete.

[Bat]

Hi,

Well I would certainly check them to make sure they haven't closed up and holding a valve slightly open...

It should be done as part of a service.

Has the oil been changed recently? That could have fuel in it and be getting dragged in via the breathers.

Cheers 

[pete l]

The engine was built up so that I could get the MOT on it. So yes, new oil, head gasket, tapets set but I did set them to the tighter value rather than the loose noisy value.

[r3k1355]

You said you've fitted a new fuel pump?? and a new fuel filter??

 

Have you checked fuel pressure is within spec?

Have you cleaned and flow-benched the fuel injectors?

Edited by r3k1355, 18 July 2018 - 09:53 AM.

[pete l]

Yes a new minispares universal pump and filter. No I've not tested the pressure but we did pinch the pipe whuke testing the emissions. Didn't really improve before the engine stalled.
Not touched the injectors.

[r3k1355]

Not touched the injectors.

 

Are they original?  20 some years old right?? 

Did you replace the O-ring seals for em??  They go hard over time and don't really seal very well.

 

Just seems like you're chasing problems with a freshly rebuilt head while not looking at the un-touched original parts that are attached to it???

[pete l]

Hi. Yes original. Not touched them. Not removed them.
The head is not freshly rebuilt. It was removed just to see if anything was drastically wrong. I got the car in bits. Never saw the engine running etc.

[Bat]

Hi,

Pete it sounds like someone else has had this problem, tried everything they could think of then pulled the head off as a last resort.

Sorry to say but it looks like an uphill battle, it could be something really obscure

Did you check the vacuum pipe to the FPR for any fuel? If you pull it off with the engine running does it speed up a little?

Have you got a vacuum gauge?

Cheers