12 replies to this topic

[Alice Dooper]

Have a strange one, here’s the history -

1. Barely made it through MOT with high CO and hydrocarbons. Car was running fine, good idle etc...
2. With an ARC4 reader, discovered that the O2 sensor was reading a constant 1.94v
3. Changed O2 sensor with what I was told is the correct one and came with the right 4 lead plug attached
4. Still same problem with the O2 sensor reading a constant 1.94v. The inlet air temp seemed a bit high at 55c. CO and hydrocarbons getting slowly worse.
5. Tappets adjusted right, vacuum hoses all good, changed out the coolant temp sensor, cleaned up the inlet air control valve (a wee bit dirty), tried forcing a lean and a rich mixture all win no effect on the emissions.
6. Did have an issue with the fuel pump relay so have changed the big engine relay pack with a genuine rover boxed one - it also has the relay for the O2 heater.
7. Car running very nicely, plugs aren’t fouled up, perhaps slightly cold/rich mixtur indicated
8. Wiring checked for continuity with a meter and all seems fine! Don’t hav acc XX to a fancy rover diagnostic kit.

Am scratching my heads, anyone any ideas please?

[Steve220]

Is this all from cold or did you take the car for a good thrash before hand?

[Bat]

Hi,

What was the coolant temperature?

What was the map sensor reading at idle?

Cheers 

[Alice Dooper]

Hi,
What was the coolant temperature?
What was the map sensor reading at idle?
Cheers 

Hanks fo looking at this.


Results were taken from the car after a ‘spirited’ good warm up every time.
Coolant temp by the dash gauge and the arc4 reader were fine. I could only see the oil temp by the dash gauge and it seemed up to temp and fine too. I had replaced the coolant temp sensor for the ECU just in case, it’s an easy and pretty cheap one.
The MAP sensor is reporting as working fine but I can’t remember the reading.

[Bat]

Hi,

A fault code reader will obviously read fault codes.

The codes are stored by the ECU when a sensor goes out of range, eg short or open circuit.

The rest of the time the ECU will believe that sensor to be telling the truth and calculate on the information given. 

Mechanical infulences and sensors giving inaccurate results cannot be detected by the ECU.

For example a small air leak in the exhaust will just tell the ECU that there's too much oxygen and it will add more fuel to compensate.

It doesn't know why it just does it and over time it will trim the fuel correction value until it matches the incorrect amount of oxygen in the exhaust...

Cheers 

Edited by Bat, 07 August 2018 - 07:38 PM.

[Alice Dooper]

Hi,
A fault code reader will obviously read fault codes.
The codes are stored by the ECU when a sensor goes out of range, eg short or open circuit.
The rest of the time the ECU will believe that sensor to be telling the truth and calculate on the information given. 
Mechanical infulences and sensors giving inaccurate results cannot be detected by the ECU.
For example a small air leak in the exhaust will just tell the ECU that there's too much oxygen and it will add more fuel to compensate.
It doesn't know why it just does it and over time it will trim the fuel correction value until it matches the incorrect amount of oxygen in the exhaust...
Cheers 

Sounds like I need a session on one of the old proper Rover diagnostics.

[Bat]

Hi,
It's not just diagnostics, it's still an engine not a computer.
You still need to check fuel pressure, vacuum with a gauge, compression, timing, etc.

For example, I used to do some work for a suzuki car main dealer. They called me in to look at an engine management fault, none starter.
From the fault codes and live data at cranking I told them the cambelt had jumped some teeth.
Their mechanics were sure it was an electrical problem, that's why they got me in.
Sure enough a couple of days later they call to say they've had the head off it to replace the bent valves caused by the jumped cam belt...
Cheers

Edited by Bat, 07 August 2018 - 10:13 PM.

[pete l]

Bat, when you say an air leak in the exhaust will let tell ECU to richen things up a bit, where would /could this air leak be ?

 

I ask because I'm still having problems with mine, the same as Alice Dooper.

 

I'm tearing my hair out trying to fix mine.

[Fast Ivan]

Bat, when you say an air leak in the exhaust will let tell ECU to richen things up a bit, where would /could this air leak be ?

 

I ask because I'm still having problems with mine, the same as Alice Dooper.

 

I'm tearing my hair out trying to fix mine.

before the Lambda

[pete l]

Before the manbda but after the inlet ?

 

That only leaves the exhaust manifold and gasket.

 

I can't see anything there that could leak air into the manifold, appart from the gasket which is new.

[Fast Ivan]

the object of the lambda is to measure the amount of oxygen in the exhaust gasess and feed that to the ecu, the ecu then richens or leans accordingly.

So if there is an air leak in the exhaust system somewhere before the Lambda, then the lambda will read this as a lean situation and the ecu will richen accordingly.

 

The lambda on the MPI is quite close to the inlet on the cast manifold, further down on an LCB, so its limited on where a leak could be, compared to more modern cars where there can be a second lambda further down the exhaust system

[Bat]

Hi,

There's the manifold gasket on the head there's the downpipe gasket between the manifold and the down pipe and also the gasket on the front of the cat.

It's not impossible for a pinhole in the back box to give an MOT fail as theie machines measure at the tailpipe...

Cheers 

[pete l]

OK, but a pinhole or leak after the lambda won't cause rich mixture though would it ??