Discussion
Hello all,
I am new on this forum but I hope that doesn't affect you helping me with the CO problem I have with my 93 Grif 4.3, with cats. MOT limits here in Holland are about the same as in the UK, so 0.5% @ idle and 0.3% @ fast idle. My car was always just around these limits, sometimes a bit under it, sometimes a bit above, and then some tricking under the bonnet gave me a pass. But this year things were different. CO changed to 0.8% @ idle and 0.5% @ fast idle. In this case the trick didn't work anymore. Lambda and HC are ok. All with a hot engine of course.
Now my garage says to me that the injectors could be the reason, but I doubt that. I have cleaned the injectors in 2004, about 20.000 km's ago, and before and after the cleaning the CO didn't differ very much. So why would cleaning now help? Above that, if they aren't clean CO should be lower and not higher in my opinion. Or am I wrong in that?
Normally I would let the garage exchange the injectors but I expect that this will cost me a lot and will give no results, so in such a case I'd rather look further what can be the cause. I got an advice to turn the screw in the air flow meter but that didn't change anything in the CO level. They also exchanged temporarily the engine management computer, also no change. This was done because I have a Mark Adams ECU in it. Stationary stepper motor has been cleaned also.
Anyone got a clue? I appreciate your advice.
By the way, I have some other nuisance with the engine, maybe it has something to do with this. When I start the engine it is running idle ok, but when I turn it off and on again, idle sinks to about 400 rpm and it just stops. I have to drive heel and toe then to keep it running while braking and shifting down. It's a real annoyance...
I am new on this forum but I hope that doesn't affect you helping me with the CO problem I have with my 93 Grif 4.3, with cats. MOT limits here in Holland are about the same as in the UK, so 0.5% @ idle and 0.3% @ fast idle. My car was always just around these limits, sometimes a bit under it, sometimes a bit above, and then some tricking under the bonnet gave me a pass. But this year things were different. CO changed to 0.8% @ idle and 0.5% @ fast idle. In this case the trick didn't work anymore. Lambda and HC are ok. All with a hot engine of course.
Now my garage says to me that the injectors could be the reason, but I doubt that. I have cleaned the injectors in 2004, about 20.000 km's ago, and before and after the cleaning the CO didn't differ very much. So why would cleaning now help? Above that, if they aren't clean CO should be lower and not higher in my opinion. Or am I wrong in that?
Normally I would let the garage exchange the injectors but I expect that this will cost me a lot and will give no results, so in such a case I'd rather look further what can be the cause. I got an advice to turn the screw in the air flow meter but that didn't change anything in the CO level. They also exchanged temporarily the engine management computer, also no change. This was done because I have a Mark Adams ECU in it. Stationary stepper motor has been cleaned also.
Anyone got a clue? I appreciate your advice.
By the way, I have some other nuisance with the engine, maybe it has something to do with this. When I start the engine it is running idle ok, but when I turn it off and on again, idle sinks to about 400 rpm and it just stops. I have to drive heel and toe then to keep it running while braking and shifting down. It's a real annoyance...
More knowledgeable people will be along soon, but I wouldn’t start with the injectors.
Presumably being a Cat car you will have lambda (oxygen) sensors fitted, one in each exhaust manifold. These report the mixture back to the ECU and allow it to trim the fuelling to keep the CO close to target. The sensors used are a very basic type and simply report Rich or Lean. If you look at the output signal they should be constantly switching status as they report Rich so the ECU backs off the fuel it then becomes lean. The ECU only uses the sensors to control the mixture at ticks over and on a light throttle up to 2000 or so RPM. So checking the Lambda sensors would perhaps be the first thing. They are of an unusual type (three wire and small bore fixing) so you may need to go to a TVR parts specialist if you need one. As they become old they become very slow to react so the switching rate slows down and the mixture wanders off target.
The other fault symptom you report could be caused by the wrong mixture but more likely is a problem with the stepper motor throttle bypass. It is a black screw in valve at the back of the plenum with which the ECU controls the tick over. It is very common for it to stick and very easy to remove and clean. Do a search for stepper motor and you will find loads of stuff.
If you simply want an MOT one fix is to deliberately cause an air leak into the plenum. By adding some air that has not been measured by the Air Flow Meter you can lean up the engine. It will run poorly but gets the pass sorted out. I have done that a number of times on a pre cat car by removing the bypass pipe (connected to the stepper motor) and replacing it with one that had a valve fitted, I used a heating system drain tap. By opening the drain tap slightly you can let in a bit more air and lean up the mixture. When you have the MOT simply pop the original pipe back on. Not certain it would work so well with a cat car as the ECU would try to cancel out the extra air using the Lambda sensor signals.
Presumably being a Cat car you will have lambda (oxygen) sensors fitted, one in each exhaust manifold. These report the mixture back to the ECU and allow it to trim the fuelling to keep the CO close to target. The sensors used are a very basic type and simply report Rich or Lean. If you look at the output signal they should be constantly switching status as they report Rich so the ECU backs off the fuel it then becomes lean. The ECU only uses the sensors to control the mixture at ticks over and on a light throttle up to 2000 or so RPM. So checking the Lambda sensors would perhaps be the first thing. They are of an unusual type (three wire and small bore fixing) so you may need to go to a TVR parts specialist if you need one. As they become old they become very slow to react so the switching rate slows down and the mixture wanders off target.
The other fault symptom you report could be caused by the wrong mixture but more likely is a problem with the stepper motor throttle bypass. It is a black screw in valve at the back of the plenum with which the ECU controls the tick over. It is very common for it to stick and very easy to remove and clean. Do a search for stepper motor and you will find loads of stuff.
If you simply want an MOT one fix is to deliberately cause an air leak into the plenum. By adding some air that has not been measured by the Air Flow Meter you can lean up the engine. It will run poorly but gets the pass sorted out. I have done that a number of times on a pre cat car by removing the bypass pipe (connected to the stepper motor) and replacing it with one that had a valve fitted, I used a heating system drain tap. By opening the drain tap slightly you can let in a bit more air and lean up the mixture. When you have the MOT simply pop the original pipe back on. Not certain it would work so well with a cat car as the ECU would try to cancel out the extra air using the Lambda sensor signals.
You can try a simple test, get a test meter across the back of the lambda probe connector, across the white and black wire and start the car. After about 25 seconds (from cold) you should see a 0 - 1 DC volt signal switching at about 1/2 - 1 second intervals if the ECU is controlling the mixture as it should be. Check this on both sides. If its not switching, either the mixture is very lean (like an air leak in the plenum) or the lambda probe is dead. If the voltage is stuck at 1 volt or more its running rich. I realise you say the lamda reading is OK, but still do the test to start with as it will prove the ECU is working as it should. If the signal is switching very slowly (about over 2 seconds) it can indicate other mechanical or ignition issues. Your garage may not be wrong about injector cleaner, as high CO can justy be a catalyts not working correctly, so you can use injector / cataysts cleaner in the fuel, and take the car for a good hard run down a motorway to get the catalyst really hot to burn off contamination.
iede said:
Thanks all for your replies.
@blitzracing: does the engine have to be warm or can I also check this with a cold engine?
The lambda feedback starts as soon as the lambda probes themselves are hot, typically after about 25 seconds when the engine is started from cold (This is due to the probes having their own electrical heaters). The ECU will then constantly adjust the mixture to keep the probes switching every time the engine idles, however hot or cold the engine. Realisticaly, if there is a termperature point the engine is bad at, then test it there.@blitzracing: does the engine have to be warm or can I also check this with a cold engine?
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