shunting and hunting
Discussion
Hi,
my Griffith 500 (94 serp, 42 thou on clock) has always had a mild shunt at low revs when driving in a high gear. I have also noticed that when holding the accelerator rock steady at 2000 rpm in neutral there is slight hunting - I can hear the changing engine note and see it on the rev counter (approx wavering of 100 rpm iether way). However holding above 3000 rpm no hunting at all. I'm thinking the two are linked and this this is something to do with the oxygen sensors. Has anybody experinced these symptoms and or resolved similar problems, appreciate any feedback.
thanks,
Ozstyle
my Griffith 500 (94 serp, 42 thou on clock) has always had a mild shunt at low revs when driving in a high gear. I have also noticed that when holding the accelerator rock steady at 2000 rpm in neutral there is slight hunting - I can hear the changing engine note and see it on the rev counter (approx wavering of 100 rpm iether way). However holding above 3000 rpm no hunting at all. I'm thinking the two are linked and this this is something to do with the oxygen sensors. Has anybody experinced these symptoms and or resolved similar problems, appreciate any feedback.
thanks,
Ozstyle
Ozstyle said:
Hi,
my Griffith 500 (94 serp, 42 thou on clock) has always had a mild shunt at low revs when driving in a high gear. I have also noticed that when holding the accelerator rock steady at 2000 rpm in neutral there is slight hunting - I can hear the changing engine note and see it on the rev counter (approx wavering of 100 rpm iether way). However holding above 3000 rpm no hunting at all. I'm thinking the two are linked and this this is something to do with the oxygen sensors. Has anybody experinced these symptoms and or resolved similar problems, appreciate any feedback.
thanks,
Ozstyle
my Griffith 500 (94 serp, 42 thou on clock) has always had a mild shunt at low revs when driving in a high gear. I have also noticed that when holding the accelerator rock steady at 2000 rpm in neutral there is slight hunting - I can hear the changing engine note and see it on the rev counter (approx wavering of 100 rpm iether way). However holding above 3000 rpm no hunting at all. I'm thinking the two are linked and this this is something to do with the oxygen sensors. Has anybody experinced these symptoms and or resolved similar problems, appreciate any feedback.
thanks,
Ozstyle
Pretty sure this is a problem with the lambada sensors as I believe they are cut out of the equation at above 3000 revs
Hi Ozstyle,
I think you have opened a can of worms here....from my many hours of research, I believe that they all do that to some degree, some a lot more noticable than others. On my car it really did Pi** me off, when driving through 30 zones the car would shunt big time, it was even accountable for me having to get the drive train reconditioned (Ujs-Cvs-Diff).
From research there are a number of cures all of which have worked for some and not others. Indeed it is an 'issue' with the llamdas, that below 3000rpm they are being used and above that they are not, and a standard fuel map is used. I was told that somewhere between 2000 and 3000 there is a 'no mans land' where the llamdas are just cutting out and the fuel map is changing and it is this no mans land that the ECU has an 'issue' with.
To solve the problem you could:
1. get a Mark Adams chip and tune.
2. get a new cam (i have heard this can cure the problem).
3. try new llamda sensors.
4. change the tune resistor to a pre-cat green type.
5. Remove the precats full decat.
I tried option 1 first, which did help but the shunting still occured, however there were many positives such as fuel economy was better, drive was smoother etc....
In the end I have stuck with option 4. The resistor cost me £30 and initially it transformed the car, no more shunting at all. Basically it tells the ECU that it is a precat and eliminates the llamdas completely, running a richer fuel map, so it does use more fuel but for me this isnt a problem as i only use the car for weekend fun!
However I read bad things about this as the cats could get too hot and explode!! So in the end I have totally decatted the car and it runs great! Although having done this, the cats were in perfect condition even after a year of 'rich running' so maybe they could have stayed in??!!
For the cost i would deffo recommend trying the green tune resistor as I am sure you will instantly notice the difference.
Drop me a line if I can be of any more help.
Regards,
Lig.
I think you have opened a can of worms here....from my many hours of research, I believe that they all do that to some degree, some a lot more noticable than others. On my car it really did Pi** me off, when driving through 30 zones the car would shunt big time, it was even accountable for me having to get the drive train reconditioned (Ujs-Cvs-Diff).
From research there are a number of cures all of which have worked for some and not others. Indeed it is an 'issue' with the llamdas, that below 3000rpm they are being used and above that they are not, and a standard fuel map is used. I was told that somewhere between 2000 and 3000 there is a 'no mans land' where the llamdas are just cutting out and the fuel map is changing and it is this no mans land that the ECU has an 'issue' with.
To solve the problem you could:
1. get a Mark Adams chip and tune.
2. get a new cam (i have heard this can cure the problem).
3. try new llamda sensors.
4. change the tune resistor to a pre-cat green type.
5. Remove the precats full decat.
I tried option 1 first, which did help but the shunting still occured, however there were many positives such as fuel economy was better, drive was smoother etc....
In the end I have stuck with option 4. The resistor cost me £30 and initially it transformed the car, no more shunting at all. Basically it tells the ECU that it is a precat and eliminates the llamdas completely, running a richer fuel map, so it does use more fuel but for me this isnt a problem as i only use the car for weekend fun!
However I read bad things about this as the cats could get too hot and explode!! So in the end I have totally decatted the car and it runs great! Although having done this, the cats were in perfect condition even after a year of 'rich running' so maybe they could have stayed in??!!
For the cost i would deffo recommend trying the green tune resistor as I am sure you will instantly notice the difference.
Drop me a line if I can be of any more help.
Regards,
Lig.
Edited by lig on Tuesday 19th September 13:04
It's normal for the engines to get a bit lumpy at low revs, typically it comes in at somewhere around 1500 - 2000 rpm although it varies between cars and even from day to day. Backlash in the transmission can make the effects more pronounced to e.g. loose diff mounting bolts can change the problem from an occasional grumble to a hideously unpleasant fault.
Apparently, disabling the vacuum advance can make a significant improvement. However, it'll also make the engine run hotter at idle. Given that some cars are a bit marginal on cooling, this might cause worse problems than it cures.
The problem clearing above about 3000 rpm indicates that the problem is associated with the lambda feedback. It may be that one or more of the sensors are misreading, or that the ECU has detected a serious fault and is running in 'limp home' mode. In the latter case, disconnecting the ECU for 30 seconds will clear the fault code and may restore better operation. Note that if the fault is still present then the engine still won't run properly, and the fault code will return after a while. In that case you're left checking the ECU sensors and reading the ECU fault code to see what it thinks is wrong. There's a chap on PH who rents fault code readers out, or take it in to a TVR dealer/specialist.
Apparently, disabling the vacuum advance can make a significant improvement. However, it'll also make the engine run hotter at idle. Given that some cars are a bit marginal on cooling, this might cause worse problems than it cures.
The problem clearing above about 3000 rpm indicates that the problem is associated with the lambda feedback. It may be that one or more of the sensors are misreading, or that the ECU has detected a serious fault and is running in 'limp home' mode. In the latter case, disconnecting the ECU for 30 seconds will clear the fault code and may restore better operation. Note that if the fault is still present then the engine still won't run properly, and the fault code will return after a while. In that case you're left checking the ECU sensors and reading the ECU fault code to see what it thinks is wrong. There's a chap on PH who rents fault code readers out, or take it in to a TVR dealer/specialist.
Remember that the lambda sensors are narrowband and I think this is where the problem lies, they can't measure an absolute mixture accurately, they just change voltage rapidly at 14.7-1.
In order that the ECU can work out whether the mixture is at "ideal" 14.7-1, it must contiunally richening the mixture 'til it crosses 14.7-1 and then weaken it until it crosses back. Stick a scope on the output of the lambda and you can see this (0.2 - 0.8v swing). It's pretty slow at tickover too, around 3 seconds for a cycle from memory. I can feel the change in engine note on light throttle cruising and with a scope hooked up I reckon it would coincide with the weakening of the mixture.
A fresh set of lambdas may react quicker and mask the effect to some degree by preventing the ECU trying so hard to force the mixture weak then rich while waiting for lambdas to react. It may be that Mark Adams can tweak the mixture step size and max deviation from map to reduce this effect. A lot of programmable ECUs have this feature.
I've monitored other cars using an OBDII scanner (mondeo v6, Esprit v8) and they seem to cycle more quickly than my Grif.
If I was replacing the lambdas, I'd be tempted by the build-in Innovate wideband that has a narrowband output... you then have an accurate mixture reading with the aid of a multimeter (or dash mounted gauge if you want) and the narrowband output can be fed into the rover ECU. They probably react more quickly than the narrowband too so may reduce hunting.
In order that the ECU can work out whether the mixture is at "ideal" 14.7-1, it must contiunally richening the mixture 'til it crosses 14.7-1 and then weaken it until it crosses back. Stick a scope on the output of the lambda and you can see this (0.2 - 0.8v swing). It's pretty slow at tickover too, around 3 seconds for a cycle from memory. I can feel the change in engine note on light throttle cruising and with a scope hooked up I reckon it would coincide with the weakening of the mixture.
A fresh set of lambdas may react quicker and mask the effect to some degree by preventing the ECU trying so hard to force the mixture weak then rich while waiting for lambdas to react. It may be that Mark Adams can tweak the mixture step size and max deviation from map to reduce this effect. A lot of programmable ECUs have this feature.
I've monitored other cars using an OBDII scanner (mondeo v6, Esprit v8) and they seem to cycle more quickly than my Grif.
If I was replacing the lambdas, I'd be tempted by the build-in Innovate wideband that has a narrowband output... you then have an accurate mixture reading with the aid of a multimeter (or dash mounted gauge if you want) and the narrowband output can be fed into the rover ECU. They probably react more quickly than the narrowband too so may reduce hunting.
Thanks for for the replies,
I reset the ecu but no change (same symptoms from idle through to 3000 rpm). Being in Australia I can't get to Mark Adams (not with car anyway). I have found a lot of threads on shunting but not a lot on hunting. The shunting is only mild and I can live with it but I guess a new set of oxy sensors are on the cards in the future.
Ozstyle
I reset the ecu but no change (same symptoms from idle through to 3000 rpm). Being in Australia I can't get to Mark Adams (not with car anyway). I have found a lot of threads on shunting but not a lot on hunting. The shunting is only mild and I can live with it but I guess a new set of oxy sensors are on the cards in the future.
Ozstyle
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