Drive line shunting / overfueling
Discussion
Chaps,
Any ideas what could be the cause on my Dads 4.3 Chim which is proving to be a pig to drive on anything but open roads?
The symptoms are unless under power and reving up the rev range you get a snatching / overfuel feeling when you take your foot off the gas.
It really is quite lethal because it feels as if the thing has a mind of its own on the overun.
To date, plugs, filter, etc has been replaced but no joy.
It feels fuel related ? or could it be ECU ?
Any positive feed back would be appreciated.
Just to close the car has a bit of a chequered past and not sure if cam has been changed or something else which could be the problem?
At this rate we will need to change the clutch every couple of months as this is the only way to control the thing smoothly off power.
Any ideas what could be the cause on my Dads 4.3 Chim which is proving to be a pig to drive on anything but open roads?
The symptoms are unless under power and reving up the rev range you get a snatching / overfuel feeling when you take your foot off the gas.
It really is quite lethal because it feels as if the thing has a mind of its own on the overun.
To date, plugs, filter, etc has been replaced but no joy.
It feels fuel related ? or could it be ECU ?
Any positive feed back would be appreciated.
Just to close the car has a bit of a chequered past and not sure if cam has been changed or something else which could be the problem?
At this rate we will need to change the clutch every couple of months as this is the only way to control the thing smoothly off power.
Ive been looking at this very issue on a 4.3 recently, its quite specific. I have to assume at this point its not failed component, but more a "design" issue as all the obvious bits had been changed. It occurs at around 1500 rpm in the higher gears under light throttle, and starts as a slight shudder and then gets worse. On the car I looked at, its caused by the engine running very lean. You can see this from moitoring the Lambda probe outputs when driving the car. This is because on this TVR's airflow meters output at 1500 rpm, is about the same as a Range Rovers is at tickover, so the ECU is only supplying a tick overs amount of fuel, even with the TVR chip in the ECU. I think this is due to the cam profile TVR use, being less efficient at tickover. I think the snatching gets worse once its started, as cars with this sort of problem develop play in the drive chain, and a mechanical resonance is then set up that makes things worse. The fueling issue can be overcome by raising the airflow meters output by about 200mv at 1500 rpm, so the ecu thinks theres more air going in, and accordingly puts in a bit more fuel. You can do this by putting a 1 to 1.2 ohm resistor in the earth line (red and black)from the AFM, as this effectively adds a small voltage to the the overall AFM output in relation to the ECU ground. This can only be described a low cost "work a round".
There is a problem doing this however. The extra 200 mv has a large fueling effect at tickover, that can cause the ECU to overfuel to the point the Lambda control can no longer control the mixture, and could put unburnt fuel into the catalysts, which can destroy them over time. So you need to monitor the lambda probe outputs when selecting the resistors, to check they are still switching the required .3-.7 volts. The resistor value is CRITICAL to get this to work. I found .8 ohms had little effect, and 1.5 ohms caused overfueling, so you cant simply slap a resistor in and hope it works, as Im sure it will vary from car to car. (I awaiting the results from another couple of cars). The ECU also sets up its overall fueling correction at tickover, so this mod' also distorts this process a bit, but basically a long as Lambda correction can still keep the mixture in range up to 3000 or so RPM, and the car drives OK, this is not a problem. Above this point the ECU goes open loop, and the extra 200 mv is insignificant compaired to the 7 or so volts the AFM produces at full load.
On the particular car I looked at, even with the fueling now spot on, (you could definately feel the difference by switching the resistors in / out when driving the car) the transmition backlash was still a problem, so it still snatched somewhat, but Im pretty sure this could be overcome by changing the sloppy components.
SO basically dont just throw money at it (LIKE A CAM ) as there are plenty of people out there who will take your money and not fix it, and know diddly squat about whats actually causing it.
Mark
There is a problem doing this however. The extra 200 mv has a large fueling effect at tickover, that can cause the ECU to overfuel to the point the Lambda control can no longer control the mixture, and could put unburnt fuel into the catalysts, which can destroy them over time. So you need to monitor the lambda probe outputs when selecting the resistors, to check they are still switching the required .3-.7 volts. The resistor value is CRITICAL to get this to work. I found .8 ohms had little effect, and 1.5 ohms caused overfueling, so you cant simply slap a resistor in and hope it works, as Im sure it will vary from car to car. (I awaiting the results from another couple of cars). The ECU also sets up its overall fueling correction at tickover, so this mod' also distorts this process a bit, but basically a long as Lambda correction can still keep the mixture in range up to 3000 or so RPM, and the car drives OK, this is not a problem. Above this point the ECU goes open loop, and the extra 200 mv is insignificant compaired to the 7 or so volts the AFM produces at full load.
On the particular car I looked at, even with the fueling now spot on, (you could definately feel the difference by switching the resistors in / out when driving the car) the transmition backlash was still a problem, so it still snatched somewhat, but Im pretty sure this could be overcome by changing the sloppy components.
SO basically dont just throw money at it (LIKE A CAM ) as there are plenty of people out there who will take your money and not fix it, and know diddly squat about whats actually causing it.
Mark
Fair comment, but Ive now come across 3 cars showing the same problem. It could be tolerances changing as components age as Im sure no one would have accepted it straight from the showroom. Interesting about the injectors, as I guess poor spray patterns at low air flows would show up as lean on the lambdas, as the fuel was not being burned effectively. I guess you really need to look at the hydrocarbons as well. Having said that tickover fueling was fine on these cars, so it looked just like poor mapping at 1500 rpm. I did try three different maps, and it was the same for all of them. Hence the physical voltage changes to the AFM, but it could be covering something else up.
Thats the point, idle mixture just fine, mid to full throttle just fine, its only that very light throttle opening at low revs (read low flows) that causes the problem, and its pretty subtle (fuel wise that is.) Thats what makes me think its not something sticky injectors plus both banks show the same fault, as I monitored them both during the adjustments. It takes 200mv more at 1.7 volts AFM output to clear the fueling issue. Thats only 11% more. Can anyone check the airflow output on a working car at idle with a test meter and post the results ? (more the merrier) Take two readings. One between engine earth and the blue green wire on the AFM, and one between the red/black (AFM earth) and the blue green wire. They should be the same and in the range of 1 to 2 volts. Its possible that something else reduces the airflow through the AFM, and hence the available fuel.
Mark
Mark
Actually Spend, you are the ideal candidate to measure the AFM outputs between the 4.3 TVR and 3.9 Range Rover and see if it replicates my results. Try and get them to idle at the same speed when measuring the voltages, so get a representitive value of the airflow. (assuming the AFM's are simular)
thanks
Mark
thanks
Mark
Can you just get somebody with an adjustable fuel pressure regulator to wind it up for a while and see if the shunting gets better or worse? Seems easier that playing with sensors etc. Alternatively, if you know anyone with an aftermarket engine management system they could test your theory by seeing what happens if you run it lean at low revs/off load.
and I'll bring you a tvr bible!
Its me that was lost Mike, thats why I posed the question 
The AFMs are the same whichever engine they are fitted to, and simply measure the amount of air going in - which presumably is quite different depending on the engine size / cam / induction. The chip in the ecu is mapped for the various different engine states of tune/size... I can't see how you can correlate anything about the ecu voltage across different engines, maybe swapping AFM's to see if one is giving erroneous readings would work?
My fag pack calc was simply 400cc capacity difference, at tickover say 800rpm is 400x400 cc's every minute hence the 'sucking in @160000cc more air every minute' comment.
Dave
PS: I'm good early next week, if we can have a good boob-watch like last time all the better
The AFMs are the same whichever engine they are fitted to, and simply measure the amount of air going in - which presumably is quite different depending on the engine size / cam / induction. The chip in the ecu is mapped for the various different engine states of tune/size... I can't see how you can correlate anything about the ecu voltage across different engines, maybe swapping AFM's to see if one is giving erroneous readings would work?
My fag pack calc was simply 400cc capacity difference, at tickover say 800rpm is 400x400 cc's every minute hence the 'sucking in @160000cc more air every minute' comment.
Dave
PS: I'm good early next week, if we can have a good boob-watch like last time all the better
This is what im getting at. My 3.9 running a piper 270 gives a reading of 1.7 volts at around 800 rpm. On the 4.3 BV I looked at that was snatching I got the same 1.7 volts at 1500 rpm. So its not as simple as simple displacement calculations, as with longer duration cams the valve overlap period is longer, and dont believe the volumetric effciency is as good at low RPM, so you dont get a full cylinder charge. Thats why race engines dont tickover well. I think the problem of snatching is down to extra cam duration causing low airflows at low speed to reduce the fueling, but I would really like more readings to confirm this. Only trying to help......
Mark
Mark
GreenV8S said:
Can you just get somebody with an adjustable fuel pressure regulator to wind it up for a while and see if the shunting gets better or worse? Seems easier that playing with sensors etc. Alternatively, if you know anyone with an aftermarket engine management system they could test your theory by seeing what happens if you run it lean at low revs/off load.
Interesting bit to add to this. I tried adding resistance to the water temeperature sender to put more fuel in over the whole RPM range, but Lambda control just kept pulling it back, and the shunting remains. I think the setting of the overall mixture at tickover would compensate for the extra fuel pressure, and the "flat" spot remain at 1500 rpm. The same does apply to a certain extent by applying resistors to the AFM, but its a more controlable adjustment at very low air flows. blitzracing said:
GreenV8S said:
Can you just get somebody with an adjustable fuel pressure regulator to wind it up for a while and see if the shunting gets better or worse? Seems easier that playing with sensors etc. Alternatively, if you know anyone with an aftermarket engine management system they could test your theory by seeing what happens if you run it lean at low revs/off load.
Interesting bit to add to this. I tried adding resistance to the water temeperature sender to put more fuel in over the whole RPM range, but Lambda control just kept pulling it back, and the shunting remains. I think the setting of the overall mixture at tickover would compensate for the extra fuel pressure, and the "flat" spot remain at 1500 rpm. The same does apply to a certain extent by applying resistors to the AFM, but its a more controlable adjustment at very low air flows. I thought you have been trying to say that all this shunting is down to the engine running too lean as it richens & weakens due to the lambda control. I understood it that you were attempting to fool the ecu into richening the mix up? Adding resistance to the AFM is surely going to tell the ecu there is less air than it has - and you think that it will learn to compensate for this down to the lambda sensors. What if your lambda sensors are foobarred in the first place or you are using well crap aftermarket jobbies?
I'm sorry but it seems to me you are trying to work around a problem rather than fix it
As Mark Adams often commented, most of his work was replacing duff sensors and just getting the EFi system in working order before he started any of his personal mapping.
Dave
Dave
No im 100% confident of my readings. Im was monitoring both Lamda outputs with custom built LED bar graph displays. It goes something like this. Reset ECU and start the engine. The lambda probes are cold, so no output. The car runs a little rough as the ECU has no lambda correction to set the base fueling at this point. After about 25 seconds the probes heat up and you start to get a reading. The ECU will then tweek the average base fueling depending if the lambdas average rich or lean. This takes about 20 seconds to average out. The idle then becomes stable, and the lambda probes give the normal .3-.7 reading. All is hunky dorey. Drive the car normally, and you can see lambda being maintained up to the open loop point, where probe output goes to .7 volts and stays there. Again spot on. At the shunting point the ECU can no longer maintain lambda, and the probe voltages drop to below .3 volts and stay there. Airflow reading is about 1.7 volts. The resistance is put in the earth line, NOT THE OUTPUT. As there is about 150ma flowing to earth, a 1 ohm resistor will lift the earth point by 150 mv, in RELATION TO THE ECU ground point. So the ECU now sees 1.7 airflow volts plus the 150 mv, so hey presto, it thinks there is a bit more air going in. Crude I admit, but easy to achieve the gains in airflow voltage. You have to realise just how small this adjustment is to bring it back in to lambda.
Mark
Mark
It sounds as if your goal is just to bring the base fuelling back close enough to L1 that the lambda control can achieve L1. I would have thought that anything causing extra enrichment would do the job - whether that's making the MAF sensor read high, raising the fuel pressure, or making the coolant temp sender read cold.
By the way, have you found that the MAF earth current is constant with load? I've never tested it, but I would have guessed that the hotwire heating current varied with load.
By the way, have you found that the MAF earth current is constant with load? I've never tested it, but I would have guessed that the hotwire heating current varied with load.
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