Low speed shunting /snatching problem
Discussion
Firstly I I know this is on another post, but its worth its own separate posting.
Had a long close look at the fuelling on a 4 litre to try and eliminate the snatching you can get around 1500 rpm in higher gears. Having set up monitoring on both banks, it immediately became clear the Lambda probe outputs where showing very lean, and with a touch more throttle the mixture corrected, and the snatching cleared. So simple, get more fuel in. We put a 1k ohm variable resistor inline with the temperature sender, and increased the resistance to make the ECU think the engine was cold and put in more fuel. This setup worked perfectly on idle, to the point you could get it so rich, the ECU could no longer compensate to keep it at Lambda 1. This however made no difference to the snatching at all, and it still showed lean ???. OK, tried 2 different fuel maps, a 3.9 TVR and an Adams 4.2. Again hardly any difference, although the Adams chip was fractionally better. Last attempt, switch it to the non cat map, and wind up the CO setting to put more low speed fuel in. No lambda feedback to get in the way now. Same result???? I can only assume that due to the cam fitted that some unburnt mixture is being dumped through to the exhaust at these low RPM due to lack of inlet velocity, and causing the lambda probes to read the unused oxygen as a lean mixture. This would explain why some inlet modifications help reduce this, as it improves the cylinder fill and burn. I don’t get the problem on my engine, but I have cast exhaust manifolds and a slight softer cam (Piper 270.) Comment welcome !!
Mark
Had a long close look at the fuelling on a 4 litre to try and eliminate the snatching you can get around 1500 rpm in higher gears. Having set up monitoring on both banks, it immediately became clear the Lambda probe outputs where showing very lean, and with a touch more throttle the mixture corrected, and the snatching cleared. So simple, get more fuel in. We put a 1k ohm variable resistor inline with the temperature sender, and increased the resistance to make the ECU think the engine was cold and put in more fuel. This setup worked perfectly on idle, to the point you could get it so rich, the ECU could no longer compensate to keep it at Lambda 1. This however made no difference to the snatching at all, and it still showed lean ???. OK, tried 2 different fuel maps, a 3.9 TVR and an Adams 4.2. Again hardly any difference, although the Adams chip was fractionally better. Last attempt, switch it to the non cat map, and wind up the CO setting to put more low speed fuel in. No lambda feedback to get in the way now. Same result???? I can only assume that due to the cam fitted that some unburnt mixture is being dumped through to the exhaust at these low RPM due to lack of inlet velocity, and causing the lambda probes to read the unused oxygen as a lean mixture. This would explain why some inlet modifications help reduce this, as it improves the cylinder fill and burn. I don’t get the problem on my engine, but I have cast exhaust manifolds and a slight softer cam (Piper 270.) Comment welcome !!
Mark
Isolating the inlets from each other is a good albeit expensive way of taming down the low speed effects of a cam. Ive got a car in at the moment with a VERY wild cam in it and on a plenum it's almost undriveable at low rpm, put it on individual throtles though and it's become like a normal car again 
So yes inlet configuration (ie getting the throttles downstream of the plenum) is one way of sorting it.
So yes inlet configuration (ie getting the throttles downstream of the plenum) is one way of sorting it.
Mark,my brother also a Mark as this problem Chim 400,it varies and on a bad day it drives him mad(yes it does it for me also,horrible)he is in the beleif that its a big engine in a light car and you should be able to use the low down torque without stirring the gearstick constantly(he as a good point,and its not the HC model)mine does it to a much lesser degree,and its never a problem, because my HC model is flat that low down anyways,next spring i am going to have another go at this for him,so far its had the lot checked, throttle TPS settings reset/timing(different settings make no difference to problem)airmass reset(changed)plugs,leads,bosch coil,i must admit he/me done all this but i knew deep down this was not going to cure fault(been round engines to long to build up hopes)and can see this lean spot on the 4 gas,and the test you have described as saved me another job,although i was hoping for more fruitful results so am unsure which avenues to pursue next
if nothing comes thru,then we are both looking into Megasquirt/Emerald with my car being the guinea pig
probably not such a bad thing,time is tight for me though(running my own garage)i had all the time in the world years agowe did consider swapping ECU,s even though my one as the HC mapping(only make things better worse i guess)i have followed your findings so far Mark
and they have been useful,two heads better than one and all that
if nothing comes thru,then we are both looking into Megasquirt/Emerald with my car being the guinea pig probably not such a bad thing,time is tight for me though(running my own garage)i had all the time in the world years agowe did consider swapping ECU,s even though my one as the HC mapping(only make things better worse i guess)i have followed your findings so far Markand they have been useful,two heads better than one and all that
blitzracing said:
Another thought...If a gas analyser shows it as lean, then can it be unburnt fuel causing low lambda, or will the gas analyser pick up the unburnt element ?
yes,well it should and the hydrocarbons should rise accordingly(the main cat will affect this somewhat though) but i have come to the conclusion its a lean lurch,the car in question drives fine on warm up(better/ish),its when its up to op temp,and it does not always go on outside air temp sometimes when the air is cooler(cool evenings for example)most cars feel better(better cylinder filling etc)he as the benefits of this (midrange & top end) but it does nothing for the shunting (its worse),so its not leaning out a Rich mixture its leaning out a Lean onemaking things worse,he as driven over to me after a ECU reset(makes no difference)on a hot day and its been fine,but may have a different drive home
i do feel however that the stock ECU can not meter the fuel properly for this tune of engine(the old bobweight dizzy does it no favours)i believe that the ECU is not very flexible in the correct department i.e fueling necesary for a cat system,the fact some cars are better than others for this problem is just luck i guess,and the fact that the hardware may be working in harmony better,you may find you could swap a good ECU and Airflow meter off a car thats not affected and the lurching one would be fine,,,maybeI've seen this shunting problem on lots of cars, as I'm sure lots of you have judging by the posts. I don't know about most of the cars but I've seen this on cars that have definitely had the cams dialled in properly, but if memory serves all have been on the hotwire setup.
Have you have checked the AFM reading during shunting, could it be that the hotwire AFM is getting upset by the "low" frequency pulses back up through the inlet tracts?
Regards
Iain
Have you have checked the AFM reading during shunting, could it be that the hotwire AFM is getting upset by the "low" frequency pulses back up through the inlet tracts?
Regards
Iain
Well in the unlikely event of my theory being correct, there's so much that could potentially upset things, things that have been mentioned like "wild cams", cam timing etc to the state of the hydraulic tappets and I guess the oil (less lift if followers are being crushed, or more if the oil drain is blocked), the length of trumpets setting up harmonic frequencies etc. I don't have enough enough experience though to say for sure, except to know I've been in a Griff 500 running hotwire with a Stealth cam from V8D and there was not a hint of shunting.
Regards
IAin
Regards
IAin
Chassis 33 said:
Well in the unlikely event of my theory being correct, there's so much that could potentially upset things, things that have been mentioned like "wild cams", cam timing etc to the state of the hydraulic tappets and I guess the oil (less lift if followers are being crushed, or more if the oil drain is blocked), the length of trumpets setting up harmonic frequencies etc. I don't have enough enough experience though to say for sure, except to know I've been in a Griff 500 running hotwire with a Stealth cam from V8D and there was not a hint of shunting.
Regards
IAin
I would say that sounds accurate enoughRegards
IAin
lots of varibles
hence why some do and some dont.Edited by That Daddy on Monday 19th November 10:36
Theres a couple of easy things to try still. One is to try fitting a varaible resistor (1 k ohms is good)inline with the temperature sender, and then switch to the non cat map. You can then easily tweek the overall fueling to see if you can get it rich enough to reduce the problem. I think this would be better than when I tried changing the mixture with the AFM setting. Have to be carefull not of overfuel the pre cats however, so you need to monitor the lambda output. Also has anyone tried a cuople more PSI on the fuel rail with a variable regulator? Also try running without the vaccum advance by clamping the pipe (if the cars use it.)I assume there is some advance applied under this light load, and just see what effect it has if you remove it. At least this way we would know if any changes in timing or fuel effect it at all. None of these things are a permanant fix, simply a way of trying to isolate the problem. As for the airflow meter "pulsing", Im sure I can knock up a LED display to read the 0 - 8 volts or so it swings through (or even just an analogue voltmeter) so see if the ouput is unstable.
Mark
Mark
FlipFlopGriff said:
Probably well off track but change the rotor arm to a geniune Lucas item. The pattern/Intermotor arms did nothing so we thought it must be something else. We've had some rocking horse running which was a PITA but was cured by changing the rotor arm - strange but it's true.
FFG
Wish that one worked for me bruv,,,,but it dident,good heads up thoughFFG
penno said:
I have two AFM's one the car shunts and runs lean apparantly. The other replacement secondhand one cures the low speed shunt but the car doesnt seem to perform as well.
Just had an AFM refurbed, borrowed a Disco AFM and it was OK but no performance. Have been told by the refurbers there are 4 types, so best to get yours refurbed me thinks.How much is an AFM replacement only Del?, ie no plenum (got that), and no chip. e-mail me off line if you want.
Anything for pre cats yet?
FFG
Edited by FlipFlopGriff on Tuesday 20th November 17:41
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