Fault speculation thread... Lumpy running on light throttle?
Discussion
As the title really, I've noticed over the last few days driving the car that its developed a bit of strange running at low throttle openings. All is fine while driving normally but when the revs drop right down and I use very low throttle openings (just a movement of the toe) such as in traffic or moving up to a junction, I'm getting shunting and what feels like a bit of a dip in power. Its quite hard to describe but it feels like when I ran the car with a lambda sensor bung fitted, but not quite tightened all the way, causing a small air leak - except this caused the car to do it all the way up the rev range in a much more severe manner with pops and farts thrown in. So perhaps it could be a small air leak somewhere, but in this case surely it would affect more than just the very bottom end?
The manifolds definitely aren't blowing, checked by holding the revs at around 3k with no tapping at all. So before I start going through the engine bay to check things, has anyone got an incling as to what the cause of this may be?
To make things a little harder, its not really standard with an Emerald ECU and intercooled supercharger in the mix. Nothing has changed on the Emerald map, and both leads and plugs were brand new about a month ago so these should be fine. The shunting only really occurs when the engine is up to full temperature, around when the fans come in (85 degrees). Any ideas would be welcome :-)
The manifolds definitely aren't blowing, checked by holding the revs at around 3k with no tapping at all. So before I start going through the engine bay to check things, has anyone got an incling as to what the cause of this may be?
To make things a little harder, its not really standard with an Emerald ECU and intercooled supercharger in the mix. Nothing has changed on the Emerald map, and both leads and plugs were brand new about a month ago so these should be fine. The shunting only really occurs when the engine is up to full temperature, around when the fans come in (85 degrees). Any ideas would be welcome :-)
Shunting is not unusual at low speed and low throttle openings, is this definitely a new problem? It may be that you have extra backlash or worn bushes etc that are making the symptoms more pronounced. You could try richening the mixture in that region and see if it helps, also back the ignition off a little to raise the air flow (but note this will make it hotter). Are you seeing any variations in AFR or pulse width at the frequency of the driveline shunt?
I'm pretty sure that its a new problem. I'm aware that its pretty common to happen to a degree, but it has developed a bit further than usual. It feels like there is a bit of hesitancy in the throttle somehow. I'll have a play with the settings on the ECU and see whats what. I haven't looked at the AFR trace yet but that may give some clues.. I'll report back.
What about the old stepper motor ?? Had similar syptoms recently - as soon as you lifted off there was a drop in power (quicker than normal) and then it would hesitate before taking off again.
Pulled stepper motor out and had a clean and chaecked it wasworking and then no problem.
Worth a try before anything else ??
Pulled stepper motor out and had a clean and chaecked it wasworking and then no problem.
Worth a try before anything else ??
Hmm, I will check that but it's not the same as the original. I run a 2 wire Bosch stepper inline with the throttle bypass pipe and the old hole is plugged up so unlikely but its possible that something is wrong. If it were to stop, it should make a big difference as it is the only source of air when the throttle is shut. I havent had a chance to look at it today but I'll have a crack in the morning!
450Nick said:
Hmm, I will check that but it's not the same as the original. I run a 2 wire Bosch stepper inline with the throttle bypass pipe and the old hole is plugged up so unlikely but its possible that something is wrong. If it were to stop, it should make a big difference as it is the only source of air when the throttle is shut. I havent had a chance to look at it today but I'll have a crack in the morning!
I’m not sure that the only way for air to get in with a closed butterfly is the stepper.I have an Emerald too, but there isn’t a stepper fitted, just a blanking plate. The tickover is controlled by ignition timing only.
Is this problem different when the engine hot or cold?
The problem seems to become apparent when the engine is hot. It does seem like a mapping problem but nothing on the map has changed since it has started happening. I've re-calibrated the throttle pot in the Emerald software to make sure its giving the right numbers, and all sensors etc appear to be working in live adjustments. No misfires or other noises, just this slight hesitancy and jerkiness when crawling in traffic. My CV joint is buggered and has a lot of play in it which as Peter has said, probably exaggerates the problem but again, this isn't new and I will be replacing it soon anyway.
Regsrding the stepper, I agree there are usually more ways for air to get in, but my plenum has all other holes blanked off as it has no dizzy/carbon canister/throttle bypass and the idle screw is closed, the crank case breather goes through a catch tank and into the charger plenum and throttle shuts very tight (72mm one). The only feeds are stepper which replaces the throttle bypass, and recirc valve vac/boost feed.
Regsrding the stepper, I agree there are usually more ways for air to get in, but my plenum has all other holes blanked off as it has no dizzy/carbon canister/throttle bypass and the idle screw is closed, the crank case breather goes through a catch tank and into the charger plenum and throttle shuts very tight (72mm one). The only feeds are stepper which replaces the throttle bypass, and recirc valve vac/boost feed.
No its not in adaptive at the moment as I've swapped for a Dynojet unit but haven't yet got around to sorting out the analogue link. I have a good gauge reading on the dash but it currently doesn't talk to the ECU. Hopefully I'll be able to get this running in the near future though as it would be a great help in times like these!
sounds like its not a mapping issue, and beware of masking the symptoms by trimming the map to compensate. just sounds to me like you need to go and do some old fashioned basic detective work .. you know : plugs, leads, air leaks that kind of thing. if you've not changed the map, then the map hasn't changed! something else has ....
Well I took it out earlier and around the problem areas, I noticed that it was running very lean, causing the shunting. So I took it out for a drive with the laptop connected and with the live adjustments screen active, I trimmed the problem areas up a bit. It took a couple of runs up and down my road but after a bit of trimming, it seems to be working fine and smooth - I even got the idle down to 850rpm whilst keeping it nice and strong!
So I'm very pleased that I managed to do some successful mapping, but why would it be lean in the first place? As you say Joo, it shouldn't have moved. It still looks nice and rich when I boot it so it seems that only these areas were affected... I'll check the usual stuff over and keep an eye on it but if nothing becomes apparent then maybe I'll pop down to Austec after new year and have them check out the AFR trace.
I'm so pleased that I managed to map successfully though! It's really nice and smooth now :-) Right I'm off to get drunk
Happy Xmas everyone!
So I'm very pleased that I managed to do some successful mapping, but why would it be lean in the first place? As you say Joo, it shouldn't have moved. It still looks nice and rich when I boot it so it seems that only these areas were affected... I'll check the usual stuff over and keep an eye on it but if nothing becomes apparent then maybe I'll pop down to Austec after new year and have them check out the AFR trace.
I'm so pleased that I managed to map successfully though! It's really nice and smooth now :-) Right I'm off to get drunk
Happy Xmas everyone!
Wow, praise indeed! Seems I'm a regular pro now 
Think I found the original cause this morning, the throttle cable appeared to be sticking so that it was hanging on to the bottom two load sites for longer than it should on light throttle. A bit of WD40 down both end of the cable and my throttle pedal has suddenly become really light and free moving, my throttle response has improved and any attempt at shunting has vanished. Seems the new throttle cable I put in in August needed lubricating!
The fact that I didn't notice must mean that both the new and old cables have been doing this for several months... Doh! Worth looking at though everyone, my pedal didn't feel sticky at all I thought, but the way it moves now is soo much nicer. Love it when its an easy fix
Think I found the original cause this morning, the throttle cable appeared to be sticking so that it was hanging on to the bottom two load sites for longer than it should on light throttle. A bit of WD40 down both end of the cable and my throttle pedal has suddenly become really light and free moving, my throttle response has improved and any attempt at shunting has vanished. Seems the new throttle cable I put in in August needed lubricating!
The fact that I didn't notice must mean that both the new and old cables have been doing this for several months... Doh! Worth looking at though everyone, my pedal didn't feel sticky at all I thought, but the way it moves now is soo much nicer. Love it when its an easy fix
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