problem on light throttle

[predator]

Noticed a bit of a problem with my car last night, and its been doing it a bit this morning. Basically with a constant very low throttle applied, the car appears a little stuttery...not massively, but i can feel something.

Its a bit hard to explain, but its basically feeling a little lumpy and not otherwise as smooth as it should be around 2500rpm and 5-10% THR (on my S-AFC) just cruising around at 60km/hr or so. On heavier acceleration its fine, at idle its pretty much fine, and if i foot to the floor it, it goes at its usual rocket pace with no problems. Its almost the is like a "gurgle" or choke in the exhaust - the sort of thing you get before a backfire.

So any suggestions?

Could it be timing?

Could it be a vacuum leak (doubt it, checked all that, building up boost fine)

Could it be S-AFC settings running to rich or something? (hasn't been tuned since my new 910 fuel pump)

Could it be spark plugs? although these were only changed around 5-8k ago

Could it be something like my accelerator cable? I do notice there is a bit of slack on it. Is that normal?

[KamikazeR33]

my first guess would be that ur safc might be tuned a little rich on lower rpms

mine was like that, was a little sluggish and felt like it wanted to go but couldnt till i hit boost

a tune fixed it right up, also fixed up the loss of pressure between gears somehow

try that first then go from there

remember ur car hasnt been tuned in a while and should be retuned every 2-3 months if it only has a safc

[LukeR33GTS]

my cars an r33 gts and i get that same problem after i have been driving for a while. is a safc worth the dollars for the gain? would it help this problem?

[predator]

S-AFC helps a lot of things although it will be less useful on a GTS, it may help it run a little smoother.

Think it could have been a bit of dodgy fuel (Caltex Gold out country way.. is that only 94RON ? ), seems to be running a lot better today, and i've still only got fairly low octane stuff in there. Its tuned for 98RON and hard use, probably a good idea to get it back on the dyno like Kami was saying and get it retuned.

I also tightened/checked all the vacuum points, so it may have been that too.

[Guest Robo's]

I too get some splutter low down, under medium throttle. Im putting it down to my SAFC settings. It was only ever tuned on full throttle. So when i get some time (very soon) im hiring a dyno for an hour, ($50) and going to try and squeeze a little more out of her plus do some lighter throttle testings.

[predator]

You raise an interesting point. That is the big problem with dyno tuning I think. A lot of it is just done to get the highest peak power reading, which isn't necessarily good for daily driving (or in fact most driving, even on the strip or track). Some tuners are more realistic and realise you're not going to be running at full boost all the time, and tune for more across the range.

I am a believer that dyno tuning is useful to sort some things out, but it also needs a good road tune as well to tidy things up. After all, you drive on the road, not a dyno!

[Guest Robo's]

I asked about the low throttle settings and they said nah they are ok. I noticed at light throttle the A/F ratio was 14.2 whilst on the dyno. To me this is still rich. I think 14.7 is optimum on light throttle.

Anyway, i will throw it on a dyno and do the changes myself, may pick up some economy too.

[KamikazeR33]

guys i *always* get mine tuned on the road as it is just more realistic

also i get mine tuned on bout 1/3 - 1/2 throttle

it boosts up so much better - and i still get full boost

there is no need for full throttle imho

btw gordo if u want it retuned for the street it will cost ya bout $20

but melb is about 9% difference in the way that the environment is compared to here

still a tune up here should be good for down there too

[Boost King]

Originally posted by predator666

Noticed a bit of a problem with my car last night, and its been doing it a bit this morning. Basically with a constant very low throttle applied, the car appears a little stuttery...not massively, but i can feel something.

Its a bit hard to explain, but its basically feeling a little lumpy and not otherwise as smooth as it should be around 2500rpm and 5-10% THR (on my S-AFC) just cruising around at 60km/hr or so. On heavier acceleration its fine, at idle its pretty much fine, and if i foot to the floor it, it goes at its usual rocket pace with no problems. Its almost the is like a "gurgle" or choke in the exhaust - the sort of thing you get before a backfire.  

So any suggestions?  

Could it be timing?

Could it be a vacuum leak (doubt it, checked all that, building up boost fine)

Could it be S-AFC settings running to rich or something? (hasn't been tuned since my new 910 fuel pump)

Could it be spark plugs? although these were only changed around 5-8k ago

Could it be something like my accelerator cable? I do notice there is a bit of slack on it. Is that normal?

Well there,I have exactly the same problem and my mechanic thinks I am wierd. I just had my standard fuel pump replaced with a 800 horsepower bosch unit, and its doing what yours does. More so when its cold. They put the car on dyno when warm and they said all the mixtures are fine etc. New plugs, cleaned injectors, cleaned fuel lines, new fuel filter, cleaned fuel rail, cant be the fuel system. I am taking it in wednesday night, returning thursday morning and taking them for a drive to show them so they dont think I'm mad. Some others here have advised they check the throttle position etc and see if that helps.

I'll let you know how it goes.

Paul

[Busky2k]

Dont bother setting any low throttle mapping on the SAFC. The ECU simply relearns around it to maintain the stoich AFRs. Trust me i've tried... You can even wind in 30% richer at a cruising rpm and within 10 seconds its back to stoich AFR by looking at the o2 output...

I reckon you should get your injectors etc checked out first. Just leave the low thr map empty.

[predator]

so full aftermarket computer might be the only solution?

Thanks for the advice bugsy2k..so you just mean its in closed loop mode at that point?

[Busky2k]

The ECU runs closed loop all the time, except for when you got your foot flat to the floor above ~3000rpm. Its constantly recorrecting its sensor parameters to stay at stoich AFR so by trying to fool it with the SAFC inside of closed loop operation is futile.

You shouldnt need an aftermarket comp to fix these problems. Its most likely a vacuum leak, very dirty throttle body, clogged PVC, leaky pipes, malfunctioning EGR/Canister control, plugs etc etc. The list could go on. I suggest you get ur hands on a workshop manual and try and diagnose a few things...