Help with surging idle

[Metro]

Engine Specs:
Nissan L28et - 2.8L inline 6
46mm GSXR ITBs with DSM 450cc injectors in the ITB (about 6" away from the head).
Vacuum manifold with equal length hoses connecting each TB
30-1 crank wheel with hall sensor
Gutted GM-DIS coilpack with BIP373s inside.
Innovative LC-1 wideband O2 sensor.
MS2/Extra v3.1.1

surging01.png

I'm having a problem getting the idle to stabilize, as you can see in the log, it just hunts from 700 to 900 rpm. I seems to lean pop in the intake occasionally, but the AFRs are usually super rich.

I've tried massively increasing and decreasing the fuel with no apparent change in idle quality. It also seems to exhibit the problem at higher RPMs, but I figured it's more important to fix the idle. I'm new to MS2 and the config is quite a bit more detailed than MS1. I'm hoping I have something dumb enabled that might is causing problems. Currently I'm running Speed-Density just to get it idling, but within the next week I'll have a plenum and a Ford/Hitachi MAF sensor to play with.

Previously, the engine ran beautifully with MS1-extra, Speed-Density, and a single TB. There's probably something screwy with my bodged together ITBs, but for now I want to rule out any software issues.

I've verified my timing at idle and higher. I've swapped in Suzuki 240cc injectors which are high impedance vs the DMS 450cc injectors which are low impedance. I've used a carb synchrometer to adjust all the throttle bodies to draw in the same amount of air. There's no apparent vacuum leaks I can find. The map signal is pretty stable with and without a restrictor.

The only other things I can think of trying would be to check the cam timing and see if the exhaust is leaking into the intake manifold through the new gasket.

[dougs951s]

I'm having a similar issue, slight missing and roughness/hunting at idle as well as severe hesitation and missing under load despite being extremely rich. Changes to fuel and spark make no difference, wish I could offer some advice but I'm stumped as well. Hope you figure it out!

[Matt Cramer]

Your spark advance is dropping as RPMs drop - instead, try setting it so that the spark advance is at its lowest at your target idle speed, and increases if RPM drops below that. This can stabilize the idle a lot.

[Metro]

On a previous attempt I had fixed the timing to 15° and that didn't help. I had set it up at one point to fall into a higher timing advance in order to help stabilize the idle, but it didn't have much of an effect.

I was messing around with it yesterday, trying to hold the RPM at 1500rpm and it's doing the same thing. The AFRs would jump around from 12.5 to 14.0 or so when held, just like it's doing at idle. Could fuel pooling in the intake be a problem? When I've shut it down and felt behind the throttle plates, the intake is noticeably wet.

Well anyway, I have my MAF setup and the plenum arrived today. I can't imagine changing the fueling algorithm will help me, but at least I can start tuning with the correct equipment.

[subwoofer]

Going MAF will most likely cure your problem in minutes. I never managed to get a good stable idle on SD without richening it up, it was just too unstable.

The root cause of your problem is the varying AFRs. If you make it between 11.5 and 13 there is very little difference in power output. Once you get over 13 power output drops quickly and the engine decelerates. Then the AFR falls again, power per stroke increases, engine accelerates. And it keeps going like that.

[jsmcortina]

Your spark advance is dropping as RPMs drop - instead, try setting it so that the spark advance is at its lowest at your target idle speed, and increases if RPM drops below that. This can stabilize the idle a lot.

Also adjust the VE table in the idle area so that the actual fuel pulsewidth doesn't change too much. (This means you'll want the VE numbers to increase as you go down the table.)
I was working on a car today with idle oscillations. The timing didn't have too big an effect. Changing the fuel solved it.

James

[rickb794]

Under engine constants "control algorithm" select ITB

This will give you a blend of Alfa-N and Speed Density and should help with ITB's.

Unless you have a well engineered balance tube with the MAP source connected there it is difficult to tune ITB's under S/D alone so this control algorithm was created to help smooth this out.

[Metro]

Under engine constants "control algorithm" select ITB

Thanks for the suggestion, if I were running N/A I would certainly look into ITB mode.

Last night I connected the MAF and it fired up first try, so that was promising. It's running a little bit better, but it's still behaving the same way.

Surge might not be the correct term. The idle is oscillating up and down, but it's running rough, like it's dropping a cylinder or two intermittently. At this point, I'm confident it's something mechanical, but I'm running out of things to troubleshoot.

[Metro]

For everyone following along: it looks like my intake/exhaust gasket was letting exhaust leak into at least one runner, possibly two or three. Combined with the amount of fuel in the runners (the gasket is soaked with fuel days later) would explain the popping in the intake even with the AFR is rich.