jawatkins95

I'm working on tuning my MSM with a freshly built motor and new turbo setup. Motor has forged rods and pistons, non-vvt bp4w head, 36-2 trigger wheel, LS coils using waste spark, FF640cc injectors, and a squaretop intake manifold with an FM turbo/mani/dp. Car is running on an Elite 1000 with a PNP harness, and I'm using the latest Nexus software to tune.
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I'm having an issue where the car will continue to pop, crackle, and make unsavory loud noises when the exhaust manifold gets hot enough to combust unburnt fuel. I have enabled the Decel function in the tune using a MAP threshold of -21 in/hg, and the log shows that the Decel Cut State is activating once the MAP target is reached, however the Injection Time never drops to zero. In the log the injection time will immediately drop to ~1.5ms when Decel Cut State is active, but it will never turn the injectors off completely.

I am not using a zero demand fuel table, I do have O2 control active, and there is an AFR target of 14.7 assigned to the high vacuum columns to prevent weirdness while cruising.

Has anyone else experienced this issue or can anyone confirm that the Decel function cuts fuel completely on their tune?

Mr Plow

Not had this issue when decel fuel cut has been enabled - but also not had the requirement to look at actual injection timing during same event.

Maybe also check your primary injection duty cycle as you may have an injection timing target but no actual injection - although you describe events that need some fuel. Apologies - don't have car/laptop to hand.

jawatkins95

The pops and crackles on decel appear to be related to my fuel tank vent routing and are not being caused by a decel fuel cut issue.

thebeerbaron

iTrader: (1)

Very interesting. I’ve had some vent issues as well as some pops on decel. I’d be interested to hear more, because I wouldn’t have connected these two issues.

jawatkins95

I actually just posted about this in the Megasquirt section regarding fuel tank venting:

"Thought I might chime in with my recent experiences:

I just got my car running with the new built engine and turbo setup; car is still technically a MSM, but is built like a run of the mill turbo NB. BP4W with forged rods and pistons, flattop intake, LS coils, FMII 2560R kit, with AC/PS

In the process of removing and re-installing the engine I decided to delete the EVAP canister and solenoid in the engine bay to make room for a catch can. Everything in the rear of the car remains the same, except with a DW200 fuel pump.
I originally did as a couple of users suggested and ran the fuel tank vent line straight to the intake manifold with one of the crappy little plastic check valves inline to prevent pressurizing the intake. I found that after cruising around in high vacuum for a long time, the car would start to crackle and pop on decel as if my decel fuel cut weren't working properly. Additionally, the car would begin to mysteriously and intermittently idle on 3 cylinders when hot and would have a dead miss on cylinder 1 under very light throttle. When this occurred my AFR's would go pig rich and the car would idle like garbage as my O2 control tried to pull the fuel back. As soon as I dipped into boost, cylinder 1 would wake up and everything would be fine.

My investigation proceeded as follows:
Disconnected the coil for cylinder #1 while idling on 3 cylinders and found no change in idle; pulled the plug and it was black, fouled and reeked of fuel. Coil appeared to be firing with a healthy spark jumping to the mounting bracket
Fresh plugs, new day... no change. After driving for about an hour the mysterious misfire and bad idle had returned. Pulled plug #1, fouled again.
Checked compression suspecting bad rings or valve problem; all cylinders were an even 150 psi.
Swapped coil 1 and 2 suspecting intermittent coil issues with heat soak... No change.
Changed fuel injectors from FF 900's to FF 640's, suspecting a leaky injector... No change.
Removed PCV and vented both sides of the valve cover to breather tank, suspecting oil flow from PCV to be fouling out the plug... No change.
Disconnected fuel tank vent line and capped the intake port... PROBLEM SOLVED

This past weekend I drove 200 miles and ran an autocross event and experienced no issues while the vent line was disconnected and venting to atmosphere. So far the only plausible explanation I have is that the engine was sucking extra fuel from the tank under high vacuum and at idle, fouling the plug and causing the obnoxious burble tune noises. As soon as I got into boost, the check valve would do its job and the issue would disappear.

Right now my plan is to either run the vent line back down into the frame to keep the fumes out of the engine bay or to implement a solenoid that is only open for a few seconds at a time during steady state cruise."