(Tried searching, couldn't find anything, but if there's a thread somewhere on this I more than appreciate a posted link to it)

So I have a boosted D16Y8 engine with vitara pistons, forged rods, and a built up head.

The car was tuned and ran fine for about 5000 miles on 16psi with a Turbonetics T3/T4 T04E .63 A/R 50 TRIM turbo (making ~380whp), then one day I boosted and my afr went down to 10.0 on my gauge (which is the lowest reading it puts out). Car would idle rough and get really rich when I gave it throttle. Sometimes it would go really lean and hesitate a bit too. Why would this happen?

What I know it isn't due to logging on neptune rtp:

• ECT sensor puts out the right values.
• AIT sensor also has the right values.
• Spark plugs were replaced and gaped accordingly and the old ones looked brown.
• Distributor doesn't seem to have any problems and was replaced less than 5,000 miles ago with a Duralast Gold distributor. Supposedly it's a rebuilt distributor with bland new parts.

What I think it might be:

• I never modified my valve cover air inlet to match the high crankcase pressure created by boosting. So maybe there isn't enough air flow to counteract excessive crankcase pressures? Would this mean my piston rings are shot and allow combustion past the rings under boost? The problem goes away if I don't boost the car, so I'm not sure if that makes sense. And when the afr drops excessively rich it will also go excessively lean sometimes as well, though not as much.
• Could also be a map sensor? I regret buying a cheap 3bar sensor off ebay and think maybe that is my problem as every part I cheaped out on in this build has come back to bite me in the *** at one point or another.

So I'm replacing my map sensor with a 3 bar omnipower map sensor and upgrading my catch can setup with -12AN lines from the valve cover and crank of the engine and allowing the catch can to breath and help release excess pressure that isn't alleviated by the -12AN valve cover line.

Does that seem about right? Anyone have experience with this sort of problem?

 

My money is on the map sensor. When it goes really rich it's probably reading a higher boost pressure then you're actually hitting. I've seen a lot of problems caused by using cheap map sensors. It's the most important sensor in speed density based efi, so never ever cheap out on it. I've personally never had an issue with Omni or hondata map sensors.

 

Alrighty, I'll see if that fixes it when it comes in next weekend and update the thread. *fingers crossed*

 

Make sure you change the settings in your tune to match your new map sensor scaling. Anytime you change map sensors your tune might need slight adjustments. Just keep that in mind.

 

Yup, it seems it was the map sensor. Changed it out and put in the -12AN catchcan setup, tuned the afr maps, and it's just fine again.

Thanks for the help.