Erratic Hz counts at part throttle

[coltboostin]

I am working on a 1g (93) and have encountered a strange issue I have not been able to figure out. When cruising, the Hz count will abruptly jump, for example form 250hz to 700hz, back down and so on. What I have found is that this only occurs between 14% and 20% throttle conditions, and this jumping causes the car to stumble as if it was dumping fuel. Has anyone ever heard of or experienced this issue, I have not and tuned many cars with similar set-ups and have never seen anything similar to this. It holds boost with no issues and is fine at anything under or about these TPS values, including WOT.

Car’s set up
93 FWD ;
Intake/IC-2g mas, CAI, 1g BOV recirc, side to side FMIC w/ 2.5 inch pluming.

Turbo/exhaust-SB exhaust manni, 60 trim bullseye turbo, 3inch shearermotorsport O2 (no exhaust installed yet)

Fuel-780cc injectors, AFC II
Random gauges, 2600, fidanza…

So far I have check the 2g mas plug,- it was fine. Used a different mas- No change. Used a known good ecu-no change.

One thing I have found interesting is how damn fast the turbo spools. It thing is at 20psi by 3700rpm, so this leads me to think it is some sort of surge issue. The car does surge a little with the crushed 1g, but the bov does function properly. I don’t see surge causing a 500hz mas count fluctuation though. If anyone has seen this, or has any ideas of what it could be, I would appreciate your suggestions.

Thanks in Advance

JP

 

[jfdid]

The only thing I can think of is your thresholds on the AFC, because it's a piggyback, it alters the signal. But jumping 500Hz from one threshold to another is ridiculous. Nevertheless, check it out.

 

[lambertvr4]

It sounds like a MAF issue to me. The MAF is designed to read laminar airflow and tell the PCM the volume of air entering the engine. If the MAF is alterred it may not accurately report the true airflow. In your case it seems that the MAF is seeing a lot of turbulant air which is causing the large fluctuations in the readings. There are several possible causes for this:

1) If the MAF is closer to the turbo than stock. You'll want >8" between the turbo and the MAF in my experience or you can get erradic readings.

2) If the you've got some crazy contraption for an air filter. You may have some weird airflow going on there b/c of turbulance caused by that thing. I highly doubt this one, but I figured I'd mention it b/c I haven't seen the car, etc.

3) If the BOV is plumbed back in too close to the MAF. I know the BOV should be closed under the conditions you are describing, but you never know. You should be able to test this easily. Disconnect the BOV from the manifold and drive at the conditions you are describing. Keep out of boost and you should be fine.

4) If the honeycombs are damaged or removed. The intake air could be affecting the highly sensitive MAF sensor b/c it is not laminar and is no longer straightened by the honeycombs. Due to the space constraints in the DSMs there simply isn't room to have a smooth intake system so we need the honeycombs to do the job. Yes, they are a restriction at WOT but it's not too bad for the stock car Mitsu built.

 

[coltboostin]

The only thing I can think of is your thresholds on the AFC, because it's a piggyback, it alters the signal. But jumping 500Hz from one threshold to another is ridiculous. Nevertheless, check it out.

The Hz count is f-ed befor the AFC modifoes it.

The problem turned out to be a god damn bad ground. I put a few good grounds on the motor, and all is now good. I even reinstalled the old maf, and it to is good. I am glad I got it taken care of. It is always the simple shit.

Thanks for everyone’s thoughts on the matter.