Thread: Boost Pressure Fluctuation

This is a 3rd gear pull with an SR20DET in an S13, stock (including WG and BOV, no boost control, other than WG), with FMIC, MAP and IAT sensors. What might be causing the boost fluctuations circled?



Thanks,
Dave

Looks like the MAP sensor is not wired securely. The "MAF V" and "Load" lines both show steady air flow (how is the load line sensed or calculated?). I'd be inclined not to trust those fluctuations because the other lines aren't showing that kind of change in air flow.

If the fluctuations are real, then I'm kind of at a loss for ideas right now.

Load is still off the MAF. I'm in the process of doing a MAF to speed-density conversion, using these data logs to build a MAP to Q table (like a MAP vs. RPM VE map). I've got to get these fluctuations resolved before switching over. I'll check the wiring. Keep scratching your head!

Thanks,
Dave

WG could be flapping a bit up top. Happened to me and it was visible on the dyno chart. Stock GTiR setup (B13 SE-R w/ UK highport motor). Vacuum lines crushing? In my case it helped to install stiffer silicone lines and a manual boost controller. Boost gauge was stabilized. YMMV.

If it was WG it should see the fluctuation also in the MAF, Injectors, Load, etc.

I think Ben is onto it. Could also be the sensor itself. Depending on how it is mounted, shaking some sensors can lead to erratic output.

It's definitely not the waste gate as mentioned. All things point to a MAP sensor that isn't very consistent. What MAP sEnsor are you using?

GM 3bar MAP sensors are cheap and work great as I'm sure you're aware

It's a GM 3bar type, but not a genuine GM.....ebay replacement
I'm getting ready to do some testing to see if it's wiring or the sensor itself. I have it mounted vertically, and it's supposed to be mounted horizontally with the port pointing down to keep condensation out. We'll see.

Dave

I'm still thinking. Maybe if you gave us some more input. Is this a street pull you're doing? Does the car feel like boost is fluctuating by 4-5 psi like the graph shows? It looks to hold 6 psi pretty well and then it starts going up and down by around 35% absolute pressure. You'd feel that on the road. If that's a dyno pull, maybe you could take it on the road and get a real impression? I'm still thinking sensor error.

I am also thinking the same thing as Ben. A little more info would be helpful also. I would never use an aftermarket Map sensor and would also mount it the way as instructed to eliminate that variable. I don't think it is a wiring issue because it would happen more inconsistantly. I would still check the wiring but wouldn't imagine I would find anything.

Why not run it with a maf and no map and see what happens?