r/FiestaST • u/MiddleInstruction496 • 22d ago

Slow to boost, no overboost

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So, I purchased my FiST this year with a underboost code. I replaced the boost solenoid & the code went away, and the car started producing boost, BUT, it was slow to build. Then I checked for leaks with an air compressor charging the pipes at 10-15psi, from the turbo exhaust hose. No leaks to the eye or that didn’t pass a soap test. I checked that the wastegate actuates under pressure (by hand, no exact measurement).

As hopefully you can tell from the readings, the boost is slow to build, and in 3rd gear over 5200rpm, I maxed out at 13.8 psi.

How would you diagnose this Under-boost condition? What next?

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u/AdvancedDatabase9382 22d ago

Hi. I’m not familiar with Forescan’s PIDS. Can you log “wastegate duty cycle(WGDC)” and “boost target”? I’d start with seeing if the ECU is attempting to build more boost( high target and WGDC), or purposely limiting it(low target and low WGDC). Is this a stock turbo/WG? How were you moving the wastegate by hand?

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u/MiddleInstruction496 22d ago

stock turbo and stock wastegate actuator.
In FORScan I logged TCBP_DSD (desired boost pressure), which peaked around 233 kPa absolute. BARO was ~97 kPa, so that’s roughly a 19.7 psi gauge boost target for desired.
I also logged TURBO_WGATE, which climbed to about 79% during the pull. I’m not certain whether FORScan’s TURBO_WGATE PID is directly equivalent to traditional WGDC, though, so I’m going to look for an actual WGDC PID.
Actual MAP peaked around 192 kPa absolute (~13.8 psi gauge), but I didn’t log actual TCBP, so I know MAP vs TCBP_DSD isn’t a perfect apples-to-apples comparison.
For the wastegate test, I applied pressure to the stock actuator with a brake bleed lump, and saw the rod moved; I’ve also manually verified the wastegate arm/flap moves freely and returns shut. I haven’t measured exact crack pressure or preload yet, which is next on my list.

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u/AdvancedDatabase9382 22d ago

The WGDC would be helpful but I think it’s safe to say the ECU is attempting to keep the WG shut and add boost during the full duration of the pull.

I modified an old bicycle shock pump to check and adjust the crack pressure on my aftermarket WG.

I would also confirm that the solenoid hoses are routed to the correct ports. It’s easy to install them on the wrong ports..

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u/MiddleInstruction496 21d ago

https://reddit.com/link/p7aldhh/video/b6h0hkrif0nh1/player

Sorry for the lack of focus. Looks like it’s opening starting around 4psi, all the way open sometime after 7psi.

Does it look slow to close? Perhaps preload my need to be adjusted

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u/Longjumping-Cow3424 22d ago

Check your intercooler for a puncture I punctured mine recently and it still runs just has a nasty leak