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Discussion Starter · #1 ·
As an OG owner for 18+ years now, I feel like I should already know this, but I don't, and it seems to be a hard thing to search for. Also, I ran a S2 PCM for much of those 18 years, and I've only more recently been running a stock PCM (with a custom calibration), so maybe it's a stock PCM issue I hadn't experienced.

The question is, does the stock PCM limit boost to 3-4 psi or so at startup until the engine has been running for some amount of time, maybe a couple of minutes? I've noticed that there is a weird fluttery noise if I reach that boost level, when the engine hasn't been running very long. At first I thought it was until the engine warmed up, but now I've seen that it also does it if the engine is already warm, but hasn't been running long yet.

I don't beat on it or anything until it's warmed up in any case, but it's easy to hit a few psi with just minor acceleration from a stop. I was at a loss about what this noise could be, especially since it goes away after a couple of minutes, but it occurred to me that maybe it's the stock PCM using the surge control valve to limit boost for a fairly short time after startup. Anybody know?
 

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The PCM controls 3 different levels of boost, if you still have it connected in with the stock wastegate under control of the ECU. Right at start up it is significantly limited, then once the car reaches a certain temperature I believe it goes to 9, then once fully warmed up obviously the 15-16 you would normally expect.

As an OG myself, I had to really think about this, and I couldn't find it in a quick search, so I'm not entirely positive on the info but this is the best from memory without going into a deep dive search.
 

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Probably agree it's the stock PCM in nanny mode.
That's a good thing though.
Not a great idea to boost before engine is warmed a little.
And has a chance for the OIL to start reaching normal operating temps.
I'm always gentle with my car when it first starts up on a cold day.
 

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100% confident that it is NOT based on time. Consider how many people drag race theses cars and they arent sitting around idling for a long time before they do a pass. They will make sure the car is up to temp, but they dont want it heat soaked before the pass.
 

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100% confident that it is NOT based on time. Consider how many people drag race theses cars and they arent sitting around idling for a long time before they do a pass. They will make sure the car is up to temp, but they dont want it heat soaked before the pass.
It is temperature, it is something that people would just about get into full blown arguments about at Fontana back in the day, even the other turbo car guys would be talking about it. The debate was about whether a car should be kept running for a specific amount of time, or if the temperature of the car mattered at all.

The stock version of the SRT-4 was calibrated from the factory for 3 separate boost levels based on engine temperature, so you are right about the time part. I will maybe do a deep dive and see if I can find more info about it, can't this week though, we have training this weekend.
 

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Discussion Starter · #8 ·
Maybe a PCM reset could help? (disconnect -ive battert terminal, press brake, reconnect, restart).

Can you please be more specific about the "fluttering sound"?
It almost sounds like a "raspberry". It's weird. I would think maybe it's a boost leak at a loose clamp, except that I can't figure out why it only does it for a short amount of time after startup. After that, everything is perfectly fine. My best guess is that it's the rubber diaphragm in the surge control valve vibrating weirdly. Does the PCM do that? Use that valve briefly after startup to recirculate boost, instead of using the wastegate? The surge control valve sounds normal otherwise, when I let off the gas, for example. Stock valve, no blue plate or anything. I'm using the same Stage 2 wastegate actuator I've always had. I suppose it could be the rubber diaphragm in that. But there are no other symptoms of anything wrong. I have an Aeroforce gauge, and all the parameters that can monitor look fine. The boost behavior seems normal, and it's not detecting any knock. Really puzzling.
 

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It almost sounds like a "raspberry". It's weird. I would think maybe it's a boost leak at a loose clamp, except that I can't figure out why it only does it for a short amount of time after startup. After that, everything is perfectly fine. My best guess is that it's the rubber diaphragm in the surge control valve vibrating weirdly. Does the PCM do that? Use that valve briefly after startup to recirculate boost, instead of using the wastegate? The surge control valve sounds normal otherwise, when I let off the gas, for example. Stock valve, no blue plate or anything. I'm using the same Stage 2 wastegate actuator I've always had. I suppose it could be the rubber diaphragm in that. But there are no other symptoms of anything wrong. I have an Aeroforce gauge, and all the parameters that can monitor look fine. The boost behavior seems normal, and it's not detecting any knock. Really puzzling.
Could be...but the surge valve is such a joy to get at that third bolt...
The raspberry sound does suggest something thin and rubbery flapping in the breeze...
If the surge valve is old the rubber may well have degenerated and torn...
 
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