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Intake manifold press bouncing= intake/exh valve issue or maybe some oddball blower issue.
I would remove blower belt and see if the press keeps bouncing at idle. If it does, you can rule out the blower.

Compression test would be a good idea too.

I'm assuming the other eng has a steady intake manifold press at idle using mechanical gauge.

Quote:

Originally Posted by zz28zz (Post 4802719)

That was going to be my next step is to pull blower belt and see what it does. Other motor gauges all good nice and steady. Is there a boost relief valve somewhere i am missing?

Did you ever get this figured out? I have a 700 stg 2 that is running rich at idle and it's a dog when first getting on it from a dig I am sure it's from being so full of fuel. Very short time out back of boat was getting black. Ordered scan tool today.
Thanks

Quote:

Originally Posted by F-2 Speedy (Post 4802709)

is the engine surging ? like a poorly tuned carbed roots blower.......:cool-smiley-011:

Yes the boat is surging when up and running i can see the boost gauge moving up and down at the same beat the boat is surging.
I get a boost valve error on my vessel view also but not all the time. Some times i get it if the motor i just idling and also get it some time up on plane.

picked this from my Whipple install PDF,

Supercharger By-pass system. The supercharger is installed with a by-pass system. This allows the supercharger to operate at higher efficiency under vacuum operation. It is advised to verify the operation of the bypass valve. At idle and low engine loads, the bypass will be open. At higher loads (engine in boost) the bypass will be closed. As the throttle is opened quickly the bypass valve will close momentarily. This verifies the bypass will close and is functioning. Actuator failure could lead to intercooler fires, poor performance and erratic idle. If the actuator fails, it could have an air leak which will result in poor idle qualities. A failed actuator will also allow the bypass to open it’s internal butterfly during boost, which will circulate air and reduce airflow to the engine, consequently lowering the boost level and power

see if you have something that looks like this, these hook to a diaphragm with 2 other hoses that control vacuum while in boost........... I know your blowers are different

https://cimg9.ibsrv.net/gimg/www.off...9031758d8.jpeg

Quote:

Originally Posted by F-2 Speedy (Post 4835950)

I just went through this late last season with my TCM1200 efi's. Poor idle that had a "blower surge" loading up with any amount of idling, fouling plugs etc. Tested my bypass vlaves and sure enough the diaphragms were blown, easy $300 fix. I was pulling my hair out thinking it was a MAP sensor or IAC which both tested good.

Sorry to step in, but man…,,that valve sure looks like the GM 3800 motor’s bypass valve found in:

97-08 Pontiac Grand Prix
96-03 Pontiac Bonneville SSEi
96-04 Buick Park Avenue Ultra
97-04 Buick Regal GS
04-05 Chevrolet Impala & Monte Carlo SS
95-99 Buick Riviera

https://cimg3.ibsrv.net/gimg/www.off...924f250d4e.png

Maybe ?

Supercharger Bypass Valve Actuator - GM (89018120)

Found for under $140

https://cimg1.ibsrv.net/gimg/www.off...2dab73648d.jpg

I was told by Whipple that a backfire will usually blow the diaphragm...........

Snd then this. They state they get from same manufacturer that supplies gm snd eaton with them. ??? $80

https://zzperformance.com/collection...0-bypass-valve

https://cimg8.ibsrv.net/gimg/www.off...c097481779.png