Not Holding High Boost 850 t5

[merc85]

Get flustrated, My 850 t5 hasn't been holding a high boost for a while 99% sure i've no Vac leaks,

Sure will boost to 18psi then drop.

1st boost 16 psi
2nd Boost 16psi drops to 13psi
3rd boost 18psi drops 14.5 psi
4th Boost 18psi drop's 15.5psi
5 boost 18psi drop again to 15.5 -16psi

She will not hold a 18psi constant any ideas?? i have replaced standard recirc valve with a normal reciric (known Good one) as it was even worse.

MBC is quite old now, have a forge replacement unos coming soon.

Does it sound like a weak actuator? Weak Recirc? or a Vac leak?

Appreciate any help.

[smithy]

sounds like weak actuator to me mate how hard is it to pull the actuator wilst on the car if it is easy to pull then tighten the rod if still weak get a better one.also with the forge mbc you not get boost spikes and constant boost

[merc85]

Thanks smithy, will check the Actuator rod out tomorrow will keep u posted.

Does anyone know are much psi standard recirc diaphram can take?

[gmain1967]

Not sure what engine number you have but here's a list

ENGINE PRESSURE(kPa/BAR/PSI)

B5204T 12.5/0.125/1.81
B5204T2 12.5/0.125/1.81
B5204T3 15/0.15/2.18
B5204T3 from model year 1999 18/0.18/2.61
B5204T4 12.5/0.125/1.81
B5204T5 12.5/0.125/1.81
B5234T 12.5/0.125/1.81
B5234T2 30/0.30/4.35
B5234T3 30/0.30/4.35
B5234T4 12.5/0.125/1.81
B5234T4 R-line from model year 1998 28/0.28/4.06
B5234T5 12.5/0.125/1.81
B5234T6 30/0.30/4.35
B5234T7 12.5/0.125/1.81
B5234T8 30/0.30/4.35
B5244T 12.5/0.125/1.81
B5244T2 30/0.30/4.35
B5244T3 12.5/0.125/1.81
B5244T7 12.5/0.125/1.81
B5254T 12.5/0.125/1.81

[The Flying Moose]

In all honesty those figures sound ok, its very seldom that even the strongest actuator wont drop a few PSI after peaking. Peak 18 and dropping to 16 is normal mate.

[glock19]

I would say it's the MBC. You are still using the TCV/BCS ?
If it's connected to the TCV/BCS, then you have to consider the air flow of the whole system because the ECU's boost control is taking all the imputs from the MAF