Weak Vacuum?
01-01-12, 05:19 PM
#1
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Weak Vacuum?
Got the S4 13bT up and running/idling with the Megasquirt v3.0 yesterday. However I'm getting very little vacuum at idle, in the VE table its still sitting on 100kPa. Reading through some other posts, idle at vacuum should be far less than atmospheric...? Maybe 70-80kPa?
Diagnostics so far:
- Inlet manifold is completely blanked off, all emissions gear removed.
- MAP line attached to a spare tube(?) on UIM.
- If I remove the MAP hose from UIM end and suck on it I can register a fair bit of vacuum on the Megasquirt and if i blow on it I get positive pressure - so appears MAP sensor is reading correctly. It reads atmospheric with hose disconnected - as it should. I have a vac/boost gauge T-ed into the same MAP line and that also does not register vacuum.
- There are no leaks in the MAP hose/connections/T's etc
- If I put my hand over the turbo inlet to block it off and blow into the MAP line the inlet manifold gets pressurised, and releases a rush of air (my bad breath) when I take my hand off the turbo inlet, so I'm confident there are no leaks in the system.
Other info - (not sure of its relevance?):
- The engine is freshly rebuilt with full Atkins seal kit etc and has 90psi cold compression. New engine has only idled for about 20mins since rebuild. Not driven yet. Tuning about to commence.
- Idle AFR at the moment is sitting around 12:1
- Idle RPM is around 1200
- Oil pressure at idle sits around 65-70psi
I would appreciate you guys thoughts! - let me know if you require any more info...
Sorry if this should be in the Megasquirt thread, I wasn't sure.
Thanks in advance
Diagnostics so far:
- Inlet manifold is completely blanked off, all emissions gear removed.
- MAP line attached to a spare tube(?) on UIM.
- If I remove the MAP hose from UIM end and suck on it I can register a fair bit of vacuum on the Megasquirt and if i blow on it I get positive pressure - so appears MAP sensor is reading correctly. It reads atmospheric with hose disconnected - as it should. I have a vac/boost gauge T-ed into the same MAP line and that also does not register vacuum.
- There are no leaks in the MAP hose/connections/T's etc
- If I put my hand over the turbo inlet to block it off and blow into the MAP line the inlet manifold gets pressurised, and releases a rush of air (my bad breath) when I take my hand off the turbo inlet, so I'm confident there are no leaks in the system.
Other info - (not sure of its relevance?):
- The engine is freshly rebuilt with full Atkins seal kit etc and has 90psi cold compression. New engine has only idled for about 20mins since rebuild. Not driven yet. Tuning about to commence.
- Idle AFR at the moment is sitting around 12:1
- Idle RPM is around 1200
- Oil pressure at idle sits around 65-70psi
I would appreciate you guys thoughts! - let me know if you require any more info...
Sorry if this should be in the Megasquirt thread, I wasn't sure.
Thanks in advance
01-02-12, 10:28 AM
#2
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You've connected the MAP sensor vacuum line to one of the fresh air nipples on the manifold. Move the connection to the nipple on the middle of the upper intake near the turbo area, which is the same nipple the stock pressure sensor connected to.
01-02-12, 09:17 PM
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At first I was a bit skeptical because I thought the nipple I initially connected the MAP line to was linked to the entire inlet manifold volume between throttle butterfly and intake ports. Obviously it isn't, but I still can't get my head around it?
Anyway, thanks again!
