Low vacuum at idle

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Schwamm

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2003 Mazdaspeed Protege, 2008 Mazda 3 Hatchback, 1984 RX7 GSL-SE
I have been in the long process of getting an MSP running, and after an engine swap, new head, new rod bearings, and a whole lot else I have it running again. It runs and drives OK but the idle is a little low, and my vacuum at idle is only 15psi. All the vacuum hoses seem to be connected and routed correctly and they are silicon. The car has custom 3" exhaust, a cold air intake, and Street Unit intake piping. I have sprayed berrymans everywhere and not found a leak, except maybe a little around the injector seals. Where else can I look? Is there something aside from a leak that could cause this? I have no CELs. I'm trying to get it in 100% drivable shape and as it is the low idle and occasional sputtering (probably from bad gas) are the last hurdles. Any ideas?
 
No CEL? I would suggest injector seals as you checked, the intake manifold/throttle body gaskets and any couplers/clamps after the MAF sensor. You are shooting for 21in/hg at idle, and it will be nice and steady. Keep in mind worn valve seals or an intake cam slightly out of time can also cause low vacuum.
 
I replaced all injector seals and it's back over 20in/hg vacuum. I had no idea they could go bad like that. Running great now
 
Can a leaking egr gasket or egr stuck slightly open cause a dip in vacuum? I have always been pegged at 20in/hg until a week ago, now at 16. Boosts at 6.5lbs, with an almost undetectable drop at higher rpms to 6lbs, maybe.

My idle seems to bounce a little when sitting at a light and smooths out within a few seconds. I've sprayed carb cleaner everywhere and can't find any leaks. Can really spray under the egr very well
 
Could be a dirty egr, could be a dirty maf, could be a vac leak like you mentioned.
 
In my case, cleaning my egr didn't help. It wasn't even really that dirty compared to a lot of stuff I've worked on. I went ahead and replaced a few suspect vacuum hoses and resprayed every single line and pipe. No jump in rpm anywhere.

On to cleaning my MAF. Doesn't look dirty but I know it only takes a fine dirt film to throw it off. I will report back
 
Cleaned maf, no change. I did notice that when I sawed the wheel side to side the rpms did drop when I let the wheel go. I'm idling at 800-900rpm.

Would a tired or dirty iac do this? But why is my vacuum 5hg off?
 
Your valve seals could be leaking, or IM/TB gaskets, could even be from the turbo manifold leaking
 
The intake and tb gaskets were my next move. The previous owner did do the valve cover gasket days before I got it from him and seemed like a rush job. Looks over-tightened in some places. Might do that too
 
Redid my gaskets and no change. Still at 17hg vacuum. It boosts a little on the slow side. It will sometimes boost to 8psi and then it slowly creeps to 5-6psi at full throttle. As i hold all the way through the rpm's, it will slowly drop psi.

I'm thinking it's my WGA(still on stocker). I'll have to test that next. Would a bad diaphragm in the WGA cause a small vacuum leak and erratic boost?

Anyone ever make a boost/leak tester with home depot parts?

What are the differences in symptoms with malfunctioning WGA and BPV(also stocker)?
 
A bad WG diaphragm would cause a vacuum leak, have you done a compression test? Bad WG will cause the car to not build boost at all or to build less boost than it should, it is not common for the BPV to fail but it would also cause the boost to build slowly or not at all
 
I build boost slow. It will hit 6-8 psi but creeps down as it stabilizes. I also have a hint of rpm bounce at idle. Its subtle though.
 
Could be a dirty MAF or IAC or a leak in the piping after the MAF, most likely a leak in one of the vacuum lines
 
I have cleaned my maf and replaced all my vacuum lines at the start of all this. I really have to check my wastegate. Iac could be another, but from what I've read on here, highly unlikely
 
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