I replaced my block-off plates with proper ones and new mazda gaskets. I went over all of the vacuum hoses I could get my hands on, half of them are new. I had the UIM off to do the TB mod and the gasket between the manifolds was like new. Solid, nothing stuck to the manifolds, no cracks or anything. I've checked and rechecked my TID which has new rubber couplers.

I have not replaced the vacuum hoses on the spider on the driver's side of the engine.

My question is how big of a leak does it have to be to lose 7-8 inHg of vacuum at idle? Would a low compression engine make that little vacuum? If it is a leak, and I should be making about 18 inHg at idle, how much might that translate into when under boost? Am I losing 2,4, 8 psi? I am only hitting 5 psi on cool nights at WOT in 4th.

I have a crappy exhaust, hollow cats, 3" TID and a boost controller opened up slightly over stock. I should be making about 10psi I think.

I'm just looking for some ideas on where and how to track down this leak. Primarily, how big of a leak am I looking for?

 

I have a streetported engine and it pulls about 12"-13" of HG. I think it depends on the atmospheric pressure for that small change.

James

 

I replied to this same post on the FC3S forum. Long story short, check your boost gauge to make sure you're getting a good reading, and if you are, do a compression test. A vacuum leak won't account for vacuum that low.

Dale

 

I check both so you don't have to reply on each although I appreciate it. I use 3 forums and post on all 3 for questions I really need help with.

Here is my combined answer:

I have a VDI gauge plumbed in with silicone vacuum hose. The car holds a fairly steady idle around 825 rpm give or take a few...

It no longer floods on me either. It used to flood quite often, like every other time I'd start it so I installed a switch. After a month, I had to use it less and less frequently and now I rarely ever use it. It just starts right up every time.

It also used to give off some nice white smoke on start up. Not thick... and it would last for 4 or 5 minutes untilt he car was warmed up. Doesn't do that now.

Also the car was compression tested just before I bought it last fall, 5000kms ago. It bounced 6.5 and 6.6kg/cm^2 on each rotor face which converts to 92.4psi-93.8psi. It's a little low but not in the danger zone at all. Anything above 85psi is doing fine. Also it's running better now than it did this time last year since it stopped flooding and the white smoke has lessened.

I had it on a dyno 3 weeks ago and it pulled 198HP and 183 lbs of torque at the wheels at 7psi.

 

^ bump ^

 

I'm assuming you meant a VDO gauge? I've had not so good luck with VDO's over time - they're not very reliable IMHO.

Might want to see if there's someone local to you with a boost gauge you can try. Might also want to hook the boost gauge up with a short piece of hose right to the car - to the BOV vacuum line or something. That will rule out any issues with the line.

Generally, a car with 10" of vacuum should flood on a regular basis. You definitely don't have a vacuum leak if your idle is that steady - maybe a VERY tiny one, but not enough to drop the vacuum that low.

Dale

 

come to think of it, that nice big thick gasket was UIM to TB. I don't recall seeing a gasket between UIM and LIM. I'll see if I can find a better guage to test with.

I had it running although the idle was really crappy when it made only 5-6 inHg. At 10-11 it feels quite smooth but not rock solid. It has some afterburn and it fluctuates with a 25-50 rpm range. Not a hunt, just little stumbles and such. It's basically pretty smooth.