Low Vacuum but drives great?

[flaatr]

OK I did a search and everything associated with low idle vacuum usually follows with drivability problems... I don't have that.

Car starts instantly cold or hot and when cold does the normal fast idle until warm... drives/pulls great... BISS is set... idles normal when cold or hot *except* when I pull off a freeway after an extended period. In this case the idle fluctuates a bit more than it should. The only thing I see is that idle vacuum is around 10-12 in.Hg where according to the factory manual (and others) say it should be 20'ish.

Engine is still basically stock... cams, turbo, all smog equipment except cat removed... 3" turbo back. Could the low idle vacuum be due to the no cat and 3"? Under deceleration vacuum goes to about 22-24in.Hg

Any thoughts?
-Raffi

 

[boost97gst]

I can tell you for stock thats not normal. Im not sure what vac a 1g pulls, seeing how it has lower compression i would think itd be slightly less than a 2g stock. How old is your gauge? If you are sure you dont have a vac/boost leak then maybe look at the gauge or the line from the mani to the gauge.
Check timing too obviously. If its a tooth off then that would explain it as well.

 

[Boostdriven]

Check the timing marks. Line up the crank pulley to TDC mark on the timing cover and check to see if your cam gear marks line up. It's possible that you skipped a tooth on the crank pulley.

 

[flaatr]

The gauge is new and I believe accurate... I will confirm this with a second gauge later.

Can the timing be off one tooth and still have the car run normal? Starts fine, No knock etc... That seems like a lot!

-Raffi

 

[Boostdriven]

Can the timing be off one tooth and still have the car run normal? Starts fine, No knock etc... That seems like a lot!

-Raffi

Yes its possible.

 

[boost97gst]

Agreed, one tooth off and you wouldnt feel a whole lot. When you get it dialed in right you will find a little better response and power and know what you were missing.

 

[95DsManiac]

When my 2g gsT was completely stock I put a brand new glow shift boost gauge in and no matter what my vac would be 10-12 in vac. I don't hint you have anything to really worry about. Every Dsm I have ever been in, it was the same as far as the aftermarket boost gauge. Ill look up a little about it and post back, but personally I wouldn't worry about it. But hey, that's just me.

Anthony B.

 

[DSM1G90]

Yep, need that higher vacuum for better operation.

Bet you jumped a tooth, or a very small vacuum leak is hiding. I had the similar and found some minute vacuum leaks with this test below:

To test with the motor in idle: Can of WD-40 and spray around the INT manifold and hit hoses, gaskets, throttle body and similar.

If idle suddenly ramps up, there is your vacuum leak.

I had to replace 4 hoses to get my vacuum up to where it needs to be. And I really could tell the difference !

Good luck - DSM

 

[flaatr]

I confirmed last night the accuracy of the VDO gauge... my two gauges, VDO boost/vacuum and commercial vacuum gauge connected at the PCV valve (like the manual recommends) match. There is a bit of jitter on the fast acting vacuum gauge but this is normal with the pulses from the engine.

Thanks for all the good tips

I'll start this weekend by checking the base timing and make sure the cams et al. are where they should be and I haven't skipped a tooth somewhere... I seriously doubt this is the problem based on how well the car drives. I'm pretty confident it's a small leak that shouldn't be too difficult to find. I want to delete a bunch of hoses anyway so this is a good time to do that.

-Raffi

BTW, I normally use carb cleaner for the quicky leak test in place of something like WD-40... less lingering smell and less residue.

 

[T is for TURBO]

I get better vacuum with my 272/272 than that.

You said it idles "normal" But what RPM is it idling at?

 

[flaatr]

750 RPM... set BISS recently. TPS is working.

-Raffi

 

[T is for TURBO]

No CELs?

 

[flaatr]

Nope

 

[T is for TURBO]

If you have a mechanical boost gauge, check the line feeding it. If it is a nylon line, they can some times kink and crack.

 

[flaatr]

If you have a mechanical boost gauge, check the line feeding it. If it is a nylon line, they can some times kink and crack.

See post #9

-Raffi

 

[T is for TURBO]

See post #9

-Raffi

HAH, I missed that post

 

[Boostdriven]

Don't be in denial my friend, check the timing alignment before you start replacing parts

 

[XiKeiyaZI]

For the record - idling at 800 with my BC 272's I see 10-12. No boost leaks and proper timing.

 

[flaatr]

Don't be in denial my friend, check the timing alignment before you start replacing parts

Denial?

See the plan in post #9... specifically the first thing I plan on doing...

For the record - idling at 800 with my BC 272's I see 10-12. No boost leaks and proper timing.

Lower vacuum with big cams is not uncommon... I'm on stock cams.

-Raffi

 

[Boostdriven]

Denial?

See the plan in post #9... specifically the first thing I plan on doing...

-Raffi

I didn't mean it in any insulting way, I see all these other ideas of a bad gauge or boost leak so I just wanted to make sure you eliminate the easier things to check first. I missed what you said in post 9, my bad.

 

[DRTredHEAD]

Going with gauge. Sounds like your vacuum line to the guage may be leaking. If you over tighten the compression nut on the back og the guage sometimes it will cut the nylon tube and cause a leak.

 

[boost97gst]

OP: if you arent idling lean and possibly throwing a generic lean code CEL then you most likely dont have a vac leak. Boostdriven is right(i was right first jk), it sounds like you jumped timing a bit. Just a little bit mind you.

FYI, Crower 272s, stock CR 2g, 185 across the board and i see around 12-14in/hg idling 850-900rpm.

Something IS wrong in the system bud, just check timing maybe do a compression test while your at it. Make sure all 4 are within spec and not way low. Either you have a leak, hose is bad, jumped a tooth, or all 4 cyl are on the low side compression-wise.
Me thinks it be the timing


Edit: Just reread first post. You see 24in/hg on decel? Sounds like the gauge is good. Maybe just a hair off but not bad.

 

[flaatr]

I'll post back once I've tested things...

The gauge was tested with a pump and it's accurate.

-Raffi

I didn't mean it in any insulting way, I see all these other ideas of a bad gauge or boost leak so I just wanted to make sure you eliminate the easier things to check first. I missed what you said in post 9, my bad.

No worries... it's all good!

Going with gauge. Sounds like your vacuum line to the guage may be leaking. If you over tighten the compression nut on the back og the guage sometimes it will cut the nylon tube and cause a leak.

Been covered in the now infamous post #9... thanks.

-Raffi

 

[Boostdriven]

Did you end up figuring out what the problem was?

 

[Xploitn]

So referencing post #9, are you connecting the gauge (T'ing it or otherwise) in between the PCV valve and the intake manifold?

Would a clogged PCV valve lower the reading for vacuum? Have you taken that PCV valve off to see if it still operates properly?

I don't disagree with the crowd of gents saying to check the timing, but our PCV valve is crap and replacing it wouldn't hurt (they are a couple dollars)

Have you tried T'ing into other spots? My setup is like the image below:

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