My vacuum sucks!

[EVO9_Inside]

so this is my second dsm. my first is a 91 gsx that was in a garage for 11 years with 60k miles, had to rebuild the entire thing. 2 weeks ago i picked up another gem, orginal owner, been in a garage for 7 years. 91 gsx with punishment racing intercooler system, buschur built short block with forged weisco pistons and eagle rods, and lots of other goodies like a 20g turbo.
My first issue was, when i bought the car the coolant would always boil off from a blown HG, so i replaced the head with a spare head i had, Crower valves. springs, retainers, new seats, new guides, stock cams, no miles on it, perfect out of my friends head shop. installed with arp head studs and a 5 layer MLS head gasket.
runs perfect besides one thing... my vacuum sucks!
on the previous head i had 15in/hg at idle. on this head, I also have 15in/hg of vacuum! the short block has 2k on it, and the head is brand new, so im not sure where my problem could be, i capped off all the throttle vacuum lines and nothing changed. so im unsure where to go from this point, i have no vac leaks that i can find with propane or starter fluid.
is this normal for these cars to have crappy vacuum? i had a 450hp srt4 with a built engine and fat cams and it still had 18in/hg!

thanks for nay ideas guys, just tired of sitting here trying to brainstorm myself.

 

[1davho]

Do a boost leak test. Pressure up the whole system with compressed air starting from your turbo inlet and listen for air leaks. The test pressure should be higher than the amount of boost you are running

 

[jdezmit]

Are you suggesting 15in/hg is bad? What is your idle set at?

 

[T is for TURBO]

Isn't a vacuum supposed to suck?

Boost leak.
Leak down

15in/hg isn't terrible. What is your idle?

 

[1davho]

15 inches isn't that bad but I am at 5000 feet for altitude and I pull 18 at idle. I think he has some boost leaks or somethong else going on. Its not major issue either way.

 

[ileagle95]

As others stated, whats your idle at? Boost leak test, possible your out of time as well.

 

[gsexxx]

Isn't a vacuum supposed to suck?

.. Like stated above, 15in/hg isn't bad.. I would recommend a BLT as a starting point.

 

[EVO9_Inside]

Im a toyota master tech, in my profession. if its less than 17in/hg something is terribly broken, so thats where im coming from kinda. and as i said, my srt4 with 280s had a higher vacuum.
I idle at 1000rpm, ill throw my boost leak tester on it and see what happens.

Im also struggling with a weird idle issue where it likes to surge 1000-1500 at random times, ital surge for 20 minutes, then ill stop again and it will be perfect. dsm problems. its gotta be something electrical.

 

[T is for TURBO]

Im a toyota master tech, in my profession.

Good thing you're not working on a Toyota Also this is not an SRT-4

Vacuum for a stock 1g is ~18-19 @ 750RPM
If you are idle surging to 1000-1500 your vacuum will be lower.

low vacuum does not mean necessarily mean that some thing is "terribly wrong"
I run 272/272's in my car. My vacuum is ~12 in/hg at 1000rpm. There is nothing terribly wrong with my car. In fact it runs better than it ever has.

 

[EVO9_Inside]

low vacuum on a car with stock cams is what stress me out, if i had bouncing vacuum at 10in/hg and ran 11s id be happy, but its stock cam with low vac
well since 15 is normal i guess ill just concentrate on the random idle issues and just get it ready for winter storage, its coming up fast...

 

[T is for TURBO]

No one ever said it was normal, it's just not that bad. It's indicative of a boost/vacuum leak, and/or high idle. If you're really concerned about it, then do a compression test. Because worst case scenario would be rings.

Another thing to consider is the accuracy of the gauge to are reading it from.

 

[kchaazz]

There are so many things that can effect vacuum.
1.Timing advance
2.Running lean/rich
3.Vacuum leaks
4.Mechanical timing
These are just a small few to take a look at. The idle surge is a fair indication that there is either an intake tract leak, or the idle needs to be properly set but before you mess with that, you should check that the ISC is properly functioning. Is the base timing properly set, and did you also properly install the cam angle sensor by putting #1 at TDC and lining up the marks on the CAS before installing the CAS? This wont necessarily cause your low vac, but damn itll make her run lean at wide open throttle due to the injectors firing improperly. 15 really isnt too far off, so perhaps you could attach a vacuum gauge to the PCV port on the intake manifold and take a reading there, as many gauges can be a bit off sometimes.

 

[EVO9_Inside]

compression test was done already, 180+-7psi, post head gasket.
I have the factory guage connected and my aem boost guage, both agree with each other, but are plumbed off the same vacuum line.
I must have a leak somewhere, im thinking its internal to my BOV, its the only line i havent plugged and tested.

it does bother me , DSM's always have the reputation of being broken, i have mine run as perfect as possible.

There are so many things that can effect vacuum.
1.Timing advance
2.Running lean/rich
3.Vacuum leaks
4.Mechanical timing
These are just a small few to take a look at. The idle surge is a fair indication that there is either an intake tract leak, or the idle needs to be properly set but before you mess with that, you should check that the ISC is properly functioning. Is the base timing properly set, and did you also properly install the cam angle sensor by putting #1 at TDC and lining up the marks on the CAS before installing the CAS? This wont necessarily cause your low vac, but damn itll make her run lean at wide open throttle due to the injectors firing improperly. 15 really isnt too far off, so perhaps you could attach a vacuum gauge to the PCV port on the intake manifold and take a reading there, as many gauges can be a bit off sometimes.

how do i confirm my ISC is working correctly? thats what i think most of my issues would be stemed from.

CAS was installed correctly, WOT is fine, im within my 11-12:1 range that i tune for. but i do need to get an adjustable timing light to confirm im on spec. its all just below 1500rpm problems.

fuel im reasonably sure is good for idle, bounces from lean to stoich very nicely.

 

[T is for TURBO]

how do i confirm my ISC is working correctly?

Check the coil resistance.

 

[kchaazz]

You dont need an adjustable light to check timing. Just ground the plug on the firewall, unplug anything attached to the diagnostic port under the dash (Having a logger cable attached grounds the pins on this plug in many cases and if you ground the timing plug with your logger cable also attached, it will put the ecu into idle adjust mode) and check the timing as you would on any other car.
It is very easy to check the resistance of the ISC per the above posted guide, however, if the ISC coils check out and you are still having idle issues, you will want to remove the ISC from the throttle body, plug the ISC back in, then have someone sit in the car and turn to ignition on and off (Dont start the car, just off position to on position) and watch the pintle on the ISC move up and down and verify that it is moving smoothly.

 

[T is for TURBO]

Also assume that every one of these cars has had the BISS screw tampered with. ISC position must be verified by a logger.

Also, here is a good read regarding idle surge.