I have low idle vacuum, and I'm trying to get to the bottom of it. My latest thought is that its in the tune. I'm seeing ~80 kPa at idle, however during part throttle cruising I often see vacuum in the 30-40 kPa range.

Here is what I've done.

I took off the intake manfold and replaced all 8 port gaskets, throttle body gasket, injector o-rings, MAP sensor seal. I started the engine and allowed it to warm up. My idle vacuum was still very low (or high kPa). That lead me to remove and plug the two PCV type lines. First I removed and capped the one from the valve cover. Then I removed and plugged the one from the lifter valley. No change in vacuum, so I reconnected them. I inspected my manifold for cracks also. I feel very confident that there is no leaks in the intake system now. I have drilled my throttle body plate to a larger hole. I went 1/32 larger diameter than stock. I did this because I couldn't get the idle steps low enough at warm idle by only using the set screw/tps reset method.

I have attached my latest log, where I was logging some AutoVE stuff to get the low rpm area tuned. Basically I'm driving around in 1st through 4th gear just crusing. You can see the vacuum characteristics when I'm crusing in that log. There isn't a good idle in that log, since I'm havin trouble having it come into idle when I stop, so I have just been feathering the gas for the time being. I'll also attach an idle log so you can see idle vacuum.

My latest thoughts are that the low vacuum at idle is due to idle tuning issues. Does the tune have a large effect on idle vacuum? Possibly increasing timing and decresasing the desired airflow table will bring vacuum back to normal? I'm not sure, just throwing ideas out there. During cruising the vacuum seems very good and normal. Here are my cam specs:

Its a TSP MS4: 239/242, .649"/.609" on a 111 LSA.

My engine is a 2004 GTO, so it has the LS6 manifold. My exhuast consists of 1 7/8" long tubes to a true dual 3" exhaust through an x-pipe. The intake consists of a dual 3.5" setup, each going to a filter behind holes in the bumpercover. I am running in open loop speed density right now. I have no MAF sensor at all.

If you require any more details, please let me know.


Attachment description:
SDAutoVEFixed_0004.tun - Tune used on those logs.
Log_0033.efi - Idle warmup log. From cold engine to operating temp.
Log_0035.efi - Cruising log. 1st through 4th gear, about 10 minues worth. I was logging to do some VE mapping.

 

And one more attachment. Here is my proposted next tune I'll be uploading on the car.

I applied the BEN factors, and did quite a lot of manual smoothing, then used the smoothin button afterwards. Then I added 3% to those cells to make sure I'm not over a value of 1.0. I'd rather take baby steps.

Then I modified the table B3605 to command richer values in the high kPa (high load) columns.

I also returned the B3618 curve to stock GTO settings. This will allow for safer ratios during some of the high rpm, high load VE mapping.

I also made a few small tweaks here and there as well.

 

Its a TSP MS4: 239/242, .649"/.609" on a 111 LSA.= Low Vacum

 

I know that it will hve low vacuum, but is ~6 inHg at idle normal? That is equivalent to the 80 kPa I'm getting on my Map.

 

It's going to be a pig at idle. But you can still put the pig
on the Stairmaster and get that bacon lean and crispy.

The idle AFR and spark advance want tightened up, get
the mixture roughly right which will probably be a lean
wideband reading because you blow air through the
big overlap. You have to apply some "windage" to the
speed density tuning (or just go with indicated-stoich
which will be rich in reality). Use bidirectional controls
to push the AFR and note the point of minimum MAP
(max vacuum) as-commanded and as-read from the
wideband. Get your VE table trued up as best you can
so you hit that point @ open loop normal. Then do the
same deal with idle spark advance looking for more
vacuum improvement. Modify your idle advance tables,
park and in-gear, to see that same value less maybe
5-10 degrees normally.

I'd turn off adaptive idle spark for this, though bidir
controls will override it. When you turn adaptive spark
back on, make it s smooth linear profile in both directions
and its endpoint for "Low" advance should be the same
amount as you backed off the base idle advance, so
maximum "help" equals maximum improvement. Look at
the adaptive in line-chart mode, it should be monotonically
increasing / decreasing and never back away (see Camaro
stock curve, it increases to a point and then backs off to
zero, a good way to "flip phase" and get unstable.

 

Only thing I can think of, would be a slight vacuum leak near the map sensor. That could throw off the reading at idle, but the engine would be moving enough air at part throttle that the small leak wouldn't affect the reading. Possibly a bad or missing seal on the map sensor.

 

Did you read my post? In particular this paragraph

 

it's a huge cam with a tight LSA and plenty of overlap. Low vac goes hand in hand. If your idle is excessively low (in the 800s) bringinr it up a tad may help a bit. Using bi-di controls for timing you might be able to get it down another 5kPa, but I'd be suprised if you found a 10 or greater inprovement with any method.

 

try this one when you get bored...looks may be deceiving,but it idles pretty good,just compare the idle area if you like...ve is jagged but works for me...

230/236 cam olsd mafless '04 gto here...leans nicely at around 70-85 mph so watch your wideband as it will be around 16-17 at lite load...