Low vacuum pressure

[Blaine87]

I have low vacuum pressure, it is reading about 15in hg and I am getting terrible mileage. I replaced all the vacuum lines. I have a scanmaster and the readings are
Knock 726 ret. 0.0
Maf 5.0

LV8 38
Volts 13.4
Int 128
BLM 127
Speed 0
Coolant 178
Air 119
Rpm 725
TPS .42
IAC 16
O2 55 and steady
Open loop
No codes.

Wondering what I can check, not sure if it could be internal. The scanmaster seems fine, but Power is down and mileage is terrible.
Thanks for any comments.

 

[Chuck Leeper]

Leak down and compression tests.
Who's chip?
O2 55 and steady?? Even in open loop it should cycle.
Engine specs, mileage?
More info than the SM readings.

 

[hensleyt]

this^^^
My turbo car can't make 10 in. of vacuum at idle, it runs good, never check MPG
Like Chuck said there is better was to check the health of these cars than a vacuum test

 

[salvageV6]

15 in. is about normal for the stock cam. so it could be your right foot for the gas mileage which you didn't state what it was?

 

[TexasT]

What is this "terrible" mileage as calculated? It isn't a Honda or Camry and wont get 35mpg. Be glad you are at 17 especially if you like that LOUD pedal.

 

[Blaine87]

The engine is stock with 150,000 km. I just installed a turbotweak classic v5.7 chip. I rechecked the O2 on scanmaster and it cycles while driving, but idling or at a light it will not change. I was looking into the low vacuum as it idles fairly well after warmed up, but driving down the street the rpm is hi enough that it will coast along without giving it gas. I read in posts that low vacuum could cause the ecm to add fuel to get the a/f mixture proper. Possibly why the mileage is less than it used to be when I bought it new. The IAC and TPS seemed fine I thought. I guess I will need to replace the O2 sensor.
Thanks for the replies.

 

[salvageV6]

What is the tps and rpm when the engine starts driving down the street with your foot off it? IAC numbers at that point also.
Does it always snap back to 725 rpm when you manually move the throttle open then closed in park?
I'd clean an reset the IACV if it isn't too much trouble for you to do so and also look for an intermittent vacuum leak.
Is 725 the commanded idle in that chip? Seems a bit low to me thought that was set at 800 or so in my 5.7?

 

[TexasT]

So, we are workin with a 100k mile until. That ain't exactly new.'

I think a compression check and leakdown are in order. Could have an intake leak. Some carb cleaner around the edges where it contacts the heads, and around the throttle body while it is idling might reveal something if the idle moves around during application. Those bushings in the throttle body "might" have some wear and are letting in unmetered air.

How about that calculated mileage? Can be in km/L. I'm pretty good with the goggle to make the conversion.

Also, the maf reading of 5.0 i am guessing is the af at 5 g/sec? Is this moving when you give it some beans?

 

[Blaine87]

I will check the TPS and IAC while driving as I have not. I will also try and get to a compression check as well. It will have to wait a few days. I have a feeling I may have a bit of vacuum leaking thru the front and rear seals. No leak in front really, but a small drip at the rear seal. BTW, The unit was a "new unit" when I bought it 36 years ago.

 

[TexasT]

oo here too. 36 yrs is a long time. stuff gets worn out. even if it doesn't have the miles it has the time. by that same notion I hope you have replaced the timing set and that top nylon gear has the age and is probably mostly in the pan if not. fuel pump and its wiring needs a hot wire and probably a replacement pump. especially if you are using that loud pedal .

 

[Chuck Leeper]

I have a feeling I may have a bit of vacuum leaking thru the front and rear seals. No leak in front really, but a small drip at the rear seal. BTW, The unit was a "new unit" when I bought it 36 years ago.

At 100K on the clock, I seriously doubt it's a vacuum leak at the crankcase seals. More like blowby and leaky seals. Think it's a vac leak? Put a low psi gauge on the dipstick tube and see.
The ft of the plenum behind the tb has no fasteners and is a common leak location.
The pvc system needs addressed, too.
If the valve cover is still plumbed into the turbo, that needs to go away and the turbo bung blocked off.

 

[Blaine87]

I have checked a few things that were suggested. I did get a compression check done. #1-150psi, #2-150 psi, #3-153psi, 4-149psi, #5-152 psi, #6-152psi. All fairly close. While cruising with foot off, TPS .42, IAC 51-67. Will idle along at 40km/hr? Valve cover is still stock and plumbed into turbo- why should this be changed? Checked idle rpm and idle does go back to 750 right after hitting throttle.
The timing chain has not been changed, nor the fuel pump.
Don't have a connection to put on dip stick to check crankcase pressure yet.
I ordered a vac pump to ensure vac ball is intact.
Thanks for all the suggestions.

 

[Chuck Leeper]

Valve cover is still stock and plumbed into turbo- why should this be changed?

Pulling oil fumes into the turbo, blowing them into the intercooler and into the engine.

 

[1986 Buick GX1]

^^ this.
The oil coats the inside of the intercooler and make it inefficient.

 

[Chuck Leeper]

IAC 51-67. Will idle along at 40km/hr?

Hi IAC will cause the 40KPH.
Look at the fuel map and see where the IAC is on the VE table..

 

[Pronto]

The engine is stock with 150,000 km. I just installed a turbotweak classic v5.7 chip. I rechecked the O2 on scanmaster and it cycles while driving, but idling or at a light it will not change. I was looking into the low vacuum as it idles fairly well after warmed up, but driving down the street the rpm is hi enough that it will coast along without giving it gas. I read in posts that low vacuum could cause the ecm to add fuel to get the a/f mixture proper. Possibly why the mileage is less than it used to be when I bought it new. The IAC and TPS seemed fine I thought. I guess I will need to replace the O2 sensor.
Thanks for the replies.

TT chips lock the O2s at idle.

 

[Pronto]

I have checked a few things that were suggested. I did get a compression check done. #1-150psi, #2-150 psi, #3-153psi, 4-149psi, #5-152 psi, #6-152psi. All fairly close. While cruising with foot off, TPS .42, IAC 51-67. Will idle along at 40km/hr? Valve cover is still stock and plumbed into turbo- why should this be changed? Checked idle rpm and idle does go back to 750 right after hitting throttle.
The timing chain has not been changed, nor the fuel pump.
Don't have a connection to put on dip stick to check crankcase pressure yet.
I ordered a vac pump to ensure vac ball is intact.
Thanks for all the suggestions.

At that mileage you can be sure there are cracks in the plastic teeth of the timing gear. That's a disaster waiting to happen. Same for fuel pump. They were too small to begin with. Add age and crappy wiring and again a disaster waiting to happen. No mention of a hotwire kit. You need to do basic TB maintenance. Timing set, fuel pump, hot wiring kit, fuel filter (good luck with that if original), afpr if not on there. TT chip wants 43psi, clean the intercooler, rid the turbo of the vc vent, check turbo for play, check headers for cracks, valve cover vents, Power Plate, check throttle body seals, check upper plenum for vacuum leak, check vacuum block for leaks/replace with metal one, check for oil in the plenum, replace all rubber lines, replace all check valves, replace pcv (the Supra one is a positive seal against boost). MAF translator, LS MAF, solid intake pipe with air cleaner, wires, NGK UR5 plugs, check coils, cat delete if not done, coolant flush, trans fluid and filter change, rear diff oil change. Probably a few other items I've forgotten but making sure those systems are up to snuff with change the whole way you car runs.