Vacuum - input wanted!

[ksimm92]

well some of you may remember me mentioning this in the exhaust thread I started the other day, I just figured It would be better to start a new thread specifically for this issue.

I am suspecting my cat to be clogged, when accelerating from a stop it is VERY sluggish until around 2500-3k RPM then it kinda takes off. I have replaced vent solenoid, purge valve, cleaned TB, cleaned MAP sensor, cleaned IAT sensor, Sea foamed threw the vacuum line earlier this year & fuel treatment in the tank. I have reset the PCM by taking off the negative terminal on the battery after cleaning and replacing parts.

I have ordered a new upstream O2 sensor & and gauge that goes into the upstream O2 sensor port that will tell me if there is any back pressure... it won't be here till thursday so just trying to get some ideas before hand.

One thing I did notice today, I hooked up my actron scanner that reads live data, I was looking at the MAP HG readings... at idle it was sitting around 9.8HG and when accelerating from a stop it would go up to around 28.6HG ... from my understanding on vacuum, it should sit around 17-18HG at idle, and go down from there as I accelerate correct???

As the throttle opens, more air/fuel mixture is being allowed in which should lower the vacuum closer to ambient pressure right? so if the vacuum is going UP when I accelerate my suspicion seams correct?


On a second note, what do you guys thing of this? Amazon.com: MagnaFlow 23967 Direct Fit Catalytic Converter 49 State (Exc. CA) 2002-2002 Chevrolet Trail Blazer: Automotive

again, any and all input is welcomed!

 

[jimmyjam]

if your cat was clogged, it wouldn't go faster at higher rpms. it wouldn't even make it that high

if your cat is clogged, and the air has nowhere to go, how would it produce vaccuum? vaccuum means the air is getting pumped somewhere.

 

[Wooluf1952]

if your cat was clogged, it wouldn't go faster at higher rpms. it wouldn't even make it that high

if your cat is clogged, and the air has nowhere to go, how would it produce vaccuum? vaccuum means the air is getting pumped somewhere.

^^That^^

 

[ksimm92]

if your cat was clogged, it wouldn't go faster at higher rpms. it wouldn't even make it that high

if your cat is clogged, and the air has nowhere to go, how would it produce vaccuum? vaccuum means the air is getting pumped somewhere.

any ideas on why the vacuum is so high? or why it gets higher as the RPM's go up?

 

[CaptainXL]

One thing I did notice today, I hooked up my actron scanner that reads live data, I was looking at the MAP HG readings... at idle it was sitting around 9.8HG and when accelerating from a stop it would go up to around 28.6HG ... from my understanding on vacuum, it should sit around 17-18HG at idle, and go down from there

Well lets see.

1 bar = 14.5 psi = 30 in hg

What you are seeing is the difference between outside atmospheric pressure and engine vacuum.

So at idle you should see 9.8 hg or 21 on a gauge. The scan tool is right it just reads the raw data from the sensor but doesn't convert for you. I suggest a haynes manual they explain it

28.6 hg at wot is right as your allowing more positive atmosphere into your engine

 

[ksimm92]

my scanner gives me inches of mercury, I do have a haynes manual. I understand what it is showing me... I just don't understand WHY it's showing me. when I accelerate the vacuum should go down, and at idle it should go back up. It is doing the opposite.

View attachment 21761

 

[CaptainXL]

my scanner gives me inches of mercury, I do have a haynes manual. I understand what it is showing me... I just don't understand WHY it's showing me. when I accelerate the vacuum should go down, and at idle it should go back up. It is doing the opposite.

View attachment 7953

I thoroughly explained it in my previous post. does anyone else not understand this? It looks pretty cut n dry

 

[Joe]

The sensor reading is an absolute pressure value. Referenced to zero pressure as in outer space. Vacuum readings are generally gauge readings - referenced to atmospheric pressure. Your + 9.8 absolute sensor reading is equivalent to -20.2 in Hg gauge if your local atmospheric pressure is 30 in Hg absolute.