I've got the dreaded po440 code on my truck but, I can't seem to find this anywhere. I have an equus 3160 and I am able to read the freeze frame data with the SES light comes on. And it's telling me that at 791 RPM, I have 13.9 in HG at the MAP. When I do a live data scan, it says the same. I've googled this question and for other cars it seems to be between 15 and 22 in HG. So, am I really low or am I in the ballpark?

 

You have an evap system fault
https://blazerforum.com/forum/diagno...plained-35145/

The vacuum is low. Like oyu mention usually over 15 in

 

Generally my truck has always ran the best when vacuum was 17-18 mm Hg. Check all vacuum lines, make sure they aren't cracked or all oily.

 

Just thought of something.. when you're looking at it with scan tool and goose it, does the reading increase or decrease, numerically?
Remember that thing is in absolute pressure. Which means a pure vacuum is 0, as opposed to about 30 inches Hg which is a vacuum metric, meaning negative of 15 psi / 30 inches Hg /1 bar {atmosphere at sea level}
Duh.. what that means is with key on engine off the MAP would read 29 something near sea level. If so subtract 14 from that to get your actual manifold vacuum.

Back to your other problem. Check all tubing from rear of engine, generally along the fuel lines path, back to the canister at rear axle. The purge solenoid on rear of manifold applies vacuum and looks for a result reading from the tank pressure sensor mounted on fuel sender assy. That's why gas cap affects it.

 

Ok, so, I just bought a vacuum gauge because it was really bothering me and apparently you can tell alot from what it does. It said to warm up the engine. I drove an hour on the freeway from work. Does that count? Anyway, It's a rock solid 17 in. When you open the throttle, it drops to over 20 in. However, pettyfog said something that made me think. Does anyone know at what part of the drive cycle the computer calls for an evap system check? Also, I'll check my lines but, how can you test the purge solenoid?

 

What?!!!! okay.. so where does it start? Where is it attached?Dont test it, not likely to fail... to start: Just hook a mity vac or tee off manifold vac to line toward back and then read with your vac gauge, most likely split or broken somewhere..
If that line holds a vacuum then we'll check the tank pressure sens

 

I plugged my scanner back in and at idle it read 10.9. I goosed it and it dropped to 6.9. I shut it off and it jumped to 29.2. Not really sure where the 14 comes in but, it appears that when the code sets at 13.9, I have a massive leak. Correct? But, only when it tries to do an evap check. So, therefore I need to check my lines from the purge valve and possibly think about my cannister vent valve?

If this is the case, could this also cause a po300 code? BTW, I have replaced the gas cap.

 

Just replaced the cannister vent valve. Went to pick up the kids and the MIL went out on it's own. Came home and verified it with my scan tool. That was the problem! BTW just looking inside the old one, it was rusted and full of insects. Must be a favorite place to hide. Plus it fell apart as I was taking it out.

Oh, and pettyfog, It's 30, not 14. You were right that the scan tool reads the MAP which is absolute pressure. Not vacuum. So, if you were to take your MAP reading (in my case 13.9) and subtact it from 29.92 (atmospheric in inches of HG) you would get 16.02 inches of Hg of gauge vacuum. It is not accurate but, will give you a ballpark figure. In my case I take the map add 2 and then subtact from 29.92 it gives me a more accurate gauge vacuum. At least that's what I came up with after playing with both my scan tool and my vac gauge. It seems pretty consistant for me.