Hello

Using an OBD II reader, it shows a pressure of 3.9 PSI at idle, is this too low?

I'm trying to find out what's causing a intermittent p0171. It will suddenly show up, then go away after a few hours. I checked the hoses/lien and can't find any appear cracked or damaged. I can't check for a vacuum atm, so figured I would use an OBD to get the manifold pressure, it that is different from normal that should indicate a leak correct?

 

you can use a vacuum pump to check vacuum at the manifold

 

What happens to the vacuum reading when you rev the engine?
3.9inHg at idle is NOT normal at all. And the engine would probably be running strangely at best.
Are you sure you are not looking at the VOLTAGE output?

 

Converted it's around 8in Hg. I'm confident it's the pressure and not the voltage. Could the scanner I'm using cause a artificially low reading?

 

8-10in hg, at idle, is normal.

 

Thanks. Since that reading is normal does that mean there no vaccume leak? I'm wondering whats causing the i intermittent p0171 too lean code

 

Unless the vacuum leak is at the base of the intake runners, it will not cause a lean condition or code,on a speed density system.



Id look else where for the source of your lean code......

 

Put a vacuum gauge and see what that reads from the scanner you are using. Easy test, 8-10in hg is not normal at idle that is to low. Are you using a low end scanner or a high price scanner not that it matters but if you don't know how to use a scanner and pull up all the data pits and know what they are reading than you may be looking at the wrong right data information.

 

Please dont confuse people with misinformation.


As mentioned earlier ,the reading of 8 inHG is a MAP reading,not a vacuum gauge reading. It is perfectly normal.

Learn the difference before giving advice.....