1998 S70 P0171 vacuum leak, bad hoses 9155862 Topic is solved

[goVolvo]

I really want some smke test to cnirm the problem before draining coolant and poking arund that small corner. This car had been messed up before. One example is MAF wire had been cut off. Don't want to take much risk.

The part #17 is the elbow 9155862. Where does the other end go on a non-turbo engine?

[dosbricks]

The part #17 is the elbow 9155862. Where does the other end go on a non-turbo engine?

If '99 is not substantially different than '98, to the flame trap on the plastic intake coupler at the throttle body--via a hard plastic vacuum line.

[erikv11]

That small line connects to the flame trap housing in a non-turbo. There are a lot of pictures over in this thread, where it is called "vacuum line number 3" https://www.matthewsvolvosite.com/forums ... =1&t=65002

If you want to do a homemade smoke test, hook up a hose to the vacuum tree (there are unused ports available on your non-turbo) and blow smoke in there, it will circulate around.

[goVolvo]

Thanks. Very good pics for vacuum lines. Also is the short-term fuel trim and long-term fuel trim number the best indication of vacuum leak? Is the normal OBDII reader capable to scan these numbers?

Last month it passed emission test with the old way: I rev up the engine to about 2500RPM and hold for several minutes, and exhaust air is measured at the tail pipe. If it was running too rich it could have failed the emission test.

Anyway it also idles a little rough sometimes. Seems also points to vacuum leak.

[draser]

P0172 is for fuel system too rich. The way this works is your oxygen sensor detects too much fuel in the air/fuel mixture. Your Long Term Fuel Trim should be highly negative as ECU is trying to pull fuel and that triggers the code. One of the culprits could be the MAF but it needs diagnosed first rather than just replacement, as you found out. Other causes are the coolant sensor operation, thermostat which both signal the ECU a cold engine when it's not. However first thing to check is the O2 sensor operation to make sure it does indeed sends correct signal to ECU. Any good code scanner will provide that.

[erikv11]

Almost always the p0172 is triggered by a vacuum leak, after the MAF and before combustion (i.e., on the intake side).

First thing is to check carefully and thoroughly for vacuum leaks.

[mecheng]

I had the same code on two different P80s and both were the elbow, check the elbow and report back.

[draser]

How would unmetered air getting into engine generate a rich (too much fuel) condition?

[misha]

O2 sensors detect that unmetered air and send info to ecu to inject more fuel.Simple as that.

But...my car does the opposite.It runs rich without vacuum leak.

With vacuum leak introduced,ecu reset....no rich exhaust and normal fuel consumption.As soon as i plug port onto a vacuum tree,car begins to run rich and fuel consumption gets higher.

With or without vacuum leak car runs like a dream and no signs that something is wrong.It's just that it's running rich when there is no vacuum leak...no extra air drawn in engine.

[draser]

Actually no, the ecu adds fuel to reach stoichiometric. It's a closed loop system.