Vacuum line

Tiny
COREY H JENKINS
  • MEMBER
  • 1996 CHEVROLET TAHOE
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 253,000 MILES
Computer showing codes P0171 and P0172 banks 1 and 2 running to lean.
Truck has new fuel pressure regulator, new fuel pump and filter so it must be a vacuum leak.

Never traced one before, where should I start and how do I do it? Maybe a vacuum line diagram would be nice.
Thank you
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Friday, March 31st, 2017 AT 2:32 PM

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Tiny
CARADIODOC
  • EXPERT
The vacuum hose routing diagram is on the emissions sticker under the hood. There are always a lot of them with slight differences for any model and year, so we always get the wrong one if we try to post it here. The first way to find a leak is to pinch off the hoses, but you need another symptom so you have something to watch for that changes. Most commonly that is idle speed that is too high. If the leak is small enough, the Engine Computer can compensate for it and bring idle speed back down. You can use a scanner to view live data and see what the computer is doing in regards to idle speed. If there is a vacuum leak, the computer will be requesting a lower idle speed.

There can also be a vacuum leak in the intake manifold gasket or throttle body gasket, as well as a vacuum hose. The most effective way to find those is with a smoke machine. You might find one of those at an auto parts store that rents or borrows tools. Inject the smoke in a vacuum port, then watch for where it sneaks out.
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Friday, March 31st, 2017 AT 2:53 PM
Tiny
COREY H JENKINS
  • MEMBER
Any certain port? And where is one?
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Friday, March 31st, 2017 AT 2:59 PM
Tiny
STEVE W.
  • EXPERT
P0171 - Bank one running too lean.
P0172 - Bank one running too rich.

I would check the O2 sensor wiring first then the exhaust on that side for any leaks, then the EGR and EVAP purge solenoid.
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Friday, March 31st, 2017 AT 3:08 PM
Tiny
CARADIODOC
  • EXPERT
Vacuum appears at every port, just like water pressure appears at every faucet in your house. If you inject smoke, it does not matter which port you use. I pick the hose that is easiest to get to or the fastest to remove. Pull that hose off, then use a rubber tipped adapter to push the smoke into that port. If there is a leak, you will see the smoke appear, then you can follow it back to the source.

The only part of the system not included in that test is the hose you pulled off. Inject smoke into that hose and search the same way.

Smoke machines are typically used in fuel vapor recovery systems because the leaks can be much too small to find any other way but still be big enough to be detected by the Engine Computer, which will turn on the Check Engine light. They can work in the vacuum supply system too, but those leaks have to be rather substantial before they will affect engine performance. If the leak is that bad, you can often find them by simply pinching hoses while the engine is running. This is where you need to have something you can watch to see when it changes. High idle speed is the most common symptom. Pinch off one hose near the intake manifold. If idle speed stays too high, the leak isn't in that hose. Move on. If you pinch a hose, and the idle speed drops, follow it to its branches, then pinch off each hose, one at a time. Idle speed will drop when you pinch the hose with the leak. Follow that branch until it branches off again or comes to a part that uses that vacuum. Once you pass the leak, pinching off the hose will not lower the idle speed.

Pinching hoses works well if the leak is in a dry-rotted or split hose, but you will not find a leaking gasket that way. The smoke machine is best for that, but you might also try running water over the running engine while it is still cool. If you are lucky, you will see or hear the water get sucked in. It will go in slowly enough to go harmlessly out the tail pipe.

Keep in mind you are assuming there is a vacuum leak, but you never mentioned a symptom that would point to that. You are just assuming there is a leak based on the lean fault codes. The Engine Computer controls idle speed, in part, by adjusting a variable, controlled vacuum leak. There are other things that can cause a lean mixture to be detected. One is a spark-related misfire. If that is bad enough, you may smell the unburned gas at the tail pipe, but there is also unburned oxygen in the exhaust. That is what the properly-working oxygen sensors detect, and that is what sets the "running lean too long" fault codes. Usually the Engine Computer will detect which cylinder is misfiring and set a fault code for that. A set misfire code may inhibit a lean fault code from setting, since the false lean condition can be expected.

A better suspect for a lean condition is a leak in the fresh air tube between the mass air flow sensor and the throttle body. Chrysler is the only manufacturer that never needed a mass air flow sensor to make their engines run right. Everyone else uses that sensor on most or all of their models. The leak in the air tube only applies to those car with the mass air flow sensor. Air sneaking in through a leak in that tube still has to go through the throttle body, like normal, but when it does not go through the sensor, it does not get included in the fuel metering calculations. You will have air the computer does not know about, and no fuel to go with it. That can show up as a lean condition with a normal idle speed.
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Friday, March 31st, 2017 AT 5:17 PM

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