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pc codes 0171 and 0174 appeared within an hour after I changed my starter.

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I recently just changed the starter on my 96 Cadillac Deville It really wasn't that big of a deal besides not being able to disconnect the fuel lines, I disconnected all hoses and electrical connections, I lifted it gently and propped it up with a hammer (just enough room. I never thought to check the gaskets, but if they weren't leaking before hand, why would they now. I changed the starter, took a bit longer than it would have if I were able to disconnect the fuel lines put everything back where it, and checked over 3 times. One of my vacuum hoses did break in the process. I fixed that using a flexible rubber hose that fit over the small plastic hose (something I have done before that worked) not sure what the hose is for, but it connects to the passenger side of the manifold right under the fuel rail and leads to a series of hoses connected at the passenger side wall. all the other hoses were fine, I know both codes lead to a vacuum leak, and I hear this happens often after removing the intake manifold, but why?? I really don't notice the difference while driving and my engine is rather quiet while Idling. I even monitor the RPMs on my dash while I'm driving, and when I'm stopped I'm steady at between 625-650 with no engine hesitation.... I have full power when I accelerate, my MPG rate while I'm driving is still in the mid twenties when holding a constant speed.... If there is still a leak in the hose I fixed it's not a bad one, would be rather small at that.... Could a leak that small bring up both codes???

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There is a note in the 1997 FSM (99% accurate for the 1996 models) for the sections on both P0171 and P0174 says:

Important: An improperly installed air cleaner or a distorted/missing Intake Air Duct may cause this DTC to set. Ensure air cleaner assembly/IA duct is installed correctly and that there are no kinks, folds, or damage to the IA Duct or possible debris in the air cleaner assembly or the MAF sensor inlet screen that may distort the air flow.

In the troubleshooting instructions, the FSM says that if there are other DTCs, to fix those other problems first.

The list of things to check if both P0171 and P0174 are set is:

  • Vacuum hoses; look for splits, kinks, and good, tight connections.
  • Throttle body, intake manifold and EGR valve
  • Crankcase ventilation valve and/or system for leaks including a loose or missing Oil Fill cap or oil dipstick.
  • Contaminated fuel.
  • PCM and sensor grounds.
  • Air induction system after MAF sensor for vacuum leaks.

If that doesn't do it, the FSM says to do a diagnosis on the fuel system.

Note that cracks in the rubber mount between the throttle body and the intake manifold, and in the manifold gaskets, are included in the last bullet.

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Did you reuse the intake gaskets? It is OK to reuse them but you may have damaged one and it is sucking air.

'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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You are getting a lean condition on both banks. There are about 3 vacuum hoses and if you left one off that may be the problem. Check the brake booster line also.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Like the previous posters started with the P0171 and P0174 it's affecting both banks. So that's easier to narrow down. It's going to be a leak before the intake. So from the rubber boot at the coolant crossover backwards to the mass airflow meter, including all the vacuum connections at the throttle body and the throttle body gasket/o-ring.

Bad news is that since your idle isn't higher than normal the leak is small enough that the idle control valve and by adjusting fuel trim the car can keep the idle normal. A big leak would be idling over 1000rpm.

A can of throttle body/ carb cleaner and spring around should make leak obvious.

Also look at the vacuum house diagram socket under the hood and double check all houses and connections. Sometimes the rubber boot connectors or the rubber part of the lines will be gummy and cracked. By removing the intake it might have caused a hose orboot to split.

Good luck

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