98 DeVille Vacuum Leak Partially Fixed

[Luke]

I drive a 98 DeVille with 151,000 miles. In times past, I would get 27-30 mpg but in recent months, that dropped to 21-23 mpg.

The car has apparently had a vacuum leak since I bought it several years back but it has gotten worse recently. With the air conditioning off, it would idle very smooth but with the air conditioning on, idle would bounce back and forth between 650 and 900 RPM. To me, this would indicate a vacuum leak. So, a few days ago, I hooked a vacuum gauge to a vacuum line. It was showing about 13 in/HG at idle. I started poking around and found that my intake bolts were very loose. One could have been easily removed by hand. So, I checked the factory service manual and torqued to factory specs. After tightening the intake bolts and studs, vacuum went up to 17 in/HG at idle! BUT, when I turn the air conditioning on, it drops to 15 in/HG.

Does anyone know.... is vacuum supposed to drop with air conditioning on or does this indicate a vacuum leak within the dash?

[Texas Jim]

I don't know what the exact numbers should be for your car...but...

A slight drop in vacuum is normal when you place a load on the engine.

When you turn on the AC...it has to open the throttle body a tiny bit to add more fuel and air to keep the idle where it should be.

Adding more fuel and air...equals a small drop in vacuum...

[winterset]

On the passenger side near the firewall is small length of vaccum hose that (on my car) seems to be getting a little brittle or crumbly. it's unlike any other vaccum hose on the car where they are made of a more rugged/hard plastic material. I'm not sure if that line only get's used when the AC is on, but I's worth checking it's condition for leaks.

[Ranger]

That is the cabin vacuum supply line. It supplies vacuum to the HVAC and the parking brake release.

[PAUL T]

If you did not take the intake off and clean the gasket and block surface you still could be leaking vacuum. Dirt could have been sucked in under the intake causing the gasket not to seal. I would take the intake off clean the gasket and mating surface then reinstall. I know my mileage jumped from 18-19 mpg to 25-26 mpg average.

[Luke]

If you did not take the intake off and clean the gasket and block surface you still could be leaking vacuum. Dirt could have been sucked in under the intake causing the gasket not to seal. I would take the intake off clean the gasket and mating surface then reinstall. I know my mileage jumped from 18-19 mpg to 25-26 mpg average.

Okay, I pulled my intake manifold off yesterday and replaced intake gaskets and throttle body gasket. I thoroughly cleaned the gaskets surfaces and cleaned my throttle body so well you can eat off it. I also replaced the two short rubber vacuum hoses in the engine bay by the right strut tower as those were getting very bad. I started the engine with air conditioning on and still the RPM's bounce between 625 and 950. It's sporadic but still does it some. I'm not sure where to go from here. All I can figure is that there is a vacuum leak inside the dash. Not looking forward to digging for that.

There was a time when I got 29-30 mpg with this car and now I am lucky to get 21-22.

[BodybyFisher]

I did not see that you have checked for DTC codes, have you checked?

A bad TPS or IAC can do this, check the codes. Clean the IAC, check the throttle body gaskets and seals.

[Luke]

I did not see that you have checked for DTC codes, have you checked?

A bad TPS or IAC can do this, check the codes. Clean the IAC, check the throttle body gaskets and seals.

Yes, I have checked for codes and none exist. I'm thinking you're right and either the TPS or IAC is bad. I was reading another post about a MAP sensor and someone said that 30-50 kPa at idle is normal. Well, mine is about 50 kPa. Apparently a vacuum leak isn't my problem.

I had my throttle body off yesterday and put in a new gasket. The IAC isn't dirty. One thing I noticed is that the TPS is not spring loaded which seemed odd to me. I checked one at the parts store and it was spring loaded. Perhaps it is the problem.

[BodybyFisher]

Typically a bad TPS will set a code.

So your main problem is that your idle is ranging up and down?, sort of a surge? Have you cleaned your throttle body throat and plate?

Maybe since you dont have codes, you do have a vacuum leak and the IAC is trying to control it.

[BodybyFisher]

How about this

Rough, Unstable, or Incorrect Idle and Stalling

Fuel System

Perform injector balance test.

Check for low fuel pressure.

Check for restricted fuel filter.

Check for contaminated fuel.

Check for incorrect minimum idle speed.

Check each injector harness is connected to the correct injector/cylinder according to the firing order. Relocate injector harnesses as necessary.

Check for injectors that are shorted internally. Compare injector resistance.

Check for fuel in pressure regulator vacuum hose. If fuel is present, replace regulator assembly.

Check for foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft. Also inspect for throttle body tampering.

Check items which can cause an engine to run rich (long term fuel trim near -99%).

Check items that can cause an engine to run lean (long term fuel trim near 100%)

Sensor/System

Check Crankcase ventilation valve for proper operation. Place finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.

Using a Scan Tool, monitor TP sensor voltage. The TP sensor voltage should be less then 0.9 volts at rest. If the TP sensor voltage is greater than 0.9 volt, inspect the throttle body and accelerator cables for binding.

Using a scan tool, monitor KS system for excessive spark retard activity.

Check the EGR system for proper operation

Ignition System

• Check for proper ignition voltage output with spark tester J 26792.
• Remove spark plugs and check for the following:
  • Wet plugs
  • Cracks
  • Wear
  • Improper gap
  • Burned electrodes
  • Heavy deposits

• An improper spark plug gap will cause a driveability problem. The spark plug gapping should be done using a wire gauge gap tool (J 41319). If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
• Visually/Physically inspect Secondary ignition for the following:
  • Ignition wires for cross firing.
  • Ignition wires arcing to ground.
  • Ignition wires for proper routing.
  • Ignition coils for cracks or carbon tracking.

  [*]Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.[*]Check for loose ignition control module ground

Additional Checks

Check exhaust system for possible restrictions. Check for the following:

• Inspect exhaust system for damaged or collapsed pipes.
• Inspect mufflers for heat distress or possible internal failure.
• For possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine.

An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits).

Check PNP switch circuit and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive

[BodybyFisher]

With all of the above, have you checked the adjustment of the TPS, minimum air, etc? Not sure if the TPS from 96 forward is adjustable but the minimum air is.

[Ranger]

Check the PCV grommet for a nice tight fit.

[Terrence Mayes]

On 8/2/2010 at 9:19 PM, BodybyFisher said:

With all of the above, have you checked the adjustment of the TPS, minimum air, etc? Not sure if the TPS from 96 forward is adjustable but the minimum air is.

How do you set the minimum air

[BodybyFisher]

17 hours ago, Terrence Mayes said:

How do you set the minimum air

I took a second look at this comment I made and reviewed the service manual.  I don't see a minimum air adjustment and am not sure what I was referring to.  

If the throttle body or throttle position sensor is replaced there is a Throttle Bidy Assembly (TP Sensor Learn Procedure) in the manual on page 6E-443.

What are your symptoms?