Huge Vacuum Leak and MPG Loss - Need Help

[Meridivs]

One of the '92 Pontiac Trans Sports (3.1L V6 engine) had developed a huge thirst - the fuel consumption had doubled along with a serious loss of power. All ignition components - distributor cap, rotor, spark plug wires and oxygen sensor are new ACDelco's.

ECM is not throwing-up any fault codes but we suspect a vacuum leak since this engine runs better with the air intake covered-up almost completely.

Is there any way to trace and fix this kind of issue? Does anyone have a vacuum diagram for this car - I couldn't seem to find it in the GM Service Manual... How would you suggest we proceed?

[LMP]

Conflicting symptoms.....
Well...because of increased fuel burn, I'd suspect the TBI....there are 2 injectors working together...and I'd investigate if one of the 2 could be blocked opened...throwing a constant fuel flow where not needed. I think you can disconnect the injectors one at a time...so try that to see what happens...

This does not explain why covering up the air intake would make it run better...there is no air flow meter on this engine...

Verify also if the ground side of the wiring to the injectors is shorted to ground...this side of the injector is grounded by the computer when needed....(the +12V is always HOT).

www.avigex.ca/xport/fuelcontrol31.jpg
www.avigex.ca/xport/TBI.jpg
www.avigex.ca/xport/injectors31_1.jpg
www.avigex.ca/xport/injectors31_2.jpg




The other possible cause is the manifold pressure sensor....i'd do a test with this sensor disconnected in case it gives a false "wide open throttle" reading (reading low vacuum = atmospheric pressure = wide open throttle). ONce disconnected the computer should fall into "default mode" ..probably better than a false reading.
Well.. go with these first.... report findings ....and other symptoms..does it have the smell of rich mixture, unburned fuel ???

BY the way...a "huge vacuum loss" with increased fuel burn should give an uncontrollable high idle..which you do not report....so I have doubts about a stray air inatake....

[LMP]

mmm....let me think....isn't your TS fueled with propane ????...in that case all of the above about injectors is no use...except some form of propane throttling is present nevertheless.....
...verify the temperature sensor....if by mistake it is disconnected, it sends a "very cold" message to computer..which then makes the mixture much richer....

[Meridivs]

Thank you LMP. We had a faulty temperature sensor, now it's new and working properly.

Fuel consumption on both gasoline and propane has doubled so there's something causing this.

I'm thinking about putting the computer in the Open loop mode by disconnecting the MAP sensor to see what happens with the mileage and acceleration... Any other ideas on what we can try on this engine?

[WelmoedJ]

Originally Posted by Meridivs

Thank you LMP. We had a faulty temperature sensor, now it's new and working properly.

Fuel consumption on both gasoline and propane has doubled so there's something causing this.

I'm thinking about putting the computer in the Open loop mode by disconnecting the MAP sensor to see what happens with the mileage and acceleration... Any other ideas on what we can try on this engine?

Why not use an ALDL reader? This way you are not wild goose chasing.

[Meridivs]

I already scanned it with ALDL, there's no obvious errors or codes showing so that's a dead end for this issue.

Still working on tracing that elusive vacuum leak. Will keep you posted.

[WelmoedJ]

You could try and trace the vacuum leak using smoke.
No cigar or so, but perhaps you can borrow a smoke generator.
There are such devices specifically designed for car shops to aid in solving vacuum issues.
If I recall correctly there are also spray cans that spray smoke-like stuff for this purpose.

Hope this info helps.

[Jeffrv]

Not familiar with 3.1 engine, but a technique I have used in the past involves starting the engine, and when will idle decently disconnect IAC so computer can not longer control idle. Introduce a fuel along all hoses and intake manifold etc. An unlit propane torch or some flammable spray lubricant such as WD-40 will work; once you have found the vaccuum leak the addition of fuel will cause the idle to change. Of course the usual precautions about risk of fire etc, but if careful can be helpful
Jeff

[autojoe]

a spray bottle with water will also work.....you will notice an rpm change.spray along intake gaskets.

[jamesslcx]

I agree with Autojoe, spray along all connections and intake gasket. You can also use carb cleaner. Do you hear any hissing with the engine running? have you done a fuel pressure test. When was the last time you replaced the fuel filter?

[LMP]

ANyway I contemplate this, I see no physical relation between a vacuum leak and doubling fuel burn.....there is something else....

[jamesslcx]

Lmp, a vacuum leak causes a lean running condition which the computer senses, it then compensates by enriching the fuel mixture. The bigger the vacuum leak the more the computer compensates.

[LMP]

Yes.....but this increased air flow and increased fuel delivery are just like opening the throttle...it would idle very fast............which is not reported
...or could Merydivs come back and tell about it....
..and at cruise speed, it would have no effect at all on overall burn as this stray "addition" would be compensated by decreasing throttle to keep same speed...