Gas Mileage problem

[Maymybonneliveforevr]

I was getting approximately 500 miles per tank which equates to approximately 34 mpg highway. Immediately after doing the intake manifold gaskets I now get 406 miles per tank highway which equates to 29 mpg. Around town it's much worse. The car tends to run well but I do notice now that my boost gauge no longer rest at the bottom left hand, which would be -7. Keep in mind that this is a Canadian car so that would be what............-10 in the US? While at idle and in gear it's at approximately -5, which would be your -7.5? I've checked several times for vacuum leaks and couldn't find any. I also notice that when I put my foot on the brake pedal the boost gauge moves a bit more towards center. Any ideas?

[bill buttermore]

Double check anything you may have disturbed in changing out the LIM gaskets. I would take a close look at the vacuum lines and fittings looking for a leak, and maybe check the brake booster to make sure it holds a vacuum.

[Maymybonneliveforevr]

Thanks for the response Bill, I triple checked each vacuum line as well as the brake boost and can't find any leaks. I did a visual check as well as with brake cleaner spray and found nothing.

By the way, it wasn't my imagination that the boost gauge used to sit at the left lower end while in gear before was it? l believe that would be your -10?

[clm2112]

Hmm.. unless there is something going on with the boost valve or it's controller, I would also lean towards a vac leak or egr gasses getting into the intakes.

The gauge reports manifold vac, so after your LIM gasket work, you are seeing less vac across the board. The motor's displacement hasn't changed, nor has the cam, so it must be trying to draw the same displacement...yet you are seeing less vac with the throttle plates closed. It has to be getting air in from somewhere.

If the BCR doesn't open to allow the boost pressure to recirculate, would the gauge show less vac at idle?

[Billha]

Yep, sounds like a vacuum leak to me.
In addition to checking all vacuum lines, including those to the charcoal canister, also check the Boost bypass vavle and the PVC plate gasket on the supercharger.

[Maymybonneliveforevr]

The items that you mention were never touched during the LIM change but I will check it just in case.

Thanks

[Maymybonneliveforevr]

In checking for the vacuum leak I did notice one line that was not connected to anything at the rear of the engine. Please see photo for more detail. It is coming off of the black plastic small canister on the rear side of the motor and exits with a 90 degree rubber fitting. Is this a vent tube or should it be connected somehow? I didn't see any unconnected vacuum line that it could connect to. Sorry but I don't know how to photochop or hightlight. In both photo's it's in the center.

[sonoma_zr2]

here is a little diagram

[Echo SSEI]

Frank, that black cansiter is your EVAP purge vent switch/actuator. The line that is going to nothing, actually goes to nothing. Looks normal to me and to other '97 L67s I've seen. I can't recall exactly if the curved, 90* fitting goes to the EVAP purge valve (the rectangle black box near that canister) or if it goes to the hard line that connects to the throttle body.

[Maymybonneliveforevr]

Travis and Bob, according to the drawing everything off of the evap goes somewhere but Bob you mention that everything looks ok?

So Bob, if I understand correctly, that black hose off of the evap that is touching the spark plug wire plastic shielding is supposed to go nowhere and be left open.

If so then I still need to find my vacuum leak.

[Echo SSEI]

Correct, Frank. What is not shown on Travis' diagram is the EVAP vent switch (the canister in your pic), only the EVAP purge valve (rectangle box). From what I have seen, that unconnected hose is a vent to the engine bay and is not connected anywhere. The diargam in my FSM shows the connection of the 90* hose you mentioned, and that is it. I know that I and a couple other '97's I have seen, all have that little hose just sticking out connected to nothing. I plan on doing some more research in the FSM, and I expect that I will find out one of two things. One, that under some, certain condition, that switch opens and "vents" EVAP vapors to the engine bay OR, two, that it draws in air from the engine bay into the EVAP system. I'll let you know when (or if) I find some text that explains it and if I can decipher the engineering jargon and figure it out.

[Maymybonneliveforevr]

Thanks for the reply Bob. I've left that hose as is and will now concentrate my efforts elsewhere.


Let me ask though, is it normal for a boost gauge to go up slightly when you put the car in gear and even higher when you turn on the A/C while at idle? Due to the fact that the boost gauge is not resting at the lower most negative spot during idle, this leads me to believe that it has a vacuum leak, that and the fact the gas mileage has decrease approximatly 20% after the LIM change.

[Echo SSEI]

Let me ask though, is it normal for a boost gauge to go up slightly when you put the car in gear and even higher when you turn on the A/C while at idle? Due to the fact that the boost gauge is not resting at the lower most negative spot during idle, this leads me to believe that it has a vacuum leak, that and the fact the gas mileage has decrease approximatly 20% after the LIM change.

I honestly don't know, but mine does that, too. Only time it sits at the bottom peg is with the A/C off and not in gear.

[Maymybonneliveforevr]

I know where not comparing apples to apples since mine is stock and yours is............well not stock, but what is your average highway mpg Bob?


Oh ya, before the LIM, the boost gauge never moved until we gave it throttle.

[Echo SSEI]

Back when I was stock, I usually got around 425-440 miles on 17 gallons, so around 25 mpg or so, all highway driving at 65-75 MPH. Best I ever got was 552 on about 17.6 gallons, but a lot of that was downhill. Last year I was still getting about 430 miles per tank during a trip, all highway, but quite a few hills. Not sure what I am now getting, but will soon find out. I'd be tickled pink if I got what you are getting now.

[Maymybonneliveforevr]

Best I ever got was 552 on about 17.6 gallons, but a lot of that was downhill.

That being the case then 500 miles per tank was easily obtainable, if I read that correctly. Another 52 miles per tank is about 3 more miles per gallon, sounds like you were gettiing what I did.

[Echo SSEI]

That was the BEST I ever got, and I have only ever seen 500 from one tank, once, heading home from WCBF '05 from Willows, California, to Pasedena, California. Again, lots of downhill and only one real climb into the LA area. The only other times I have had ALL freeway miles on a tank has been my other trips to WCBF over the last few years and each year, every one of those trips, I got about 435-445 on 17.5-17.7 gallons (of an 18 gallon tank) between fill-ups. I am not really sure why your mileage has dropped after the LIM gaskets. Has anything else changed? The place where you are buying gas? The blend of gasolines (my gas blends change twice a year for polution controls in October and March). Maybe the weather is affecting your mileage? Only other obvious thing I can think of that would affect mileage (but not vacuum) would be if the 02 sensor is failing.

[BillBoost37]

Did you check the EGR tube as suggested? Those are prone to cracking near the LIM and it was moved during the LIM gasket change.

[Maymybonneliveforevr]

I did a visual without removing the thottle body and couldn't find anything out of the ordinary.

[Maymybonneliveforevr]

The EGR tube has been JB welded since it was found to have a leak and I believe there are no leaks at the moment but a replacement has been requested. Further to this thread a coil has gone bad and replaced. We will monitor the gas mileage to see if there is any change and at the moment I can't remember if the boost gauge has now gone back down to the most negative boost.
I'll update soon.