Ok so I have a 2001 monte ss non supercharged that I bought 6 months ago. It has 42,000 miles on it and I love it. I'm gonna let you know what I've done to it (no real mods) so perhaps someone can suggest something that I've overlooked. It has the full cat-back borla exhaust system (which was already on the vehicle when i bought it). I did the LIM gasket and all that's above it a week after I bought it, including a new plenum (fuel injectors looked clean). New spark plugs, wires, fuel filter, k&n cai, tie rod ends... and I'm replacing all 4 strut assemblies next week. Also already ordered a overkill pcm

I keep track of my mileage by watching how much it takes to fill at the pump.. then calculate that with the trip mileage that i reset during refueling. I don't really drive on the freeway at all. But it seems recently I've been getting even worse mileage. I just filled up with 14.4 gallons, and my trip mileage for those 14.4 gallons was 225 miles. Is that normal?

Also not sure if it's related but my "service vehicle soon" light came on twice in the past week. It happened first the day after I was flushing (bleeding) my brake lines.. I held the brake pedal.. when it dropped to the floor my car would drop rpm's and almost stall out..?

If anyone understands what I just wrote and has any advice I'd really appreciate it!

 

my monte has about 3.5 gallons in the tank when it hits E and it hits 220 mi which is about 19mpg i have a 15.5 gallon tank

 

Check out the lights on a scanner. A small vacuum leak can throw things off, so could a bad maf. Might be making you dump more fuel then you need to.

 

The only way to get the light checked would be with a tech2 scanner correct? since it's not a cel? I tried to use a reader at O'reilly auto parts.. But the message wasn't on at the time i checked, and found nothing. The light is intermittent, it didn't come on today at all.

 

Hmm, well the best way is to get it on a hptuners or dhp if you know anyone with one. If not take it to a shop and have them check the AF ratios. If its rich that would explain your issue.

 

1 - Find out what your code is. That could be critical and answer everything. The car is telling you it has a problem, so this is hopefully this is easy.
2 - With the LIM work, you may have a loose, dry rotted, or otherwise not connected vacuum line (basically what Enzo suggested). Again, this can cause a variety of issues, bad fuel economy being one of them.
3 - You mentioned you did plugs. Based on my experience, if the plugs are BOSCH, get rid of them. They will be a problem now or shortly down the road.
4 - In your re-worked exhaust, you still have the factory cat? The '00-'01 W-Bodies had a problem with the stock cats. The supplier for GM had a production problem resulting in cats clogging up. This could be the source of your whole problem with fuel economy (and eventually performance problems, which may be getting covered up by some of the mods).

 

A good scan tool can check current and historical codes. So even if the light is not currently on and active, it will retrieve codes recently thrown. I luckily have access to a Snap-On ODB-II scan tool and it does the trick with no problem. I would image the service at the parts stores should do the same.

HP/DHP tuning scan tools should not be required to just retrieve the error code.

 

Thanks for the reply and help. I'll give ya some quick responses to your thoughts.
1. The message I'm getting is from the message center window not from a cluster icon. I assume you know that so I'm gonna have to try to scan it somewhere else since theirs didn't work for me. (goes with your second post xD)
2. The first thing I did was to look at all the vacuum lines which as far as i can see are all good. Am I missing any besides the ones right on top?
3. I'm pretty sure i got the ac delco ones which I think are the same ones the car comes with?
4. Nope it has a full cat-back borla exhaust with resonator

Thanks for the input!

 

Not that it helps where you are at, but I believe the Snap-On scan tool I have access to can get access to the "Service Vehicle Soon" message light codes and if the little "Check Engine" light pops up. I'm fairly certain an HP/DHP scan tool is not required. Worst case, you can always go to a dealer and have them scan it with a GM Tech II, that should be able to access all those codes. Again, even if the code is not "active", a good scan tool should be able to check the historical data.

As for the vacuum lines, you are correct, there are not many of them. A couple on the throttle body, one the goes to the right of the engine and loops into a little box (I had the loop dry rot and brake apart on my '04 Monte, I had to replace that). And a couple others. Not too many in all. You can try running the car and mist starting fluid around all vac lines and connections. If there is a leak, the starting fluid will get sucked in and the car's idle will change a little. If this happens, you found where you should focus on your leak search.

At this point, without knowing more about the code you are throwing, I don't think there is much we can offer at the moment. BUT again, the mods you did should not have that dramatic of a negative impact (if any negative impact at all) on your MPG.

 

The only reason I mentioned those tuners is that he can use them to check all of the numbers flowing through, its easier then just getting a code.