Heres my Delima:
Before this recent repair, The Monte was performing flawlessly.. Fuel mileage
was normal... (300+ miles to a tank)..

I was loseing coolant from the elbows, deceided to replace both UIM,LIM gaskets. Since it still had the Plastic Lower intake gasket.. with 163,000 miles I figured Id fix it before it fixed me...lol

The repair went flawlessly... The Monte seems to perform even better now..(Probably just me) The Loud pedal feels to be more responsive...

The oil is full, theres no leaks from the R&R of the uim lim... I kept the anti freeze loss to a minimum.. (Maybe 1 1/4 quart loss of antifreeze) Rather than draining the whole system. we pinched off the upper and lower radiator hoses.. had minimal loss of antifreeze via the tensioner removal..
There was no Antifreeze in the Lower intake gasket area...which tells me it was definately the elbows that were leaking.(They were horribly deteriorated) as well as cracked.

Heres the Issue Im now haveing..
There are no leaks anywhere on the vehicle, trans,power steering,oil or fuel.. are all at there full levels...

The mrs drove the car today a total of maybe 8 miles (round trip)
I had Half a tank of fuel in it... Whe she got home..it was down below a quarter tank!

So, I opened the hood checked all the connections with the fuel system..looked under neath at the fuel lines and Nothing...
I then took it to the gas station and filled it up... (thinking it was a faulty guage,sensor etc...
But wouldnt you know it.... I thought maybe $10.00 or $12.00 bucks would fill it...

The jokes on me! I ended up putting $30.00 friggn bucks in it to fill it!

So obviously, I am Stumped on what could have possibly changed by merely putting LIM,UIM, gaskets and elbows on it... To have it useing up so much Fuel.
It idles soothly, The Exhaust doesnt ,look,smell like its running rich...
Theres plenty of power pushing the Loud pedal, At posted speed limits it runs perfectly smooth.. Not even a hint that theres anything wrong with it performance wise..

Has anyone else experienced this issue? if so what was the final resolve?

Refresher:
1999 MonteCarlo Z-34 ... 3.8 series II Non Supercharged with 164,000 miles...
New MAF last spring,along with IAC and TPS. and fuel pump
Oil gets changed regularly at 3months 3000 miles religiously.
I never let it get below 1/4 tank on fuel (Gas)

Thanks in Advance!

 

This is not expert advice, just a guess....

I just have to take a guess here.....I'm going go with the fuel sending unit is bad, it's probably a long shot. But my 99Z eats through gas also, I have't done anything to it since I bought it on e-bay 2 summers ago. I hadn't realy planned on doing much either maintenance wise because it was not gonna be a long term 2nd car.

I put about $25 a week in my tank and by Thursday the needle is struggling to get above a 1/4 tank and I hate running it that low but money is tight. My commute back and forth to work is only about 22 miles a day also. My friend who is a mechanic also said it's missing but it's only at higher RPM's so I just blew off changing the plugs/wires.....I'm sure that may bite me in the *** one of these days, but like I said it's just a beater for about another month or so.....

 

That was the point of why I filled it up..To dbl check the sending unit...Since I put a new pump/sending unit last spring..

Lizzy usually drives it to and from work useing a mere $30.00 bucks a week. She drives 12 miles there and 12 miles home. to and from work.
At the end of the week she still has between quarter to a half a tank at weeks end. Which is mostly highway driving. Its a mile from my house to the highway. and less than a quarter mile from her work back to the highway...

I drove us yesterday to the gun range for practice.. It was just above a quarter tank when we left... Once we arrived at the gun range..about 6 miles from home..I didnt even bother to check the gauge...once we got there...
However, when we left I happen to check the gauge and it was nearly on E in the red...(which I never let it get that low) Haveing made it to the gas station a 1/4 mile away I put $20.00 bucks in it..($3.68 gal) which gave me a little over a half a tank... When we arrived home .. It was right at 1/2 tank.. Which I also thought was excessive.. for the short distance we traveled to get home.

So, I know after haveing this Car for14 yrs that is not its "Normal" Mileage/Fuel usage....
I now wonder if that cracked, Intake Duct has contributed to the loss of fuel mileage..despite it being Taped shut?

 

Best thing you can do is plug it into a scanner and look at the 02 sensor voltage values. Also you can pull the plugs and see if they are black or soot covered, this indicates a rich situation. Also while you have the scanner plugged in I would check to see what the engine temp sensor is showing. If it is not reading accurately, it will cause the computer to stay in open loop mode thinking that the engine is still cold. This is not the same one your temperature gauge reads from either. It will be in the intake manifold coolant passage somewhere. The temp gauge sender is in the head. All of these things should trigger a SES light, but sometimes it won't.

 

Thats exactly what we did last night...

Had the scanner on it...all the O2 sensors (2) were at there proper voltage.
after driveing it un til it reached operateing temps.. My gauge was just about dead on with the scanner..

Plugs look a nice tan color with no spotting.. No black looking soot anywhere tail pipes are arent black ...
Even the exhaust still smells like it did when it was new..(rotten eggs) lol


So obviously.... Im STUMPED!

 

Do you do mileage based calculations of fill-ups? How's the weather been?

12 mile trips at a time can eat up fuel. You would probably be around 21mpgs that way. Figure the car is taking a while to get up to operating temp, so it's running pretty rich until the oxygen sensor is up to temp.

While taking everything apart, you also could have torn an o-ring on a fuel injector. Maybe the injector is just leaking.

Also an exhaust SHOULDN'T smell like rotten eggs.

 

My '98 3.1 was going through the gas like that & occasionally the "Service Engine soon" light would come on & off. I took the EGR valve off & clean it & the ports with Sea Foam spray, then the Throttle body cleaner... Now it gets mileage like it did before & the performance seems to be back... (120,000 miles.)

 

The Temps yesterday were in the upper 20's..
Lizzy hates the cold feeling of leather seats in the Monte ..LOL
During the winter we always preheat the car (auto start)..
So, It is pretty much warmed up before driveing it... (it had heat
and was up to running temp)
When we left for the gun range like I said earlier, I really didnt pay attention to the gas gauge...Once we arrived.. It was when we left(The gun range) that I noticed we were on E... after haveing a quarter a tank before we left.(The house)
We did put new O rings on the injectors which came with the gasket set... Theres no odor of leaking gas (you would definately smell it) I didnt see any signs of any leaks from the injectors as well.

There isnt a CEL .. Thats what makes it so hard to diagnose

However, I did read on the net about the IAT sensor.
My intake duct is cracked. (used electrical tape as a temporary fix)
I recently removed it to take to the boneyard to find a replacement..
(The dealers have discontinued it) So, The boneyard is my only place to find a replacement..
I removed the IAT from the duct in order to not lose it and also to put back in the replacement. I did have semi greasy hands when i removed it and put it in my pocket. Since, The boneyard didnt have a replacement..
I put the duct back on the Monte. Also put the IAT back in it...
I could have possibly contaminated it with my dirty hands...
The IAT is part of controlling the amount of fuel needed to run the fuel system via the Internal (Intake) air tempature going into the Plenum. possibly causeing it to sense that the air is colder and needing it to produce more fuel to run efficiently.
So, i got the idea last night to clean it.. (I used brake clean) since I didnt have any carb cleaner.

The mrs took it to work this morning, Then she informed me that the car was running Great and fuel gauge showed it didnt use any excessive amount of Fuel...
Shes been driveing it to and from work for the last 9 yrs.. So, I would say she has a good idea of how much Fuel it should be useing.
Upon arriveing at work..Which is all highway pretty much..

We generally just set the trip meter as a check for keeping an eye on the fuel mileage...So, I guess after this tank runs out I will know for sure if the fuel mileage is back to normal...

I guess the exhaust doesnt actually smell like rotten eggs... But it does stink...Especially when you pre start it in the garage despite the garage door being open while its running...and warming up...
It tends to warmup quicker in the garage vs being outdoors

Thanks Mike(Chibi) for your help!

 

MPGs in the winter suck. Just a fact of life. Especially with warming the car up first haha.

it sounds like it's nothing serious though, which is good

 

So, Would you say the IAT is a non issue?/ or the taped up intake duct?