Calling all experts and guru's
 

Ok so I've seen this problem on here before but no real diagnosis or solution. Three months ago I bought a 2001 Super Sport convertible with 127000 miles I changed all fluids to Royal Purple engine,trans,diff,power steering even purple ice instead of coolant also replaced the motor mounts the alternator a new K&N Filter for the SLP cold air spark plugs thermostat 4 New tires and new AC compressor and condenser OEM... once all the work was done I started using it as my daily driver and man it drove like a new car the engine runs strong with no tapping or knocking or any problems whatsoever the engine truly feels 100% now here's a problem I'm getting about 140 miles for an entire tank and that's driving super conservatively and not going Full Throttle once...my current full tank gave me about 120 so I'm basically averaging six to eight miles per gallon on a car that supposed to get I think it's 16 City 28 Highway... something is obviously wrong but the car is running so well and theres no check engine light on which is also why I have no idea where to even begin any help would be greatly appreciated

thanks in advance

Does the car have any mods? I mean anything, including different wheels/tires (what size are they?), exhaust? intake? possibly a cam?

What transmission, auto or manual?
If you are an auto what rear gear do you have? (check RPO codes on door) and what RPM are you at when at ~60mph?
Ive seen this before with newbie V8 owners with manuals that came from driving imports and think they need to be at like 3000rpms when cruising because that is what they are used to (gutless engines).
If thats the case you need to drive at as low of an RPM as possible, these things have gobs of torque everywhere, use it to your advantage and drive at low rpms. Cruise at 1000-1500rpms and get into 6th gear by ~50mph.

6-8 mpg is ridiculously low....except possibly for a high HP drag car. Even 426 Hemi's from 1966-1971 got better mileage than that. Your car could have a rich tune or be sending excessive fuel down stream. I'd look into PCV valve for starters. Other choices would be each part of the fuel management system. Something appears to be acting as "stuck open" and dumping fuel. If you're maintaining typical rpms at 1500-2200 for around town driving and shifting as appropriate....15-18 mpg would be normal for a stock vehicle. I find 6th speed in an M6 to be essentially unusable at less than around 60-65 mph (1360-1475 rpm)....unless you're running on a dead flat highway for long periods at a time.

Well, the fuel is going somewhere. Assuming you don't have a leak, and assuming you're not cruising around at much higher than normal rpms, then something isn't right.

Do you have access to a comprehensive OBD scanner? I'd want to look at some data, such as fuel trims specifically. The front O2s may be having issues that don't yet throw a code but still result in poor MPG. The MAF may need some cleaning. There could be a leak in the PCV system as mentioned above (the lines and rubber boot for the valve tend to deteriorate on the higher mileage examples.)

One last thing to consider - when you say 120-140 miles for a "full tank", how many gallons will it take when you refuel after those miles? I wonder if the gas gauge is giving you bad info. Your 2001 has a 16.8 gallon tank.

Get rid of the junk K&N filter, put a clean paper filter in it and clean all the oil and dirt off the MAF sensor that the K&N put on it. Check all the PCV piping for deteriorated rubber hoses or broken plastic piping causing a vacuum leak. Especially where it runs behind the intake manifold and where it connects to the front.

Bad maf or 02s. I had an old 305 tpi motor with a bad maf that got 80 miles a tank.

123456

First thing comes to mind is oxygen sensors. You got anything you can hook up to read your fuel trims? If not, find a friend with HPTUNERS to see what your fuel trims and O2 voltages are.

Check your brake calipers. Make sure they're not dragging. Lifting the front and spinning the wheels should be good enough. Same for the back, but both tires gotta be off the ground.

What do you mean by this. Did you take the coolant out and replace it with only their coolant additive???


Many of us have been suckered by K&N's propaganda, (I am one) and had their filter nearly ruin our cars. Do some research on how it fouls the MAF sensor and you'll be looking for another person to push this off to...


I get anywhere from 8 to 29 MPG on a stock car. 29 is on the highway at 55 mph on a flat road with a fresh tune-up and carbon cleaning. 8 is with a dirty engine and REAL city driving in a dense urban environment.

As stated above, we need more specific numbers on gallons, etc. to confirm what mileage you area actually getting and need to understand more about your driving conditions.

With 127K miles it just might be best to change out your O2 sensors, clean MAF, replacing PCV valve and line, etc. You could also check the CAT inlet/outlet temps with an IR heat gun to ensure it's not fouled. While a blown or clogged fuel filter wouldn't cause excessive fuel use, I'd inspect it and change if it looks old. Could be the factory unit, especially if you can't make out the part # on it (I think my orig one was an AC Delco BF 621 - and not fully legible). The newer ones are 578.

If you still have the stock power steering cooling in the upper radiator hose, that needs to come out before power steering fluid finds its way into your coolant system. That's not fun to fix.

The Purple Ice coolant additive sounds similar to Red Line's water wetter. Both are good to have if you're running straight water on the track/drags. But for a normal street car I see little use in it with today's higher tech OAT/HOAT/PHOAT coolants....GM's DexCool is an OAT. While a coolant additive claiming to reduce surface tension might seem to make your coolant temps run a bit lower (I used to use that stuff in my '68 GTX which ran hot)....that might be fools gold. It's not the coolant temps we're trying to keep low, it's the cylinder wall metal temps we're trying to keep low....even if that means slightly hotter coolant temps. That "observed" lower coolant temp could be a result of high cylinder wall temps where heat is not being radiated away as fast as before. No way to know without installing thermocouples in the engine metal walls and bearings.

Replace both front oxygen sensors and go back to a stock paper filter. Working a parts counter it's ridiculous how many times I see a maf code from a freshly cleaned k&n filter.

My gauge does seem to be a bit off cause when it says I'm completely empty I fill up with 15 gallons meaning I still had 1.8 in the tank so my gauge might be a bit off but even at 15 gallons 140 miles is nothing.

wouldn't those parts failing make the check engine light come on? And if they were bad would they decrease the mileage by that much and still run so well?

purple ice can be added to coolant or by itself with water... I got 152 miles on 15.4 gallons 70%highway and i baby the throttle the whole tank

You just took it to a whole new level lol

I want to thank everyone for giving their 2 cents....i think I'm gonna start with replacing the O2 sensors PCV valve and cleaning the mass air flow hopefully it'll get me somewhere...ill keep y'all posted

Might be a stupid question but...when reading the fuel trim and O2 sensors what numbers are we looking for ? .

Excellent question.

When oxygen sensors begin to fail, they read low voltage. If it's not too low, the ECU tries to add fuel to bring voltage back up to 450 mV.

So, you'd be looking for high positive short term and long term fuel trims with O2 voltages still persistently lower than 450 mV or even centered on 450mV.

Point is that the ECU would be dumping fuel.

If you are only getting 6 miles per gal I'd say your car would smell like fuel coming out the tail pipes and your cats would be melted into cat heaven. How are you actually calculating your mileage?

Worse tank ever I've got an 1/8 tank left and only 81 miles driven!!!!