Terrible MPG

[Padgett]

OK at first I thought it was just me but just made a round trip to Tampa which I do quite often. My other cars all get 22-26 mpg at 70 with AC on including the Crossie which has lower gears than the GTP.

 

Average (same gas station, same pump) for 183 miles, 16.7 mpg. And this is running very smooth mostly in cruise and 15.4:1 (Highway) A/F ratio enabled.

 

Any ideas ? Can anyone tell me what advance, pulse width,  and MAP you are seeing at 70 in top ? (LQ1/284). Know turning 2500 at 70 isn't helping but Crossfire is turning 2600 and gets 25.

[Nas Escobar]

My LQ1 cruises at 2200 at 70MPH and I get 20mpg if I cruise and not dog the car. I got more distance with my Z34 than my CSC but that's another story.

 

I'm surprised with an extra gear you're not at 1500 at 70.

 

I get 17mpg in my Cutlass mixed but that's when the car drops me to 3rd gear (sometimes my transmission kicks me out of 4th gear).

[Imp558]

Converter locking up? You should be under 2000 rpm on the highway.

[Nas Escobar]

Converter locking up? You should be under 2000 rpm on the highway.

 

Padgett's is a 5 speed.

 

Unless GM put a torque converter on a manual trans and I didn't know about it. I know my Infiniti doesn't have a torque converter.

[Psych0matt]

I usually get around 24-25 under idgaf driving. I can get above 30 if I try.

[Galaxie500XL]

My LQ1 averaged around 26-27 on the highway, around 20 on my daily commute of about 50-50 highway/surface streets. It didn't when I first got it, but did after new plugs, wires, and especially a new OEM oxygen sensor.

[Imp558]

My L67 gets 28-30 highway with the new tranny, the old one had some TCC problems and it was about 24 unless I drafted semis and stuff.

[RobertISaar]

 

^{I'm surprised with an extra gear you're not at 1500 at 70.}

 

 

LQ1 final gear is short, something near 3.7:1 IIRC. 5th gear is a short ratio too, I think near 0.8:1.

 

 

anyways, less than 17MPG? something is definitely up, that's barely beating EPA revised city numbers.

[Padgett]

Exactly and why I'd like to know what others are seeing for a pulse width in cruise. May just change the O2 sensor and be done  but the transitions look good.

[RobertISaar]

what kind of BLM and INT values are you seeing at on flat road at a steady-state 70MPH? going into lean cruise, your INT should lock at 128, but BLM should still be where it was before switching into open loop.

[Nas Escobar]

LQ1 final gear is short, something near 3.7:1 IIRC. 5th gear is a short ratio too, I think near 0.8:1.

 

 

anyways, less than 17MPG? something is definitely up, that's barely beating EPA revised city numbers.

 

So the 5th gear is worthless in a LQ1 5 speed?

 

I always thought LQ1s werent meant to be gas sippers. I average 15-18 mpg 80% of the time. I have never seen anything past 22.

[Psych0matt]

I seem to get better mileage closer to 75-80 than at 70

[Imp558]

I can do 30 miles per gallon all day at 45 miles an hour

[Padgett]

I had a herd of rental Centuries with 3.1s that always got mid twenties on the road. Agree 2500 vs 2000 at 70 has an effect but would not expect an 8 mpg effect. Hate to just throw parts but an O2 sensor is easy/cheap.

[RobertISaar]

So the 5th gear is worthless in a LQ1 5 speed?

 

I always thought LQ1s werent meant to be gas sippers. I average 15-18 mpg 80% of the time. I have never seen anything past 22.

 

worthless? no, 4th gear with a LQ1 car at 55MPH is 2800RPM with 26.3" tires. at 70MPH, you would be looking at 3600. looks like I had the ratios from memory off by a little. the 284 is definitely not an economy minded transmission though.

 

http://www.dohcfiero.com/Gearing%20Getrag%20284.htm

http://efi.moonwell.com/gears.php

 

I seem to get better mileage closer to 75-80 than at 70

I seem to remember Buck saying this before as well... I take it either LQ1 "low" RPM fuel mixing isn't quite where it needs to be or the calibration may have some room for improvement.

[Nas Escobar]

worthless? no, 4th gear with a LQ1 car at 55MPH is 2800RPM with 26.3" tires. at 70MPH, you would be looking at 3600. looks like I had the ratios from memory off by a little. the 284 is definitely not an economy minded transmission though.

 

http://www.dohcfiero.com/Gearing%20Getrag%20284.htm

http://efi.moonwell.com/gears.php

 

I seem to remember Buck saying this before as well... I take it either LQ1 "low" RPM fuel mixing isn't quite where it needs to be or the calibration may have some room for improvement.

 

That's rather interesting. 3500 RPM is what I get in my 94 Cutlass at 70 when I get kicked out of 4th gear. I know the 4T60-E changes into 4th gear around 45mph. So that's why I'm confused as far as the 284 goes. One would think that being able to control the shift points and RPM that you would be able to achieve low RPM at cruising whether or not the trans was built for economy.

[Padgett]

I consider 1900-2000 rpm at 70 to be the "sweet spot". 2500 is not. OTOH 6th in the Crossfire is worse, 2600 at 70 and a 3.2 liter 6 cyl and gets mid 20s at 70. 16-17 is close to what the 400RA in the Judge gets with a 3.55 and 1:1 4th.

[RobertISaar]

That's rather interesting. 3500 RPM is what I get in my 94 Cutlass at 70 when I get kicked out of 4th gear. I know the 4T60-E changes into 4th gear around 45mph. So that's why I'm confused as far as the 284 goes. One would think that being able to control the shift points and RPM that you would be able to achieve low RPM at cruising whether or not the trans was built for economy.

 

with a 3.43 final gear and some torque converter slip(I'm assuming TCC is disabled when 4th gear is), 3500 seems a bit high with a 1:1 gear ratio. if no slip in the converter, looking at around 3100, 400RPM worth of slip at that speed would heat the transmission up fast.

 

I think GM(or somebody within Getrag) was just not too concerned with the 284 doing anything other than taking 7000RPM shifts and having a reasonable for the time cruise RPM.

[Schurkey]

Exactly and why I'd like to know what others are seeing for a pulse width in cruise. May just change the O2 sensor and be done  but the transitions look good.

I have a K1500 that threw a SES light on the highway for two years--lean fuel mixture code.  I dicked with it on-and-off looking for any reason for the fuel mix to be that lean.  I could watch the speed creep up--the short- and long-term fuel trims started going rich at about 45 mph, by 70 the SES light was on and it had set a code.  Eventually, the SES light was on at 55 mph.

 

NOTHING made a difference.  I changed TBI units, I played with the AIR distribution system.  I looked for air leakage into the exhaust system.  I watched the same shit on the scan tool over and over.  O2 crosscounts looked wonderful at low speed, but went to zero as the speed increased.

 

In desperation, I ripped-out the O2 sensor and replaced it.

 

End of problem.  Defective O2 sensor showing false lean exhaust--which drove the computer to richen the mixture--as vehicle speed increased.  From that point onward, I didn't put too much faith in O2 crosscounts as a measure of O2 sensor efficiency.  If in doubt, sh!tcan the O2 sensor and see what happens.

 

For the record, both my auto-trans Euro 3.4s get about 24 mpg at 75 mph.  The '92 used to do better than that, before the engine started knocking from piston slap.  I believe the piston slap is jacking with the knock sensor to retard timing and reduce fuel mileage and power.  The '93 has always knocked since I bought it (used), therefore I believe it would have gotten better fuel economy before the knocking, but I have no proof.  I kinda remember 28 mpg with the '92 at 75 mph, the first few years we owned it.

[Padgett]

Well I remember my 72 tow car getting 15mpg and was turning 3200 at 70 mph (400 4bbl/THM400 and "economy" 3.08 gear).

 

Seems like all of my fun cars have turned too high, typ  2500@70 (1900-2000 is the best for efficiency)  but still get mid 20s (except the Judge)

 

OTOH my Crossie has a 3.25 axle now and have a 2.87 in the garage I want to try, mainly to see if the 4.46 1st gear (284 is 3.36) will become usable (punch it and makes one jump and is on the limiter @30 mph. Usually start in 2nd (2.61) but often bogs.  First is mainly good for malls and teaching someone a clutch since impossible to stall.

 

Think next thing I'll try is a different O2 sensor, have a few new 14s. Am seeing transitions at 70 so no light (saw a O2 go ded at 70 before though).

[pshojo]

I'm just now reading, this is interesting. My LQ1 only got max 22-24 mpg highway.  Mix 18, 19mpg.  I try to do only 65-70 mph.  Maybe i should drive faster?

[19Cutlass94]

Even with my lightly modded 3400, with a very unfinished tune, (tune and engine parts have since changed) and a 284 still managed 27mpg on the highway.

 

 

 

Sent from my iPhone

[Schurkey]

I'm just now reading, this is interesting. My LQ1 only got max 22-24 mpg highway.  Mix 18, 19mpg.  I try to do only 65-70 mph.  Maybe i should drive faster?

Had a fuel pump failure on my '93 while I was 1500 miles from home.  The folks that replaced the pump said it was down to 7 psi when it was running.  It was nearly impossible to start the engine, I had very little success keeping it running below 40 mph.  I spent 8 hours driving 170 miles to Dear Old Dad's home, because the engine would stall and not restart for minutes or hours.

 

OTOH, IF (big IF) I could manage to get the car up to highway speed, it ran pretty good at 70--75 mph.  There was some minor misfiring, but overall, I could imagine a non-car-guy not feeling the surges.  Clearly, fuel demand at 70--75 was less than at lower speeds.  I never tried to go faster than that while the pump was wounded.

 

For many years, I did my best to drive the speed that produced the intake moan.  My two cars show different speeds for that moan, but I think that's an inaccuracy in the speedo.  I get that sound at something like 72--76 mph.

 

As I understand it, the '95 and newer 3.4s don't have that intake moan.  Different tuning of the intake ram length, I guess.

[pshojo]

maybe the intake in pre 95 helped gas mileage