Fuel economy low on OBD I car - Gremlin?

[The_Eric]

I've been chasing my tail trying to find the cause of the lower than expected/normal fuel economy in the wife's car. It's a 1995 Grand Am with the 3.1 V6, which was rebuilt recently (4k miles and some change) and the details of the build and back story can be viewed HERE. The only other thing not mentioned in that thread is that I replaced the fuel pump with a Delphi unit because the Carter replacement was making intermittent noises. The drop in fuel economy started before replacing the engine, but I chalked it up to old or bad sensors/high mileage and didn't investigate, knowing the engine would be replaced soon.

Once the new engine was in place, the mileage never came up. She swears up and down her car used to average 27-30 mpg in combined driving (approx 80% hwy) and now it's getting 22-25. It's achieved 25 only a few times and 26 once. Otherwise it's 22-23mpg over the course of 4k plus miles in fairly consistent conditions.

I've been waiting to purchase my latest toy (Snap On Solus Ultra) to go over the various values hoping to spot the darn gremlin... Now that I have it, I still can't find anything too out of the ordinary, but I do have a few questions...

-Fuel trim. I'm not very familiar with the older systems. I know that it's based on a range of 0-256, with 128 being neutral. Going down the road, cruise on 55, fuel trim is around 125-130ish, so pretty normal?

-LT trims - hovers in the high 1teens to low 120's, so around 117 to 123ish. Looks like the computer is trying to take some fuel out.

- Trim Cell????? No clue.. Any input?

-Throttle position is only around 12% cruising at 55mph, so it doesn't appear to be laboring.

Other sensor values appear normal. Intake air and coolant temp are accurate within reason and the o2 sensor shows lots of activity. Switching to graph, it appears to have an even mix of rich and lean counts.

My only other observation is that the exhaust does have a strong odor to it, but I don't know that it means too much because the car has no catalytic converter.

 

[johnachak]

I know when we went to 105 gasohol my mileage dropped approximately 2-3 mpg and the idle roughened. Even in my 2006, when I visit family in SC and use real gas I get a better feeling idle and a couple/few more mpg's on a tank.

 

[Rand]

its winter time.

 

[mechanicx]

22-25 mpg in mixed driving does not sound far from normal for a V6 especially in Winter time. I think these cars can approach high 20's mpg strictly freeway. But with city driving very low 20's is pretty common. Similar cars I've seen would drop under 20 mpg in city driving in Winter with E10 gas.

I think your scanner values seem normal. It's possible the O2 sensor or plugs are not causing a drivability issue or showing problem on a scanner but could effect milage. I don't know if these have been changed. Sometiems differnt tires can drop milage some.

 

[chrome]

Once you have checked the obvious (sensors etc.), check also that the torque converter is locking up properly in the higher gears. I am not sure if OBD1 will send a code if a TC doesn't lock up, but driving with it unlocked all the time can affect fuel economy.

 

[The_Eric]

I can't verify, but like I said, the better half is sticking to her guns on this. She has always ran E-10 (170k+ miles) and still claims that in spite of it being winter (I educated her on loss of MPG in winter), she is saying the same thing. Maybe it's possible she's dreaming... IDK...

Plugs and wires were replaced with the engine swap (correct # Delco Pros and Denso wires) and the O2 sensor was replaced shortly thereafter because I was afraid that coolant from the blown head gasket may have damaged it.

chrome: good call, I didn't think of that! My scanner does have a pid value for TCC slippage. I'll check that and report back. Any idea what is considered acceptable?

 

[mechanicx]

I hate to state something obvious that you might have checked already but as you know ocassionally a brake can drag without showing much symptoms other than running hot. I'm guessing there's probably nothing wrong with the car that's not FI related that would effect gas milage or you'd spot something pretty obvious

.

 

[Rand]

she is dreaming
summertime 65 on highway it might approach 30mpg

 

[jetmech1]

Here's something to consider. It is something I read awhile back it made sense to me. I think it was either in one of my performance mags or an AA1car.com article.

I've read through you old thread on the swap. It would make sense that the mpg was dropping in the old engine and agree with your reasoning. There is a possibility that the mpg never came back is because you deleted the cat converter. Here is the reasoning given.

It has to to with volumetric efficiency. When you deleted the converter, you lowered the back pressure. When the back pressure is lowered, there is less "pressure" in the cylinder at TDC of the exhaust stroke thereby creating less distance the piston has to move down the cylinder (on the intake stroke) before it starts to draw in the new charge of air. Since you are able to draw in more air, the computer is naturally going to adjust (read increase) the fuel charge to compensate and MPG drops. Everything will still look normal on the readouts because the change is still within the operating parameters (hence no CEI) even though you'll have a net loss of a few mpg's.

It's a little counter-intuitative because one would think that the more "efficient" an engine breathes, the better the mpg. This seemed to hold true in the old '70's & '80's smog engines, but not anymore. Those were heavier carburated vehicles and freeing up HP allowed the vehicle to run at less throttle and get better mpg especially on cruise control. My son installed a cat-back flowmaster on his vehicle and it's mileage dropped. I've had several others tell me similar stories when they gutted their converters or installed a "cat-back" systems. It appears nowdays that when you alter anything on an OBD-I or OBD-II, you'll pay a penelty. Anyway, something to consider........

 

[NHGUY]

I thought the thread was about an AMC Gremlin! lol

 

[The_Eric]

Originally Posted By: mechanicx
I hate to state something obvious that you might have checked already but as you know ocassionally a brake can drag without showing much symptoms other than running hot. I'm guessing there's probably nothing wrong with the car that's not FI related that would effect gas milage or you'd spot something pretty obvious

.


You're right, I did think of it and I forgot to mention it... My bad. Brakes (calipers, pads and rotors) were replaced 2 years ago, and then much more recently I replaced the front left (caliper) because of unevenly worn pads and warped rotor, as well as both rotors and new pads. I've since checked them with an IR thermometer and they're both very close and still no change in MPG.




Originally Posted By: jetmech1
Here's something to consider. It is something I read awhile back it made sense to me. I think it was either in one of my performance mags or an AA1car.com article.

I've read through you old thread on the swap. It would make sense that the mpg was dropping in the old engine and agree with your reasoning. There is a possibility that the mpg never came back is because you deleted the cat converter. Here is the reasoning given.

It has to to with volumetric efficiency. When you deleted the converter, you lowered the back pressure. When the back pressure is lowered, there is less "pressure" in the cylinder at TDC of the exhaust stroke thereby creating less distance the piston has to move down the cylinder (on the intake stroke) before it starts to draw in the new charge of air. Since you are able to draw in more air, the computer is naturally going to adjust (read increase) the fuel charge to compensate and MPG drops. Everything will still look normal on the readouts because the change is still within the operating parameters (hence no CEI) even though you'll have a net loss of a few mpg's.

It's a little counter-intuitative because one would think that the more "efficient" an engine breathes, the better the mpg. This seemed to hold true in the old '70's & '80's smog engines, but not anymore. Those were heavier carburated vehicles and freeing up HP allowed the vehicle to run at less throttle and get better mpg especially on cruise control. My son installed a cat-back flowmaster on his vehicle and it's mileage dropped. I've had several others tell me similar stories when they gutted their converters or installed a "cat-back" systems. It appears nowdays that when you alter anything on an OBD-I or OBD-II, you'll pay a penelty. Anyway, something to consider........



Interesting and definitely food for thought. My boss had mentioned this before but could never articulate anything that sounded like a well reasoned answer, so I dismissed it.

I does make sense since this particular engine has no mass airflow sensor to note the potential increase in airflow.

Now, how to test?

 

[cchase]

Originally Posted By: Rand
she is dreaming
summertime 65 on highway it might approach 30mpg


We average 32-34 with our vehicle which is functionally identical. I would say 22-25 is pretty low for highway. Even in the winter we stay above 30 for 65-70% driving.

 

[eljefino]

Can you get knock retard out of that scanner? I think something is off in the engine making it pull timing and hurting your power. 12% throttle is actually a lot IMO at cruise.

I had a 2000 century 3100 that I hoped would get great MPG but blame its piston slap for getting it down near 25 driven very gently.

I'm not one to compare it to others online, I compare it to EPA estimates with my own driving style which usually exceeds those estimates handily.

Modern cats are not restrictive at modest power levels, you can look right through the honeycomb! I don't really see that being a factor. (A universal from rockauto is $40, clamps in, and will keep you out of the 1:1000000 chance of trouble with johnny law)

 

[OVERKILL]

Originally Posted By: NHGUY
I thought the thread was about an AMC Gremlin! lol


Thank you!! I thought I was the only one! LMAO!!

 

[lexus114]

Originally Posted By: OVERK1LL
Originally Posted By: NHGUY
I thought the thread was about an AMC Gremlin! lol


Thank you!! I thought I was the only one! LMAO!!





I thought so too.

 

[Vikas]

Originally Posted By: eljefino
Can you get knock retard out of that scanner? I think something is off in the engine making it pull timing and hurting your power. 12% throttle is actually a lot IMO at cruise.


Really? What would you expect to see at cruising on flat road?

 

[mechanicx]

Originally Posted By: jetmech1
Here's something to consider. It is something I read awhile back it made sense to me. I think it was either in one of my performance mags or an AA1car.com article.

I've read through you old thread on the swap. It would make sense that the mpg was dropping in the old engine and agree with your reasoning. There is a possibility that the mpg never came back is because you deleted the cat converter. Here is the reasoning given.

It has to to with volumetric efficiency. When you deleted the converter, you lowered the back pressure. When the back pressure is lowered, there is less "pressure" in the cylinder at TDC of the exhaust stroke thereby creating less distance the piston has to move down the cylinder (on the intake stroke) before it starts to draw in the new charge of air. Since you are able to draw in more air, the computer is naturally going to adjust (read increase) the fuel charge to compensate and MPG drops. Everything will still look normal on the readouts because the change is still within the operating parameters (hence no CEI) even though you'll have a net loss of a few mpg's.

It's a little counter-intuitative because one would think that the more "efficient" an engine breathes, the better the mpg. This seemed to hold true in the old '70's & '80's smog engines, but not anymore. Those were heavier carburated vehicles and freeing up HP allowed the vehicle to run at less throttle and get better mpg especially on cruise control. My son installed a cat-back flowmaster on his vehicle and it's mileage dropped. I've had several others tell me similar stories when they gutted their converters or installed a "cat-back" systems. It appears nowdays that when you alter anything on an OBD-I or OBD-II, you'll pay a penelty. Anyway, something to consider........



I think the most likely scenario for why removing the cat would instead of improving milage reduce it might be in the EGR. Less backpressure could mean less EGR and so the computer is pulling more ignition advance. I've heard of cars losing gas milage when the EGR is uhhooked or inoperative. If Eric can check for knock sensor retard that might be the telling factor. If there's no timing being pulled then I don't know.

 

[jetmech1]

Quote:
I think the most likely scenario for why removing the cat would instead of improving milage reduce it might be in the EGR. Less backpressure could mean less EGR and so the computer is pulling more ignition advance. I've heard of cars losing gas milage when the EGR is uhhooked or inoperative. If Eric can check for knock sensor retard that might be the telling factor. If there's no timing being pulled then I don't know.


This is also correct. Here's the explanation for that from a online source:

"The purpose of EGR (Exhaust Gas Recirculation) is to reduce the NOx emissions. Air is mainly made of oxygen and nitrogen (O2 and N2). At temperatures above 1300°C (2372°F), these molecules split apart and rejoin with each other to make nitrogen oxides (like NO, NO2, etc...). The nitrogen oxides contribute to smog formation.

EGR puts a portion of the exhaust gas back into the intake manifold, so it mixes with the fuel and air. (Note that the exhaust adds to the fuel and air; it doesn’t replace any of it). The added mass in the cylinder is harder to heat up, so the combustion events have lower peak temperatures. The lower temperatures prevent the O2 and N2 from splitting and combining. Even though the exhaust is hot, about 600°C (or 1112°F), it's much cooler than the 1300°C required to make NOx.

In summary, the exhaust adds mass, increasing the heat capacitance of the mixture (i.e. making it harder to heat up the mixture in the cylinder). Peak temperatures are lower, reducing NOx formation, which ultimately reduces smog in the environment.

The reason EGR improves fuel economy is because it reduces the engine's pumping losses. For the cylinder to move down on the intake stroke, the piston is working against the intake manifold vacuum. Another way to say it is that the vacuum above the piston tries to prevent the piston from going down. EGR increases the mass in the intake; more mass means higher pressure, or less vacuum. Now the piston has less resistance during each intake stroke, which results in better gas mileage."

Note: EGR valve is supposed to be closed at idle and WOT. Highway cruise is when it operates at it's maximum effect.

 

[mechtech2]

She did not get 30 MPG in winter with E10 with that car.
Maybe you should not argue with her about that!

Any tire size changes? Is the pressure OK? How about brake drag? And alignment is a hidden but crucial factor!

 

[mechanicx]

Since there's no cat The_Eric should now just tune the prom for lean cruise maybe 17:1 AFR, this will lower cylinder head temperatures, allowing blocking off the EGR and allowing more ignition advance timing. Then it'll be getting 30 mpg

.