poor city gas mpg unless i drive on highway for a bit?

[zedjr10]

I got a 92 ex coupe. I get good city mpg for about 100 miles after i drive on highway for about 60 or more miles. Full tune up and so many other things have been done to car in last year. So it isn't something simple(or maybe it is). When i first bought the car years ago it would do the same thing i realized. I figure it might possibly be the ECU but i don't know. Has anyone else experienced this? It seems after driving on highway for a bit(60 or more miles) the air/fuel mixture am guessing is being set differently. Then if i just drive in city it will wear out and go back to the inefficient air/fuel mixture until i drive on highway again. It doesn't have to be a highway. Just above 45mpg for at least 60 miles. Any help is appreciated.

[F22Chris]

It’s because the engine has to work harder to putt around in the city. On the highway, maintaining speed is completely different than gaining it.

Going 65mph on the highway uses significantly less power. It’s actually pretty impressive the amount of power the little 2.2 has to put out to maintain the 65. You’re really only using about 15-20 of its 115hp to maintain speed. Whereas the stop and go city driving uses a butt load more to get going again.

Objects in motion tend to stay in motion.

[zedjr10]

Originally Posted by F22Chris

It’s because the engine has to work harder to putt around in the city. On the highway, maintaining speed is completely different than gaining it.

Going 65mph on the highway uses significantly less power. It’s actually pretty impressive the amount of power the little 2.2 has to put out to maintain the 65. You’re really only using about 15-20 of its 115hp to maintain speed. Whereas the stop and go city driving uses a butt load more to get going again.

Objects in motion tend to stay in motion.

Yes i know all this. Of course u get more on highway than city. That's not what i was asking. If i just drive in the city i get probably 16 mpg. But if i drive on highway for an hour or so i then drive in city i get better mpg for a period of time. Usually lasts about 100 miles or so. THis has been going on since i bought the car. So by driving on the highway for a period of time i then get better city mpg for a period of time. Not lasting more than 100 miles if i just drive in city. And it is a dramatic shift. 17 mpg if i just drive in city with no previous highway driving. 22-23mpg in city after i drive on highway for at least 40 or 50 miles. Something is causing this as i thought it would go away after all the work i've done on car but it hasn't. The ecu has cycles i know reading driving styles and adjusts the air fuel mixture. I am assuming it reads probably last 50 miles and makes the settings based on that but not sure on how Honda ECU's work. Obviously my air fuel mixture isn't right unless i drive on highway for a while then it makes an adjustment which it will keep as long as i keep driving on highway. Once i drive in city for 60 miles or so it reverts back to something that gives me crappy mpg in city.Like i said this has been going on since i bought the car over 5 years ago and hasn't gotten any better after all this work done to car. My intake manifold i am assuming has alot of carbon deposits on it and this could be a cause. AS highway driving doesn't require much air. But city driving it does. SO maybe cause of the highway driving it lowers the fuel output(for a period of time) which then corresponds more closely to what my intake can deliver for a 14.7 to 1 ratio. This is my best guess to what is going on. Car has 247k miles on it. I did a couple of those seafoam intake treatments a couple months ago but hasn't seemed to help. I am replacing my rings and head in a couple months and then i will also burn off all that carbon that am sure is in my intake. But wanted to see if anyone else has experienced this and what they thought was going on.

[sonikaccord]

How are you measuring "instantaneous" fuel consumption?
How is your driving style after driving on the highway?
Is your engine cold before getting on the highway?
Denver is also 5280 ft above sea level.

If you have a vacuum or exhaust leak, it could affect your part throttle fuel consumption. Causing your O2 to read more oxygen, and therefore, inject more fuel. After driving on the highway for x amount of miles, the long-term fuel trim could be set to add more fuel to level out the air/fuel ratio. Then when you get back in the city, it is trying to lean it back out by reducing the fuel injected. This cycle would obviously repeat.

[Jon]

No, I know what you're talking about.
I experienced almost the exact same thing last winter/earlier this year. Seemed as though the car just performs better after a stint on the highway (I was seeing this after 20-30 minutes @ 60-80mph).
I was driving 60+ miles a day for two months. My car LOVED the highway. I'd say I was driving 90% highway, 10% city. Great MPGs.
At the end of this period I was doing about 50/50 highway/city, getting about the same gas mileage.

Now that I'm seeing nearly zero highway miles, I see the lower MPGs as before and without the better performance I was seeing.

HOWEVER if I go for a cruise out on a country road at 45-55MPH for a 10-20 minutes, I see that pep that was missing and MPGs start to balance out.


I think this all boils down to the ECU's ability to get a better read on how the engine is performing on these highway trips vs. puttering around the city.
The engine is probably running at 2000-2500 (depending on if you have a 5 speed or auto and what cruising speed you're going at) for several minutes. This RPM range is what most people would call the sweet spot for getting good MPGs, efficiency (less harmonic vibrations), and so on.

This is all a guess though. Nothing scientific to back it up.
For all I know my better MPGs and performance could have came from the winter tires I was running during this period, different fuel, cold weather, etc... hell I know the leaky ass Bisi header was detrimental to my MPGs, especially in the city.

MPGs were calculated based on gallons put in the vehicle, miles driven on that tank of gas (tripmeter), and gallons put in it at fill-up. Repeat this process over and over, you can tell when something isn't running quite right.

[zedjr10]

Originally Posted by sonikaccord

How are you measuring "instantaneous" fuel consumption?
How is your driving style after driving on the highway?
Is your engine cold before getting on the highway?
Denver is also 5280 ft above sea level.

If you have a vacuum or exhaust leak, it could affect your part throttle fuel consumption. Causing your O2 to read more oxygen, and therefore, inject more fuel. After driving on the highway for x amount of miles, the long-term fuel trim could be set to add more fuel to level out the air/fuel ratio. Then when you get back in the city, it is trying to lean it back out by reducing the fuel injected. This cycle would obviously repeat.

I have checked for vacuum leaks(on several occasions) and i have none as far as i know. But i like your explanation and a partially clogged intake due to carbon build up could cause same thing, right? Wouldn't it be reverse though? Injecting less fuel and then trying to rich it back out. I am not measuring instantaneous. I wish i knew an easy way to do that.Denver and cold engine has nothing to do with this issue. AS i had this same issue at the beach. I just accepted it then cause i drove alot more highway miles then and i thought this problem would be fixed after doing a shit load of work to this car but it hasn't at all. But now i drive almost entirely in the city. So it has become annoying. So now am thinking i either missed finding the vacuum leak or too little air flow from intake. Maybe i should try finding a vacuum leak while engine is revved up a little. As i can't find one with it at idle.

[oni_cb7]

Isn't this just normal?

All cars get better gas mileage when on the freeway/highway. That's just how cars work. If you are in city traffic, starting and stopping, then the acceleration is taking up more gas. That's normal.


Jon - I hope you are basing your measurements on a full tank. The only way to correctly gauge MPG is to put in as much as you used. The only real way to do this is to top of the tank, unless you can somehow magically measure the exact amount of gas in the tank at any given moment. Or, your explanation was just confusing.

[sonikaccord]

He isn't comparing highway mpg vs city mpg.

He is saying that he notices a difference in city mpg before long runs on the highway vs city mpg after long runs on the highway.

[sonikaccord]

A clogged intake would give you the same effect as a smaller TB/Plenum/Runners/Port/Valves depending on where the buildup is and how evenly distributed it is.

Partially blocked cylinders, where some are blocked more than others would probably be detected as rich by the O2. Because that sensor averages all the cylinders, it may lean out the others and have a rough and uneven powerband.

[zedjr10]

Originally Posted by sonikaccord

A clogged intake would give you the same effect as a smaller TB/Plenum/Runners/Port/Valves depending on where the buildup is and how evenly distributed it is.

Would this be a smaller powerband in lower rpm's like idle to 2k. Can you elaborate. AM assuming if it is clogged a bit it would run rich natively and ECM would try to lean it out as it wouldn't detect as much oxygen as it should. SO then it would lower fuel output, right? This would be more city driving while on the highway it would detect a closer rich mixture to 14.7 to 1. Maybe 12 to 1 or something. This gets a little confusing cause i have no ideal the algorithms the car uses for fuel output. And i have no way of testing instantaneous air/fuel mixture. My explanation really makes no sense as why doesn't it then lean it out more in city. I am sure i have a rich condition and not lean causing this issue. As i can sometimes smell a light raw egg smell and my engine doesn't overheat as it should in a really lean condition. I have oil temp sensor installed on car also.

Originally Posted by sonikaccord

Partially blocked cylinders, where some are blocked more than others would probably be detected as rich by the O2. Because that sensor averages all the cylinders, it may lean out the others and have a rough and uneven powerband.

Car runs pretty smoothly and powerband seems consistent. So i am leaning back to what you said earlier about vacuum leak somewhere. I have checked everywhere except the back bottom of plenum(hard to reach) and the butterfly plate gaskets(2 of them) have been replaced recently so i kind of ruled that out since this problem existed before.

[oni_cb7]

Originally Posted by sonikaccord

He isn't comparing highway mpg vs city mpg.

He is saying that he notices a difference in city mpg before long runs on the highway vs city mpg after long runs on the highway.

And how exactly is he measuring that?

gotta drive a full tank one way and a full tank the other way to see a real difference. I really hope you aren't telling me that he is accurately measuring in real time what his current MPG is. This whole thread is confusing as hell.

If his car smells like eggs, then his cat is bad.

[illinois_erik]

test your air intake temperature sensor (visual check for cracks, melted probe end/remove it), sounds like its "normal" after everything is heat soaked. also for good measure test the ECT sensor.

[zedjr10]

Originally Posted by oni_cb7

And how exactly is he measuring that?

gotta drive a full tank one way and a full tank the other way to see a real difference. I really hope you aren't telling me that he is accurately measuring in real time what his current MPG is. This whole thread is confusing as hell.

If his car smells like eggs, then his cat is bad.

Yes of course. My cat isn't bad. I have tested that. Intake Temp sensor has been replaced and the one i replaced wasn't bad either. ECT sensor has been cleaned and reinstalled. Are we going to get into how i measure this issue. Seems pointless as i am sure in what i am describing. This has been measured many times over years but like i said now that i drive in the city almost 100 percent of time it is annoying as hell and not the way it was intended to be. I am leaning towards some vacuum leak somewhere or even still possible the ECT sensor. I have tried to find a vacuum leak on 2 occasions and can't. I am not sure how an exhaust leak would effect the o2 sensor so not sure on this one. And yes i do have a slight raw egg smell as that is not a surprise as i am pretty sure my poor city mpg is due to my car running rich when cold. Running too lean i would have a hotter running engine which i don't have. I have oil & coolant temp sensor installed in car to know this.

[zedjr10]

Could worn valve seals be causing this. I doubt they have ever been replaced and i have never thought on checking them. IS there an easy way? A lot of the symptoms seem to align with worn valve seals. I think my next project will be replacing valve seals and whatever else in there i should do when doing that.

[oni_cb7]

What is your city mpg currently?

Worn valve seals will cause oil build up in cylinder and burning oil smoke from exhaust.

[Rilas]

If your car is smelling like fouled eggs, you need to check the catalytic converter. You mentioned you already tested this? How did you test this? Considering that your car has most likely never had the catalytic converter changed. On the 5 Accord's I've owned, all had the original catalytic converters in them still. Chances are high that yours still has the stock one in it as well. The extra back pressure could be causing extra strain on the engine in stop and go traffic causing the MPG's to go down.

[zedjr10]

Originally Posted by Rilas

If your car is smelling like fouled eggs, you need to check the catalytic converter. You mentioned you already tested this? How did you test this? Considering that your car has most likely never had the catalytic converter changed. On the 5 Accord's I've owned, all had the original catalytic converters in them still. Chances are high that yours still has the stock one in it as well. The extra back pressure could be causing extra strain on the engine in stop and go traffic causing the MPG's to go down.

My city gas mpg is horrible, like 16 or so. Fuel consumption is just horrible when idling.Cat is fine, tested back pressure and working operating temps. BOth are fine. I think the worn valve seals are messing with the air fuel mixture. Confusing the computer into making it more rich. I read an article about this a couple days ago how the oil seeping thru causes wrong o2 readings.

[sonikaccord]

Originally Posted by zedjr10

Would this be a smaller powerband in lower rpm's like idle to 2k. Can you elaborate. AM assuming if it is clogged a bit it would run rich natively and ECM would try to lean it out as it wouldn't detect as much oxygen as it should. SO then it would lower fuel output, right? This would be more city driving while on the highway it would detect a closer rich mixture to 14.7 to 1. Maybe 12 to 1 or something. This gets a little confusing cause i have no ideal the algorithms the car uses for fuel output. And i have no way of testing instantaneous air/fuel mixture. My explanation really makes no sense as why doesn't it then lean it out more in city. I am sure i have a rich condition and not lean causing this issue. As i can sometimes smell a light raw egg smell and my engine doesn't overheat as it should in a really lean condition. I have oil temp sensor installed on car also.


Car runs pretty smoothly and powerband seems consistent. So i am leaning back to what you said earlier about vacuum leak somewhere. I have checked everywhere except the back bottom of plenum(hard to reach) and the butterfly plate gaskets(2 of them) have been replaced recently so i kind of ruled that out since this problem existed before.

My bad, I thought I responded to this. Clogged intake system would affect your mid range and upper rpm performance due to restriction. If it was really bad and uneven, you'd feel a sort of vibration in the upper rpms.

You are right on the ECU lean/rich mixture, but you are also running in a steady state condition on the highway...blah, blah, blah essentially. Have you read your spark plugs?

Your best option would probably be to pick up A/F gauge of some sort and verify if you really are running rich.

[Rilas]

Unless they are excessively leaking, they will only build up enough oil on the backside of the valves to burn off after sitting 8+ hours. If they are excessively leaking then I could follow that train of thought.

I still feel as though you are grasping at the wrong things hoping it will help. Or possibly are just as stubborn as I am in certain things and are not willing to listen to some sound advice.

Second if there is a rotten egg smell its one of 2 things. Either way your going to need to fix either of them.

1. Battery - Your battery is on it's way out and can give off this smell.
2. Catalytic Converter - You don't live in California, buy an $80 Magnaflow Hi-Flow catalytic converter that is bolt on. No welding of flanges, just cry as you cut your rusted on catalytic converter studs/nuts off. (I can usually get 1 of 3 on average, even with soaking it in penetrating oil for days ahead of time. Re-coating a few times a night, as I have my own garage it can just sit in. Even with a pneumatic impact gun.)

Vacuum leaks can occur in a ton of places on the intake manifold. You have something like 10+ o-rings/gaskets that are at best 25 years old, at worst 28 years old. All of them can be brittle and cracked causing minute air leaks. I've recently been rebuilding an F22A6 to put back into an Accord. I replaced every non paper intake gasket with OEM gaskets. I have absolutely no doubts that the intake is properly sealed. Every gasket that was replaced broke upon coming out due to shrinkage and brittleness. I'm sure some of them were leaking.

You have to remember that the valves on the intake also come apart and have internal o-rings to themselves that need to be replaced as well.

Third if your so overly concerned about things such as an air intake leak, you need to build yourself a smoke tester and come up with some way to provide some air flow to it. Borrow a friends pancake air compressor if possible, if you don't already have your own. This will undoubtedly let you find any leaking spots. (I had an air leak on my 3rd gen TL, and with the air leaks it had, I would have paid thousands of dollars for the amount of time/labor it took to find them all and fix them. I did it myself with a homemade smoke tester, I believe Sonik did the same thing trying to find an air leak on his Prelude.)

Fouth, have you adjusted your valves recently? If so have you verified that it was done correctly if not done by yourself? If not you can check it in a few hours with basic hand tools and a feeler gauge.

[Rilas]

Also read your spark plugs as Sonik mentioned it will tell you exactly what the engine is doing.

Look up some guides online to read spark plugs.