Poor Mileage

Yoda · Post by **Yoda** » February 20th, 2006, 10:08 pm

Before someone ask what's the deal with the leaking exhaust, especially between the engine and cat, has to do with gas mileage and power. Exhaust flow is not a constant flow but is actually a series of pulses. Between pulses is a slight reverse wave which pulls Oxygen rich air from outside the exhaust system which could make the O2 sensor think the engine is running lean there for richning the mixture even though the engine is running stoichiometrically. As we all know to much fuel on the fire kills the flame. When burning rich the engine runs cooler when the ECU sees the coolant sensor temp is lower it advances the timing and adds fuel. The other thing that happens as exhaust leaks out it disrupts and weakings the pulse waves. The stronger the pulse waves the stronger the scavanging effect which in turns equals less exhaust gasses left in the cylinder and in some cases if the conditions are right you can actually achieve positive manifold pressure. Another thing to be aware of is 1 wire O2 are slow and dependant on exhaust gas temp to get it up to operating temp. Trying to read the output voltage at idle is almost useless which is why many engine manufactures recommended running the engine at over 2000rpm for about 5 mins before testing a 1 wire sensor.

projectmx316 · Post by **projectmx316** » February 21st, 2006, 1:56 pm

i get about 300 miles to the tank on a 1.6 dohc with and auto does anyone think that is good ir can it do better plus i have a small exhaust leak.

corbanbrook · Post by **corbanbrook** » February 24th, 2006, 2:05 pm

I figured out that my tank is probably a 10G, i hear everyone says they have 13Gallon tanks. But i was bone dry today and could only squeeze in 38 litres into my tank.

Still getting about 350 kilometres to the tank, or 9 kpl = 21 MPG. Guess thats not as bad as i origionally thought.

Went to PartSource today and bought a brand new air filter, fuel injector cleaner, new fuel filter and new universal O2 sensor. Also bought a litre of acetone to add to the fuel. 2 oz per 10 Gallons of fuel. Supposidly it adds between 4%-20% MPG based on some internet "facts" i came across.

So one big whammy.. next time i put gas in, i will check to see how the MPG is and let you guys know if any of this crap actually did anything.

Read an interesting distributer hack the other day. Venting it and piping the Ozone created as the rotor spins and sparks away on the contact points into the vacumm line into the engine. When the ozone mixes with the fuel it burns it a lot hotter and completely. Guy said he increased his MPG by 4 on one of his cars.

One final question i guess would be, What MPG are some of you getting on your car? I have seen on a new MX-3 4 Cyl, about 25 City and 32 Hwy. combined 29. much better then the 21 im getting.. What is an acceptable goal to aim for on a car with 215,000 kilometres on it?

Thankyou for your posts already.

highlife · Post by **highlife** » February 24th, 2006, 3:48 pm

Definitely post back if you discover anyway to boost that mileage, I will keep checking. I understand how the MPG's drop in cold weather but I wouldnt expect the 6 MPG I am experiencing. I found another thread on here a while back (may have been tunes67) which mentioned adjusting the timing to 12 BTDC and getting better performance, they were going to post back with mileage results but never did. Here's a fuel mileage site for comparison http://www.fueleconomy.gov/feg/sbs.htm

Tunes67 · Post by **Tunes67** » February 24th, 2006, 4:16 pm

I do run my timing on my RS slightly advanced. And it does help with mileage.. slightly.. as for economy.. not so much. See in order to take advantage of the advanced timing.. you HAVE to run 92+ octane fuel.. which costs more cash.. so while you will pick up 15-20 more miles to the tank.. you pay more at the pump. Do not advance your timing if you think you will ever put anything less than 92+ octane gas in your tank. It WILL knock and ping and you could have detonation occuring. Detonation will destroy your rings, pistons and if severe enough.. can damage valves too.

As for the cold weather giving poor gas mileage.. I am of two minds on this one.. For our MX-3's.. its true. I average less per tank in the winter than I do in the summer. However.. I dont believe this has anything to do with the temperature of the air. As the air temperature deceases.. the density of the air increases. The Air Temp Sensor and the VAF & MAF (whichever a car has) will detect this and the ECU will compensate with a slightly richer fuel mixture. However.. this would only consume noticable amounts of fuel during cold starts. And I dont think it would impact mileage as drastically as I see in my car in the winter months.. No.. I think there is another reason..

See with my car.. I started noticing a drop off in fuel mileage BEFORE it got cold here.. which is what convinced me it wasnt the air temp.. but there is one other factor that winter driving brings to our cars besides air that is more dense.. and that is Night time driving. We have our headlights on, we use our Rear window defrosters, Wipers, heaters.. heck we use ALL of our electrical features far more in the winter than we do in the summer.. Why should this matter? Because the ignition systems of our beloved MX-3's were very badly designed.. and as the electrical demand of our cars increase.. there is less juice available for the ignition system. So our spark is weaker.. combustion isnt as complete and as a result.. our mileage decreases since the ECU has to respond by dumping more fuel to meet the demands of our lead feet.

I freely admit that this is speculation on my part. However.. I know with my carbuerated truck.. my gas mileage increased by an average of 2mpg this winter while the air was cold. Granted in my carbuerated truck.. the mixture isnt as directly controlled by the ECU as it is in a fuel injected vehicle.. so.. the colder air provided a denser oxygen content to my cylinders.. this resulted in a leaner air/fuel mixture. Now in my truck.. the Carb has a computer to control the idle mixture of the carb.. but it does nothing for the mixture when the truck is cruising along down the road.

As I have stated.. most of this is speculation on my part about the MX-3.. but it does make more than a bit of logical sense. I would be VERY interested to know how those with upgraded alternators in their cars fare for gas mileage in the summer and winter. Just a couple of random thoughts.. cheers

Tunes67

Tunes67 · Post by **Tunes67** » February 24th, 2006, 4:20 pm

Oh and Btw.. 300 miles to the tank with a DOHC B6 /ATX is about 10 miles less than I average with my DOHC B6/ATX. I would consider that pretty close to normal. Cheers

Tunes67

projectmx316 · Post by **projectmx316** » February 25th, 2006, 4:59 am

oh ok i thought it would be alittle low but thats good miles then thanks

highlife · Post by **highlife** » February 25th, 2006, 10:36 am

Tunes.. One thing I dont get is the weak spark theory. Seems If the fuel detonates it detonates, and if the spark - whether weak or strong - does this it has done its job. I can see a partial (weak) burn if say your valves opened early or your rings were bad, but a weak burn due to a weak spark ?

Tunes67 · Post by **Tunes67** » February 25th, 2006, 11:10 am

You have to understand that the hotter the spark, the more complete the combustion.. and a weak coil output can cause an engine to seem like its running rich. The engine isnt running rich because of the ECU.. its just not burning all of the fuel in the mixture. This goes down to a molecular level... and I am not a scientist LOL so I am not sure how to really explain this. But maybe this analogy will do.. If you have a cigarette lighter lit... and you dump a bucket of gas on top of it.. what do you think will happen? Likely it will extinguish the flame. But.. if you dump that same size bucket of gas onto a bonfire.. BOOM. Same sort of thing happens inside our cylinders.. if the fuel isnt atomized properly or there is more fuel than what the spark can fully ignite.. your engine just isnt going to operate at its peak efficiency. Again.. I am speculating a bit.. its even possible that the power draw is effecting the performance of the ECU or other components.. not just the ignition system. There are two reasons I speculate its the ignition system though.. One.. poor placement of the distibutor & coil. In my DOHC B6 engine.. the distributor is mounted on the end of the exaust cam... to an aluminum head and the distributor body is aluminum as well. So the distributor is getting more heat than some engine designs I have seen. No big deal except in our cars.. the coil is also inside the distributor. Heat has a way of distrupting magnetic fields.. well magnetic fields are what the coil uses to produce spark. Granted.. the effect would be minimal... but consider this as well.. Most of our distributors are now 12+ years old. My second point: Look at how often our coils and distributors in the MX-3 fail. Its so bad that folks here on this site have even found a way to modify their distributors to run external coils and ignitor modules. I would also love to hear from those folks as to how their winter mileage works since their modification.

I admit that I am speculating... But I think my theory is strong enough to validate future experimentation. But since I just bought a GS car.. my RS isnt likely to see much work for a while. But I would love to see winter gas mileage from a RS that is running a GS alternator and has converted their distributor to an external coil & ignitor module. I'd be willing to bet that their mileage would be closer to or the same as their summer mileage. Just a few more random thoughts.. Cheers

Tunes67

corbanbrook · Post by **corbanbrook** » February 25th, 2006, 3:31 pm

ran into some trouble after the O2 Sensor install.. seems my half turn wasnt tight enough.. and after driving 30 km or so it loosened and i suffered massive loss of power and it stalled almost ever time i disengaged the clutch... so everytime i stopped the car i had to apply gas just to get it to idling.

Got a proper tension wrench and o2 sensor socket and that fixed it up good.

im hoping to see some improvement in MPG now. Ill let you know next time i fill up.

Shades · Post by **Shades** » February 25th, 2006, 11:08 pm

Also fixing the EGR passage and valve helps with mileage.

I just unclogged mine due to air care restrictions here in BC, Canada... and this not only got me thru air care it also helped my gas mileage and made it idle more stable.

corbanbrook · Post by **corbanbrook** » February 27th, 2006, 12:38 pm

Well, just checked the MPG so far. Doesnt look good

17 MPG.

Looks like I will try what shades suggested and clean and replace the EGR valve. Then i dont know... New fuel tank and fuel lines should only set me back $200.

What are the best spark plugs to get for the MX-3? I had new spark plugs put in when i got the safety done. But im sure they are cheapest stuff the mechanic could get his hands on.

corbanbrook · Post by **corbanbrook** » February 27th, 2006, 12:46 pm

BTW the 17 MPG was calculated off the first 100 kms i put on the new tank.. and that was all with the o2 sensor not properly tightened.

Hopefully the next 100 Km will be better.

Tunes67 · Post by **Tunes67** » February 27th, 2006, 1:27 pm

General things to check that effect fuel mileage.

Timing ~ should be 9-11 degrees BTC.
Tune up ~ Distributor Cap, Rotor, Plugs, Plug wires, Air Filter, O2 Sensor
Exaust leaks will effect mileage.
Gummed up or otherwise malfunctioning Fuel Injectors.
Failure in fuel delivery system - Gas Cap, FPR, etc..
EGR operation.
Weak Coil Spark (This will effect mileage, the question is.. is it an issue with our cars)
Vacuum Leaks in air intake system. Unmetered airflow will affect your mileage and will affect your idle.
and of course.. your lead foot

The Gas tank in the MX-3 holds 13.2 US Gallons. Only about 10 show on the fuel gauge. I think the conversion to imperial gallons is like 10 or 11 imperial gallons.

NGK Spark Plugs & Plug wires are generally considered the best choice for our engines.

5spd cars will get better mileage than ATX cars. The ATX robs a fair amount of power from the engine just to operate.. this takes extra fuel. So you can expect to see 5spd cars get an average of 2-4 mpg better fuel economy than a ATX car. Both in city and on the highway.

SOHC engines get better mileage than the DOHC engines. This is because the DOHC heads have better flow characteristics and you get more aif/fuel flow per cycle with the DOHC than you do with the SOHC head. There could also be compression ratio differences. So the engine makes more power but at a sacrifice of fuel economy. The MX-3's that get the best mileage are the 92-93 SOHC RS cars with the 5spd trans.

Tunes67

Bochek · Post by **Bochek** » February 27th, 2006, 3:40 pm

All these numbers are starting to scare me.

Like my dads '92 protege with the 1.8L BP-ME (16v sohc) gets allmost 50mpg in good conditions (summer time)

Maby it has something to do with the magnets on his fuel line

Deffently going for fuel milage with my mx that little 1.6L should be able to do better then the heavier protege with the bigger engine no!?!?!?!

Bochek