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Fuel Economy in my DSM

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mikelv

15+ Year Contributor
566
10
Jun 17, 2006
Columbus, Georgia
Ok, I know this is a boring topic probably over-work on these boards, but I'm looking for some fresh ideas to help me out here. I'm pretty sure I have an idea of what's going on but some backing would most def be appreciated.

Okay, I just bought myself another daily driver so I can build a FWD monster out of the GS-T. I found a 98 GS-T with the following mods for pretty cheap.

98 GS-T Burgundy w/ Tan Leather
16g turbo
Full Exhaust
550's w/ an AFC
255 pump
1G CBV crushed
MBC
some other basic stuff...nothing fancy.

Anyways, my goal is to completely get this thing stock. Maybe some nice breaks/suspension, stiffer shifter bushings, ect... and wheels but that's about it. I've got two other power houses, it's not what I need.

Here's the issue, this car has 164K on the clock, the most I have ever ran or owned on a DSM. I have an idea that maybe the rings are a little worn on it, and I'm losing some gas....not to big on this, but during oil changes, I don't smell any fuel in the oil, and the car doesn't have tuning issues.

The highway mileage on this thing is actually great. I can easily get 400 miles out of the tank or close to it if I drive nice and easy...this is typical of all my DSM's tuned correctly for mileage on the highway. The problem is in the city...I literally am lucky to get 220-230 out of it. I can easily get 100 more miles out of a tank on my other cars in similar conditions. Anways, I wanted to list a couple of ideas that I'm going to do/try and I just wanted to use this thread to double-tap my ideas, get some fresh ones and make sure I'm not missing anything.

So far, I haven't done too much, but I'm just bouncing ideas.

  • Removed the 550's
  • Zero'd out the AFC
  • New Tires
  • Replaced both O2 sensors
  • Reguar oil changes and premium fuel is used always.

These are the things I suspect & plan to do:

  • Fix a small flange exhaust leak in the down pipe
  • Run a boost leak test, (holds 17 psi without drop/creep ect... fine right now)
  • Take out the 255 pump
  • Try running something thinner than 10W30---maybe 5 weight, but it's too hot in SC, I actually would like to continue to run 15W as I do in all my other cars in the summer.
  • Completely remove the AFC and make sure ECU wiring is completely fine.


Look, I'm not a stock guru with these things...I can't even remember the last time I had a DSM this close to stock, (probably 10 yrs ago) so feel free to chime in if I've missed some ideas. I'm use to being able to tune with full or almost full control of the ECU parameters/systems. I mean, I get better mileage out of the street beast than this thing.

One last thing, I have a CEL, but it's for a BCS malfunction....I put in a MBC, so I don't think there is an issue there, but someone re-fresh me if the BCS malfunction will get the ECU fuel maps going haywire. If I remember correctly, the maps might max out fuel if they think there is a turbo-overload/fuel cut-type issue. Been a long time man, hit me up!

Thanks,
Mike
 
The BCS malfunction is probably from the BCS not being plugged in. It needs to still be plugged in or you'll get a CEL.

If you're really looking to get the best gas mileage out of this, you'll need some way to tune it. Since it's a 98, check and see if it has a black plastic case ECU. If it does, for ~$200, you can buy the cable and software needed to flash that computer. Then you can do things like lean out the fuel maps. I'm not sure if Ceddy has made a patch for it yet, but there's also the possibility of running leaner than 14.7AFR for closed loop.

http://www.dsmtuners.com/forums/tun...first-flashed-dsm-ecu-98-99-flashing-faq.html
 
sounds good but there is no need to replace both o2 sensors since the second o2 only monitors if the cat is working so if you don't have a MIL then i wouldn't bother.

also if you plan to run fairly stock boost i would suggest stock plugs to ensure a more complete burn.

iirc the bcs may affect timing maps but im not sure if its only with higher load only.
 
Ha, ha, sorry didn't post an update this weekend.

Took a look under the hood, and laugh out loud. Good thing I have the excuse that I have a "slipped" disc in my neck, because not knowing that there isn't even a BCS on a car I bought 3 months ago is just embarassing.

The guy I bought the car from, (who I do know well, and know what the car has been through) took off the FMIC he had to make a little more money from the sale. Like I said, I wanted a stock machine, so I wasn't too concerned about it.

Well, when he took off the UICP, and MBC off, he just ran a line off the wastegate to the BOV and didn't hook up the BCS. I guess for good reason, because there isn't even a BCS solenoid on the car. Makes me wonder how long he drove the car with a BCS malfunction. I never thought anything of it because I had an extra MBC kicking around and threw it on the car, day #1.

Anyway, I'm not going to worry about it, I think I have a spare kicking around I'll throw on today, I didn't have a chance this weekend. 99% sure it's the problem, so we'll see this week. Thanks for the help guys.
 
Ha, ha, sorry didn't post an update this weekend.

Took a look under the hood, and laugh out loud. Good thing I have the excuse that I have a "slipped" disc in my neck, because not knowing that there isn't even a BCS on a car I bought 3 months ago is just embarassing.

The guy I bought the car from, (who I do know well, and know what the car has been through) took off the FMIC he had to make a little more money from the sale. Like I said, I wanted a stock machine, so I wasn't too concerned about it.

Well, when he took off the UICP, and MBC off, he just ran a line off the wastegate to the BOV and didn't hook up the BCS. I guess for good reason, because there isn't even a BCS solenoid on the car. Makes me wonder how long he drove the car with a BCS malfunction. I never thought anything of it because I had an extra MBC kicking around and threw it on the car, day #1.

Anyway, I'm not going to worry about it, I think I have a spare kicking around I'll throw on today, I didn't have a chance this weekend. 99% sure it's the problem, so we'll see this week. Thanks for the help guys.


I'm not completely sure that is going to solve your mpg problem in the city. Yeah you boost a bit IF your heavy on the pedal. But I can easily stay out of boost in the city. Entering highway is the issue :rolleyes:.

But I would point to NOT having an AFPR (adjustable fuel pressure regulator) for that 255 you have pushing all that fuel. Just another thing to look at. Walbro 190, evo9 fuel pump or stock one. That should help your mpg as well. I would think the FMIC would Help. But whatever.
 
I'm not completely sure that is going to solve your mpg problem in the city. Yeah you boost a bit IF your heavy on the pedal. But I can easily stay out of boost in the city. Entering highway is the issue :rolleyes:.

But I would point to NOT having an AFPR (adjustable fuel pressure regulator) for that 255 you have pushing all that fuel. Just another thing to look at. Walbro 190, evo9 fuel pump or stock one. That should help your mpg as well. I would think the FMIC would Help. But whatever.

yeah, im ripping that 255 out soon.

the bcs malfunction is causing timing and fuel issues in the ecu. when its malfunctioned, the ecu basically thinks the turbo is over-boosting. its really a fail safe like fuel cut. so it dumps max fuel no matter what adjustments are made to airflow (AFC). you can tell when your bumper is black from running rich as hell all the time regardless of tuning settings, plus WOT is horrible, causing huge puffs of black smoke, indicating really rich mixtures.


that's what im thinking. ive never had major problems before with 255's and stock AFPR's, as far as fuel economy.
 
I drive both my DSM's mainly in the city and they both have injectors and fuel pumps in them etc and I get around the same in the city and both cars are tuned, just trying staying out of boost more. I'm always at atleast 2psi because traffic is a little faster here than in other places, but that doesn't sound too bad in town..
 
I wish traffic was faster here. I hate the northeast with its 65mph speed limit on the interstates. I get better gas mileage at 80 than I do at 65 because my off-boost low end torque isn't the greatest. 65mph is about 2700rpm and 80 is about 3300rpm. My engine doesn't really come alive off boost until about 3200rpm. I think I need to adjust the cams.
 
i hate speed limits at all, no one obeys them except old people who are making love to the steering wheel. 55 is a snail's pace. it really makes me go insane when i'm going that slow on a highway. 65 is ok, still go crazy. 75-90 is where i like to cruise.
 
I wish traffic was faster here. I hate the northeast with its 65mph speed limit on the interstates. I get better gas mileage at 80 than I do at 65 because my off-boost low end torque isn't the greatest. 65mph is about 2700rpm and 80 is about 3300rpm. My engine doesn't really come alive off boost until about 3200rpm. I think I need to adjust the cams.

What is your vacuum at idle and what is your idle set at?
 
It's not driving habits, I have the same habits in/out of the city, in/out of this car. I can hyper-mile this car and get crappy mileage.

I have 3 of these things, and I only go fast in the ones that are fast, even in those, I can tune them well enough to get great mileage out of them. 25 or so in the city, and 40+ on the highway. This one gets okay highway, horrible city.

300whp is boring to me, I don't boost it at all. I'm pretty sure its a ECU idiot light for the BCS killing me, but I'll let you know.
 
What is your vacuum at idle and what is your idle set at?

Vacuum is about -15inHg at a 750rpm idle. I've had it hold a steady idle as low as 600rpm before when I tried. The slushbox doesn't help. There's just a very pronounced increase in torque at 3200rpm.
 
Maybe returning to a smaller t25 would be better for your mileage as opposed to using the 16g? Even staying out of boost for most of the time, smaller turbo means less air and in turn means less gas. I mean, if your going back to a "stock" setup that would only make sense...
 
That might sound like a good idea, but think about it really. It doesn't matter what turbo you have, or what setup you have period. Just because you have larger stuff only means you have the capability to run big.

For example, my built GSX, stroker, 3065, 1600's, ect...I can get 41mpg highway as long as I manage the engine correctly.

This car has all stock fuel systems and an AFC set at zero.....there is no way there should be less gas mileage.


Okay, perfect example and an idea of what is going on. On the highway last weekend, I approx 41mpg highway with the car, like 400 miles out of the tank. I am literally getting 15mpg city if lucky, maybe 16. I go from 400 miles a tank to like 210, there is no way that is suppose to be happening.

I have put a BCS back on, and the only difference is that I can get clean pulls out of it and no CEL. It did nothing for the gas mileage.

I have 3 cans of sea foam, fuel, air, and crankcase. The previous owner neglected basic maintainece, and I'm going to take care of that. Otherwise, I'm stumped.

The car doesn't leak boost, but has a small exhaust leak. After that, I got nothing besides worn pistons.


I'm going to take my Link ECU and see if there are any other electrical problems going on.
 
I get 17 mpg city on a basically stock car and I just attribute it to the engines age and my driving habits. If you want a 40mpg car maybe consider a prius? hahaha. only kidding. As for heavy duty tech advice, the turbo was my only idea.

Are you tuning it yourself or having a shop do it? Maybe there is something basic that is just getting over looked? Timing advanced or retarded?
 
These cars are only rated at 20mpg city new, not really sure what more you are trying to get because it sounds like you are getting near that which can be attributed to age. :idontknow:
 
Have you done a compression test to verify the rings are getting wore down? I feel like your FPR is being over run by the 255.
 
These cars are only rated at 20mpg city new, not really sure what more you are trying to get because it sounds like you are getting near that which can be attributed to age. :idontknow:

Makes sense dude, but I've never gotten less than half between highway and and city mpg. When you get about 200 miles out of a tank something isn't right. I have 3 of these cars currently, and built or not built, I've never had this bad of a mpg problem.

I get 17 mpg city on a basically stock car and I just attribute it to the engines age and my driving habits. If you want a 40mpg car maybe consider a prius? hahaha. only kidding. As for heavy duty tech advice, the turbo was my only idea.

Are you tuning it yourself or having a shop do it? Maybe there is something basic that is just getting over looked? Timing advanced or retarded?


Nah man I tune my own cars, and this one is pretty simple, that's why I wasn't thinking a tuning issue.


No, this car is only running an AFC, zeroed out with stock injectors. At worst the pump is doing me no justice, but there shouldn't be any advancing, ect... I was thinking something electrical, hence wanting to put Link on it. But who knows man, I'm pretty sure it's worn pistons, which mean new shit again.


Have you done a compression test to verify the rings are getting wore down? I feel like your FPR is being over run by the 255.

Yep, this is my only option left, and I'm pretty sure I'm not going to like what I see. This is the oldest motor I've ever run, so I'm pretty sure this is what is going on.

I have run a stock AFPR on a 255 before without issues, but it's coming out anyway just to eliminate the thought. If it was simply the pump, why would my highway mileage be so damn good? Or worn rings for that matter? I mean seriously, from 41-42mpg to below twenty?

I've always gotten 300+ miles out of a tank regardless of red light traffic, or not. I'm going to do a compression test this weekend so we'll see, like I said probably not going to like what I see. If there is 180 across the board, then I'm totally ####ing stumped.


One thing that leads me to worn pistons, well 2 actually....1) is that the wideband on this car is reading perfect, 14.7 in flux, and 2) the oil is smelling like gas big time during the oil changes. Now #2 is only when I don't have highway trips.....change the oil after a long trip and no problem at all.

So I guess where I'm at, stable RPM = stable compresion? Hell man...
 
Yeah the piston rings might be dead by now. If the oil smells like gas at all, that's blowby right? Unburned gas slipping past the pistons/rings... How old are the spark plugs? Maybe they aren't burning enough of the fuel.
 
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