90 SOHC Gas milage

[skylinekin]

1990 Nissan 240SX SOHC 5-speed

my gas milage is crappy and im looking for some advice. Im getting 200 miles to a tank of gas. Ive done the following

UPDATED

-NGK plugs and wires-Cap and Rotor-Fuel Filter-K&N Cone Filter-CTS-Thermostat-O2 Sensor-Cleaned MAFS-Timing is set at 15 degrees-Doesnt use oil, compression is fine

When i start my car up after its been siting is shoots out black sludge, and there are black specs all over my rear bumper. I dont drive it hard, shift around 3k rpms, all city driving baisically, sometimes I can smell alittle gas when driving.

So im wondering what i should do next, someone said my cat converter might be clogged, would that make me get that crappy of mpg?? Please help me out!
Modified by skylinekin at 12:36 PM 11/22/2006

[Q45tech]

Do you have a recent dyno emission test showing HC, CO, CO2?

What about code checks from ecu............compression and leak down readings, MAF cleanliness?

[rawill240sx]

Might check your O2 sensor, if it is bad that might be the cause of it.

I believe that the timing should be 15 BTDC instead of 20 BTDC.

[skylinekin]

i live in michigan, no emission testing, I advanced the timing alittle and run 89 octane, thats why its at 20 degrees. I dont have a CEL since its a base model and i have never checked codes from it. Compression is 165 psi on all 4.

So far here is the list of what the problems may be

-O2 sensor-MAFS-Cat Converter

[skylinekin]

any other ideas??

[180fan]

bring the timing back to 15btdc, replace the o2 sensor and clean the mafs. another thing to check out are the plugs at the injectors.

[silvia707]

it could be the gas tank. maybe the car got rear ended and it pinched the tank. it could cuase a leak. park your car somewhere there is not any leaks on the ground and leave it for awhile and check the ground a couple hours later. you never know man . i had the same problem and it was the gas tank.

[skylinekin]

okay, I will get a new O2 sensor and reset the timing, is it alright to clean the mafs with carb cleaner?

[180fan]

brake cleaner on the mafs. Don't spray directly, spray on the metal housing of the mafs then let it flow into the hole where the actual mafs element is, that bugger is pretty fragile.

[skylinekin]

okay, I cleaned out the mafs, replaced the O2 sensor, the one that was in the car was totally black, and i set the timing to 15 degrees. I just filled up my tank, 12 mpg , we will see if it gets any better.

[skylinekin]

wow, the O2 sensor definitly made a difference, i havnt checked the mileage yet but just by watching the gas gauge its alot better. Thanks for the help!!

[lbrowne]

all in how you drive too - shift before 3k and don't be flooring it to get to that point. I'm trying to increase my consistent 280-300 miles to the tank!

[BMAR240SX]

Does your engine hesitate when it shoots out the black smoke?

[skylinekin]

doesnt really do it anymore, the 02 sensor has seemed to fix it.

[BMAR240SX]

Thats cool but you might want to give your exhuast system a check up never know what you can find

[skylinekin]

yeah, it wouldnt hurt to get a new cat converter, it has a newer catback on it.

[lbrowne]

21 mpg all city on my last tank. I don't have the lightest foot either, which surprised me since I thought this tank was going to be worse than others.

[carcrazyguy]

My last 240 was a 90 with SOHC (of course).Anyway, initially it got really poor gas mileage (for a 4 cylinder). I would guess I was getting around 28 on the highway, and low 20's in town.Also, sometimes the car would idle really bumpy, and even when it didn't do that, the idle was not that great.One day I had the hood open and smelled fuel very strong. After poking around, I noticed a fuel leaking from an injector o-ring. I went ahead and put all new o-rings in but it still leaked at the same injector. So out of curiousity, I switched the leaking injector with the very front injector. Once I did that the front position was leaking - so I had isolated the problem as the injector. Out of curiousity, I pulled a the plug where the bad injector was and it looked to be very rich. I pulled another for comparison and it was night and day.What was occuring is that the injector was stuck all the way open, so even at idle and normal driving, one injector was operating as if the pedal was to the floor. Once the car was turned off, the injector was still open so any remaining fuel pressure would push its way out since it could not be burned. Since the day I bought the car I had noticed a fuel smell after the car was off, but always thought it was my imagination.Anyway, I priced injectors and they were relatively high - seems like about $75 each for remanufactured units. What I ended up doing is buying a used set of 4 injectors on Ebay for $20. I figured at least one of them would be good. I put one in place of the bad one, and fired the car up,Immediately, the car idled perfect...well, as perfect as the truck engine can. Also, once I drove it, the car was much smoother and had more torque at lower rpms - due to the fact that with the stuck injector, the o2 sensor was surely backing the fuel ratio off at anything less than full throttle because of the unburnt fuel. Anyway, I began to notice a HUGE improvement in fuel economy after that. At the time I was driving most highway, and taking it easy, but I remember averaging 35.X on the first tank I averaged after the repair.So this may not be your problem, but I do know for a fact that this is a common problem with these cars, particualrly with the SOHC, because they have so much immediate low end torque that many people drive them at 1/4 throttle or less most of the time. Then when they "kick it" every blue moon the dirty injectors can stick open. Hopefully this may help your problem.

[skylinekin]

ill have to look into that, thanks for the heads up!!

[lbrowne]

Hrmm. I wonder could this be another thing to check on mine.

I'll check for gas smell... Its been a while since I drove her.

[180fan]

carcrazyguy, the 40 iirc should have gotten 20 in city and 25 highway. looks like you were in the ball park for mileage. You could have squeezed out more mileage by leaning out the mix ala afc.

[carcrazyguy]

carcrazyguy, the 40 iirc should have gotten 20 in city and 25 highway. looks like you were in the ball park for mileage. You could have squeezed out more mileage by leaning out the mix ala afc.

A couple factors to keep in mind. For one, I am not in CA so fuel and fuel systems are calibrated somewhat differently.Also, when I had my SOHC 240, this was right when gas peaked around $3 a gallon. Due to that I was driving really slow and carefully - in other words, running around 58 mph on cruise control on a long straight highway driving 50 miles each way daily was 3/4 of the tank I averaged. But if I truly believed that 240's only get 25 on the highway, I would get rid of mine...my previous gas guzzling RX7 got 23-24, and my Z32 gets as much as 26 on occasion, and never less than 24. So I guess there may be some serious differences as to what we consider highway driving, and as to what speeds and throttle pressure we use. Even with cc set at a constant 75 or 80, my current 240 will make 30 mpg.

[180fan]

good stuff that you're doing about 30 on the freeway.

http://www.fueleconomy.gov/feg/noframes/5356.shtml

it's not gonna make much of a difference with CA emissions controls added or if it just passed federal emissions regulations, the maps were for the most part the same.

In your case, I think it was more the driver being careful and mindful of the way the car was being driven that squeezed more mileage out, not really what other stuff you did to the car.

[lbrowne]

well well well. My front injector is leaking.

Just pulled the car around back to do the pads, I haven't driven her since the last time I posted. For kicks I popped the hood since I could smell a hint of gas as I was placing a board under the front end (changing the oil too)

Sure enough the front injector is wet with gas. All others are dry as a bone. You can also see traces of where gas has stained or burned off on the motor around the injector port.

I'm going to check online for some rebuilt or nos injectors. (can't spend much)

[carcrazyguy]

good stuff that you're doing about 30 on the freeway.

http://www.fueleconomy.gov/feg/noframes/5356.shtmlit's not gonna make much of a difference with CA emissions controls added or if it just passed federal emissions regulations, the maps were for the most part the same. In your case, I think it was more the driver being careful and mindful of the way the car was being driven that squeezed more mileage out, not really what other stuff you did to the car.

I just looked up the original EPA figures from 89, and they say 23/33 for a 240SX so I am not totally sure where the figures came from on that site. I have a little foldout thing that has all US cars for that time period, and I also looked in a couple 89 240 articles and they also show the same numbers.It may have something to do with an adjustment that the EPA made for "real world conditions". I read an about this recently in CR (while waiting at the basrber shop). In other words, the EPA, at some point changed how they test cars, but simply subtracted roughly 20% from older tests, as if that would make it accurate?Anyway, Road & Track's long term 240 got 25 mpg overall, and they don't drive too conservatively, ya know, so I would still stand by the original 33 highway.

Regardless, I looked up 91-93 on that site and their numbers are spot on...I still have the window sticker from my 92 showing 21/26.I do agree with the latter however - the DOHC cars do well to see much more than 26, maybe 28-30 at the most on slow highway driving, as is the case with my 92. But with a (properly running) SOHC, and careful throttle, you can easily get well into the 30's...

[carcrazyguy]

well well well. My front injector is leaking.

Just pulled the car around back to do the pads, I haven't driven her since the last time I posted. For kicks I popped the hood since I could smell a hint of gas as I was placing a board under the front end (changing the oil too)

Sure enough the front injector is wet with gas. All others are dry as a bone. You can also see traces of where gas has stained or burned off on the motor around the injector port.

I'm going to check online for some rebuilt or nos injectors. (can't spend much)

As I mentioned above, I simply bought a set of used ones on Ebay for $19. They are often on there since people are upgrading injectors with turbo conversions or whatever. I didn't see the need to spend $75 for one injector when the other 3 would still be used. In other words, if I couldn't buy 4 new ones, I didn't see the need to buy just one. So my suggestion is to get some used ones if money is truly an object as you mentioned. There never seems to be more than one or two sticking, so chances are you would get at least a couple good spares if you buy a set of 4. Then you could simply run acetone or similar for a while once you get all good injectors until everything is clean again.

[lbrowne]

I got 4 coming from a ka with 125k miles on it. They're going to be good. (its my bro's parts car)