Mileage Question Gen II

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olddavid

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I have had two other Marks - both Gen I's. They were high mileage cars (150k+) without modification. I always run Mobil 1 and install a K&N in any car I drive. Those cars would average 19-24 mpg on the readout with a 50-50 hiway city driving mix. My 98 has never exceeded 14, and struggles to attain 20 when on the road. At first I thought it was because of the old plugs and my lack of discipline to give it the de-carbon treatment, and general cleaning of all fuel-related devices, like the IAC and MAF, etc. I finally did all these things and also installed new Motorcraft plugs, yet no mileage improvement. I cannot recall the number of the sparkplugs, but they were for one of my older cars that I had not used. Does the Gen II take a different plug than an older car? This car is just getting to 100k, and has been so well maintained by the previous owner that the under carriage has no oil leaks whatsoever. I also put new wheel bearings in both front hubs and changed the trans fluid and filter. I have 225 60 17's all around with 32 lbs. What am I missing? Don't get me wrong - even 14 will do for the comfortable pleasure I get from this car, but previous experience says I have a fuel or air restriction of some kind. Any tips from you experts out in LVCLand?
 
First of all, don't trust that on board mileage. Do hand calculations to get your MPG. I have noticed on my brothers car that the instant MPG has always read higher than my 97. Although it does get 1-2 MPG more than I do still but the instant read out is always giving like 3-5 MPG higher than it really is.

Maybe pull codes to see if you have any stored coded.
 
Try replacing the O2 sensors. They are also a maintenance item. They don't have to throw a code when they are going bad.
 
I agree with mark0101 and roadboss. Gen1's always get 1 to 2 mpg better than gen 2's and the on board mileage computers can be way off. If your mileage is really that bad, clean mass air flow sensor and replace 02's.
 
My mileage is also horrible and I have cleaned the MAF already.
Have the O2's handy - just not looking forward to getting the old ones out.
Probably will pay someone to deal with that mess.
 
I've noticed all my cars have been getting worst milage for a couple of years. I think it's all the junk they put in the gas to make more $$$. 14 is a little low. Reset the computer, it might have bad knowledge in it. I'd check the air box intake, maybe a critter was moving in. I took about a gallon of crap from under the intake manifold, where somebody was building a nest, talk about a fire hazard. One car I bought had a K&N air filter in it. The bottom of the air box was filled with oil. All my cars have K&N's. I'm getting an average of 18/25 on my cars. Best I've ever got was 28/29 on a road trip. Never going to see that again.
 
My air box was my first suspect. Clean. I do not want to change the O2 sensors, but they have been on for 16 years, so bi#*@ing will not get it done any faster. I did not know they were a maintenance item. To re-set the computer, just disconnect the battery, right? I have also done the math on my mileage, and 1-2 miles better (on the readout) was normal and I am accounting for that in my 14 mpg quote. I am also going to relieve the airbox and check under the intake. No stored codes, either. Thanks for the tips. PS- does the MAF from any other car interchange with ours?
 
If I remember the talk on here right, it's possible to do the driver side O2 from above in the engine bay if you have skinny arms. The passenger requires going in from below and unbolting everything. Or the other way around. It was one of them.

And yes, generally speaking O2 sensors begin getting lazy after about 100k. They can suck for a while and give you poor performance before they suck so much the computer starts complaining and throws a CEL.
 
Typically a second Gen base will do 27ish on the highway and an LSC around 25. There are several things that will hurt these cars mileage in a bad way so here is a list of everything to check:
1. Change O2s. They should be done every 50k anyway. Don't use the cheap ones, use quality versions.
2. Fuel filter. They clog much quicker now and hurt mileage.
3. Tire pressure.
4. Dirty MAF will make the car run very rich so use some MAF cleaner on it.
5. Dragging brakes. Seen this on many marks. The brakes are inadequate for the car and tend to heat up. If the calipers piston deforms it will drag. That will eat away lots of mileage.
6. Stored codes. Just because the MIL is not on does not mean there are no codes stored. Scan it, find the problem and clear it.
7. Fuel. They say "Premium Recommended" but it should be Premium Only IMHO. Stay away from the cheap places like Stripes and race Track. Stick with Valero, BP, Chevron and Exxon. They all test at proper ethanol levels where as other stations don't. More ethanol means crappier mileage.
8. Cats. Make sure they are not damaged. Clogged cat will really hurt mileage.

In my experience my stock LSCs usually get an average of 22 with a 50/50 city/highway ratio when stock and 18-20 when modded. Base models tend to do 2 MPG better and the last Base I sold just came in for a spring conversion last week and his average has been 27.4 MPG and he usually has the cruise set at 80.
 
Sorry if I derail the thread a little but any easier way to take out the O2 sensors Laser? I think I read DLF say you could get the driver side O2 sensors out by removing the carpet from the driver side interior. I can't find the thread now but any tips in replacing them would be helpful.
 
I was taught to disconnect the battery whenever I do any electrical work. When done I start the car and let it idle for 5-10 min's, or drive it for a few days. Can't take it to smog check until you have at least 100 miles on it, they can't get good readings. I've found that I get 2-3 more mpg at 68 mph, than at 73, on the same 80 mile run. Laser gave you a good list. There's a special wrench/socket made just to change the o2"s.
 
mark0101 said:
Sorry if I derail the thread a little but any easier way to take out the O2 sensors Laser? I think I read DLF say you could get the driver side O2 sensors out by removing the carpet from the driver side interior. I can't find the thread now but any tips in replacing them would be helpful.
Click to expand...

Doug's trick was to remove the steering shaft and boot from the firewall.

I did this myself since then, it works great.
 
Sapperfire just did his last week and I don't remember him having to remove anything but we do have O2 sockets at our shop. After I diagnosed the issue and he replaced them he did tell me the car is behaving much better now with better throttle response.
I know in my truck when I changed them out I instantly picked up 2 mpg and have held it since.
 
I will check it out tomorrow. I have the slotted socket to remove, and I don't need to pass smog for 18 months. The negative cable is always removed when anything electric gets worked on at our house. I will need to look at it and decide for myself if any steering device gets removed. I am grateful for all tips from you veterans. :)
 
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