bad fuel economy again....what to look for?

bigcletus94

Registered User
I said again but it actually never got better. It just keeps getting worse. Now I am down to 200 miles to the tank when It is just about empty and I have to fill it. This is starting to piss me off. My 5.0 mustang gets this kinda mpg. `This is with a new motor (rebuilt ford) and MANY other new things. Too much to list. I used to get over 250 to a tank. What is the deal? What are some things I can check/replace to improve this? I haven't changed the O2 sensors since I have owned the car (2 years now) and they are probably the originals. I have heard mixed things about changing them. Some say changing those improves fuel economy and I have also heard that it is a myth. I dont race the car ever and almost always keep it out of the boost. I haven't checked the codes in a while so I guess I could start there. Any other ides? Thanks
 
o2 Sensors...

Good place to start...If the HG's have ever went they should have been changed...I know my 94 had been back from ford for only 830 miles with a new Reman in it after HG's, and they only replaced one fo them...I replaced the other one and it improved the MPG by quite a bit...

EGR is another one...

Injectors...Were they properly cleaned during the Eng. change?

Plugs obviously can play a role here, as well as wires...I noticed that my plugs were gapped at 60-68 after ford did the eng. change...I was blowing them out at high RPM's...Duh...

I got all of this stuff sorted out and the Rod bearing went in the new reman. eng. after 8K miles...Not very impressive huh...

cheers
 
EGR is new. It was previously giving me 'check engine' lights until I put a new one in.

Plugs and wires are recent. Ford parts.

Injectors...they were not cleaned or replaced. Just removed from fuel rails and reinstalled...I have my doubts about there condition but dont feel like tearing apart the top of the motor on my suspecion alone. I also have a hard time getting the car to start after it has run recently, when making quick stops around town. It will crank for a long time then it finally starts. I think it is fuel related and maybe even linked to my poor fuel economy? It has always done this but it is getting much more frequent.

It idles good but I have noticed with my new exhaust that I can hear an irregular deep tone come out from the back randomly while it is idling. It almost sounds like a bass note from a sub. Kinda like a backfire. I am not sure if it did this before the exhaust was put on and maybe it just makes it more apparent. Don't know if it could be related to anything though.

Does that fuel injector cleaner stuff that you poor into your gas work at all? Thanks for the help.
 
Your comment about difficult starting when the engine is warm triggered a bad memory. I chased a similar problem on my Mustang a few years ago. The culprit turned out to be a bad ECT (Engine Coolant Temperature) sensor.

It seems that when this little bugger fails it signals the computer that the engine is cold all the time. The computer responds by providing a very rich mixture to the engine. That's why the mileage will go down severely; it's like driving a carbureted car with the choke closed all the time! That's also the cause of poor starting when the engine is warm. Again it's like trying to start a carbed engine with the choke fully closed; it'll flood every time. The temporary answer to starting a flooded engine with the Ford FI system is to floor the accelerator while you're cranking it. The computer recognizes that and shuts off fuel so that further cranking will dry out the flooded cylinders.

Hope that helps!

Regards,
Bob P.
 
exhaust = lower mpg

I got my exhaust redone last summer, 2 1/4 inch straight pipes into magnaflow resonator to 3 inch to 2 1/4 inch magnaflow mufflers. The mpg before I got this done was never lower than 24 mpg and the tank right before the exhaust work I managed a 26.8 without pampering it. I figured the new exhaust system may help my mpg but in fact it lowered it to 20-21 mpg. I hardly ever used boost and even tried fourth gear on the highway to get away from the supercharger as much as possible to no avail.

I have also noticed a sporadic low note coming from the exhaust. It is a random sound and it is almost exactly like a mini backfire. I am going to try the o2 sensors next to try and figure out how to get my mpg back.

94 5speed
 
I am going to order a ECT sensor today. I don't remember if I have ever replaced it before or not. Not even sure where it is located. I have changed a handfull of sensors but not sure it about that one. Thanks for all of the input. It is greatly appreciated!
 
ect sensor

I was just told that there are two sensors involved in this, one for the guage and one in the system or something of that nature. Do both need to be replaced if the above symtoms are present? The system one cost about 38 bucks, does that sound right? and how are are they to replace? Is there any way to check all of the senors on this car w/o just replacing them- or taking them out? Thanks a lot. Dan
 
Only the "system" sensor, the one that tells the computer what the engine coolant temperature is, needs to be replaced. The sensor that feeds the gauge has no effect on the fuel strategy.

Regards,
Bob P.
 
Dan,

According to Probst's book on the Ford Fuel Injection and Electronic Engine Control, the ECT is a thermistor device that behaves opposite of a resistor. That is, its resistance goes down as the temperature rises. At room temperature its nominal resistance is 37,300 ohms and it goes down to about 2,000 ohms at 212°F. With the engine fully warmed up, at around 190°F, the ECT resistance should be somewhere around 3,000 ohms.

Also, a KOEO (Key On Engine Off) check for codes would reveal a code of "21" if the ECT is out-of-range.

Hope that helps!

Regards,
Bob P.
 
just wanted to update anyone interested. After having the new ECT sensor in for a whole tank of gas, I have noticed no improvement. It still cranks long sometimes and my gas milage still sucks. No changes.
 
Hoooouuuugghhh, young grasshoppa'. You have much to learn!

My list of economy:

1. Use Berry man's B-12 Fuel Injection cleaner in your gas tank (little green monster in a supercharged corvette) this stuff melts styrofoam cups and will melt the deposits on your injectors and back of exhaust valves.

2. Throttle body cleaner, or GM top engine cleaner. Hooough, you must remove your air intake hose and spray this stuff down the throttle body with the engine running and you cracking the butterfly valve open (to keep the engine from stalling). Funky smoke will exit your tailpipes. PooooH, out with the bad air my son.

3. Think light warrior! Remove dead bodies from thy trunk to recieve a 1mpg bonus. Dead wieght will have a negative impact on economy, as well as acceleration, braking, handling etc.

4. Hot air keeps you afloat. And it also benefits your tire life, by inflating them to maximum pressure. Properly aligned tires will also help improve the rolling resistance situation. Swwerrrk, in with the good air!

5. The fire within cleanses you. Use a propane torch and Throttle body cleaner to clean your O2 sensors. Wire brush them and then burn the carbon deposits off. O2 sensors are like spark plugs, they send voltage across a sensor gap, it is the voltage that it recorded across the gap. Too much and it's running lean and too little means running rich.When the sensors provide a clearer signal to the computer, it can alter fuel flow much more efficiently.

6. One cannot live without the living air. And that means you should check your air filter and make sure it is clean. Otherwise you will run richer than normal.

7. Do not cut off one's supply. So replace your fuel filter so that way fuel flow is optimized and there aren't supply issues.

8. Never allow thy enemy to see you break apart at the seams. And check for boost leaks at the intercooler connections while you are at it. When there are air leaks after the MAF there can be economy issues.

9. Plugs are wires are not a bad idea to revist, even when you *think* you may have done them, you really have not. Ahhhhhh....

10. Know thy enemy and you can beat him from within. Learn to shift at 1800rpm and never go past the "0" on the boost guage when accelerating. When on flat open road, use cruise control. If going on hilly winding road, down shift, rather than run boost to get up a hill for better economy. Ford's fat tuning while in boost kills economy.
 
I would say that you should check the regular stuff (fuel leak, plugs, dist cap), but I have another idea that may help you out.

My 1985 Camaro IROC-Z started getting terrible fuel mileage (4mpg). The problem turned out to be a bad MAS(mass airflow sensor). The TPS(throttle position sensor) could also be it. If it is out of adjustment the car may think you are accellerating when you are cruising.

Good Luck!
 
As for the hard starting, I would check to see if your crank sensoring is taking a dump. My old sc would crank for a few seconds before it would catch. Once I replaced the crank sensor, the car would start in a split second. Also, if the motor is low on break in time, that will use extra fuel. I'd check the MAF and throttle body based on what you've said. Just my thoughts.
 
wow, thanks for all of the info guys. :cool: Now I have lots of fun things to check on my days off this week.

The crank sensor was replaced with a new one from Ford when I put the new motor in. The car has always (at least ever since I have owned it) started hard randomly. So it must still be something that I have not yet tracked down:

Things I have replaced: Cam, Crank, Knock, EGR, IAC, ECT, fuel filter, regular oil/filter change, clean air filter, 6 month old ford plugs wires, new long block, harmonic balancer....that is the big stuff.

Things I have never addressed: Fuel injectors, MAF, TPS to name a few.

The MAF is something that I have never messed with. It is the original and I have never cleaned it. I don't have a K&N air filter so I never felt the need to clean the MAF until now. I think I will clean it soon.

A co-worker of mine thought he heard an exhaust leak when my car was idling the other day. I sure hope he was wrong. I don't know what an exhaust leak sounds like so I would never have noticed it. Don't know if that could be related to fuel economy or not...

I have not pulled the KOER codes since I got the new motor in so I guess that would be a good place to start before I start throwing more parts at it. I called the dealer and they told me there is only one O2 sensor? I know there is one sensor on the header and the other side has it on the down-pipe. Is that one a different sensor? Thanks again for the help.
 
Be careful when cleaning your MAF sensor. The one in my IROC-Z wasn't cleanable. I had to replace it. I haven't checked how my TBird's MAF works, but in the IROC-Z it had a filliment in the center that would burn out. They said I would damage the filliment if I touched it. So maybe you should find one you can test with first before possibly damaging a good MAF sensor.

Good Luck...
 
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