What Could Be Causing My Horrid Gas Mileage?

Venom351R

Venom351R

Founding Member
Apr 27, 2002
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#1
I know how often we hear "dont mod a mustang if your concerned about gas mileage" but there's a line and this is a little to much.


I drove the car 60 miles and used half a tank of fuel. The 60 miles was split between interstate driving of 65-70 MPH with the RPMs around 2,300-2,400 and the other half being roads going around 35-55 MPH. I was not hammering on it at all or leaving in to high of a gear and taching it out. At the end of the drive I had used a half a tank of gas in a total of 60 miles and I had just filled up before I got on the interstate. Its never been that great on gas to begin with but I dont ever remember it being that bad before. It should not be running rich it has an SCT chip and the fuel filter was changed in the past 2 years, #36lb injectors and 255lph in take pump. I never really checked my MPG before but I do know it was not this bad.
 

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#2
I'd go fill it up again just to be sure your gauge isn't acting funky. Many of them are not calibrated all that well. Something might have gotten sticky over the winter (I seem to recall you just pulled it out of storage).
 
#3
yeah it has not been out for that long. When I did fill it up before it was almost on E according to the gauge and it took a lot of fuel so I now the tank was almost empty and then full. Be awhile before I can drive it again it residing at Rio5.0's residence for a few wks.
 
#5
Ive always used stabil fuel additive when storing mustangs for the winter. Done it many years. Any that was left in the tank should have been used up by now. Ive filled it up a few times since Ive taken it out.

Inflated the tires to the proper PSI when I first took it out and I cleaned the air filter one day when I was bored and it was far from even needing it.
 
#6
If the engine runs too cold, the computer will dump fuel in to drive up the temperature. Kind of like a choke. I would try and figure out what temperature the computer is showing the engine at when it is fully warmed up.

Kurt
 
#7
the temp gauge always shows that its running between 190-215, not sure if that relates to what the computer is seeing though.

PS: I just remembered this as well Im not sure if it matters but I thought I would bring it up.

the ECU that was in the car was a custom program computer. When I had the SCT chip put in there was not specific code he could use to sync the chip w/ the ECU. He just put in a regular code for a 94/95 ECU. Since then I had picked up a 94 Cobra ECU and have put that in and been running that so far this year
 
#9
Couple of things...

Is it possible that the synch code for the "regular" 94/95 ECU is different from the Cobra ECU? If so would that make the chip non-synched in the Cobra ECU? I have no idea, but given the two different ECUs, I would at least check it out. Maybe your chip isn't doing anything?

One thing to also consider that has nothing to do with the car...

Could it be that you got some gas with ethanol content on the "high side" as opposed to the "low side?" I don't know what your gas is like where you're at (Maine), but I know that here (Michigan) I get a tank every once in a while that just runs through. In my wife's car, in my truck, in the 'stang. It doesn't seem to matter, but a couple times a year for each vehicle I seem to get a crappy tank of gas. If the ethanol content fluctuates from, say 10% on the high side to 2% on the low side, I would think that could make a marked difference in fuel economy. I have no idea if this is the case, but it happens. My wife's car has a Northstar engine in it and it is really noticeable when you get a bad tank of gas in it. It has no guts at all, gets pingy, and gets about 16 mpg instead of 23-25. That's a 25% or so decrease based solely on the gas in the tank.

Of course, all of this is pure speculation on my part...
 
#10
Let's see if I've got this right. You have an ECU from a '94 Cobra with a SCT chip from some other computer plugged in, 36# injectors and a PMAS 95mm MAF with a tune based on a a regular 94-95 ECU (I assume you mean for a GT) and you wondering why the fuel consumption is horrid. Sounds to me that you need to take it back to a tuner to get it set up for what's in the car now. BTW where did the rwhp and torque numbers in your signature come from? Are they based on the tune from the installed SCT chip with the other ECU?
 
#11
there is no gt ecu involved in this. The tune for the injectors i assume runs through the chip. The ecu that is in there now is what the chip was programed for. There was no code for the ecu that was custom programed. The times in my sig are from the custom ecu. I'll email the tuner and see what he says about it
 
#13
I think we (or I at least) am still confused on the EEC and chip stuff. When you noted that there was not a Cbaza-specific puter in there (they all have a catch code), I assumed you had an aftermarket EEC.
Richard and folks like that can answer better than I, but if you went from one of those to a CBAZA EEC (kind of the reverse of what Richard and others have done), I'd bet some of the parameters the EEC looks for have changed. It would be the same as if someone had an RT and a tune for a Cobra EEC and then went to Brand A aftermarket EEC, the RT tune would probably have to change for things to remain the same, as both EEC's would not be looking for the same values and parameters. They might not even use the same reference values.

:shrug:
 
#15
The ECU that came in the car was not a standard ECU. It was a prototype unit that had a custom tune in it for the 351 swap that was done in the car. It was programmed by Craig Barker who at the time was the power train manager at Roush industries. All ECU's from the factory have have as you stated the catch code that tells what type of ECU it is. I cant remember what the catch codes are off the top of my head but there are a few different ones for 94/95 ECU's. When my SCT chip was made it could not be synced with any of those factory catch codes b/c none existed for my prototype ECU so he just put in a code for a standard 94 Cobra ECU. Since then I had bought a 94 ECU and have since decided to run it in the car. It runs fine, idles fine I did not think anything would change at all and I dont even know for sure if its the cause of my poor MPH as far as the chip and the actual 94 cobra ECU not being synced with each other.


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003-31.jpg
 
#17
no its def not that. Not in this case.

I contacted the tuner last night and told him about the ECU swap and he said that the only way to tell would be to put it back on the dyno and check the cruising A/F ratio b/c it could be running rich at idle and that it would be an easy fix.

A vac line has been mentioned a few times. I do know I have one thats messed up b/c only my vents work for blowing any air. All of the settings lead to it only blowing out the vents by my windshield but that has been an issue for awhile now that I have yet to take care of but perhaps it could have gotten worse??
 
#18
they don't really get worse, its just one small line. I doubt that will effect the vacuum of the car much. i vote the tuner route after you've tested it with another tank of gas from another place.

That prototype computer is pretty cool.
 
#20
ProKiller said:
they don't really get worse, its just one small line. I doubt that will effect the vacuum of the car much. i vote the tuner route after you've tested it with another tank of gas from another place.

That prototype computer is pretty cool.
Click to expand...

You'd be surprised what a crack in a vacuum line can do. The vacuum line for the internal vents is a tiny line running from the vacuum tree in the driver's firewall corner (smallest hose) to a spot by the A/C equipment on the passenger side of the firewall. If that one is broken or has a leak, you'll only get air out of the defroster vents. It's a fairly common problem.

To test if that's the issue, just disconnect the hose at the vacuum tree and plug the hole. If your gas mileage improves, you're set. Our cars are sensitive to vacuum, so it is likely affecting your idle. I also recommend fixing the vacuum leak before the tuner gets to the car, or he'll tune the car for a vacuum leak. As soon as you fix the leak, your tune will be off.
 
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