FUEL MILEAGE PROBLEMS---HELP!!!

1

1992MustangGT

15 Year Member
Sep 6, 2008
166
14
19
Washington State
#1
I have a 92 Mustang Gt 5.0L that gets 23 mpg on the highway after installing a 70mm throttle body and a 76mm MAF. It got 26 mpg on the highway in stock form. But now for some reason the mileage has went to crap!!!! I've used almost a half a tank of gas and I've only went 86 miles and 45 of those miles were highway miles.

The engine has a new ignition pickup (installed for another reason, not because of gas mileage), platinum plugs not too many miles ago (maybe 2000 at the most) along with cap and rotor. I also cleaned the Mass Air Flow Sensor Wire.

My question is this....will the Idle Air Control cause bad gas mileage if it's malfunctioning? This is the only other thing I can think of that might cause this problem. What do you guys think? Is there another possibility I'm missing???
 

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#3
Pull the codes. With mileage that crappy, you should be throwing O2 and other codes.

Down the road you might want to switch to copper plugs if your idle is a little funky or you get a higher RPM cutout.

Good luck.
 
#5
LOL, gas leak would do it.... What kind of plugs do you have in there? I hope they are not bosch, those plugs are expensive and are crap plugs. I tried to use them in my 96 and they made that car run like a 2 legged dog. Switched to motorcraft and cured the problem.
 
#6
stangfanatic said:
LOL, gas leak would do it.... What kind of plugs do you have in there? I hope they are not bosch, those plugs are expensive and are crap plugs. I tried to use them in my 96 and they made that car run like a 2 legged dog. Switched to motorcraft and cured the problem.
Click to expand...

I ran the codes a few months ago and the only two codes that came up was 14 (Ignition Profile Pickup (PIP) circuit failure & 34 (insufficient EGR flow or EVP voltage high (Sonic) or PFE sensor voltage high or out of specification).

I have since then replaced the ignition pickup because it was cutting out every now and then. A few months prior to replacing the ignition pickup I replaced the EGR valve and the electronic part that bolts to the EGR valve. I But for some reason it still throws that EGR code. However, I don't think that the EGR valve will cause this bad of gas mileage, but if I'm wrong I hope one of you please correct me and set me straight.

As far as the plugs are concerned...I think they are Bosch platinum plugs. I was considering replacing the plugs again just to rule that possibility out.

Regarding a gas leak....I don't see any gas leaking anywhere or on the ground nor do I ever smell gas so I'm not sure if that's the problem. But logic would lead to that, but I would think I could smell it if that was the problem. Am I correct?
 
#8
pudman said:
Get rid of the platinum plugs. Mustangs don't like them. You said you replaced the EGR valve. Might check the EVR solenoid. This helped my crappy gas mileage on my '86
Click to expand...

What is the EVR solenoid? Where is it located? What is it's function? As far as the plugs are concerned....I'll get rid of the plugs, but what plugs do you recommend I replace them with?
 
#11
The EVR is an electromagnetic device which controls vacuum output to the EGR valve. The EVR replaces the EGR solenoid vacuum vent valve assembly. An electric current in the coil induces a magnetic field in the armature which pulls on a disk, closing the vent to atmosphere. The ECU outputs a duty cycle to the EVR which regulates the vacuum level to the EGR valve. As the duty cycle is increased, an increased vacuum signal goes to the EGR valve. The vacuum source is manifold vacuum.





I agree on ditching the bosch platinum plugs. I would not run a bosch plug in my lawn mower!!!
 

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#12
hotrodredneck said:
The EVR is an electromagnetic device which controls vacuum output to the EGR valve. The EVR replaces the EGR solenoid vacuum vent valve assembly. An electric current in the coil induces a magnetic field in the armature which pulls on a disk, closing the vent to atmosphere. The ECU outputs a duty cycle to the EVR which regulates the vacuum level to the EGR valve. As the duty cycle is increased, an increased vacuum signal goes to the EGR valve. The vacuum source is manifold vacuum.





I agree on ditching the bosch platinum plugs. I would not run a bosch plug in my lawn mower!!!
Click to expand...

Where is this EVR solenoid located? Is it located by the EGR valve?
 
#14
hotrodredneck said:
The EVR is an electromagnetic device which controls vacuum output to the EGR valve. The EVR replaces the EGR solenoid vacuum vent valve assembly. An electric current in the coil induces a magnetic field in the armature which pulls on a disk, closing the vent to atmosphere. The ECU outputs a duty cycle to the EVR which regulates the vacuum level to the EGR valve. As the duty cycle is increased, an increased vacuum signal goes to the EGR valve. The vacuum source is manifold vacuum.





I agree on ditching the bosch platinum plugs. I would not run a bosch plug in my lawn mower!!!
Click to expand...


Excellent definition on the EVR solenoid. I'm glad you came aboard!!
 
#15
stangfanatic said:
LOL, gas leak would do it.... What kind of plugs do you have in there? I hope they are not bosch, those plugs are expensive and are crap plugs. I tried to use them in my 96 and they made that car run like a 2 legged dog. Switched to motorcraft and cured the problem.
Click to expand...

What's your opinion on Autolite Platinum plugs? I no you're recommending copper plugs, but aren't platinum plugs supposed to be better?
 
#16
Copper conducts better than platinum. The advantage of plats is that they last longer. Great for engines that have hard to reach plugs and that call for plats from the factory (thought even some of those run better on copper).

The downside to copper is that you simply have to change the plugs more often.

In EFI 5.0's, using plats has been correlated to a low idle and/or cut-out in higher RPMs.
 
#18
HISSIN50 said:
Copper conducts better than platinum. The advantage of plats is that they last longer. Great for engines that have hard to reach plugs and that call for plats from the factory (thought even some of those run better on copper).

The downside to copper is that you simply have to change the plugs more often.

In EFI 5.0's, using plats has been correlated to a low idle and/or cut-out in higher RPMs.
Click to expand...

I put Autolite platinum plugs in my 67 Cougar I'm building right now. Haven't fired it up yet, but after reading this advice I'm having second thoughts. Should I put copper plugs in this engine also? The 5.0L Mustang we have been talking about is computer controlled and has alot more sensors and solenoids, etc.... on it, but the Cougar is a basic engine with a carbuerator. Does that make a difference on using platinum vs. copper????
 
#19
1992MustangGT said:
I put Autolite platinum plugs in my 67 Cougar I'm building right now. Haven't fired it up yet, but after reading this advice I'm having second thoughts. Should I put copper plugs in this engine also? The 5.0L Mustang we have been talking about is computer controlled and has alot more sensors and solenoids, etc.... on it, but the Cougar is a basic engine with a carbuerator. Does that make a difference on using platinum vs. copper????
Click to expand...

Since most of the cars in 5.0 Tech are EFI, I use that as the frame of reference.
To be more concise, the biggest issue IMHO with a car's ability to handle plats vs coppers is the ignition. If you're using the TFI ignition, it's stout (good for 400 HP+) but it still doesn't like plats (as well as Factory coil pack cars).

I would run a copper plug in your situation, esp if you don't run a TFI ignition in the car (an older style ignition will like plats even less). And if you have an ignition box, they don't like plats either (the companies often explicitly say not to use them).

Anyhoo, I'd swap the plugs out for some cheap coppers (my preferences were listed in an earlier post).

JMHO. Good luck.
 
#20
Did you start getting bad mileage right after the TB and mass air? if so put the stock junk back on and call it a day, i would guess this is your problem
 
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