Poor Mileage

OK so this is not SC related, but my post is not showing up at tccoa. I figure quite a few people here also have V8 birds/cougars. So on to my issue.

As of lately, my mileage has been terrible. Like 10 mpg, whereas my lead foot can usually squeak out at least 15. I've also noticed my exhaust smells like a charcoal grill. Not super rich, but charcoal-y if that makes any sense. The cats have now started making pinging noises after I shut the car off. Something that it never did before.

The car is a 93 LX 5.0 M5R2 swapped car. SCP down tube with cats, tru-bendz 2.25" dual exhaust, cobra intake with stock throttle body. EGR and thermactor have been deleted and it was tuned by Dave Dalke after all of the current mods were done. O2's have less then 10,000 miles on them and are Motorcraft. Rotor and cap, plugs and wires were also done at that time. I cleaned the MAF, even though it wasn't dirty. IAT is clean. ECT tested good. I ran a cylinder balance test and that checked out. Fuel pressure is within service manual specs. The only thing that's changed is I replaced the stock fuel pump with a Aeromotive stealth 340 in preparation for some forced induction. It runs fine, I haven't noticed any loss in power.

So after checking all of that stuff, I'm officially stumped. Any ideas are appreciated!

If you have an IR temp gun, I'd check the operating temps on the cats.

Might be raw fuel is being dumped into them... They should be 350° max, generally, I think and most importantly no big temp offset from one to the other. Are they glowing red?

No, they aren't glowing and don't seem abnormally hot. No discoloration of the metal. I had an injector get stuck open on my 90 SC and it turned the cat blue from the excess heat.

Good morning


I would check for air/vacuum leak based on the description of gas mileage. Sounds like a "lean" indication. Areas such as the exhaust manifold gasket and exhaust flange is a good place to start.

The MAF sensor could still be bad. There are tests to verify. Check the MAF housing at the sample tube. Does it need a good cleaning? A good "heads up" visual check I use is how "shiny" the MAF sensor is after cleaning. After cleaning if the cold/hot leads are dull it is a good indication that the dirt has "baked" on the MAF sensor.



Good Luck.

I'm about 99% sure it's not a vacuum leak. Idle is nice and low around 650. There are only three vacuum operated things left on this car: brakes, cruise, and hvac. I pulled the cap off of the former egr feed and plugged in the vac gauge from one of my SC clusters and it's sitting a hair past 20" of vac at idle.

Rechecked the maf and it looks like new.

Good afternoon


Locate a scanner (discount auto parts stores rent them) and do Key On Engine OFF/KEY On Engine ON test. Check the live data. With the scanner 1989 - 1995 Thunderbird EEC IV are not

programmed for Short and Long Term Fuel Trim (STFT and LTFT) data. This reading is valuable for to checking a lean/rich condition.

However by monitoring the oxygen sensors you can verify the voltage switching is within limits. With the engine running check the oxygen sensor output signal rapidly changing from (0.1 to 0.9 VDC).

Start at idle and go to @2000 rpm. Sensor(s) voltage switching slow and or reading low is possible bad sensor and or lean condition. If the sensors are good the MAF could be sending the EEC a incorrect reading causing the lean condition.

You could back probe the MAF signal at the connector and check the voltage. At idle the voltage is @ 0.6 VDC. Don't forget the to look at the BARO sensor. This would require a Hertz tester

and a vacuum pump to verify function. If unavailable you could just swap it out.


Good Luck.

Unfortunately, none of the local part stores carry anything obd1 related. I can bring up o2 output on the vmm while the car is running. It is limited in the information it gives me however. All it displays is rich or lean. It does switch back and forth rapidly while idling as I would expect.

There is no BARO or MAP on 5.0 thunderbird's. So there is nothing to check there.

I will test the maf tonight.

I pulled the maf out to test it last night and decided to pull the upper half of the airbox and intake tube, just to look them over. I discovered the bottom of my tb gasket had blown out. So lots of nice unmetered air getting pulled in. Must not have had a ton of air coming in since the IAC was still able to control the idle without throwing a code.