Backfire through exhaust under load...

[jrichker]

Code 98 - basic computer internal tests not passed before it started processing the request to dump codes. Do a Key On Engine Off test before starting the engine. Wait until you see the 11 flash before pressing any buttons if you have a hand operated scanner. The computer is operating in Limp mode, so fix any codes it dumps prior to doing anything else.

Code 66 MAF below minimum test voltage.
Insufficient or no voltage from MAF. Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing +12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for computer power relay switched +12 volts. Check for 12 volts between the red and black wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF. This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.

There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater heats the MAF sensor element causing the resistance to increase. The amplifier buffers the MAF output signal and has a resistor that is laser trimmed to provide an output range compatible with the computer's load tables.

If you have a K&N flat panel filter or other filter that requires oiling, excess oil may coat the MAF sensor element and cause problems.

The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element, remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable brake parts cleaner (same stuff in a bigger can and cheaper too).

Changes in RPM causes the airflow to increase or decease, changing the voltage output.. The increase of air across the MAF sensor element causes it to cool, allowing more voltage to pass and telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins 50 & 9 on the computer. Be sure to measure the sensor output by measuring across the pins and not between the pins and ground.

At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt

Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer (dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the computer (tan/light blue wire) should be less than 2 ohms.

There should be a minimum of 10K ohms between either pin C or D on the MAF wiring connector and ground. Make your measurement with the MAF disconnected from the wiring harness.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring Mustang FAQ - Wiring & Engine Info

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

 

[cougar_68]

Went for a test drive this morning. It ran like a complete dog as expected. I really don't understand how it can run perfectly fine one time and like complete crap the next.

I tried to check voltage at the MAF, but for some reason, my multimeter doesn't want to read voltage now? It will do continuity though. I have continuity from A & B to battery. Also, I decided to check and see if the MAF had any voltage using a test light and the safety pin method mentioned above. The light did not work, so I assume there is no voltage there. I did this with Key off, Key on, and Engine running (wasn't sure when I was really supposed to do it, would assume Key on is adequate?).

The codes I get when I do the KOEO are 81, 82, 84, & 85. I did not get 11 (pass). How can I fix these codes when these systems are not present or are you saying I need to fix the MAF before it will do the test properly?

Would the no voltage be due to the wiring harness, MAF, or computer?

 

[cougar_68]

Got a new fuse for my multimeter. There was no voltage between A&B with the connector plugged in and key on engine off.

With the connector unplugged I did not get anything when measuring ohms between C or D and (body ground, battery or B), it just said .0F.

I measured 3.9K ohms between D and B on the MAF sensor itself. Nothing between C & B.

Does this sound like MAF, ECU or Wiring issue?

 

[cougar_68]

I checked the four fuses that came with the new EFI harness. The 15 amp fuse was blown. So I replaced it and found that I had 12 volts at the MAF.


However, when I did a Key On Engine Running test, the fuse blew.

I did a KOEO test first and got the same codes as before 81, 82, 84 & 85. Then went to do the KOER and the car died the first time I pressed the test button. I restarted the car and tried again, that's when the fuse blew. What would cause this?

 

[cougar_68]

After further inspection, this 15 amp fuse is prior to the Power wire for MAF and the Left Oxygen Sensor. I don't have the Left O2 sensor hooked up, not sure if that or the MAF is causing the problem?

Come to think of it, I guess that the computer could have also caused this? Maybe there is something shorted in there?

 

[BiminiBlueLX]

My '91 lx has done something similar. I saw this thread before posting my own so here's my story: I had set up my video camera to record an exhaust sound clip and as I revved the car (cold start up) everything seemed fine. I let it rest a few seconds at idle, then hit it again and *POP* the noise came. On later review of the video, I saw after the *POP* that the left side tailpipe had a small amount of bluish white smoke come out from it. All I can think of is that maybe an injector isn't working properly. If anyone knows anything more about these issues, I'd appreciate any advice. if you want to see the video for clarification i can post it to youtube.

I feel your pain cougar_68.

 

[TenorPlayinGuy]

After reading through the plus 60 amount of replies, I cannot believe no one thought to ask if he'd pulled the distributor at any time. It sounds alot like when you put the distributor in cockeyed. Do your self a favor and crank the timing pointer over to about 10* BTDC, and make sure your rotor is pointing at the #1 for the cap. If not, you need to re-clock the distributor. I'm not saying this is exactly what is wrong with the car, but everytime I pulled my distributor and put it back on a little...off, it would buck, and only run okay under about 70% throttle. Not to mention the indescribably annoying backfire. So check it out, let us know.

And if you don't already have it... 1,3,7,2,6,5,4,8 the good ol' firing order for our 5.0s (thats going counter-clockwise on the distributor.

48
37
26
15
^^

 

[cougar_68]

Have never pulled the distributor. Will check firing order...

I had an idea. What if I replace the fuse then unplug the MAF and try to start it again an run the test? If the fuse still blows, I will know the problem is with the Left MAF (or a short in the wiring). If the fuse does not blow, then the MAF is causing my problem and I can replace the sensor.

I think it is pretty much down to the MAF or Left O2 sensor wiring.

How many Amps should these systems pull? Technically, I would say the left O2 would have none unless the connector is arcing across.

 

[cougar_68]

Well I narrowed the fuse blowing problem down to the power wire for the left O2 sensor. I disconnected the wire, replaced the fuse, and the car ran great. The O2 wire must be grounded somewhere.

I took the car for a drive it ran great. So I brought it back and ran the codes. Got 81, 82, 84, & 85. Then I got 10 (moving onto continuous memory test) and 11 (pass!!!). So I ran the KOER test and the only codes I got were 91 (Left O2, duh) and 33 (EGR, what EGR?). So it looks like it should be all good. It was purring like a kitten (except for the squealing from the PS pump, seriously, will it ever end?).

As per usual, I will do a test drive tomorrow. Cross your fingers for me. I think I'm going to wire tape mine crossed until tomorrow, hope it's not too late already.

 

[cheese_05_70]

sorry i lost track of the thread. open the computer and check the capacitor if they are leaking there will be black goop under them. my car was running great one day and like **** the next. also its hard to understand did you get the o2 fixed? basically what's all the problems that aren't fixed right now? thanks

 

[cougar_68]

The only problem with the fuel injection that I can think of is the Left O2 sensor is not installed. There is a plug in the O2 bung that is stuck. Also, the fuse for the power wire of the Left O2 kept blowing. Since that O2 isn't hooked up anyway, I went ahead and disconnected the wire until I can figure out where the short is.

Day 2:

Car ran great. I took it for a long drive and not one backfire. The real test will be trying to drive it to work tomorrow. If it will fail, it will do it when I actually have somewhere to go.

 

[Lawbreaker5.0]

I didn't read all the posts but I had his same problem. Mine wasn't a full out backfire but was more of a muffled backfire. Anyway, I was checking compression cuz I never had after ali freshened up this motor I bought and when I pulled #4 plug was cracked. These were brand new plugs maybe 400 miles on them. I replaced it and no more backfire. Check your plugs and if you alfeady have disregard my post. Good luck

 

[aar0s]

I didn't read all the posts but I had his same problem. Mine wasn't a full out backfire but was more of a muffled backfire. Anyway, I was checking compression cuz I never had after ali freshened up this motor I bought and when I pulled #4 plug was cracked. These were brand new plugs maybe 400 miles on them. I replaced it and no more backfire. Check your plugs and if you alfeady have disregard my post. Good luck

and make sure your gap is right on them! i had that problem before

 

[cougar_68]

Day 3:

The real test, drove the car to work (man I really, REALLY, need to get the power steering pump fixed). It ran great again! Dare I say it, I think it may be fixed?

 

[stang9150]

http://www.stangnet.com/mustang-forums/809062-strange-misfire.html

Turned out to be bad o2 sensor for me.Sounds like the same issue minus the blowing the fuse deal