Engine is 4.0L from a 94 Ranger.

Engine misses during acceleration, surges at times, and backfires at times. Lacks power. Engine comes alive when I "floor it."

Timing is ok. Fuel pressure is ok. New EGR valve. Most, if not all, sensors have been replaced. New plugs and plug wires. New 02 sensors.

Pulled codes, cleared codes, and pulled codes again. I get the same codes everytime (with both old and new 02 sensors). The codes I get are:

172, 173, 176, 177

These codes reference the 02 sensors and address both rich and lean conditions.

I've checked VPWR and VREF and both seem to be fine.

I'm really stumped here. Anyone have any suggestions? Help!

Mark

 

Exorcism.

I'm sorry. I realize that is no help.

. . but I just couldn't stop myself

 

May be a Bad ECM?, and is instructing the sensors incorrectly?

 

have you tried dumming the computer un hook the positive cable for about a minute then hook it back up and let the computer reset sometimes it takes out the bad codes

 

I'm hoping it's not the ECM; however, that's a real possibility.

Yep, I disconneted the battery to clear all codes and settings. I've done this at least a half a dozen times now. After test driving for 10 - 20 miles, I always end up with the same codes.

I've been testing all the wiring and may have found a potential problem.

With the ecm harness disconnected from the ecm, and using a digital volt meter, running one of the probes to pin 46 of the ecm harness (sgn-rtn) and the other to battery positive, I should be getting a voltage reading of 12V +/- 1V. I don't. I get 0.03V. With the O2 sensor disconnected, I get 0.0V.

Any idea what this indicates? Any suggestions where to start looking?

Thanks!

Mark

 

I just checked on my Mitchell On Demand Service manuals and there is a TSB on your problem, you might have a MAF Sensor contamination, which will give you the symptoms you have, plus the codes you're getting. The easiest whay to fix this is by replacing the MAF sensor.

 

I would certainly clean the maf before you replace it. Disconnect it and then just undo the 4 security torx on the black box on top and spray down the thermisters with electrical contact cleaner. Reinstall, reconnect.

 

Thanks guys. I had previously cleaned the MAF; no change. I eventually replaced the MAF; again, no change.

Mark

 

yuo said the problems start after you have drive it your ecm may have a broken board in it some older ford ignition modules when they get hot they quit as to your electronic troubles thats why i drive a 71 f250 no ecm the points slip sometimes no big deal

 

Start by checking your fuel pressure. You may have a bad pump or regulator. I think you're running off in the wrong direction here.


BTW your pin 46 shouldn't have 12v on it. It is the sig ret meaning that the comp sends out 5 volts and this is the line that it returns on. It is not connected to the pos cable though so it shouldn't see anything going there.

 

I had previously checked fuel pressure and it tested ok. Based on your feedback I went ahead and checked again. Pressure is between 30-35, both with key on and with engine running. This is with both the old and new pressure regulator.

With pressure on the fuel system and key off, should the pressure be relieved when applying vacuum to the pressure regulator? When I do this, the pressure drops slightly, then returns to a higher reading when vacuum is removed.

Mark

 

That' sounds fairly normal. When you turn the key on, you should see35-45. When you start the motor, it should drop a little. (should still be 30-45) When you remove the vacuum line at the reg, you should see an increase.
Do you get these codes in KOEO?
Things to check:PCV valve and hose, MAP sensor, and coil to start with. Sounds like you may have a bad coil.