Hi eveyone,
I have a 1990 4.0 V6 Ranger that I'm going to start using as my winter vehicle. The problem is that anything over light throttle will cause the truck to back fire through the intake and into the air cleaner. I first thought it was the plugs and wires. Although they are pretty new they were the cheap wires from advanced auto. I then scanned the computer and got a code for a bad A.C.T. sensor (Carbon fouled from the back firing), a code for low voltage from the map sensor and also a code for a bad fuel pump circuit. Just for fun, I changed the plugs and it really didn't do much to improve it. The funny thing is that about 1.5 years ago the truck started to "loose" a cylinder when cold, so I replaced the plugs and wires then it was fine. Then about 2 weeks later it would do this backfire thing at full or near full throttle. So that leads me to believe that the computer is giving a code for a bad M.A.P. sensor because it is miss firing and thus the O2 sensor is thinking it is righ. What do you guys think?
Thanks,
-Phil

 

The MAP sensor can be checked with a DVOM that has a Frequency position. It would read around 150 Hz at sea level. It would cause you to have a nasty idle that wandered all over the place, and could give you a lean condition, I spoze, so that it backfired on acceleration. Your condition sounds like lean backfire..
Too bad this board does not let you see the original message when replying, I forgot what else you said.
opened another window and looked.
The fuel pump circuit problem could lead to lean miss.
The ACT can get carbon fouled from the PCV stuff being bled back into the intake. It tells the computer how warm the Air Charge is to account for cold air operation (like a choke on a carb'd car) and enrichen the mixture as apropriate. They can be cleaned sometimes, but if they throw a code, they are generally toast.... Warm toast?
If the MAP is throwin a code, check for loose wires. Apply a variable vacuum to the MAP and see how the engine responds. It should get very lean on high vacuum and richen up as the vacuum decreases. You might be able to see it while idling, but if you could monitor the O2 sensor, you would definitely see it...
Lucky you

tom

 

Thanks for your info. I re-read my post and realized I put M.A.P. instead of Mass Air meter. I'm sorry, I was writing about my truck and thinking about my 87 'bird.
-Phil

 

Have you tried cleaning the MAF? mechanicmatt.com has a page on this.

 

[updated:LAST EDITED ON 08-Sep-02 AT 00:29 AM (EST)]I have taken the sensor out of the housing and inspected it, I will try to clean it though. Also, I'd like to mention that the truck is hard to start when it is cold. Also, I had suspected low fuel pressure so I cycled the key to on then off then back to on to build up pressure. But the truck still took a while to start. I also took the vacuum line off the regulator and plugged it while the engine was running and it still does the back firing/popping. So I don't think the fuel pump is to blame. I know it is probably on its way out though although when I changed the filter there were no metal shaving in it.
Thanks,
-Phil