NEED HELP! BAD SENSOR,VACUUM LEAK.

[BRONCO BILLY'95]

I have a '95 XLT with a 302,4.10 gears,35.12.50 BFG's,6"pro comp lift,3"body lift.I've been rebuilding the whole truck.I repaced everything in the front end.New Warn hubs,bearings,brakes,ball joints,rod ends,shocks.I have now moved on to the engine.The problems I have are when I rev the engine it will want to almost stall when I let off.It will drop down to about 100-200rpm,then return to normal idle.Also at highway speed 45-55 it will sometimes almost sound like its running out of gas,almost stall, I'll get a check engine light,I'll then give more pedal and it seems to clear up.I did notice a exhaust smell at the same time.(had the rear window down)I had it scanned today and got a KOEO 157(mass air)I replaced it tonight and it still wants to stall after revving.Do I have a vacuum leak?Is there an easy way to find leaks?Maybe EGR?

 

[Justshootme84]

Did you disconnect the battery before replacing the MA sensor? The ECM needs to reset and clear the code(s).

 

[Seabronc]

Try this, it won't hurt http://www.fordmuscle.com/archives/2002/01...pin/index.shtml

Good luck,

>-

 

[miesk5]

right,

a JSM advised, clear the codes and then try self terst or get it scanned again if CEL comes on

if no go, clean connectors as Seabronc advised, and inspect MAF connectors,

& do you have a K&N or other aftermarket filter; could be over-oiled...

also, a recent post elsewhere:

"Code check: Key On Engine Off (KOEO) provided a code 157 Mass Air Flow (MAF) sensor. The only way a code

157 can be set is if the MAF voltage signal to the PCV drops below .400 Mv while the engine is running.

With the engine at idle ~nd transmission in gear, MAF signal voltage was at .410 M v. The common cause of low

voltage readings at the MAF is due to vacuum leaks, but none could be found. It looked like a costly MAF sensor

was called for. The engine was having another fit; a noticeabler attle seemedto be coming from the PCV valve.

Hitting the PCV valve made the engine's idle change.

Pinching the PCV vacuum line with pliers corrected the problem instantly. The PCV valve was new. When

removing the valve to check the part number, none was found. An OEM replacement PCV was installed. The

replacement PCV valve corrected the drivability and stalling problems."

""-

The customer had purchased a generic PCV valve at a DIY parts store. The incorrect PCV valve nearly cost a

misdiagnosedM AF. Just becauseit looks the samed oesn't mean it works the same!"

L - Article contributed by Jim Grant of the Council of Advanced Automotive Trainers, www.caat.org,

producers of the Automotive Emissions Repair Network (AERN)

---

and,

From the Ford Manual: DTC 157 indicates that the Mass Air Flow (MAF) sensor signal went below 0.4 volt sometime during the last 80 warm-up cycles. Possible causes:

- Open MAF SIG circuit.

- Open MAF RTN circuit to MAF sensor.

- Open VPWR circuit to MAF sensor.

- Open PWR GND circuit to MAF sensor.

- MAF SIG circuit shorted to ground.

- Damaged Powertrain Control Module (PCM).

- Damaged MAF sensor.

- Air leak before or after MAF sensor.

- MAF sensor disconnected.

- Idle Air Control (IAC) system failure (at closed throttle position).

When the PCM determines that the MAF circuit is malfunctioning, it will cause the EEC system to go into "failure effects mode" which results in the PCM using substitute values from tables stored in memory rather than using "live" signal feedback from the MAF sensor. This could cause doggy performance.