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1992 f 150 6 cylinder cutting out

 
  #1  
Old 02-07-2016, 12:11 PM
Tommy28097
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1992 f 150 6 cylinder cutting out

my 1992 f 150 4.9 is cutting out around 1500 rpm.There is no CEL on and I can`t get a code.I have checked for vacuum leaks and checked the wiring.I have checked the fuel pressure on the rail,45 lbs running
Can`t find the problem.My timing is off a tad bit and I am supposed to get it set tomorrow or Tuesday.
Any suggestions in the meanwhile?
Thanks
 
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Old 02-07-2016, 02:03 PM
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Does it cut out under load, or even revving it in park/neutral?
 
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Old 02-07-2016, 03:06 PM
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just sitting in park.It barely moves in the driveway
 
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Old 02-07-2016, 06:54 PM
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Try unplugging the electrical connection to the manifold pressure (MAP) sensor, and see how it runs. It's a little black rectangle on the firewall with a vacuum line going to it.
 
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Old 02-07-2016, 07:43 PM
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I cranked it up,pulled the MAP electrical plugin and the rpm fell but it kept running
 
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Old 02-07-2016, 09:49 PM
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Okay, but how did it react giving it throttle? With the MAP sensor unplugged, the computer will go into a "default" mode where it substitute the MAP sensor with some default values.
 
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Old 02-07-2016, 09:53 PM
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it did about the same,not much difference Ian
I have just replaced the PCM because the old one had a few burnt circuits,which helped get my CEL off but the skipping and bogging out was still there.
 
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Old 02-07-2016, 10:16 PM
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One of the neat things about these trucks, is the computer will generally compensate for bad sensors if you unplug them. My favorite method for trying to quickly narrow down something down is to simply unplug things until the problem goes away.

Assuming your fuel pressure is good (45 PSI @ idle is normal for a 4.9, but it should go up to 60 PSI when you crack the throttle wide open. Have you tested the fuel pressure under high throttle):

There are four main sensors involved with fueling the engine:
MAP sensor
ECT (Coolant) temperature sensor
TPS (throttle position sensor)
O2 sensor

Since unplugging the MAP made no difference, I'd plug it back in and next try unplugging the coolant temperature sensor (it's the two wire sensor located next to the upper radiator hose). If no difference, plug it back in and try the TPS, and then once again with the O2 sensor.

This is just a fast way to quickly and roughly eliminate things. If none of that made a difference (and your fuel pressure goes up to 60 PSI when giving the throttle a quick crack wide open), I'd suspect an ignition issue.

Plugs, wires, cap and rotor should be changed if they haven't been done in a while. After that, you have the ignition coil, TFI module, and the PIP module inside the distributor. With no erratic PIP signal code, I'd suspect a failing coil or TFI module (assuming no other sensors are acting up)
 
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Old 02-08-2016, 06:30 AM
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thanks Ian,I`ll go down the line and see what happens today,I think I will hook my fuel pressure gauge first and see how it looks with throttle wide open.Thanks for everything
 
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Old 02-08-2016, 12:02 PM
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I did what you suggested
MAP sensor-unplugged,motor idled up a tiny bit but run smooth at idle
ECT (Coolant) temperature sensor - same thing,no change-run smooth at idle
TPS (throttle position sensor) motor cut off when unplugged
O2 -same thing,no change-run smooth at idle

the fuel pressure was 48 when idling,when I tried to throw the throttle wide open,it cut out,bogged down and quit running.The fuel pressure was dropping down to 42 during this before it quit
the CEL was not on,but I tried to get a code with my OBD 1 reader,it said 332,insufficent egr flow detected

the distributor ,cap and rotor are new.
 
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Old 02-08-2016, 12:35 PM
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Sounds like a fuel issue.
On 332 what was that code from (CM or KOER)?
You have cleared the codes before you ran it , right?
 
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Old 02-08-2016, 02:32 PM
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If the fuel pressure was dropping when you snapped the throttle open, I would suspect a plugged fuel filter or failing fuel pump. The pressure should have instantly snapped to 60 PSI or so when you cracked it open.
 
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Old 02-08-2016, 03:21 PM
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Bill,I did clear the codes
it came from a KOEO and it means insufficient egr flow.I may try to clear it & do a retake later today.It don`t hurt to check all my fuses,I haven`t done that yet but I will today.

Ian
I checked the fuel filter and it was putting out a good stream of clear good gas.It looked good.A friend of mine asked a fella mechanic he knows and he suggested I start looking at ground wires.He first asked me to unscrew the TPS and remove it from the throttle body and leave it plugged in and try it and it did the same,cutting out around 1400.

this has been a good learning tool for me and I appreciate the help
 
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Old 02-08-2016, 04:18 PM
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Before messing with any grounds or any more sensors, you must get the fuel pressure problem addressed.

45 PSI at idle, and 60 PSI WOT is what you should have. The fact that it doesn't go up, but actually goes down as you crank the throttle indicates a serious fuel supply issue.

Cycle the key on/off several times with the engine off. You should hit 60 PSI. If it hits 60 PSI doing that, I suspect you have a failing fuel pump that is unable to keep up with the demand of the engine.
 
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Old 02-08-2016, 04:33 PM
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ok I have nothing to lose so I give it a shot

I cycled the key on/off 4 times,with out trying to start it, and the pressure was 51 lbs
 

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