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crankitup 3/25/2010

1992 Ford F-150 XL 6 Cyl 4.9L


P/U cranks/wont start/ 2 fuel tanks/pumps/ has spark / fuel rail psi=83# (no vacuum on regulator yet)

This darn engine started to run rough..2 weeks ago..chugging after a cold start, and then intermittently. Cold weather 40 degrees. Finally wont start...cranks good and has spark..occational one cylinder will fire/ no chk engine light at all but illuminates on bulb circuit check. Changed fuel filter/ old one was passible(blew threw ok). switched tank to rear tank...same problem/ no start. crank is fast and normal. I would imagine, with 80 psi on the rail, with spark and air obviously this should start. What has me thinking though (the dummy that I am) Ha!...is why is the pressure so high. IF and only IF the injectors were opening, then the fuel should ignite and the engine should run somewhat even if rough. My thoughts are, no pulse to the injectors?.....Does the ECM decide the injector full open on cold start, or is there a CSV cold start valve. I'm too fat to stand on a ladder in hang into the engine compartment easily.
It's aggravating as heck! Err..Err.Err%^puff Err. Err...puff!

2 Answers


HouseCallAuto 3/25/2010

The fuel pressure is too high but that would not prevent it from starting. You have to check for injector pulse. A simple 12 volt automotive circuit tester can be used. One side of the injector (red wire) has battery voltage with the key on. During cranking the other wire (white) should flash the circuit tester on and off as it cranks. The circuit tester should be clamped to battery + for the pulse test.

Answer this question - very important... Does the fuel pump run continuously with the key in the on position?


crankitup 3/26/2010

Thanks HCAuto... Have a Simpson 270 ....and a Fluke meter...but I will look for the test pen

BTW... The pump (fuel) did not run continually with the Ign. switch turned to the run.
I had jumpered the Ground (-) on the fuel pump relay to be sure the pump was running whilst cranking the engine. Thank you for the answer.
I'm scared the ECM may be kerflunked! ( tears) .... I'll get back


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