91 I6 Rough Start Bad Idle No power
Senior Member
Joined: Oct 2016
Posts: 3,695
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From: Parts Unknown
Posting For anyone with similar problems: 91 F250 4.9L with Hard start when cold unless feathering throttle, low idle, rough idle, dies at idle, with lean code 172.
The fuel pressure stayed constant at 50 psi and at 55 psi when the regulator vacuum line was unplugged. Which led me to rule out a stuck-open injector. When I took off the vacuume regulator I noticed the engine ran much more smooth but bogged down again when plugging the manifold port with just my finger.
Eventually I located the MAP sensor on the Firewall with a vacuume line going to it and a wire harness. When unplugging the Map sensor Vacuume line the engine runs fine.I used my vacuum gauge and measured 19Inches of vaccume at the intake manifold. So then applied the 19Inches of vaccume to the Map sensor hose with the manifold port plugged and the vehicle immediately began to run rough and die.
Check Map sensor on firewall,
Could be bad capacitor in ECU that controls air/fuel from input from Map sensor. When disconnecting map sensor the engine runs on predetermined values.
The fuel pressure stayed constant at 50 psi and at 55 psi when the regulator vacuum line was unplugged. Which led me to rule out a stuck-open injector. When I took off the vacuume regulator I noticed the engine ran much more smooth but bogged down again when plugging the manifold port with just my finger.
Eventually I located the MAP sensor on the Firewall with a vacuume line going to it and a wire harness. When unplugging the Map sensor Vacuume line the engine runs fine.I used my vacuum gauge and measured 19Inches of vaccume at the intake manifold. So then applied the 19Inches of vaccume to the Map sensor hose with the manifold port plugged and the vehicle immediately began to run rough and die.
Check Map sensor on firewall,
Could be bad capacitor in ECU that controls air/fuel from input from Map sensor. When disconnecting map sensor the engine runs on predetermined values.