p1391 random/intermitent miss/stumble Fix

This one drove me crazy and cost me a lot of money throwing parts at it, so I thought I'd share my fix that may help some of you.

'99 4.0 4x4 code: p1391 - symptoms: random misfire/stumble/stalling, usually over bumps or right turns and reverse.

After replacing:

TPS,CKP & CMP
Ignition Coil
Cap,rotor,plugs
up/down o2's

Improved but still random stumbling/stalling on right turns and bumps.

Partial Fix:

Cleaned/replaced engine GROUNDS! - Found braided ground strap from valve cover to firewall touching metal brake line on firewall and "shorting out" !!!

-Rerouted ground strap away from brake line.


Real Fix:

Also found engine wiring bundle at firewall was wrapped around the backside of the fuel-rail. The fuel rail had punched through the corogated plastic wire bundle protector... so I dug deeper... removed the plastic and found the black tape covering also worn through... dug deeper... "3" three wires in that bundle, right at the same spot, had the insulation worn through just enough to barely expose the wire. It all made perfect sense... every bump or right turn was enough to touch these wires to the fuel rail causing a short i.e. intermitent loss...!!!

-Wrapped the wires individually with tape, wrapped the bundle with tape, replaced plastic protector and wrapped with tape... Pulled the bundle up and away from fuel rail secured to the other wire bundles on the firewall.

No more codes! no more cel! all problems gone! runs better than ever!

Hope this helps someone!

Yup I had the same problem and the same fix on my '97

I had the EXACT same issue on my 99, but it has shorted out the Fuel pump/ecu constant power wire and every time I'd go over a bump it would pop the fuel pump fuse and kill the motor.

had a similar problem, but in my case the auto trans would blow a fuse, replace fuse and it would be ok for a couple of minutes and then pop again, i manually shifted it for about a year, turned out to be a chaffed wire as you described.