Misfiring a bit after warmup


Old 11-21-07, 02:23 AM
Thread Starter
Join Date: Oct 2005
Location: USA
Posts: 274
Misfiring a bit after warmup

I have a old but well maintained 1990 F150 4.9 liter manual shift. It runs fine when cold and warming up but once it has warmed up it has a slight misfire on and off and not constant
with engine speed.
New plugs,cap,rotor,wires have had little effect on the problem.
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Old 11-21-07, 07:01 PM
Join Date: Aug 2003
Location: Cape Cod
Posts: 4,320
Pulling the codes on this type of vehicle is a good DIY project, and should be the first step when troubleshooting
Even if you don't expect any codes
And even an "all clear" response tells you something

This vintage EEC-IV controlled engine can store many trouble codes w/o firing off the Check Engine Light

Here's some links on DIYing the code pull:
BAT hosted by TroubleCodes.net
Ford Fuel Injection.com

And just a heads up, this vintage and type of engine seems to be very sensitive to plugs, wires, and other ignition consumables
Stick with Ford/Autolite(OEM)
If you have replaced any/all with aftermarket parts, I'd suggest switching to OEM until the problem is solved
I wouldn't mention it, except it's scary how many times A/M ignition parts have led to chasing around the diagnosis of problems on these engines ("it can't be the plugs, they are new..." etc.)

Post up the codes, if any, when you pull them
As I mentioned, even no codes tells us something

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