I have a 99 xlt extended cab 4x4 4.6L. About 6 months ago it was running perfect.. I decided to change spark plugs . the auto zone didn't have the Bosch in stock so i got auto lite... Truck ran like carp after.. Misfire so bad I had no acceleration 0-30 in 2 mins.. So I went to another store and got the Bosch double platinum .. No change truck misses and runs horrible.. So in the past 6 months of trying to fix the problem I have put on new , plugs(5x),plug wires,coil packs, egr valve, egr tube, egr solenoid, egr pressure sensor, both o2 sensors, pcv valves and hoses, every single vaccine line, fuel filter , fuel pump. I have full compression on all cylinders. Also did upper and lower intake manifold gaskets, new battery and alternator.. I have no codes coming up , no check engine light on.. I put on 2 new catalytic converters ... I am at a loss. I can not figure it out and i have invested over 6000 so far in a truck i bought for 400 .. I don't wanna let it go, but unless anyone out there knows of something I'm missing ima have to shell out 2000 for new bottom end.... PLEASE HELP!

 

Wow... that's a lot of money to throw at it. You hould have showed up a long time ago. First off.... anything but Motorcraft plugs in these trucks is asking for trouble. But lets get to the basics first. Do you have a check engine light on, at any point during this? What are the codes?

 

Only codes where for o2 sensors but I put new ones on and light went away.. But when i try to accelerate quickly it blinks for the knock sensor saying hard miss fire

 

The only plugs I would run in a modular are Motorcraft or Denso Iridiums. Start there.You can order either at rockauto.com

 

Since it all started with a plug change, plug wires out of order seems reasonable.

 

Checked and rechecked plug wire position then replaced wires with new motor craft wires..

 

X2 go buy some motorcraft plugs and try again. You'll hear that recommendation a lot in this section. For some reasons our trucks are PICKY when it comes to plugs

 

Listen to what these posts are telling you lol.

Yea, Bosch plugs are the worst and can't handle the heat range. So bad they heat up and fall apart inside the cylinder. Most just eat a valve when that happens...snaps a piece off a valve, hacks the seat..that's what makes them the worst.

Autolite isn't as bad BUT, - whether they can handle the heat range is hit and miss. Consider yourself lucky if you get 8 good plugs in a row. This is why Autolite plugs are considered Motorcraft rejects. They are made side by side at the same factory. Motorcraft plugs are produced strictly to Ford Motor specifications. Autolites are not. All that may occur running Autolites in a Modular, and it's quite common, -the engine will suffer misfires. The bad in that, -not only have you lost money/time and have to change them out, you can also cook your converters if you run with misfire too long.

Crazy to even attempt to use those two brands in a modular. They can stab you in the back/pocketbook deeply.

The only plugs for these engines, -

Motorcraft
Denso
NGK

Nothing else is good enough.

 

These are the 3 brands I will use exclusively on anything (even lawn care equipment gets NGK).

 

It appears this statement is true, it all started when you changed plugs...... I know it was 6 month and 6K ago but what else did you do? Firing order is so inportant and you stated that you checked that several times. I cannot think of anything that would throw an engine into a powerless pc of crap faster than a spark timing issue. Best of luck to you, I hope you find the problem and share the results...we can all learn from this one.