Hi Guys,

I bought a 1996 F150 extended cab long box 2wd with the 5.8 351 Windsor less than 2 weeks ago. It has 177k on it. I have been a life-long Ford fan, having had several F150s, Crown Victorias, Grand Marquis, Mustangs, Torinos, etc.

Anyhow, the idle in this latest truck was just slightly rough....just enough to notice. I replaced the fuel filter (looked like it had NEVER been replaced in all of those miles) with a new Motorcraft filter, cap, rotor, plugs, and plug wires. Now the truck is not running well at all and I can't figure out why. The old plugs were Autolite copper 24's and I replaced with the same, gapped to 44 as per the decal on the underside of the hood. Wires are Denso, correctly routed. The only other thing I added was a drop-in K&N permanent air filter.

I have been reading a lot of arguments about Autolite versus Motorcraft spark plugs and others.

Supposedly, Motorcraft SP 501 plugs are original equipment but I would like to know if that is a true fact because 1996 seems early for a manufacturer to have been installing platinum plugs at the factory.

If it makes any difference, this truck is a very early 96, having been built in August of 1995. The first 3 digits at the end of the VIN are 009, thus my screen name.

I would just like to solve this problem and get the truck running smoothly. Any input on spark plugs and anything else would be GREATLY appreciated.

 

Advance Auto had a sale on the Motorcraft SP 501 platinum plugs, so I bought a set of those, gapped them to .044, and installed them. No improvement at all.

Any ideas?

 

Does run bed at all rpms or at idle/off idle? Have you checked to see if it has a code thrown? It's not uncommon for those older high mileage trucks to have a code and burnt or removed CEL. I would think your truck would have dual tanks. Does it run the same on both?

 

I tried an MSD cap and rotor. No improvement.

I think I may have narrowed the problem down to the plug wire routing. I've had the wires correct from the plugs to the corresponding positions on the cap, but I think the positioning of how they get there along the way may be the culprit. Going to Auto Zone today, because someone told me they will do a printout of the routing, because search engines on the internet have been no help in finding that specific diagram.

Engine running is lousy across the rpm spectrum, not only at idle.

Previous owner disabled the rear gas tank and only used the front one and I am fine with that, by the way. I had a 94 F150 about 15 years ago and I never bothered with the rear tank on that one either.

 

Routing doesn't matter, until a wire rubs through and grounds out. Firing order does.

 

Based upon what I'm experiencing, routing evidently DOES matter. What's happening seems to be crossfire.

Up until just reading about this since this problem cropped up, I had thought that covered wires were shielded from each other....done deal. It seems that is not the case.

I was thinking about one of my 3hp push mowers. The throttle cable was messed up for a while and temporarily I had to pull the plug wire off to stop the engine. The covering of the wire was perfect, with no cracks or anything. Still, if you touched that covered wire with bare hands you would get a shock. That at least taught me that the covering on a wire does not seal the electricity inside 100% as the average person would assume. It's different with engines than with household electrical extension cords and I leave the experts to explain why.

Point being, changing the plug wires on the truck created a situation that did not exist before. The plugs, cap, and rotor have been eliminated as the source of the problem. The wires from plug to cap are in the correct firing order. All that leaves is the routing along the way.

 

How old are the wires?

 

also, comparing the plug wires from a lawn mower to a car engine is pointless. Two different animals. Automotive plug wires have more insulation.

 

When I went to Auto Zone, I picked up a set of Duralast Gold plug wires. Installed those and that immediately eliminated the problem.

One or more of the brand new Denso plug wires must have been defective from the factory. That's the last time I will have anything to do with that brand.