Hi,
Trying to find a nice clear graphic for the spark plug wiring order for my truck. Info is in my signature, but to confirm its for a 91 f150 v8, 5.8l, 4x4, xlt lariat. I have a haynes manual but its not very helpful in this case. My first time changing plugs, wires, distributar cap and rotator, so ill take any experienced advice!

 

I believe your firing order is 1-3-7-2-6-5-4-8. The distributor rotates CCW as you look down at it.

 

Thanks! I was able to find a graphic that corresponded to the timing you provided. Ive added it to this message. Ok, after referencing the graphic to what im seeing on my truck, im a bit concerned there are some discrepencies here! Seems like the previous owner may have really mucked this up at some point, assuming the graphic I provided is correct. If it is incorrect, how is my truck even running well? Ive added some images of my distributor cap. You can see the number 7 written on the distributar cap is incorrect, it should be #8. The #6 written on the cap should be #4. Ive also added an image where I drew a red line over one of the wires. According to the graphic, this should be going to #3, but its going to #5. I havent checked all of the wires yet, its over 105 degrees out right now, but it looks like #1 is going to the correct spark plug. Am i just missing something here? Or can anyone tell by the images ive sent that this definetly wrong? Im a little afraid to mess with wiring now since the truck us running pretty good. If i do get the wiring wrong, can I destroy the engine, what are the risks involved? I feel like if the wiring was wrong, it would very noticeable immediately. Thanks for any help!

 

Ok so I made a diagram of my current wiring configuration and it looks like its been setup for a 5.0l engine this entire time! I double checked my work, and here is the current order on my truck:1-5-4-2-6-3-7-8

According to my Haynes manual, it says "255, 1993 and earlier 302 (5.0L) and 460 cu in (7.5L) V8 Engines = 1-5-4-2-6-3-7-8". Which is exactly how my wiring is setup.

And for 5.8L Haynes says "351W (5.8L), 351M, 400 and 1994 and later 5.0L V8 engines = 1-3-7-2-6-5-4-8". Which is what you posted and what I've seen online. So sanity check, sounds like the previous owner or their mechanic used the wrong diagram? Or am I reading the Haynes manual incorrectly? Seems like a no-brainer here but wanted to get a second opinion before I change to the 5.8L configuration. If its been wrong this whole time (1.5 yrs since I've had it, who knows how long its been though) , could there be any damage? Or just lower performance? Thanks! I've also attached my diagram.

 

Why are you interested in the firing order if you say your engine is running well? The engine would be running very rough of the firing order was off. Are you sure you have a 5.8? Maybe someone changed engines. Before you get into trouble, I would see if you can verify off a junkyard vehicle.

 

Its running pretty good but my acceleration seems to struggle at speed. So Id thought I'd give some new plugs and wires a try. The distributor cap and rotator are cheap, so might as well replace them as well. A big part of it too is I just want to learn how to do it. I wasn't really interested until I was going to replace everything and wanted to make sure I had the correct wiring diagram. After comparing my current wiring with the order from you, other resources on the internet and Haynes, I noticed a discrepancy. So just curious why that is. I'm sure it's a 5.8L, says it on the engine, and I even checked the VIN, but i guess that would be irrelevant if the engine was changed. But, I totally agree that it should be running like garbage if the wiring is off. We have a pick and pull here but they currently don't have anything that matches ( i recently checked cause i was looking for new door panels), so I have no frame of reference from a junk yard. I don't know, maybe I should just leave it be. Attached what I see in Haynes and a picture of my engine.

 

Go ahead and change the plug cables to match your findings and see how it performs.. Make sure you make a sketch of your present firing order in case you have to change it back.

 

Thanks, I'll give it a shot! Made a wiring diagram yesterday, and got pictures so should be able to change it back no problem. I'm a little nervous, won't lie! But all evidence points to my current configuration as being for a 5.0L, below is what I made yesterday, this is my current firing order.

 

Well, that was a total disaster after changing the wiring order. Ran terrible. Idled at about 500 rpm for about 5 or 6 seconds, the truck was shaking pretty good, then I just shut it off. So pretty confused about this. Is it possible the previous owner adjusted timing to make this current configuration work? Can't wrap my head around it.

After resetting my wiring to the original configuration it started up fine. I took it around the block a few times, everything seemed good, then i started to hear splashing. My coolant overflow tank was completely over flowing. Opened the hood, I could hear air bubbles in the overflow tank and main radiator hose. Now, I also changed my radiator cap this morning too, so there's that. I let the truck cool down for about 20 minutes, and took a quick trip around the block, everything was working normally again, took another trip around the block, still fine. Then took it on the highway for about 20 minutes. Everything normal. So I'm wondering if my old cap was bad and I just overfilled my radiator. With a new cap, the pressure on the system was higher and it just purged the extra coolant and possibly some extra air that was still in the system? Maybe wishful thinking but I can't think of another reason at the moment. Thoughts? As always, appreciate the help.

 

Wrong sir... pre 1994 is 1-5-4-2-6-3-7-8