I recently picked up a '91 F150 XLT Lariat with a 5.0 and E4OD transmission. I bought the truck non-running and I did what research I could here to test and troubleshoot the no-start problem and get it running. Problem 1 resolved...the truck now runs...but barely.
The truck is hard to start when cold. So hard in fact that without spraying starter fluid into the TB, I'll kill the battery trying to get it to fire. After the initial spray to get it to fire, it'll run on it's own. When it does start, it has a serious miss at idle. It shakes like crazy, and the passenger side of the engine seriously looks like it's trying to jump out of the bay. Probably a shot PS motor mount, but the shaking is extremely violent. Smooths out drastically over 1K RPM. Adjusting the timing only introduces more shaking as it becomes overly advanced/retarded. Even with the shaking, the truck will idle all day at roughly 700 RPM without having to keep it alive. The exhaust reeks, and being anywhere near the exhaust will cause your eyes to burn immediately. It's also burned through 1/4+ tank of gas, and I haven't driven it anywhere... It will go into gear fine. Sometimes the RPM will drop to 500 and the vibration will get worse but it will move under power, other times it'll smooth out completely because it stalls.
I have a few questions to help get this thing running better:
- This was originally a 2 tank truck, but over the years the front tank was removed. Is this a problem with there being no front tank/pump? Supposedly the truck was run like this for a while and did not have issues. I have no idea as to why the tank was removed, however. The switch still works properly, but pegs the fuel gauge when switched to the front pump.
- The Haynes manual has a different firing order listed for 1993 and earlier F150 5.0s than for the 1994 and later ones. The earlier firing order will not work on my truck but the later one does. Normally I'd just assume that perhaps either the motor or cam had been replaced some time in the past and was "upgraded", but I'm not looking to make any assumptions with this truck at this point. Is there any easy way to verify the proper firing order other than trying each to see which order works best?

Sorry for the wall of text, and thanks in advance for any help!

 

I also forgot to mention...I don't believe the Check Engine light works. I have never seen it light when starting. The Rear Antilock light, however, does. But that's a problem for another day.

 

Sounds like one or more cylinders aren't firing.
Could be a lot of things, bad spark plug wires, spark plug wires not all the way on the plug, fuel injector(s) not firing.
Have you checked the compression?
Have you checked for codes?

 

Thanks Subford!
The truck came with some ridiculously long MSD 8.5mm plug wires. I haven't inspected them, but I think I'll replace them anyway for peace of mind. I did have to change the plugs this weekend, as the old ones were completely fouled and smelled of gas. I'll double check to make sure that I pressed the ends down completely on the plugs.
I haven't checked the compression. I'm hoping it's nothing internal, but I'll see if I can borrow my buddy's compression tester.
I'll have to look into getting an OBD1 scan tool to check the codes. The CEL doesn't light at all so I can't do a KOEO test unfortunately.

 

Checking the codes is the best first step, The firing order has not changed on the 5.0 unless you're looking at the 5.0HO here's a link to some reading for it.

Ford 302 V8 (5.0L) | Specs & History

I personally would start by looking at the fuel injectors, everything you describe is telling me you're dumping fuel into the cylinders at least some of them are stuck open.
The other thing I'd look at is I'd pull the distributor cap and take a look at the towers and the rotor if they're corroded and the rotor edge is really rough, I'd change them. verify the firing order and timing by static, #1 piston on top of compression stroke and rotor pointed to #1 tower on cap.

 

Thanks Mike1!
It was definitely the HO firing order (1-3-7-2-6-5-4-8), and if I'm not mistaken it was listed as the correct sequence for 1994+ F150s. I'll double check once I get home. The HO firing order is also the only sequence that my truck seems to run off of.
The cap and rotor look good. I didn't see any corrosion on either (MSD replacements). One of the first things I did was to static time the motor. I verified #1 and wired everything else counter-clockwise.
As for the injectors, is there any way that I can test them to see if any are stuck open? A quick search shows that they are 19/lb injectors. Would there be any issue running a set of stock, pink-top injectors that I pulled from my 2002 Mustang GT a few years back?

 

I don't know if the stang injectors would work or not.
you can check injectors by testing the resistance in the wires but isnt always accurate you can also do it the old fashioned way and use a screw driver as a stethoscope and listen for the ticking which will tell you its working but doesnt tell you if it's actually letting fuel through. I'd go to the autozone or advanced auto and rent the tool to test them the proper way.

 

I would start with a compression check. The bad misfire/hard start and exhaust fumes could all be a result of a cylinder issue. It's always a good starting point to work from.

 

A small 9VDC battery will check a injector to see if it is working. You should hear a click each time you turn the power on an off with screw driver or stethoscope. Do not use the trucks battery as this can burn the coil open in the injector.

 

Thanks everyone for the suggestions! Unfortunately I wasn't able to dedicate any time to the truck yesterday, but I should be able to put in some time today after work. In a few minutes, I'll be heading out to pick a FP tester and possibly a compression tester as well. I also have an OBD1 scan tool and new plug wires coming in the next few days. Hopefully these parts will at least help isolate the issue.
Perhaps I should replace the coil as well as preventative maintenance. Even though I verified that I was getting spark, it wasn't anything to brag about.

 

So initial tests of the fuel system seem to check out. FP gauge shows 37psi with the key on engine off and doesn't seem to drop off when turned off. FPR vacuum hose doesn't seem to be sucking fuel either. Is this definitive enough to rule out the injectors being stuck open?
I would've tried to get FP readings with the engine running but it wouldn't fire and with daylight fading and me being new to the neighborhood, I decided to call it a night.
This is starting to bring back memories of my old 5.0 Explorer that had a dead miss at idle. Luckily that was due to a brand new plug wire that was cracked under the boot.

 

Pulled codes: 111, 10, 327, 328, 332, 334, 542, 543, 556
Stored codes: 327, 328

I believe that codes 542, 543, 556 are related to the lack of the front tank and pump.

 

sounds like a problem I had with my 94 about a year ago. In my case it was the ABS sensor. I know that sounds crazy but I replaced the sensor and things have been OK ever since.

 

Yes.

That would seem to indicate a serious EGR system problem but that should have no effect on the way the engine runs unless the PO connected the EGR valve directly to engine vacuum.. which should not be. You may want to disconnect and plug that vacuum line until you sort out the bad miss.

Yes quite possible so ignore for now.

IMO there is nothing in the codes that would cause the problems you have so go back to basics and make sure all plugs and wires are routed correctly and are in good condition, all cylinders have good compression, and the distributor is stabbed correctly at TDC on Cyl 1. And just to be thorough you are aware of how ford numbers cylinders? If it turns out your motor uses the HO firing order then it's not the original motor and that begs the question.. what else did the PO change? Hopefully he didn't use any wiring from the donor vehicle because there is no guarantee it is compatible with your truck and could cause problems with the injector firing.