91 F-150: Rehab 4.9L I6 or swap in rebuilt 5.0 V8
 

My dad bought a fairly clean 91 F-150 5-speed last year for use as the company truck and we've been going through and fixing things. The suspension is entirely rebuilt with added swaybars and steering damper, AC system totally redone with parallel flow condenser, totally new fuel system including tanks, and we had done some work on the I6 to get it running (bent push rods upon first startup after sitting forever).

We were having the classic Ford 300 issue of an exhaust leak at the manifold and that was making it unsafe to drive the thing, so we bought a set of headers thinking that we had a set of cracked manifolds. Turns out that the manifolds were not crack (very rare as I understand) but whatever idiots worked on it last didn't put in exhaust manifold gaskets. As we were installing the headers, one of the exhaust studs/bolts (which are NLA) broke in the head and we couldn't get it out. We already planned to do a clutch job and replace the rear mean seal plus motor mounts and all so we went ahead and pulled the engine, planning to have a shop extract the stud or put on a rebuilt head then replace the cam and lifters since they were starting to chatter. Turns out the head gasket had been slowly leaking at cylinder 6 (we thought this might be an issue before doing this) and eroded the cylinder wall slightly. It's going to need to be honed and probably new rings in the engine. We figure the cost of the parts we need is around $800.


So we decided to check out if there were any better engines around to start with or maybe a V8 to swap in. We found one and have been talking to a guy with a truck that has a rebuild on the V8 with not many miles on it that was totaled, for $500. Since I'm not an expert on older Fords, what would you guys do? The V8 is certainly a more attractive option, we don't need the low-end torque of the 4.9, the 5.0 seems like it would be a bit more comfortable cruising on the highway and has much better aftermarket support.

I am surprised to hear about the i6 failing - those engines have a reputation for being extremely durable.

They are. It only has 160K miles, but I think neglected/improper maintenance caused the damage to cylinder 6, having coolant going into the cylinder never is good. We've found a lot of dumb little things when fixing up the truck. The other 5 cylinders look fresh out of the factory. I'm sure we could do the rings plus the hone and it'll be fine for another 160K. It ran fine before besides needing lifters, and those were probably bad from it sitting for a few years when the owner was too old to drive it anymore.

A few things to consider-does it need to be smogged? Is the V8 EFI? You would need to transplant the wiring harness from the donor vehicle. This is probably just the beginning. I'm sure you considered the work to do an engine transplant vs repairing the 6. I would go with repairing the 6cyl. I used to have an Econoline van with the 300 6 and a stick shift. Surprisingly powerful.

A lot cheaper to keep it stock prolly. Value wise probably either way not a big difference. If you plan to drive it a lot maybe the swap is a good deal but if not probably a good idea to keep it stock. I don't think you will find the five liter v8 much more powerful than the six unless it is a newer one with all the fuel injection etc.

300 ford inline 6 with fuel injection is a good motor,trouble free in contrast to v8s.

Agreed on the 6. It wont be a speed demon (iirc they only had like 150hp) but they were torquey, probably close to 300 ft lbs. The 5.0 is only marginally better those years anyways. A 351 would probably be a good upgrade, but what is the use?

Turns out the 4.9 needs a full rebuild, I thought that only one cylinder was bad but that's what I heard from my dad who clearly didn't understand what my friend/my dad's full time mechanic was saying. Also considering an LS swap given the cost of a full rebuild.

As a Ford fanatic, go with another 4.9 I-6. Swapping out the engine to a 302 is a horrible idea, due to the poor TQ out of the non-roller 302...not to mention the added cost of swapping out the computer, harness, and mounts.

A 300, well maintained, should last over 500,000 miles...so putting money out now to have yours rebuilt by a local shop is money well spent. You're looking at nearly $2000 for a proper rebuild on that block, depending on your area pricing. But, that $2k will last you at least 20 or more years of good service.

As for the transmission...check the mounting holes when you pull the engine. The M5OD is prone to have cracks there if vibrations in the drive line are present. Since the bellhousing is integral to the case, you would need to replace the whole thing. Mine is mounted to a BW1356 transfer case, and locating then rebuilding a proper replacement ran me $1100 4 years ago at a Pittsburgh local trans shop.

Seriously...just rebuild the 300 or locate another one that looks good...best way to go with that old truck

Yeah, I am looking for another 4.9L, hopefully can find a good one in a yard locally, maybe even with a warranty to make sure we aren't doing the same song and dance in 2 months.

Seems that we are going ahead with an entirely different idea, a 4.6 modular V8 with a 4R70W out of what was my grandfather's Grand Marquis that has a wrecked body but good engine bottom end and good transmission. Probably going to do a j-mod on the transmission and 4 valve heads on the engine.

I sold my 1998 F150 last year. 220k miles and still ran great. Even the A/C. It had a 5.1 or 5.2.

Anyway, I trailered a 40 HP tractor with 5” bushhog or boxblade, etc. it was sluggish going up steep hills but aside from that, an excellent and reliable pickup for many years.


I’d put in A good solid engine. Then keep it or sell it. Your early model, IIRC, has a loyal following. I know mine did.

This is a drivetrain that my uncle declared worn out when the car broke down on him at 108K miles, we then drove it to 245K miles when it started having issues that we put a lot of money into fixing yet never did but figure it was probably a bad head. When we did the head gaskets, the cylinder walls looked perfect. Going to the updated 4V heads that don't have the problem of cracking intakes.

I had this powertrain in a 1999 Ford Expedition which I drove for 93,000 miles with no engine issues, and a minor, inexpensively repaired transmission problem. I did not use the vehicle for towing, and was manic about oil and tranny fluid changes. However, this drivetrain provided leisurely performance, especially on hills.

At least you didnt have the triton 5.4