I recently bought a 1994 F-150 5.8 auto 4x4 for 200 bucks! The guy said the truck needed a new timing chain and since it has some rust and is old, they wanted to get rid of it quickly. He said it was driving fine and then all of a sudden lost lots of power and started misfiring. It starts up great and can move around on its own. I towed it home and would start it up to move it around on my property to park it one place or the other. This week, I went to drive it for the first time on the road to get it registered. I wanted to see what the power was like since it's hard to tell when I just move it 20 feet or so on my property. It was misfiring a lot (but not very bad misfires) and had very little power. Top speed was about 40mph on flat ground, and would quickly lose speed going up the slightest incline with 20mph being about the top speed. I just pulled KOEO codes and got 111 and 334. The battery died so I couldn't get KOER codes. The catalytic converters aren't glowing after driving, but I'm not sure if the trips I've made have been long enough to heat them up badly. The engine operating temperature is in the proper range and doesn't overheat any.

I just got done replacing the timing chain and sprockets today (it seemed a little loose so it probably need to be done), but the problem has not gone away. The misfiring seemed more pronounced now, with the power being a little better possibly (very little if any) but I did hearing pinging as well which I don't recall hearing the last time I drove it. However I did check the timing and change it from 10 ATDC to 10 BTDC.

My Bronco used to have a 302 in it, so I am familiar with this setup, but not anything that has to do with automatics (since my Bronco was and still is manual.) My odometer/speedometer has the issue when it pops down to 0 occasionally and I read about this long ago and wonder if this has to do with part of the problem, or if I shouldn't concentrate on it now. I have spare instrument clusters back in Arizona, but I'll have to have my wife send them over or wait until I get back there to bring them up. could this cause total lack of power, or would it just affecting shifting (making drive-ability poor, but not lack of power?) I don't believe the speedometer issue would affect idling.

Here are some things I can think of that may be worth checking:

Fuel pressure
Plugs and wires (apparently just done but still worth checking, the wires are at least routed to the correct firing order)
EGR?
Distributor (pip sensor maybe?)

Can anyone else point me in a good direction to correct these issues? The motor seems like mechanically it should be pretty good, so I'm thinking it may be a 'simple' fix like a fuel pump/ignition/computer thing. I'd just like some insight to see if I'm overlooking anything. Hopefully I can get this figured out relatively cheaply and quickly!

Thanks for any help!

 

Similar issue here.

I have a 1996 F150 and after driving it home one day from a 2 hour ride trip I heard a light pop and then lost about half of the power barely making it home the final 5 or so miles.

Checked everything I could think of (timing, plugs, compression you name it) but nothing seemed out of place. Did notice that on the back side of the engine there is a crossover pipe going from head to head called the 'secondary air injection pipe' that spilt from the threaded plug attached to it dead center of the pipe.

Haven't replaced mine yet because of the location and am not sure if this may be your problem.

Check to see if you now have the same issue.

 

I'll check that and see, I forgot about that pipe. I did fix a vacuum leak near what I believe is the EGR solenoid. Also, my timing was actually set at 20btdc not 10, so I fixed that as well. It does drive a little better now, but still has very little power and a very rough idle.

I believe I heard the catalytic converter ratteling very slightly, but am not too sure if it was something around it. It doesn't seem to glow hot when driving, but maybe it's still clogged?

Just checked my fuel pressure. 10 at KOEO and 32 at idle. The pressure goes up when I hit the throttle as well so I don't believe fuel pressure is causing the issue.

 

Read about using a vacuum gauge to diagnose a clogged converter. As the exhaust pressure rises, vacuum goes away, no vacuum, no power.

 

I will see if I can get my hands on a vacuum gauge sometime soon then to check that.

I got back from the parts store and crawled under the truck and let it idle to see if I could hear rattling around the catalytic converts. I may have heard rattling coming from the front one. However, I saw sparking from the passenger rear spark plug itself. It was arcing through the ceramic it looked like. The plugs look new themselves, so I'm not sure why that would happen but I'm going to throw on a new set of cheap plugs now. They were apparently replaced, as well as the wires and cap/rotor (which all look semi clean so they are somewhat new), so maybe they guy just was too rough and cracked some of the plugs?

I dont think new plugs/wires will cure the sever power loss, but it should help the misfiring at least. If i'm pretty heavy on the throttle, I can go up to around 40-45 normally. After changing the timing chain and timing the truck, I think it would have power to reach highway speeds, it would just take me flooring it to get there and it wouldn't be immediate. My 91 305 camaro had about double the power before I did any work to it's engine, so this 351 should feel quite a bit peppier that's for sure. I know the Bronco felt decently peppy when it had the 302/

 

Well I replaced the spark plugs and wires. One of the wires was cracked at the boot for a plug, and one of the plugs was cracked as well.

After replacing everything, it definitely runs better, but it's still misfiring pretty badly at idle. The misfiring isn't nearly as dramatic as it was, but it's definitely idling pretty rough. It seems to clear up under throttle, but the truck is lacking significant power. It's probably 1/3 of what my 302 was stock, so something is still wrong.

Upon replacing the plugs, I noticed the passenger side exhaust manifold is cracked. I'm not sure if its leaking that badly (because it doesn't sound like it has much of a leak and I didn't feel exhaust come through the crack), but the whole manifold has a crack going down it so I'm wondering if that has any play in the drive-ability issues? I wouldn't think a cracked manifold would affect power output or cause a pretty rough idle, but maybe I'm overlooking something.

 

Yes, because exhaust leaks can screw with the O2 sensors

 

Well I finally managed to get the old exhaust down. Thought it would take 30 minutes... it took like 4 hours. That's just the Y-pipe to the second cat as well. I cut them out with a reciprocating saw which didn't go as easily as I had planned.

However, The first cat is definitely plugged. I peeked down in there and saw it was all crumbled up and in pieces. I couldn't peek down there too well but its definitely not in good shape. I have to see if I can get a new y-pipe and maybe shorty headers now. I can afford to spend a bunch though... Good thing this isn't my only vehicle. It should be up and running fairly soon though and then I'll post back if that was all it needed.

 

Did you check that the cam and crank were timed properly when replacing the timing set, or just set them how they were? A slipped timing chain could explain the misfires and also explain why your ignition timing was twenty degrees out. Running with the ignition timing retarded puts a lot of heat into the exhaust (combustion hasn't finished when the exhaust valve opens) which could be what wiped out the cat.

 

I made sure that the two marks on the camshaft/crank sprocket were aligned before taking the old chain off and when putting the new chain up. They did seem to align properly and the distributor was pointing in the vicinity of cylinder 1 when I checked so I don't believe the chain slipped. It was pretty loose though so I think it was due for a change.

 

I forgot to update this thread, I spoke some in another thread.

I ended up cutting off the exhaust and found the first cat was plugged pretty bad. I haven't checked the second yet, but the exhaust manifold on the passenger side was cracked in two (almost 3 places).

It took a map pro/oxygen torch to get the driver side exhaust manifold off fully (one of the bolts was really stuck) and I still have to torch out the stud where I broke a bolt off, but things are coming along. The passenger side came off pretty easily surprisingly. I bought some cheap pacesetter uncoated headers and stripped the paint and applied some VHT high heat stuff. We'll see how long that lasts, its cooking in the oven now. Hopefully I'll get everything back together this weekend.

 

So, the truck definitely runs a lot better now, especially after installing the new oxygen sensor.

However, it's still not nearly as powerful as it should be. I installed a new oxygen sensor, and have the exhaust go from the headers to about 2' past where the oxygen sensor is. The oxygen sensor is in the stock location (getting a reading from both sides of the exhaust), but the pipes themselves are currently separate. Unfortunately the header collectors are leaking a bit, but not enough to throw an engine light.

Would the exhaust leak at the header/pipe collector cause a large loss of power? What else should I look into? I purchased a new fuel filter and will install that tomorrow. I looked for vacuum leaks but didn't notice anything. I haven't put up a vacuum gauge yet, but I know that when vacuum is low enough, the cabin air vents will default to floor when the gas pedal is floored and there is low vacuum. It doesn't do that anymore so I'm thinking vacuum is okay (I will put a gauge on it when I get a chance though).

 

I've seen, driven and heard a lot about cracked exhaust manifolds. It's a very common issue with some engines. But I've never seen one cause a large loss of power. Even when leaking enough to hear.
That said, it's certainly possible that messing with O2 readings can do at least some of that. Just never seen it happen from a cracked manifold.

What about the cat(s)? Did you say you replaced at least the one you found clogged? I'm assuming yes since you did so much exhaust work, but I only remember you saying you did the headers and paint. Did you also say yours had a second cat? If so, did you verify that one was not also clogged?
Ran into that only one time before (single cat) and it was truly a dog on power until I got it replaced. With a normally stout running engine losing that much power, it almost had to be a clogged exhaust! Luckily I got right on the first guess, 'cause that one might have found me throwing parts at a problem instead of carefully diagnosing it first!

Good luck getting your power back.
Oh, and speaking of that.... Are you sure this '94 isn't just underpowered compared to the other engine you were used to? In other words, what year/model/modifications were you comparing it to?
And last but not least, perhaps some of your perceived loss of power is gearing. Hey, lots of these rigs came with freeway-flyer gearing, and maybe you're just fighting an uphill battle looking for more power, when what you really need is the gearing to get the existing power to the ground.

What size tires are you running? And what gearing?

Paul

 

Thanks for the response! I know for sure that the first cat (where the pipes go from both banks into the cat, then converts to one pipe) was clogged pretty bad. The second one may or may not have been. However, I currently cut that whole thing out, and am cyrrently running straight pipes (dual exhaust, they are just connected where the o2 sensor is). I welded the y-pipe back up so I could bolt it on, and then welded a 2' section of pipe to both parts of the previous y'pipe (where the ends went into the first cat) so that the o2 sensor wasn't too close to the exhaust end. The piping is pretty short now with no cats or muffler, but I plan on finishing it up when I get a chance.

The new headers are on as well, so the exhaust should be good to go (except for the exhaust leak at the collectors, but it doesn't seem super large)

I'm not 100% sure on the power, but it would be very surprising to me if this is all the 351 has to offer. My 96 Bronco used to have a 302 with 3.55 gearing and 33" tires (5 speed manual transmission though). I recall it feeling a bit more powerful than this truck which has I believe the stock tire size. Now, I did upgrade that motor with a better intake/exhaust and then eventually got AFR aluminum heads. It also was a MAF equipped truck, while this 94 is not. However, we also had an 88 F-150 years ago with a 351 that felt a bit more powerful than this truck. I never ran that truck hard though, but I thought I remember it being pretty quick for an old truck.

Also, I have a 305 camaro, and even before the engine work I did on that (cam/intake/exhaust), it seemed quite a bit more powerful than this 351 as well. I don't believe this truck could chirp the tires if I tried, and the fastest I could go in it now is about 80-85mph (there is a highway with a 75mph speed limit here so I wanted to see how it would do there). It reaches there decently enough, but it's about topped out there with the pedal to the floor basically and would struggle to keep that speed with a slight hill.

The idle is usually extremely smooth, and the motor sounds really good, it's just much less powerful than any other 90s V8 vehicle I've driven. I'm more used to the 302/305 as well so I would have believed this would be more powerful.


Edit: Again, I'm not 100% sure on the tire size now or gearing (I'll check tomorrow when it's light out), but the tires are smaller than my Bronco's and my Bronco has 3:55 gearing. The Bronco felt more powerful with the 302, 33" tires, and 3:55 gearing so even if this truck has 3:55 gearing, it should feel more powerful I'd imagine!

 

I agree then, it should feel more powerful in this application. Most people that swap a 302 for a 351 are underwhelmed by the difference. It's just not that much more horsepower than a 302, but does have more low to mid-range torque (in stock form anyway) than the 302 does. But even then, it's not big numbers when stock. Sure, you have more cubic inches, and more stroke, but that's not any recipe for a road burner. If it turns out you have something like 2.72 gearing or 3.08's or so, then those 3.55's you had would certainly have made a noticeable difference.
But yeah, if you have 3.50-ish gears in there now, then it certainly sounds like you have a wimpier-than-expected running engine. Especially with those modifications you've made!

Are you saying that this current 351 has heads, intake and now headers straight pipe duals and still can hardly get out of it's own way? That is very strange indeed.
Now I'm even more interested to hear what your gear ratio is!

Paul