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5.8 low power!!

  #1  
Old 06-22-2018, 01:11 AM
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5.8 low power!!

I figured I'd post this in here as well since it's the same motor. This isn't about my Bronco or a Bronco for that matter, it's for a 94 5.8l 4x4 F-150 auto trans with about 148k miles.

Long story short, bought the truck for 200 bucks running pretty rough with very little power. I replaced timing chain, spark plugs, and the plug wires. The truck then idled pretty good, but still had no power. I then inspected the exhaust and found the driver manifold cracked in half, and at least one of the cats clogged bad (after cutting the exhaust off). Fuel pressure tested okay (I think it was like 30 at idle, and went up when I hit the throttle in park to like 35 or 39, its been about a month since testing).

I put up some headers, cut out the cats/muffler, and welded up the y-pipe to mount to the headers. There is an exhaust leak at the header collectors currently, but it's not very large. I didn't weld the y-pipe to converge either, it's set up like dual exhaust and dumps out at the end of the transmission (waiting to fully put pipes/muffler up until I get this sorted more.) The o2 sensor is in the stock location, and is connected to both pipes. I also replaced the o2 sensor with a new bosch one.


That aside, the truck idles surprisingly smoothly after this work, but about every 10 seconds will seem to have a very small misfire when in park only. In drive/reverse, it doesn't seem to do this and idles excellent.

After swapping out the exhaust and replacing the o2 sensor, the truck has much more power back, but it still seems like it's quite low on power. The engine light isn't on, but I haven't tried pulling codes since I've replaced the exhaust (engine light works because it came on when I drove without the o2 plugged in). This truck seems like its quite a bit less powerful than my Bronco when it had the 302 (my Bronco has larger tires and 3:55 gearing, so even if this truck has 3:55 gearing, the tires are smaller so it should feel more powerful)

I took it on the interstate today and it was able to reach 80mph without a significant amount of trouble, but that is pretty much where it topped out. It couldn't really go much faster than that. If there was a slight/moderate hill, it would probably drop down to around 70-75 being the top speed, this is with the pedal floored. I don't believe I could chirp the tires if I tried either, and I'd think this would have a reasonable enough amount of torque to do that.



So, any thoughts on what to look for now? I will try and pull codes, but otherwise can't think of too much. It seems to run very smooth and well, just not be nearly as powerful as it should, and the MPG seems like it's quite low as well. I'm not too upset. I'm about 1000 bucks into it now with the registration/towing/parts, but it seems to be a reliable (but quite weak) truck now, and can take the brunt of the hail storms here in Cheyenne Wyoming when then come (so I can shelter my Bronco and Camaro.)
 
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Old 06-22-2018, 08:48 AM
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is the timing changing when you plug the spout back in. hows the compression. how about the valves rockers and stuff under the valve covers.

did you add to the timing, your at like 6500 ft right.
 
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Old 06-22-2018, 10:56 AM
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Check base timing.
 
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Old 06-22-2018, 12:31 PM
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For those without a scorecard: https://www.ford-trucks.com/forums/1...-no-power.html

Same issue posted in the OBS forum
 
  #5  
Old 06-22-2018, 01:51 PM
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Originally Posted by Redark1 View Post
is the timing changing when you plug the spout back in. hows the compression. how about the valves rockers and stuff under the valve covers.

did you add to the timing, your at like 6500 ft right.
The spout is reconnected. I haven't checked compression yet, I'm heading back "home" to AZ next week so my wife can move up with me and I'll grab all my stuff then (compression gauge etc.). Maybe the block cat messed up the valves or something and they aren't sealing as well? It idles and drives great, just not very powerful.

As for the timing, I didn't add any from what was recommended. I'll have to look into that. I didn't realize I would have to adjust for elevation. I know the vacuum will read lower, but I wasn't aware timing would change!

Originally Posted by Conanski View Post
Check base timing.
I did check the base timing. When I first got the truck, it was set at 10 ATDC. After changing the timing chain and the plugs/wire (PO just replaced those, but they were damaged, so the new ones I put in are good). I set it at 10BTDC. I'll double check to make sure I didn't accidentally move it when tightening the distributor.

Originally Posted by rla2005 View Post
For those without a scorecard: https://www.ford-trucks.com/forums/1...-no-power.html

Same issue posted in the OBS forum
Yeah I posted it there and am more active on these forums (which seemed a little more activity here as well) so I was going to move the post over here. We had to work last weekend and I'm working standby this weekend so between all that, I kinda lost track of that thread from a while back until yesterday! Either way, I'll make sure that when I find my issue, I won't have a random thread with no answer. I hate when I search for stuff online, it seems to be my exact problem, then someone says I'll do this and post back and there is no post!!
 
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Old 06-22-2018, 11:00 PM
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You don't have to adjust but it helps some in the response and drivability. It's like the 6 liter tune up. Here at elevation it's more of an advantage compared to sea level. Leave it at 10 then.
 
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Old 06-25-2018, 12:57 PM
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that exhaust leak could possibly give the o2 sensor a false reading. I assume your running just basic copper plugs correct?
 
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Old 06-30-2018, 06:31 PM
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If you don't have mufflers/cats attached yet the lack of back pressure on the exhaust can really make the engine lag.
 
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Old 07-14-2018, 03:18 PM
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I've been awfully busy the last three weeks (moving my wife up and the rest of my stuff from AZ to Wyoming) but I have some time now! I haven't had a chance to work on the F-150 too much, especially since my poor Bronco started giving me trouble on the way back from Denver. The wiring or switches to the electric "throttle" went bad or was going out (on the 7.3) and then right outside of Cheyenne it looks as if an aftermarket plug on the oil reservoir came out and dumped like 10 quarts of oil on the road. I should be able to get things running again this weekend hopefully, then I can start doing more work on the 5.8

I haven't yet put a muffler on there, but I plan on doing that soon. I was about to order one and never went through with it. As for the exhaust leak with the o2 sensor, I thought about that as well. I'm not sure if they make larger y-pipe donut gaskets but i think I need those to prevent the leaks, otherwise I'll have to cut the y-pipe at the 02 sensor collector so I can widen out the span a tiny bit more.
 
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Old 07-16-2018, 07:22 PM
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Probably need an exhaust shop to rebuild the Y pipe with the proper mating flanges to your manifolds, shouldnt be very expensive
 
  #11  
Old 07-17-2018, 08:38 PM
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Originally Posted by Kemicalburns View Post
Probably need an exhaust shop to rebuild the Y pipe with the proper mating flanges to your manifolds, shouldnt be very expensive
I may look into that. I ended up using the stock y-pipe, and the cheap headers I purchased mate up to the stock y-pipe. However, because I cut the y-pipe into two sections, I welded it together a tiny bit to short. They pipe is only connected by the O2 sensor crossover piece, so I may cut that in two, connect the pipes, then try and weld it together. If none of that works, I'll end up looking into shop prices.
 
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Old 07-18-2018, 12:37 PM
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I had my whole exhaust redone about 9yrs ago for about $400. that included a new y pipe all mandrel bent 2.5" exhaust into a 3" single out stock location. in my opinion why mess with it, whats your time worth to just have a hacked on Y pipe? get something built that will flow properly ya know.
 
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Old 07-18-2018, 10:51 PM
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Yeah that may be worth doing. I don't know if Cheyenne Wyoming has many places that would do that, but I'm nearby in Colorado I can find someone that will do that.

I haven't had much time to do any work on the truck recently. I did some work with my Bronco but the motor is still cutting out on me. There has to be some stupid electrical issue somewhere that's causing the issue. Once I get my Bronco back up and running normally, I'll probably start tackling this 5.8 a little more.
 
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