I've been chasing a rough idle for a little while. Its a 98 F150 2WD with the 4.6. Recently I've been able to drive the truck a lot more, it's actually currently my only vehicle. Rough idle, actually seems a bit worse when it's cold. Seems like it has absolutely no power as well. I've also noticed that when it runs rough it smells like it runs really rich. It's also only getting around 8-9 mpg, and I don't even drive it hard since it's such an old truck. Check engine light came on for the passenger side upstream O2 sensor, I replaced that and the light doesn't come on anymore, it seemed to fix it for a little bit but now it's back the way it has been for a while. Any tips on where to look or maybe some procedures to test different sensors that could be causing the symptoms? Any input would help greatly.

 

I'd suspect plugged cats.

 

That was a dreaded thought in my mind at one point. A while back we had to have the head gaskets replaced because the driver side one was leaking massively, I'm assuming the burnt coolant through the exhaust could easily ruin the cats? It destroyed one of the O2 sensors so it would make sense to me.

 

I don't think plugged cats would cause a rough idle.

 

Wouldn't plugged cats also cause the truck to not be able to go over a certain speed? It doesn't have a problem with that, it's just really low on power. I've been thinking maybe a sensor going crazy is making it run extremely rich.

 

If it's running that rich it should be throwing codes. A muffler shop can test for plugged cats.

Have you pulled the spark plugs and looked at them?

 

The only code it's thrown was for the O2 sensor I replaced. The plugs were replaced pretty recently, they only have a few thousand miles on them. I'll pull them when I get the chance this weekend. Last time I tried to check the fuel trims it seemed like they were normal, but I'll check again this weekend. If I get the chance I can probably take it to a muffler shop in town too to check if the cats are plugged.

 

If they are not Motorcraft plugs, I'd replace them anyway. Plug wires and/or coils could also be going bad.

 

Everything is replacement Motorcraft, I tried aftermarket once and learned my lesson. Plugs, wires, and coil packs are all recent.

 

I got home from work and the truck was still running, so I decided to do a quick test. I put my hand behind both exhaust tips, feeling for the exhaust coming out. It didn't feel like much was coming out. I know it's a small V8 but there was next to nothing, and the puffs that I felt were pretty hot. I'm gonna go ahead and guess that that points more towards clogged cats too, or possibly even a clogged muffler. I'll take it to a muffler shop and see if they can test for any blocks in the exhaust system.

 

Hmmm. 8mpg. Maybe Cats, maybe not. If it's not, well, then, how good was the gas mileage before? My 4.6 truck gets 19mpg on the interstate; so I would suspect you should get at least 16? I'm just trying to figure out how far off it is. Anyway- It's possible that if it 's not cats, you could have an injector that is staying open too long (or not closing) OR an engine that has jumped time on one side. Here's how I'd approach it; get yourself an infrared temperature checker. Start with the simple stuff. Start by checking the temperature at each exhaust port with the engine fully assembled and idling at operating temperature. Excess fuel will create excess heat. If all ports read within 20 degrees (F) of each other move on to your next test. If any one exhaust port is significantly higher temperature then write that down, and you can then proceed testing that cylinder under the guise of a leaky injector or a bad ignition coil. If not, you will then want to let the engine cool to do a compression test and spark test across all 8 on your engine to make sure the compression is even across the entire engine to make sure it's still actually capable on all 8 cylinders of burning the fuel it is given. Write down your results and make no assumptions. If compression is even and there are no exhaust hot spots, pull the valve covers so you can do a visual cam timing check. Don't assume that the computer would necessarily see or necessarily report an engine that has jumped a tooth off the timing chain. Check the marks on each camshaft and their relation to the crankshaft individually according to the manual; I have had a truck do EXACTLY what you described because the engine had jumped time on one side by one tooth while the other side was still correct. It never set a code because it never could get past open loop/warm up mode; it was that confused; it was kinda trippy to be honest, but I digress.

One more thing, the 4.6's of your era were generally good, but occasionally burnout exhaust valve seats for reasons I can't fully explain; don't be surprised if you have to remove that head and throw a valve job at it to fix the problem.

Also, your plugs have been tolerating excess fuel for how-ever long this has been going on; the likelihood that they're all still even and at least a one of them not ruined is kinda low, so keep that in mind, irrespective of how "new" they are, and that you may have to replace them again, or at least, the one or two that may be burned up from all of this that's going on. In other words, assume nothing; follow the evidence you find, regardless of what it is.

Best of luck.

 

The gas mileage before was around 13-15, and that was in town. I took it down the highway and used half a tank, I ended up getting around 9.5 mpg on that tank. I replaced the fuel injectors around a year ago because one was clogged badly and I've been keeping up really well on the fuel filter since I got the truck. I'll have to get myself an infrared thermometer, I've been meaning to get myself one anyway and that'll help me diagnose like you described. When I had the head gaskets replaced around 2-3 years ago, the shop checked the compression and said all the cylinders checked good, I guess it wouldn't hurt to double check myself though. Cheap and easy testing. I took it to the reputable shop down the street from my house and they diagnosed it for free. I guess this wasn't one of the best times I've sent a vehicle there because all they told me was "I'd replace the mass air flow sensor." I couldn't find a Motorcraft sensor, so I replaced it with a Delphi sensor that I got a good deal on. Seems to idle a little better, but I can still tell by driving it that something isn't right. I'll get that thermometer either later today or tomorrow and check the cats at that time too with it and by loosening the exhaust enough to let gas flow through. That's an interesting thought about the timing chain; when the head gaskets were replaced they said the timing chain looked brand new, but I'll double check just in case. Thank you for all the great info.

 

Went out and bought an infrared thermometer earlier. Pulled both wheel well liners out and pointed at each exhaust port as well as before and after the cats on both sides. I'm not 100% sure on what my findings mean so I'll post them here for the more knowledgeable.
1 - 350 F 2 - 370 F 3 - 440 F 4 - 480 F 5 - 370 F 6 - 450 F 7 - 450 F 8 - 450 F
Driver's Side Cats Before: 520 After: 270 Passenger's Side Cats Before: 480 After: 230
Now if I'm correct, the exhaust temps being twice as high before the cats than after the cats is a major problem. The temps of cylinders 1, 2, and 5 seem somewhat low compared to the rest, but I'm not sure if that's enough of a difference to mean there is a fuel/spark problem. I'm gonna go ahead and guess that my next step would be to either remove the O2 sensors or loosen the pipes behind the manifolds to let gasses escape and see if it performs better.

 

If you take it to a muffler shop, they have the equipment to test for excessive backpressure. Note that you should have 2 cats on each side.

 

I'll find a muffler shop nearby and try to take it there tomorrow or Saturday. The exhaust is still factory with the two cats on each side, the only difference is from the muffler back.