I have a '97 two coil 4.6L that threw a "cylinder 8 misfire" code, so I changed the spark plug and wire...no changes. Next I replaced the fuel filter and cleaned the injectors, but inserted cylinder 8 injector into cylinder 5 to see if cylinder 5 would throw a code. Erased the codes and tried again. Computer now says cylinder 4 is misfiring. So I replaced the ignition coil, and cylinder 4 is still misfiring.

Symptoms are
1. Starts fine
2. Idles ok when cold
3. After warming up, begins to sputter and misfire severely when getting up to 45-60mph and RPM's are around 2,000. After it starts sputtering, idle gets very rough as well.

I don't think it could be compression because it started at cylinder 8, then went to cylinder 4. I haven't run a compression test yet, though.

If not the spark plug, ignition coil, wire, fuel injector, or compression, what could it be?
Kinda stumped.

Thanks in advance for any help.

Joe

 

just to check...... #1 cylinder is front on the passenger side and 1-2-3-4 to the firewall ...... #5 is drivers front and 5-6-7-8 to the fire wall ?

 

4.6L V8 Ford firing order Ricks Free Auto Repair Advice | Automotive Repair Tips and How-To

 

Thanks for the info, but I know that already

After doing some reading and looking at my Foxwell code reader, I'm thinking the catalytic converter(s) is/are damaged. (It shows they are anyway)

I recently had a MAF sensor that needed cleaning because I got a "fuel bank 1 & 2 running lean" code. I drove around for a month or so with the CEL on (stupid me, but it was running fine). The codes disappeared after cleaning the MAF sensor and have not returned.

I think the engine computer may have tried to compensate for the dirty MAF/perceived lean condition by dumping more fuel into the cylinders (running rich). Over time, the extra rich condition may have damaged my catalytic converters, and now the backed up exhaust is making my engine run rough.

Does this sound plausible?

 

That's exactly what the computer does when it detects a lean condition - dumps fuel to compensate. But you need to monitor live data in closed loop to determine what the O2 sensors are telling the computer.

Check for intake and exhaust leaks - those can cause random misfires as well.

If you're concerned the cats are failing, you can find out with a few different test methods:

1) Monitor live O2 sensor data at idle and under load (using a graph if possible). The downstream O2 sensor should remain at a steady voltage with minor fluctuation if the cat is working properly.

2) Check exhaust back pressure before and after the cats. 1-2 psi is normal - anything more suggests restriction but pay attention to whether the reading is post cat - if it's downstream of the cat then the restriction is not in the cat.

3) One of Ford's specific test is the temperature differential test. With the engine at operating temperature, disable the spark to one cylinder and rev the engine to 1500 RPM and hold (you may need an assistant for this). Using an infrared thermometer, check the surface temp of the cat at the inlet and the outlet - there shouldn't be more than 50 degrees difference between them. Since you have dual banks, you'll have to disable spark on a cylinder for each bank. NOTE - Since you have coil packs, make sure to provide a path to ground for the spark or you will damage the coil. Using a spare spark plug and jumper wire works well.

 

Thanks for the feedback, DBGrif91.

Do I check the backpressure with a vacuum/fuel pressure guage, or is there some other guage I need? Guess I'll read up on it.

I May need to buy a thermal gun too.

Is it worth the trouble to just take the catalytic converter off the truck and inspect it? It's only a few bolts

 

A fuel pressure gauge won't work.

I don't know that a standard vacuum gauge will work either, but I know there are combination vacuum/back pressure gauges available. I can't tell you what the difference would be between a standard vac gauge and a combination gauge.

As to whether it's "worth it" - that's your opinion/preference. If you're confident you won't end up dealing with seized/broken bolts, then I suppose it could be a relatively quick job. To me, it's not worth it, but that's because I hate dealing with exhaust.

 

The I/M on my code reader (Foxwell NT301) says the converter and O2 sensors are bad, so I think I'll just go ahead and replace all of them. (Not so fun project, but I want it fixed.)
Need some opinions between these two sites to order from.

Rock Auto's Walker brand (shipping fees are very high!!! $140) https://www.rockauto.com/en/catalog/ford,1997,f-150,4.6l+v8,1122477,exhaust+&+emission,catalytic+c onverter,5808
or
Amazon's Evan Fisher brand (slightly cheaper and no shipping fees)
I don't know the quality of any of these. Anyone familiar with them?
Thanks

 

I'm not familiar with Foxwell scanners, so I'm not sure how it would tell you specifically the cat and O2 sensors are bad, but here's something to think about:

If the performance of the cat is measured by O2 sensor activity, how could it determine the cat needs replacement when the data it's receiving is from faulty O2 sensors?

 

That's a good question. Guess I'll find out when I pull the converters off the truck and inspect them. I will hold off on ordering new ones, but I've got a feeling they are bad.
Thanks for not making this easy for me