I have a 1998 F150 with the 4.6 V8 and I have been chasing a problem for almost 2 years now and I am left with the possibility that the issue is the knock sensor. Before I start going thru the trouble of taking the intake manifold off to access the knock sensor, does anyone know what a healthy knock sensor reading should be on a scan tool?

The performance on the engine has been degrading and now it has started to take longer and longer to start, fuel economy has dropped to 11 mpg, lunging and very poor performance. I have replaced the fuel filter twice, checked the fuel pump and pressure is fine. Replaced the EGR valve, replaced the spark plugs, wires and coil packs, replaced the O2 sensors, replaced the fuel pressure regulator, replaced the fuel injectors, replaced the TPS and the computer and the situation has been getting worse instead of improving. Everything I have replaced has been original accept for 7 of the 8 spark plugs. The one on the drivers side near the firewall was clearly never replaced.

I have asked this question on some of the Facebook groups with no answer and we are scheduled to start disassembling the intake this Saturday. So any assistance would be greatly appreciated. Thank you

 

I don't know if the PCM algorithm needs to see sensory signals like vibrations, without knocking, but you might try getting a new sensor and plugging it in to the harness with the sensor disconnected from the engine. Hanging in space. The PCM won't know but it will see a good senor. You could also fasten it to an external part of the engine so it does create a signal, just not from the same location. Might be worth a shot, to confirm that the sensor is bad at least. If there's no change you'll still be left wondering.

 

Instead of firing the parts cannon at it, why not take it to a trained tech with the proper diagnostic equipment to pinpoint the cause of your concerns? It's a lot cheaper. FYI it sounds like you have a MAF issue. Only way to know for sure is to diag the system.

 

Unless you have a code for the knock sensor leave it be
The piezoelectric sensor puts out roughly 0-5 volts depending on the severity of the knock
We used to have to smack the engine block with a long extension and a hammer during the old KOER tests.
That would stop the knock sensor from setting a code saying it was low voltage
Unless you have a code, you are looking in the wrong direction for your problem

 

Thank you for that information as I am not getting codes but just increasing poor performance.

 

No codes and the symptoms don't really point to knock sensor. Not clear how you got there.

How about air intake and exhaust? Air in, exhaust out, both important. Clogged converters or mufflers, blockage in the air intake system. Slow degradation and no codes points to something subtle. Good luck.

 

If you have real poor fuel, the knock sensor will have the computer reduce or back off the timing, and your truck will be / feel gutless
If you turn it off and restart it, it will reinitialize the timing and drive normal / good for a while until it backs off the timing again
Your 1998 4.6 is digital with nothing to adjust. The fuel filter, air filter, spark plugs and wires are the tune up. You are running Motorcraft plugs right?
Try some Chevron fuel (regular) for a tank or two and see if there is any improvement.

 

So this issue continues. I dropped it off at the shop where they worked on it for a month and it passed almost all tests. They tested the vacuum, passed. They tested fuel pressure, passed. They tested the MAF sensor, passed. They tested back pressure in the exhaust system and this failed so they gutted the cats, assuming they were clogged. This didn't address the issue either. I went to start the truck yesterday and it took longer then it ever has, almost 10 seconds before it fired and ran poorly as I mentioned in my original post on this thread. I purchased a Bluetooth ODBII reader while the truck was in the shop and had it record on my way home. What I found is that nothing was recorded for the Air Intake Temperature sensor, which I read, would cause the problems I am seeing. However, when I went out this morning, turned the key to the On position and turned on the ODBII reader, I compared the temperature the weather was saying to the temperature my Air Intake Temperature sensor was communicating and they pretty much matched. Is it possible that I have a short in the plug going to the Air Intake Temperature sensor or the sensor is failing all together?

I appreciate any additional insight.
Thank you

 

Does it still show no codes? Very unusual to have poor running and no codes. Most people get codes on engines that run just fine.

Start the engine with the scanner connected and see if parameters are normal. See if air temperature disappears.

Did the shop retest exhaust back pressure after gutting the cats? Or just assume.

 

What exactly does that mean?

 

I can’t help but think you’ve got degradation of your intake gaskets and possibly all the other connections to it…evap, blah blah,

but you should be getting codes

what does your fuel trim look like? Are you using forscan?

did the shop smoke the upper intake and check the whole truck?

 

FIXED:
Thank you everyone for your help. I really appreciate it.

Issue was the fuel pump. Wasn't throwing codes and the symptoms all fell under other causes which were addressed. In the end it was a fuel pump that was starting to go bad.