How much detonation is normal in these older trucks? I thought I've read about it being common with the 5.0L but I don't recall where or when I read that. I have a 92 F150 5.0L and it has a substantial amount of audible detonation once I get into 3rd and 4th gears at low speeds under moderate-heavy load. During acceleration, there will be detonation until the trans decides to downshift, then it will go away. I've tried 87 and 89 octane with no improvement. I haven't tried premium (91 octane here in Kansas) because I honestly don't think it'll make a difference nor do I want to pay for it. The truck has entire new ignition system, running factory 10 degrees BTC timing (with spout disconnected). Truck has 199,510 miles and I don't think it's ever been torn into farther than what I did to replace the timing chain and gears. With the high mileage, I'm suspecting possibly excessive carbon buildup? I'll mention that I did run the truck with 8 degrees and it made no difference.

Other than the detonation, it seems to run fine. I get ~15mpg highway, ~11mpg city.

However, I'm still having ICM failures, and just replaced #6 ICM today, however the last one lasted 3000 miles unlike the previous ICMs which lasted ~1k miles. The parts store has tested all the failed modules and it's been the RPM Low signal according to their tester each time. I wouldn't be surprised if the ICM had something to do with the detonation, but at this point I'm not sure. At some point I plan on refreshing my understanding of the TFI operation and go after the RPM signal circuit and see if I can locate the root cause of my ICM failures. I imagine I've got excessive amperage or a short somewhere in the harness, resulting in repeat ICM failure. As I mentioned previously, the entire ignition system has been replaced (besides the harness) and the modules keep failing. The modules fail at once. There are no signs of it beginning to die. It will be running great, then all of a sudden the truck will die as if I turned the ignition off. Then it has crank no start, with zero spark. I'm getting tired of dealing with this, because otherwise the truck is great.

Any thoughts?

 

I forgot to mention that fuel pressure and compression is good. It's been a few months since I checked both but I recall them being on the high end of specification so I do not suspect fuel or compression.

 

And a third thing I forgot to mention. I searched for a knock sensor on this motor, and according to the publications I was able to find, Ford didn't put a knock sensor on the 5.0L for 1-2 years, and my truck is one of those years. I looked on the back of the motor where it would be mounted and I didn't see one, however I didn't look very hard since it's kind of a pain. I think it's odd that they wouldn't have used one for only a year or so, so I suspec the information I found may be wrong?

 

A bad EGR valve can cause that issue.

On my truck, I ended up being given the wrong EGR valve and that caused detonation.

Also, make sure you have a Motorcraft PCV valve.

Last, check your vacuum hoses and if necessary, replace the vacuum hoses with new rubber hoses.

 

Here's another thought. What spark plugs are you using?

 

The engine light came on a while back and hasn't returned, but I'd bet it's EGR related. It came on before for EGR, and I installed a new position sensor and cleaned the EGR and manually actuated it and it seemed to work fine so I'm not entirely sure it's the culprit. However I will see if there are any codes and will revisit the possibility.

I have not replaced the PCV valve yet so I will do that. Explain to me though how that would be related to this issue?

I've gone through and repaired the vacuum hoses before and I do not believe there are any issues with them currently. I'll double check them again though.

I don't recall but they're the ones commonly recommended by people here and I've replaced them twice within 10k miles so they aren't the issue.

 

A few quirks with the EGR setup on our trucks.

1) If you use an OEM EGR Valve, you need to use an OEM EGR Valve Position Sensor. Otherwise, you will likely have a CEL.

2) If you use an aftermarket EGR Valve, you can use either the OEM or an aftermarket EGR Valve position sensor and be ok.



The PCV valve deals with crankcase ventilation and your EGR setup is plumbed into to the crankcase too (through the lower intake manifold) and this part is known to cause quirky issues that seem completely unrelated unless it is a Motorcraft. It costs a little more, but is worth it in the end.

Did you replace them entirely or fixed the cracks?

I'm going to assume you're talking about regular copper core Motorcraft or Autolite. Other spark plugs have the potential to create a detonation issue.

 

Sorry for such a late response and reviving an old thread. I haven't had the chance to work on the truck yet. I sent it to a shop to get a new transmission and I've been broke after that repair so I've been driving it how it is with gentle acceleration to avoid detonation.

To answer your question, I did not replace the vacuum lines, I only fixed the damaged lines by splicing in rubber vacuum line. I do intend on replacing all the lines someday, I just haven't had the time nor the motivation to tackle that project yet. I'm in my senior year of college and I have to relocate 1000 miles away from home upon graduation for my job, so unless something major happens, I'm not working on the truck until I'm settled into my job and new home.

I purchased and installed the recommended motorcraft plugs. They probably have close to 5k miles on them by now if not more.

I have yet to install a new Motorcraft PCV valve, it is on the list of to-do's.

I installed an aftermarket EGR position sensor so I intend on replacing the EGR valve with an aftermarket as soon as I have some cash to spare. Hopefully that will address the detonation.