Hi all,

I have a 1992 f150 that I swapped a 351w into, with gt40p heads and bbk 1515 headers.

The issue that I am having is that the exhaust detonates, but only when its warm.

Like I said, it's a 351w with gt40p heads, no smog hooked up(no ports for it), and aftermarket headers with no egr port, therefore the egr isn't hooked up, but I have a block off plate between the egr valve and intake manifold. I want to hook up egr again eventually, I just want to get this problem resolved first.

I found that several plug boots melted, so I replaced them and put heat shields on the wires. The problem went away for a bit, but then came back. I found a crack in a wire from heat. Do you think that this crack could be enough for the spark to jump and ground to exhaust, causing it to detonate?

Another thought-

Because of the gt40p heads I had to custom make my exhaust from the headers back. The collector diameter is 2.5, but I couldn't find any header reducers in the 2 bolt pattern that fit the 2.5" collector, so I got 3" and welded the explorer pipe ends inside. It doesn't leak, but with a crappy welder inside the pipe they are booger welds. Do you think these welds could be getting hot and becoming ignitors?

I'm sorry if I left anything out, there's a lot of things I've done on this project, and a lot to remember to type. Thanks for any help!

 

Detonate? You mean backfires? When? Accel? decel? Idling?

First thing I think of is an exhaust valve hanging open. Or it is misfiring.

 

Not backfiring, unburnt fuel is actually detonating. And I'm just not sure what's causing it. It's under load, so I think it's running rich due to the egr not opening.

Not misfiring.

More of an angry crackle in the exhaust.

 

The computer leans out the mixture and adds timing when the EGR is supposed to be operating, which is under cruise or low load conditions. Under load the EGR is shut off, that is not your problem. The only way I can think of unburned fuel could be lit off in the exhaust is extremely late timing. Are you sure your noise is simple pinging due to some other factor like low fuel pressure under load?

 

If you could get a video clip and post it, I'm sure it would help boat loads. I dont fully grasp what you are saying, but it sounds like all the mixture isn't burning in the cylinder and lighting off in the exhaust. But without a ignition source that's not possible.

 

Haha I don't fully grasp what's happening, so it's hard to explain, but I will get a video as soon as possible.

 

Update:
The problem still persists.

I have new ideas of causes though. I was unable to get a clear video of what the noise sounds like. It just sounds like normal motor noise on the video.

The noise sounds like a popping, loud, header only exhaust. Not raspy like a leak, just poppy. It gets faster with higher rpms. I can feel it in the gas pedal and steering wheel.

The engine is leaking oil. I can't tell if it's the valve covers or rear main seal, more investigation to come.

Oil pressure starts at 60 cold, at idle, but then slowly goes lower and lower until about 8 at idle. However, goes up to 40 past 2k rpms, and 30 at 1500 rpms, 20 at 1k rpms.

May or may not be related to the issue, but I'd like that to be a little higher at idle, otherwise its healthy. I think it may have something to do with the leak?

I found out that it only happens on acceleration when the steering wheel is straight or left. Not right at all.

I let the truck out of gear and turned left, nothing. Revved it while rolling and turned left, nothing. Only when in gear.

The noise is coming from beneath the floorboards, at the firewall. Near where the trans and motor meet.

Is it possible that the clutch is oiled and slipping? It feels like there's only grime in the bellhousing when I popped the plug and felt inside.

I no longer think this is engine related, it seems like something else drivetrain.

Sorry for the long post, but any ideas are appreciated. I'm stumped, and brainstorming never hurt, right?

 

I guess the other thing that makes it hard to diagnose is that it only happens when the engine is up to temp.

 

Any ideas are appreciated.

 

Post# 5: https://www.ford-trucks.com/forums/1...l#post18085668

No one can obviously help you on this side of the interwebs by simply describing your issue. None of us have heard of exhaust detonation.

 

...well thanks anyway.

 

Don't you have one of those fancy phones?

 

...huh? haha

 

VIDEO ->>>>> (fancy phone?)

Doesn't everybody (except me) have one?