Detonation
Thread Starter
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Mountain Pass
Joined: Jun 2009
Posts: 248
Likes: 0
From: East Wenatchee, WA
Detonation
1991 F150, 351w, MAF swap, A9L ECU, 0.030" over, stock style pistons, GT40 heads, performer truck upper and lower intake, timing set at 10* BTDC, about 10,000 miles on engin, no CEL.
Started having a detonation problem under load, basically any hill would make it start pinging like crazy. Went home set the timing back to 10*, drove it easy, figured bad gas maybe. Filled it up with front tank full of 91 octane, still did it, didn't seem to do it as much as on 87 though.
Only thing I wasn't sure about was when I set the timing, put the spout back in and revved it a bit, timing shot up to past the marks on the harmonic balancer, maybe 40* or so, about 20ish warm idle. Is that normal? Just to clarify, spout out, turn on, set timing, turn off, spout back in, rev up.
What else could be causing it? I've been running on cheap gas since I built it, no problems with detonation until now.
Started having a detonation problem under load, basically any hill would make it start pinging like crazy. Went home set the timing back to 10*, drove it easy, figured bad gas maybe. Filled it up with front tank full of 91 octane, still did it, didn't seem to do it as much as on 87 though.
Only thing I wasn't sure about was when I set the timing, put the spout back in and revved it a bit, timing shot up to past the marks on the harmonic balancer, maybe 40* or so, about 20ish warm idle. Is that normal? Just to clarify, spout out, turn on, set timing, turn off, spout back in, rev up.
What else could be causing it? I've been running on cheap gas since I built it, no problems with detonation until now.
The Southernmost Mod
Joined: Dec 2004
Posts: 6,902
Likes: 21
From: Caracas, Venezuela
Which sparkplugs are you using? what's their gapping? did you remove the SPOUT before you set the timing to 10°BTDC? Do you have a fresh/good coil?. If none of those are the causes then something is loose inside the engine (even more possible if the engine was recently rebuilt), I would take the valve covers off as a starting point.
Apparently you did disconnect the SPOUT, so cross that one out.
Apparently you did disconnect the SPOUT, so cross that one out.
Thread Starter
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Mountain Pass
Joined: Jun 2009
Posts: 248
Likes: 0
From: East Wenatchee, WA
Autolite coppers, 0.044", not sure on coil, doesn't sound like something rattling around, it has no problems on flat ground, I can get on it good, just with a hill or really lugging it in 5th, that sort of thing.
How would a bad coil cause detonation?
How would a bad coil cause detonation?
Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
Bovaloe, are you running an oiled air filter (like a K&N)? When was the last time the fuel filter was changed? The ignition advance you saw while revving the engine with the SPOUT in is normal.
Thread Starter
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Mountain Pass
Joined: Jun 2009
Posts: 248
Likes: 0
From: East Wenatchee, WA
Yes I'm running a k&n, haven't checked it in awhile, dirty air filter maybe?
Just swapped in a paper one, at work didn't have my cleaning stuff for the k&n.
Fuel filter hasn't been changed in awhile either, can't remember, bought one too,swap it on my lunch.
Wires are touching in places but they're only a few months old.
Just swapped in a paper one, at work didn't have my cleaning stuff for the k&n.
Fuel filter hasn't been changed in awhile either, can't remember, bought one too,swap it on my lunch.
Wires are touching in places but they're only a few months old.
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FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
Fire it up in the pitch dark with all the vehicle lights off and rev the motor, you'll soon see if there are any cross firing issues.
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Laughing Gas
Joined: Dec 2007
Posts: 959
Likes: 8
From: Imperial, MO
The fuel filter probably isn't the source of the problem, unless it's getting clogged/restrictive, but it certainly wouldn't hurt to change it and rule that out.
Thread Starter
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Mountain Pass
Joined: Jun 2009
Posts: 248
Likes: 0
From: East Wenatchee, WA
Ok, so changed the fuel filter, sorted disney on the MAF, checked for cross fire (none).
Still pinging, no change at all.
Put some more 91 octane in, had an STP fuel system cleaner, the seafoam looking bottle, so I threw that in too, figured it couldn't hurt.
What other things can I check for?
Still pinging, no change at all.
Put some more 91 octane in, had an STP fuel system cleaner, the seafoam looking bottle, so I threw that in too, figured it couldn't hurt.
What other things can I check for?
