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View Full Version : Engine Knocking (from LT-1 thermostat ?)


sfitzgerald351
05-01-2005, 09:41 PM
Thought I'd start a new thread...

An EFI will adjust only as much as it's programmed to. If you have detonation and it's not hot yet, try running some fuel system cleaner or something that gets rid of carbon deposits. If the motor has extremely high mileage, I would be checking compression, oil use and condition when changed and anything else that will indicate the health of the motor. The coolant temperature sensor can affect this, too. If it thinks the motor is really hot, it leans the fuel mixure and it can cause detonation on acceleration or under heavy load.

All of this is moot if there is a bad bearing or crack in the cylinder wall, though.

As for my truck (1992 Ford Bronco 351) I keep leaning towards a leaking intake manifold gasket as the explanation for the knocking. It's leaking a bit of oil up front, so who knows how well the it's sealed to the heads farther back. The truck is old, but it's a new Jaspar block with only 25,000 miles on it at this point so I doubt I have compression issues. And with the coolant sensor, wouldn't that throw a code? The computer thinks things are hunky dorry and reports no error codes. The engine only knocks under really heavy loads (e.g. foot to the floor, 4k+ rpms, or accelerating up a bit hill with the boat). Drives pretty nice most of the rest of the time, though occasionally the idle get a bit erratic and runs a little high. It's just tough with the age of the truck to put a bunch of money into it (even with a relatively new block) as opposed to just running a higher octane. So I'm always trying to find inexpensive things to check. Unforetunately, with the design of the intake plenum, replacing the intake manifold gasket (if that's the problem) is a good 4-6 hour job and until I can convince myself that that's the problem I've resigned myself to higher octane gas (since it doesn't knock with 95% of my driving).

Any other thoughts on how to go about troubleshooting this would be appreciated!

JimN
05-01-2005, 10:55 PM
Like I said, maybe try something to get rid of carbon deposits.

How new/old are your plug wires and cap/rotor? Fords are notorious for caps carbon tracing and leaky plug wires. This would cause a cross-fire condition and this will definitely be a knocking sound. Some time when the motor has been running for awhile and it's dark out, open the hood and rev it up. If you see lightning, it's time for new wires. I have also seen the spark travel down to a ground point from the tower of the coil from a leaky boot.

Leroy
05-01-2005, 11:12 PM
I just wouldn't go in that 5% :D that is some pretty tough running anyway!

Guess this didn't help much :rolleyes:

sfitzgerald351
05-03-2005, 01:16 AM
Like I said, maybe try something to get rid of carbon deposits.

How new/old are your plug wires and cap/rotor? Fords are notorious for caps carbon tracing and leaky plug wires. This would cause a cross-fire condition and this will definitely be a knocking sound. Some time when the motor has been running for awhile and it's dark out, open the hood and rev it up. If you see lightning, it's time for new wires. I have also seen the spark travel down to a ground point from the tower of the coil from a leaky boot.

Hmmm. Good thought on the wires / cap. I thought I replaced them relatively recently, but it was 18 months ago and 20,000 miles. Not a big distance, but the last set definitely had some cross-fire so maybe that issue has come up again. I've read that it's really bad on the 7&8 cylinders. Which ones are those anyway, so I can be anal about routing those wires?

I think I'll do new wires / cap / rotor and also do some fuel system cleaner. And I'll clean the IAC and throttle plates again while I'm at it. At least I can do all that in an hour for not much $$ and see if it helps!

Thanks for the advice.

sfitzgerald351
05-03-2005, 01:19 AM
I just wouldn't go in that 5% :D that is some pretty tough running anyway!

Guess this didn't help much :rolleyes:

Yeah, I pretty much avoid flooring it. But ocassionally you need to get the boat up a hill or pass a semi or have a short merge and need to get 5,000 lbs moving quickly and then you need all the ponies running.... I don't mind running 89 octane, but once you get into the 92, it starts to become more attractive to fix the issue.