clattering sound under power

[lifespeed]

After 99K trouble-free miles (except for the ignition coils) my engine has just started making an ominous sound. When I jump on it it starts making a clattering sound. Not even full throttle, just medium acceleration. Probably starts around 3500 RPM. Sounds totally normal at lower engine speeds.

Is this likely to be a timing chain tensioner problem? How would I go about diagnosing it? I am afraid to reproduce the problem too often for fear it is something serious like a dropped valve seat or some other valvetrain disaster that could lunch the motor. :(

I'll see if I can reproduce it in the driveway briefly, and post a video.

Edit: After further review, free-revving the engine won't reproduce the problem. To be honest, it sounds like pinging (pre-ignition) at throttle tip in, especially at medium throttle. If I floor it the problem does not manifest. Also, it is less noticeable with the car lightly loaded. I noticed it while driving with four passengers and a trunk full of luggage. All this seems to point to pinging, especially the full throttle lack of issue. I assume at full throttle timing would be reduced.

I run 91 octane fuel always. I guess there is the possibility of bad gas. I'll try diluting it with a new tankful of hopefully decent brew.

[Marauderjack]

Sounds like low octane.......try higher octane if you can or get some Toluene and add it (Race Gas will work if you can find it)!!:beer:

http://www.best-chemical.com/racing.htm

Good Luck!!

[JohnE]

Try to drive it easy and use your gas mostly up, before the next refill. Topping off early doesn't raise the octane enough. Most likely you got some low octane fuel, causing detonation.

[fastblackmerc]

If it sounds like marbles rattling around in a tin can than it's pre-ignition aka detonation.

Detonation = DEATH for our aluminum motors.

[SpartaPerformance]

Sounds like detonation to me too. When was last time changed fuel filter? That's cheap and easy I'd start there.

[05crownsport]

Without hearing the noise...it's difficult to discern. I would think that if it were pinging, that having a load in the car would exasperate the problem. Is there a possibility that it could be a catalytic converter that has come apart inside and is rattling?

[lifespeed]

Having been a gearhead for many years with a few motor builds and tuning under my belt (all except the most recent engine being carbureted, however) I have definitely learned what detonation sounds like.

It did detonate twice, very briefly, at part throttle. Yes, this is absolutely deadly, which is why I was so concerned about it.

I took the MAF out and cleaned it with electronic parts cleaner, and blew it off gently with compressed air. Then reset the PCM (disconnected the battery cable) and drove it around for about 30 miles. The air cleaner only has a few thousand miles on it, so I left that alone.

It is now much smoother at idle, and has not detonated once. The same fuel is still in the tank. I'm not entirely sure how the tuning got to be (I assume - based on the pinging) incorrect, but am somewhat suspicious of the 100K mile O2 sensors. The MAF did not look dirty, I am a bit skeptical cleaning it was more related to the improved behavior than the PCM reset.

At this point I think it is probably a good idea to replace all (4?) O2 sensors. I don't think they last much longer than 100K anyway.

[Curless]

O2's are done at 75,000...yes, you should do the all at once.

[lifespeed]

Just to close the loop on this bizarre detonation incident, a few details:

I had just driven the car down out of the mountains after spending a week driving around at 7,000 ft in Lake Tahoe. Is it possible the ECU "learned" more aggressive timing with the aid of the knock sensor? One can run more spark timing at altitude. Resetting the ECU would have reset the advanced timing, immediately curing my problem.

I replaced all the O2 sensors with Ford parts. There has been a noticeable improvement in driveability, smoothness, and maybe even a tiny increase in power. I have not yet measured MPG, but have hopes for improvement here as well. Gotta love computerized and injected cars.

[JohnE]

The computer learns the atmospheric pressure and uses increased timing advance when you are up a higher altitudes. The knock sensor(s) are only used to reduce timing if they detect knock with the factory programming. The O2 sensor feedback is also learned. O2 sensors cause the engine to run a little extra rich when they age, so keeping them too long will make the car more sluggish.

John