[Shotimes] Knocking help needed (long)

[Adam Parrott]

Hey all,

I haven't received many responses or ideas on this topic elsewhere, so I
thought I'd post about it here.  Below is a copy of the message that I
posted on the SHOforum for those who care to read it.

(original post follows)

On Wednesday night, I decided to remove my upper intake for some plug gap
measurements and valve cover cleaning (TB coolant line bypass was also
performed while the intake was off). I was slow and careful during removal
and installation but still ended up with a stumbling idle after
reinstallation. Figuring a vacuum leak was to blame, I double checked my
connections again this afternoon and noticed that the EGR-to-intake bolts
weren't fully threaded into their holes, resulting in a teeny gap between
the EGR and intake which created a vaccum leak. I tightened everything up
and the stumbling idle was fixed. I let the car sit at idle for a few
mintues before I left for my test drive, thinking that everything was
golden. Right?

Wrong.

After a few minutes of a seemingly perfect idle (and no more stumbling
around 400-600rpms like the night before), the stumbling returned right as
I was about to leave. The engine would never completely stall, but it
wouldn't hold an idle, either. I decided to shut to engine off and give it
a few seconds before I restarted.

That's when it happened.

All of the sudden, as I restart the engine, I hear a strange knocking sound
that wasn't present just a minute ago. The best description I can give is
that it sounds as though something is rattling around inside the block and
is not unlike the sound of a completely failed cam tensioner or chain. The
sound is RPM-matching and will very rarely fade away and come back after I
rev the engine. I've always heard a significantly softer version of this
sound since I owned my SHO (sdpatt and I noticed it after my 60k in '02)
but it's never been this loud or noticeable.

Regardless, I'm both stumped and very concerned. The engine was idling
quietly one minute and making noises the next. I welcome any and all
suggestions or ideas as to what could be causing this sound. The sound is
present as soon as you start the car, is with you through-out the rev range
and is concurrent with engine speed. It's pretty loud and, to the casual
observer, can be heard from both the top and bottom of the engine.

Based on various searches I've performed, I've read the following may be
possible:

- rod bearings or wrist pin failure
- loose spark plug that was not fully tightened
- bad or low octane gas
- hardened carbon deposits or other foreign material in the cylinder
- bad water pump or accesory bearing(s) or bolts
- other misc. things like a loose bolt, etc.

It should be noted that aside from the sound, the engine idles perfectly
now. My rod bearings shouldn't be a concern since they were changed 10k and
6 months ago by TexanTony himself. Oil level is good although it'll need
changing within a few hundred miles. Engine is otherwise perfectly healthy
(sdpatt and Tony can attest to this) and is fed Castrol GTX 10w-30 with a
Motorcraft FL-400S filter every 3k miles. The only plugs that were touched
the night before were the front three (gap was measured and adjusted, plugs
reinstalled) and the wires stayed on the coil pack. I am using 87 octane
gas with this tank, but I've already put 100 trouble-free miles on this
tank and I can't believe that the low octane would cause a sudden and
unexplained knock like I'm hearing.

Any ideas what would cause a seemingly-fixed idle to start stumbling and
then produce a sudden and consistent knocking sound?

(end of original post)

http://www.shoforum.com/showthread.php?t=28103

I welcome any and all ideas on the subject.  Hopefully it's just a case of
a loose bolt or other somesuch that I've overlooked.  Further inspection on
my part will continue either tonight or over the weekend.

Many thanks for any help offered.

Adam