GlennD

I got my new lift and put the car up high to redo the exhaust system. Well, started the car up and noticed a bit of a knock ... knock ... at the back of the engine (from underneath). Almost sounds like piston slap.

Is this a normal sound? I only had the car running for a couple of minutes so nothing really got hot (e.g. iirc piston slap starts to quiet down when the pistons get warm enough to grow a bit).

Front of the engine is quiet ... only the very back just before the flex plate area (checked flex plate - straight - the PKClamp is doing it's thing)

Any opinions on this knock knock noise? Something I need to worry about?

Thanks
Glenn

Mrmerlin

is there metal in the oil ?
what does the oil pressure run at?

take a lamp cord put it into the dipstick hole and take an oil sample drip the oil onto a black piece of cardboard look at the oil in the sun look for metallic particles (dont use the dipstick as it will pull metal from the tube when its inserted and removed)

User 4221

Could it be an exhaust leak? This can sound like piston slap.

jcorenman

Is it a knocking sound, or more like a quiet "tick"? Like a ticking lifter?

I heard this once, from our GT, about a year ago. Mileage (then) was around 180K and I had just finished some work (don't recall what exactly), it sounded like a lifter that has been sitting for a while-- except coming from down low, more audible under the car than above, and from the rear of the engine.

I decided to ignore it for a while, and it went away

(Just for the record, we also change oil regularly, usually cut open the filter for inspection, and routinely send off a sample to Blackstone).

I think piston slap would be very unlikely-- the pistons are aluminum but also run in an aluminum block, and the clearances are much tighter than the usual aluminum pistons in an iron block.

I am liking this theory! That would also explain why it would be localized under the car-- a mechanical knocking/ticking from the block would be heard everywhere around the engine, like a drum. But an exhaust leak radiates sound acoustically from the point of the leak. Take a rubber hose and use it as a poor-man's stethoscope and see if you can isolate the source. That may have been what ours was, also. Good thinking, Scott!

GlennD

Thanks for all the suggestions. Sorry for the delay in getting back.

The oil pressure is fine - 5 bar when cold and drops down when warm (still on the lift so the engine has not had a chance to get real warm yet so I do not have that number but will watch closely). But, initial observations makes me believe there is nothing different.

Exhaust leak - not any more . Just finished redoing the whole exhaust complete with new clamps, hangers, etc. There was one slight leak (just after the cats) but that is gone. Nope, I am quite positive it is not an exhaust leak.

Checked the oil on some black construction paper (not a common item in the house so that took some time to find). Nothing outstanding on the oil smear. I saw a couple of very very very small (almost dust size) particles but they did not reflect in the sun. So, that is a bit of a dead end.

The noise is loudest just in front of the flex plate housing. From the top of the car, I can hear the injectors ... and this one sound too. Sounds like one cylinder from the knocking rate.

Could be a stuck lifter. The car was in storage for 6 months (darn winters here) and then sat for 3 more months as I was waiting to find time to get to the exhaust systems. I am going to run it a bit and see if the noise goes away - will report back later tonight or tomorrow.

Other information - oil is changed one per season (approx 3,000 km or so) with Mobil 1 15W-50. No surprises in the oil catch pan last change (2 km ago ... 2 months ago). I looked closely as I drained the oil catch pan (or pail) to recycle the oil.

On the positive - flex plate did not release at all since I put on the PKClamp. All is good there ... just this darn sound

Glenn

GlennD

Back from a drive ... it ended up being a lifter. The noise was there for about 1 mile and then away it went. All the car needed was to get the oil moving through the engine. Never had a noisy lifter on this car so this noise surprised me (different than noisy lifters I had on some of my old beater from my youth).

Another example of why these cars need to be driven ... 9 months was the longest downtime this car ever had ... and hence, the new sound that had me going. Now, time to drive it some more

Thanks for all the replies and ideas. Next time I know better

Cheers