Hi All,

My Son and I rebuilt the engine in his 1988 IROC-Z.

The machine work was done by a local machine shop.

Very mild performance cam. Stock EPROM. Sounds stock at idle.

At 700 RPM, the oil pressure is 30 lbs when hot. The oil is Mobile 1 conventional 10W40. ZDDP Plus is used to push the Zn/P to SF oil specifications. A melling High-Volume Oil Pump is installed.

Presently, the engine is in the break-in stage. About 200 miles on the motor.

When starting or running down the road at temperature (180 degrees), the engine runs great. No knocking or strange noises at idle or under load.

When starting cold, a fair amount of ticking / knocking noise is heard. This noise goes way after 2 seconds of running.

When I use a rubber hammer to pound on the exhaust system, I can hear something rattling in the exhaust. I think this may be a loose Cat-Con core. This may be part of the problem.

I am somewhat worried about the motor. Not sure if the Cat-Con is the total problem?

The clearences in the motor are somewhat loose.The mains clearance is anywhere from 0.002" to 0.0035". The rod side clearence is anywhere from 0.0125" to 0.018".

Are these clearances wide-enough to cause such a start-up noise?

I would think a hot start-up would result in more noise if the mains or the rod clearances are the issue?

Please send some trouble-shooting advice.

How can I tell the difference between lifter knocking, Piston Slap, or engine bottom end knocking?

Best Regards,

Bruce

 

Did you replace parts or re-use?

 

The Block, Crank, and Rods are re-used.

The crank was turned 0.010 under. Due to wear after 150K original miles.

 

An exhaust leak will make a tapping noise and go away when it warms up.

 

^^ on other forums and other sites for different vehicles it says it gets worse when the engine is revved and warm. so which one is right?

 

Where did you come up with those clearance numbers? That is a large range for a freshly rebuild motor. You are talking .0015 of difference between tightest and loosest. Still, even .0035 shouldn't be noisy at 30psi oil pressure. Rod side clearance is out of spec but I've seen worse that didn't make noise. You're right to think that loose bearings are more noisy with thin, hot oil. I am of course assuming that the pistons are new and fitted to the newly bored and honed cylinders. Of course, with the wide range of oil clearances that you report, I won't assume that the cyls were honed with torque plates. Still, it takes quite a bit of clearance to produce piston slap and it doesn't go away after a few seconds.

This sounds more like valve train noise. Where are your valves adjusted? Did you go with OE spec of 1 turn in from zero lash? Or did you go with a high performance setting like 1/2 turn or 1/4 turn? You may have lifters that are bleeding off slightly and have to be pumped back up after startup. This is annoying and points to cheapy lifters but it's not awful. Not as long as the noise stops right away.

You can tell lifter tick from bearing knock by the pitch of the noise and its general location. Lifter noise is a higher, lighter pitch made from a lighter part and will seem to come from the top of the motor. Bearing knock is a deeper, heavier pitch made by excessive clearance between the heavy crank and a connecting rod and will come from the lower part of the motor, where you can hear it at the oil pan.

Get yourself a cheap mechanic's stethiscope at the parts store and run the tip along the valve cover on each side. The place where the noise is loudest is where the loose lifter lives. If you think it sounds more like bottom end noise, do the same stethiscope test along the side of the oil pan. Where the noise is loudest is where the loose bearing is. However, I am sure from your definition ( the fact that the noise is only there on the first cold start of the day after the engine has sat overnight, and that it goes away quickly) that this is lifter noise.