Thread: rattle at idle

I have a 1998 Gtp and recently it started making a nasty rattle sounding noise only at idle.
I replaced the coupler thinking it was the problem, it continued.
A buddy looked at it with a stethoscope and says its coming from the back cylinders and perhaps may be a main bearing.
Any ideas?

Possibly some lifter noise. I'd recommend pulling the rear valve cover and checking there or do an oil pressure test. Main bearings aren't as common as connecting rod bearings. Also, what kind of oil and oil filter are you running?

Flexplate? Bearing would be a knocking sound.

At around 1300RPM the noise is gone.
Running 5w30 synthetic

Why are you running 5W? Did you install a stronger by-pass spring? Have you replaced the gerotor or plate?

A collapsed lifter, bent pushrod, or loose rocker will make a prominent "tick, tick, tick" sound.

I second what hitman92499 says; do an oil pressure test at idle. If the motor is getting "loose" from normal wear there may not be enough oil pressure with the 5W to properly lubricate the top of the motor. If the main bearing clearance on these motors gets higher than 0.003" bad things happen. Especially when using a lower viscosity oil. You'll most likely spin a rod bearing.

If there's not something obviously wrong with the valve train I would put 10W-30 in and compare oil pressure at idle to the 5W.

And don't forget about checking the flex plate for cracks. This is more common than you might think.

Good luck and give us an update. The suspense is killing me!

Originally Posted by copgtp

Oil pressure is not going to fluctuate that much when going from 5w to 10w.....we have run 5w in our platform for years with no problems.....

Sent using my bionic thumbs from my sweet DROID Razr Maxx

I totally agree. The mil spec for clearance on the mains is 0.0008" - 0.0026". If your within this range then 5W will do fine, especially if your near the lower end of the spec and it might even do better than the 10W. But when the motor gets loose there may be quite a difference in oil pressure at idle between 5W and 10W. The gerotor pump on these motors are notoriously bad at idle rpm.

This is exactly why I'm replacing my engine right now...low oil pressure at idle and spun #5 rod bearing. Of course, I never used 5W in the motor for 200k.

iceman
The gerotor may be making enough pressure right off idle to quiet things down. This is what I saw with mine but it was to late; it had already spun a rod bearing. It tore up the crank and the rod will need resizing. I just went and got a good short block from Morad.

I will record in the morning,