2.3 Rod Knock?

[geoff c.]

I have a 94 mazda b2300 with about 175000 miles. This has happened twice now, the first time about a month ago then again today. I started my truck this morning and it was knocking very loud, I did have oil pressure. I popped the hood and tried to pinpoint it sounded like either the first or second cylinder. The first time this happened it stopped after about 10 seconds but today it knocked for about 2 minutes. Knock increased with rpm. It didn't knock again the rest of the day.
What is weird to me is it sounded like a rod knock but it went away and has only happened twice. Im going to try to record it with my phone next time it happens.

 

[b23]

I dont have a lot of experience with these engines, but have been a mechanic for cars for many years. In my experience what you are describing is likely to be a stuck valve or lifter. As the engine heats up, the valve becomes unstuck and the symptoms go away.

The rarity and irregular nature of your problem took me to this diagnosis. If it happened all the time, or at least predictably, its likely to be something else.

I would recommend seafoaming the engine, I have done it to many many cars and done properly there is no risk and it will do a lot to clear up any gunk that may be in the engine, which depending on what is getting stuck, may help a lot to prevent this in the future

 

[BRUTUS_T_HOG]

I have a 94 mazda b2300 with about 175000 miles. This has happened twice now, the first time about a month ago then again today. I started my truck this morning and it was knocking very loud, I did have oil pressure. I popped the hood and tried to pinpoint it sounded like either the first or second cylinder. The first time this happened it stopped after about 10 seconds but today it knocked for about 2 minutes. Knock increased with rpm. It didn't knock again the rest of the day.
What is weird to me is it sounded like a rod knock but it went away and has only happened twice. Im going to try to record it with my phone next time it happens.

Mine does that sometimes on a cold morning. Its piston slap caused by worn cylinder bores or collapsed piston skirts... in my case at least

When the engine is cold the pistons have more clearance to the cylinder bores and slap around against the cylinder wall, then when they reach operating temperature the piston expands and swells with heat and the clearance is reduced.

Typically a rod knock will not go away and is loudest under a light load

 

[tomw]

A collapsed valve lifter will cause a loud rapping sound until it pumps up and closes up the clearance. It is not unusual. The sound would come from underneath the cam cover. If you remove the oil filler cap, it should be louder. If it persists, it can damage the cam lobes and the follower, perhaps the valve stem tip, causing permanent noise.
If you haven't done so, you could try changing the oil, going to a lower viscosity for winter time, and changing to more viscous oil in hot weather.
tom

 

[PetesPonies]

Listen under the car. If its a reciprocating assembly noise, you will here it better near the oil pan. They will make some noises in the morning when cold as an engine wears. A rod may knock some and then go away as oil pressure is finally built up in all locations. These engines don't have typical lifter noises as do engines with lifters and pushrods in the block. But any noises from the valvetrain will be heard up on the top of the engine. Use a piece of hose as a listening device, moving it around against the engine. It really works. Has you oil been changed recently? is it full? I would change it and make sure everything is good if you have not recently done this.