Engine knocking

jbutler67

Petty Officer 1st Class
Joined
Jun 19, 2016
Messages
220
1988 OMC 5.0
502APRGDP

Engine just started making a loud knocking today while running on water muffs at low idle. Was idling fine and then knocking sound just started. Knocking increases when RPMs increase. I pulled the valve cover on starboard side and checked the push rods. All look ok. Haven't checked under valve cover on port side. Ran out of daylight.

I have videos with sound it started making today and how it sounded just 2 weeks ago, when it was running great.

What changed in 2 weeks? I just rebuilt and reinstalled carb. I also was rotating the engine by hand (to check the lifting and falling of intake and exhaust valves).

I'm not sure where to focus. Could it be rod knock or spun rod bearing and if so could it just start that quickly? Timing off from manually rotating engine? Bad lifter(s)?

No knocking when engine turns over with starter. Knocking begins once combustion in engine begins.
 
Last edited:

jbutler67

Petty Officer 1st Class
Joined
Jun 19, 2016
Messages
220
Link to videos. Video 1 is a few weeks ago when all was good.

Video 2 has the knocking.
 
Last edited:

jbutler67

Petty Officer 1st Class
Joined
Jun 19, 2016
Messages
220
Results of a compression test run today after I started hearing the knocking.

1 - 150
3 - 135
5 - 120
7 - 135

2 - 160
4- 155
6 - 145
8 - 145
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
I would pull the motor, get it on a stand and pull the pan

You have an issue with cylinders 3,5, and 7 anyway
 

jbutler67

Petty Officer 1st Class
Joined
Jun 19, 2016
Messages
220
No Title

I pulled the cylinder heads and found the source of the knocking. Small piece of metal in #4. How it got there.... through bad judgement on my part and an unfortunate series of events. I'll skip the long story.

Finding this on the top of the piston is consistent with the noise coming from "up high" in the engine.

I did a simple test of the piston rod bearing in #4. Brought piston just past TDC and onto downward stroke. Then pushed down on the top of the piston with the heal of my hand and all my weight, to see if piston would moved even a 1/16 of an inch. It did not.

Taking advantage of the situation to clean up valves, lap valves and valve seats, and replace valve seals.

Curious about something... but it may be a new thread? My question... why only the intake valves have valve stem seals? All 16 valves have the little rubber rings but only the intake valves have the rubber valve stem seals. Was this common on 1988 GM 305 / 5.0L engines?

Thanks again for all the great advice and feedback in this forum.
 

Attachments

  • photo275843.jpg
    photo275843.jpg
    62.1 KB · Views: 0
  • photo275844.jpg
    photo275844.jpg
    130.6 KB · Views: 0

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
You know we all want to heat the story lol!
I just did a top end overhaul on my old 4.3 and was constantly stuffing paper towels in the cyls, then intake ports etc after the heads were on to keep stuff out of the engine.
BTW before installing the freshened up heads, clean those head bolt threads really well with a thread chaser. After really cleaning the block sealing surface check it for flatness with a straight edge and feeler gauges (less than .003"), same for the heads and intake manifold sealing surfaces.
I think the lack of seals on the exhaust valves may be normal to make sure the valve stems are lubricated due to the extra heat the exhaust valves experience
 

jbutler67

Petty Officer 1st Class
Joined
Jun 19, 2016
Messages
220
Clean the head bolt threads with a thread chaser / thread restorer. Will do. Thanks!

I'll provide the details of how that piece of metal got in there while I'm enjoying a cold beer - after this job is complete. :)
 
Top