7.4L Knock at start-up

[dracer398]

I have a 1996 GMC Sierra with the 7.4 Vortec in it. When I start it cold, it has a slight knock for a few seconds. The oil pressure is great. The engine has been doing this since new. I've taken it in for the knock under warranty but the service people could never hear it. I always use AC oil filters and I have changed to Mobil 1 oil. It still knocks. The engine has 63K on it right now. I'm just courious what this knock is and I am wondering if anyone else has experienced this problem.

[buymeabmwm3]

Have you tried using a higher octane (93) gasoline?

[dracer398]

Yes but the knock goes away after about 3 seconds ofter starting. I think that if it were pre-ignition from bad fuel it would knock going down the road. Also it is a low compression engine probably 8.5:1 maybe 9.0:1 if it's lucky but I doubt it.

I've been working on engines for over 26 yrs and I know that a knock IS NOT normal. A knock other than pre-ignition or detionation means that there is a clearance problem possibly a lack of oil or just too much clearance.

I've talked to a few people with the same problems. I'm thinking that it's an issue that GM knows about but it would cost them $$ to fix.

Any GM Guru's out there care to add or correct me?

[Gavin Curtis]

Sounds like GM's piston knock. Some of the cylinders were overbored on the assembly line causing the knock in many of their engines. Once the pistons warm and expand, the noise goes away or becomes less audible.

Do a google search on GM piston knock.

Any problems with excessive oil consumption?

[dracer398]

No, it's very good on oil.

[BOOZER]

G.M. has had many bullitin's on this over the years.
Oil filter updates-antidrain back valves were added, PF35 has changed to PF1218 for this truck. Selective main bearings have been the latest I think? But this was mostly 5.0-5.7L. There is an update on the pcm for a reflash, bullitin
#99-06-04-014, this is strictly for 7.4-j motors. See your local dealer, hope this helps.