Cold start knock or under a load - Pontiac G8 Forum: G8 Forums - G8Board.com
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post #1 of 14 (permalink) Old 06-26-2019, 03:40 PM Thread Starter
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Cold start knock or under a load

I purchased the car this way with 130k miles. Hopefully will be taking it out soon. Its my first go on afm LS, so i'm trying to get all my ducks in a row for what's needed to delete it. First though will be finding the gremlin.... it sounds like a rod knock, hopefully not a main. Anyone run into something similar?
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post #2 of 14 (permalink) Old 06-27-2019, 05:20 AM
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Can you post up a sound clip? Might help narrow it down.

things are not as they appear to be.........

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post #3 of 14 (permalink) Old 06-27-2019, 09:49 PM Thread Starter
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It's in the bottom end for sure... i hooked my dad's scanner up it at least shows 55psi cold on the oil pressure. Time to start pulling it. Trying to make a list before hand, however who knows what i'm going to run into.... im figuring crank, bearings, oil pump, gaskets... then ive gotta figure out the whole dod delete... that's a whole other list of valve train trinkets....
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post #4 of 14 (permalink) Old 06-28-2019, 09:09 AM
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I dont think its a rod bearing. A failing DOD lifter can sound like rod knock if the lifter has completely failed.
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post #5 of 14 (permalink) Old 06-28-2019, 09:46 AM
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It's in the bottom end for sure... i hooked my dad's scanner up it at least shows 55psi cold on the oil pressure. Time to start pulling it. Trying to make a list before hand, however who knows what i'm going to run into.... im figuring crank, bearings, oil pump, gaskets... then ive gotta figure out the whole dod delete... that's a whole other list of valve train trinkets....
Holy crap are you going to have a bill of a life-time. At that point I think it'll be cheaper for you to just go find a used L99 and swap that.

But on a serious note I think its the lifters getting poopy. Mine has been making a knock like noise at startup for 2 years now. If the car sat for a while it does that. Or if ambient temp goes below 45F. It's the hydrolic lifters that release oil far to quick and don't store any. So once you go to turn the car on they tap/scrape the camshaft causing that noise.

Best way would be to look at the oil for shavings (mine has some metal specs in there every once in a while.) But no tick yet.

Oil pressure on L76 is going to be 'Weird' because of DOD. Once it goes on it'll read lowish; At first I thought same as you, then when I went to various engine forms and found out that the system tends to run at lower pressure than normal LS when DOD engages, for example I have oil pressure gauges on mine and its hilarious to see the oil pressure dial jump back and fourth at idle because of DOD. (One of the few ways you can tell its on.)

If the oil pressure drops too low you should get some sort of Error/Check engine code pop out. System should do it automatically once you go below 10KPA (15ish PSI). On cold start, high oil pressure is normal it'll drop once everything warms up.

Example: (My gauges are aussie so my readings are in KPA) At startup my engine reads around 450KPA (65PSI) but once it reaches operating temp it cruises at 200 Kpa (29psi). The funny kicker is that once I pull over and idle at about 550rpm the pressure jumps from 150Kpa (21psi) to around 200Kpa (29psi). And goes back to normal once I as much as breathe on the gas.

One last thing. In your owners manual it should actually tell you the threshold on the oil pressure (I believe on LSs 30Psa on warm oil is normal). Oil pressure should increase I think 10 psi per 1000rpm.

Last edited by Carnagexz; 06-28-2019 at 10:21 AM.
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post #6 of 14 (permalink) Old 06-28-2019, 11:29 AM Thread Starter
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I dont think its a rod bearing. A failing DOD lifter can sound like rod knock if the lifter has completely failed.


I hope your right. Ive honestly never heard one of these lifter failures soooo....
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post #7 of 14 (permalink) Old 06-28-2019, 12:28 PM
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I dont think its a rod bearing. A failing DOD lifter can sound like rod knock if the lifter has completely failed.


I hope your right. Ive honestly never heard one of these lifter failures soooo....
There are a bunch of clips on youtube. The car will sound like diesel with a very audible tik noise while its running. It's just funny that on that engine both the injectors and the valve train make it sound like a sewing machine, but once the lifter fails its a loud tik you wont miss it.

Also since the block is aluminum, if your somewhere cold the car will piston slap as it gets warm. That's a ton of fun too.

I think its the 'Eventual death' anxiety, if you know there might be an issue with lifters so you'd be 1mil times more attention over analyzing everything.

Though DoD delete would most definitely fix that.
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post #8 of 14 (permalink) Old 06-28-2019, 12:39 PM
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Sometimes even an exhaust leak can sound like a lifter tick. Clickity tickity tick.... My headers were leaking all over and once they were fixed the clicking stopped.

I'd say this would be a prime reason to pull the motor for the rebuild. Then you can inspect everything. The bottom end of the LS is pretty solid; frankly you shouldn't even have to touch it at 130k. Check compression if you think it is a rod knocking. Easy to do, and 0PSI would be a dead ringer.

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post #9 of 14 (permalink) Old 07-10-2019, 01:22 PM Thread Starter
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#7 rod bearing wasn't happy....
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post #10 of 14 (permalink) Old 07-10-2019, 01:40 PM
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Well that sucks. Its crazy how some of these stock engines just fail this way. And some engines live forever.
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post #11 of 14 (permalink) Old 07-11-2019, 08:39 AM
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#7 rod bearing wasn't happy....
How bad was it?
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post #12 of 14 (permalink) Old 07-11-2019, 11:42 AM Thread Starter
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I just started tearing the motor down. Pulled the pan and found #7 had some play. The crank isn't horrible but polishing it, i'm sure won't cut it.... hadn't mic'd it yet.
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post #13 of 14 (permalink) Old 07-11-2019, 03:00 PM
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I just started tearing the motor down. Pulled the pan and found #7 had some play. The crank isn't horrible but polishing it, i'm sure won't cut it.... hadn't mic'd it yet.
Hmm, maybe I should take a look in my engine when I get around to doing the DoD delete. I get a bit of a knock everytime I cold start her. With some metal specs in the oil everytime I change the fluids. Lifters never failed, but at 113K miles I think its time. (Just don't have garage to pull the motor out, thinking maybe a car tent would do.)

How hard is it to replace the rods/bearings?
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post #14 of 14 (permalink) Old 07-12-2019, 03:35 AM Thread Starter
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All from cylinder 7...
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