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Engine Knocks but ONLY when Hot....

Knocking engine only at full operating temperature.

Engine Knocks but ONLY when Hot....

Postby jr81menards [OP] » Apr 04 2010, 7:27am

My 1990 GMC Sierra has less than 20K on my complete new drivetrain (Engine, Trans, Rear End). The truck only has 160K anyway. The truck is professionally built and has a .30 over 350 TBI injected, teflon coated flat top pistons, Vortec cylinder heads, roller rockers, Crane Cam, MSD Ignition, Holley TBI, etc. This motor has less than 17K. Three weeks ago I heard a knocking sound coming from engine. Immediately I thought it was a rod. I immediately dropped the drain plug and changed the oil. It was still gold in color with no metal shavings. The next morning I started the truck and there was no knocking sound. As soon as I arrived at work which is 4 miles from my home I heard the sound again. The truck "ONLY" makes the noise when it reaches full operating temperature. I checked oil pressure, pulled spark plug boots while truck was running but no change in sound. The truck still runs great and doesn't effect performance at all. I was told by several people it could be a cracked flex plate. This Saturday I slid the torque converter off the flex plate and got the engine 'HOT". No noise what so ever....I took the trans out and the flex plate looked like brand new with no cracks. To be on the safe side I replaced flex plate, all bolts and the torque converter. NO CHANGE. I even adjusted timing thinking this might be problem but that did nothing either. I hate to start tearing down the motor especially since nothing seems wrong with the engine. Any help would be greatly appreciated. Seems to me if it was a bearing it would do it worse when cranking or cold....
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Re: Engine Knocks but ONLY when Hot....

Postby cobra19946 » Apr 04 2010, 11:41am

i am completely stumped. everytim i was thinking it would be something, i read further and saw that you checked that. i have a buddy at work that is a mechanic and also runs a mud bog and does all the repairs on trucks the break down out there. i will ask him and get back to you ASAP.
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Re: Engine Knocks but ONLY when Hot....

Postby koolz71 » Apr 04 2010, 1:44pm

please help keep this site clutter free, you posted the same issue/question almost word for word back on march 21, you could have just replied to that to keep the original topic going instead of adding another..thank you and i hope you find your issue

Torque converter making a knocking sound.
Crap, my hampster fell off the wheel and it broke its leg.

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Re: Engine Knocks but ONLY when Hot....

Postby lzn197 » Apr 04 2010, 2:36pm

I had a situation just like this on a 1992 Caprice 350 with TBI. With normal driving around town and even one or two hour trips the engine was quiet. After an extended expressway drive; or with a good hot run, with a short overnight cool down; this crazy engine would knock. After several oil pressure checks, detailed checks with my "electronic ears" stethescope and some serious soul searching, one day it hit me. The problem was carbon on top of a piston. I AM SERIOUS! To remove carbon, what you can do is purchace this stuff GM sells called "top engine cleaner" and follow the instructions on the can. I assume it is still available. The stuff is pretty knarly so don't get it on your hands or breathe the exhaust fumes as you run it through the engine. Most of the mechanics out there with any time under their belt know all about this liquid because they had to deal with "carbon issues" back in the day of leaded fuel. I know that many of you out there will not believe this is a situation that could happen on an engine that uses unleaded fuel but it is true.The engine you spoke of sounds like it is modified in some way and it might not apply. This procedure is a lot cheaper that pulling the engine down! I hope this helps!
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Re: Engine Knocks but ONLY when Hot....

Postby LonestarEMT » Apr 04 2010, 11:51pm

The good old "spark knock". he is right about the carbon build up. The stuff from GM can cost some cash. You should try running a product called Sea Foam. It does the same thing for alot less. I'm willing to bet you buy fuel at the same place almost every time. Bad fuel can cause this build up faster than normal. It could also be that you need to use a higher octane fuel because of the fancy new engine has expensive taste and doesn't run well on low octane.
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Re: Engine Knocks but ONLY when Hot....

Postby cobra19946 » Apr 05 2010, 2:36am

I know all about the octane thing! i made the mistake of using 87 octane in my silverado one time and my truck ran like crap until i ran it all out. ever since then, i have only run a minimum of 89 octane and it has made a world of difference.
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Re: Engine Knocks but ONLY when Hot....

Postby lzn197 » Apr 06 2010, 7:42pm

Assuming your engine is designed to use 87 octane, it should run fine (fuel without alcohol). I have found that on very hot days or when towing a heavy load, 89 octane fuel is a better choice.
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Re: Engine Knocks but ONLY when Hot....

Postby LonestarEMT » Apr 07 2010, 5:00am

The way he descibes the motor it sounds like it was built for performance and that is why I think he would need to use a fuel with a higher octane. I have an 2007 tahoe that does the same thing and the knock stops when I use a high octane fuel. The manual says I only need 87 octane.
AIR GOES IN AND OUT,
BLOOD GOES ROUND AND ROUND,
ANY DEVIATION IS A PROBLEM.
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Re: Engine Knocks but ONLY when Hot....

Postby lzn197 » Apr 07 2010, 6:15am

Interesting. Have you tried other brands of fuel? With a lot of tricks out there to get the best fuel mileage, I am sure they (engine calibration people) put the spark curve real close to the edge by design. Also if you knew someone who has access to a scan tool, they could verify your coolant and intake air temperature sensors are telling the truth. Yes they can shift values and not set codes. An intake air temp sensor that is reading 80F intake air and lies and tells the computer 50F could cause a spark knock. Rare but it happens. GM engines used to be notorious for cold engine piston slap (just an annoyance). Usually they do it in cooler weather and yes higher octane fuel helps cover that up too due to reduced combustion pressure.
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Re: Engine Knocks but ONLY when Hot....

Postby LonestarEMT » Apr 07 2010, 3:16pm

I have tried different fuel and the results have been the same. I even did a top engine cleaning to see if that would fix. That was a no go.
AIR GOES IN AND OUT,
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ANY DEVIATION IS A PROBLEM.
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