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John
John, GMC Technician
Category: GM
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Experience:  ASE MASTER TECHNICIAN/40 years experience
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1990 GMC: a bad miss and a loud knock in top end of motor

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1990 GMC Sierra 5.7 (350) Has a bad miss and a loud knock in top end of motor, like a loud valve knock. Very bad miss on idle. When put under a load, it is real loud knock and bad miss. At idle it does not knock. New wires and plugs, all getting fire. Plugs are golden color. Noise is on driver side. Took valve cover off and all looks good. Rockers are all on. Runs like it has bad gas in it, but have half tank of gas and ran other half out for 2 weeks, and ran fine. Sounds like a diesel.

Thanks,
Jim

Hello,

 

Your problem sounds like you may have a broken valve spring or a valve that is not seating due to damage.

Do a cylinder balance test on the engine to pinpoint the exact cylinder.Air the cylinder & make sure it holds pressure.if necessary remove the valve springs & inspect.look very closely at the valve guide to valve stem clearance .If you can move the valve side to side the guide is worn out.

Customer: replied 7 years ago.
Did what you said. Everything is good. Had air cleaner off and valve covers off. Started motor and flogged it. Poppin thru carb. This is a timing issue??? Where would I go from here?

Thanks,
Jim
The popping through the throttle body can be an incorrect timing or a faulty intake valve spring.
Customer: replied 7 years ago.
Will I have to take valve springs off to inspect? Will timing light tell me anything?

Thanks,
Jim
Customer: replied 7 years ago.
Will I have to take valve springs off to inspect? Will timing light tell me anything? I am not getting any error codes, either.

Thanks,
Jim

The timing light will show you a problem "IF" the timing is moving caused from a fault.

 

Tr to find out which cylinder is backfiring through the throttle body before removing the springs.Do not mix up the springs.

 

You will need a special tool & a torque wrench to test the pressure it takes to depress each valve spring.If you do not have these tools I suggest you find the cylinder that is causing this problem ,remove the springs & replace if necessary.

John and 2 other GM Specialists are ready to help you
Customer: replied 7 years ago.
Will a set of 10065202 casting heads fit on my 1990 gmc sierra 5.7 tbi motor? Found a set right out of machine shop for $375.00. Is this a fair price, or would it be cheaper to redo my old ones?

Thanks,
Jim
Yes,the heads will fit & I would use them rather than rebuild yours.
Customer: replied 7 years ago.
Thanks,
Jim
Customer: replied 7 years ago.
My last question, I promise. I can get these heads from my buddy, will they work?
Casting # 10125320.

Thanks so much,
Jim
Yes,I believe they will also work.