How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask John Your Own Question

John
John, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience:  ASE MASTER TECHNICIAN/40 years experience
6408229
Type Your GM Question Here...
John is online now
A new question is answered every 9 seconds

1990 GMC: a bad miss and a loud knock in top end of motor

Customer Question

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
Submitted: 4 years ago.
Category: GM
Expert:  John replied 4 years ago.

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 4 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
Expert:  John replied 4 years ago.
The popping through the throttle body can be an incorrect timing or a faulty intake valve spring.
Customer: replied 4 years ago.
Will I have to take valve springs off to inspect? Will timing light tell me anything?

Thanks,
Jim
Customer: replied 4 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
Expert:  John replied 4 years ago.

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, GMC Technician
Category: GM
Satisfied Customers: 9176
Experience: ASE MASTER TECHNICIAN/40 years experience
John and other GM Specialists are ready to help you
Customer: replied 4 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
Expert:  John replied 4 years ago.
Yes,the heads will fit & I would use them rather than rebuild yours.
Customer: replied 4 years ago.
Thanks,
Jim
Customer: replied 4 years ago.
My last question, I promise. I can get these heads from my buddy, will they work?
Casting # 10125320.

Thanks so much,
Jim
Expert:  John replied 4 years ago.
Yes,I believe they will also work.

JustAnswer in the News:

 
 
 
Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.
 
 
 

What Customers are Saying:

 
 
 
  • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
< Last | Next >
  • I was having a terrible problem with the wheel bearings on my 98 Chevy Cavalier and I suspect that using this website saved me plenty of time and money. Todd L Rochester NY
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland
 
 
 

Meet The Experts:

 
 
 
  • Gary

    GMC Technician

    Satisfied Customers:

    903
    15 years gm master tech/15 years ASE master tech
< Last | Next >
  • http://ww2.justanswer.com/uploads/ggrote1/2009-06-27_031011_me.jpg Gary's Avatar

    Gary

    GMC Technician

    Satisfied Customers:

    903
    15 years gm master tech/15 years ASE master tech
  • http://ww2.justanswer.com/uploads/CA/caprio23/2011-12-13_155550_20111213105344.64x64.jpg Anthony's Avatar

    Anthony

    ASE Certified Technician

    Satisfied Customers:

    476
    ASE Master Automobile Technician with over 20 years exp.
  • http://ww2.justanswer.com/uploads/Rocket2005/2009-09-13_133703_jack.jpg car tech's Avatar

    car tech

    ASE Certified Technician

    Satisfied Customers:

    448
    25 year master tech and shop owner
  • http://ww2.justanswer.com/uploads/2P/2paulm/2012-4-4_183431_justabswer.64x64.jpg Paul M ASE Master's Avatar

    Paul M ASE Master

    ASE Master Tech

    Satisfied Customers:

    187
    ASE Master Tech 20+ Years
  • http://ww2.justanswer.com/uploads/MU/muddyford/2012-6-13_1204_1.64x64.png Chris (aka- Moose)'s Avatar

    Chris (aka- Moose)

    GMC Technician

    Satisfied Customers:

    89
    16 years of experience
  • http://ww2.justanswer.com/uploads/AM/amedee/2013-10-24_23656_Amedee1.64x64.jpg Amedee's Avatar

    Amedee

    GMC Technician

    Satisfied Customers:

    360
    ASE Master Technician advanced level specialist
  • http://ww2.justanswer.com/uploads/DT/dt69nova/2012-6-15_234154_pic1.64x64.jpg Dave Nova's Avatar

    Dave Nova

    GMC Technician

    Satisfied Customers:

    2262
    ASE Master Certification. GM World Class Certification