Engine 91 LX Rhythmic Thumping sound and high idle

VSK2033

Member
May 17, 2019
24
3
13
Maryland
Hi, so the past weekend I noticed cylinder 8 failed the balance test so I did a compression test on all cylinders, checked spark plugs plus wires and took intake off to check injectors and everything came out good. Once I put everything back together and started it up without the IAC plugged in, the idle hung for a second at about 1k rpms and then shot up to 3k rpms and stayed there and at the same time a noticable thumping sound started coming from somewhere inside the engine and was pretty rhythmic where it thumped every few seconds. I am a little lost as to what this could be and was wondering if anyone else experienced anything like this.
 
  • Sponsors (?)


Some things to check. There is a checklist for this but I feel it is a reassembly issue.
throttle cable out of whack, hung up or base idle out of adjustment.
vacuum leak?
have you reset the computer?
if you didn't have the idle problem before it likely something out of whack when you installed the upper intake.
here is the checklist that will help you also.
 
Some things to check. There is a checklist for this but I feel it is a reassembly issue.
throttle cable out of whack, hung up or base idle out of adjustment.
vacuum leak?
have you reset the computer?
if you didn't have the idle problem before it likely something out of whack when you installed the upper intake.
here is the checklist that will help you also.
Yeah the idle was fine before, we had surging issues in the past and that checklist is what I used to help smooth things out, my biggest concern is the loud thump sound that I have never heard before because I haven't really had a chance to get idle sorted out because I get nervous when it starts to thump so I turn it off. Its a very deep sounding thump coming from somewhere inside the engine, At I first thought it was spark knock or something of the sort but its not a pinging sound or rattling metal. Is it possible that a major vacuum leak or something could cause this?