A Pop A Pop sound during compression test???
09-19-2017, 05:22 PM
#1
A Pop A Pop sound during compression test???
still working on the fuel injection coversion on my 81, the bang bang sound was the result of the missing fuel injector clamp screw head that broke off ending up in cylinder 8 found it after i pulled the heads, won't do that again. i now have the heads back on and have been running a cold compression test with the intake off to make sure the new head gasket has a good seal. i have noticed a pop pop sound on the cylinder that i am doing the compression test on, it seems like the popping sound happens as the intake valve opens as i put my hand over the intake passge of the head and can see my fingers sucking down to coincide with the popping so it is popping in not out. is this a normal thing? does the pistons on an 81 with 400k on the motor start dropping down before the intake opens creating a vacuum that goes pop when the valve cracks open? it seems to be doing this on atleast 5-6 cylinders. i put the intake manifold on and i can still head healthy popping sound, thought i would ask for some input on this one before i go through the long process of re-assembly
09-19-2017, 05:39 PM
#2
while i had the heads off i pulled the valves, cleaned the valves, installed new springs and new umbrella seals, the valve seats look good and my compression is good, but i am getting a popping sound which seems to coincide with the valve opening, the valves (rocker arms) were adjusted properly as well, i dont recall hearing this when doing compression tests before, did something change or is this just normal?
09-19-2017, 06:20 PM
#4
the cold compression is 120 to 140 across the 8 cylinders, 340k on the clock i was factoring in for no over drive like most vehicles have, i doubt the timing chain jumped as it is fairly new and a double roller
09-19-2017, 07:00 PM
#5
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St. Jude Donor '05
if its coming from that cyl with no plug in you could just be hearing the noise from the piston sucking air in. Makes a strange sound like you described. The compression test on cyl checks out ok? Is it coming out of the throttle body?
Last edited by cv67; 09-19-2017 at 07:00 PM.
09-19-2017, 07:43 PM
#6
the popping sound can only be heard when there is a plug in or the compression gauge and seems to be on 5 or 6 cylinders, i have 120 to 140 on all 8 cold compression test, i have not put the fuel injection system back together just have the manifold on, i suppose it would be quieter if it was popping behind a closed throttle body, like i said the popping happens when the valve starts to open and at the same time i can feel the vacuum pulling my fingers into the intake
09-20-2017, 05:51 AM
#7
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A "pop pop pop" sound makes me think you have one or more lifters way out of adjustment. If a valve begins to crack open when the cylinder is pressurized it WILL make a "pop".
09-20-2017, 07:52 PM
#8
Near as i can tell it looks like i ran the oil out of the new lifters by doing my compression test multiple times across all of the cylinders and once the oil has been squirted out of the lifters and not replaced the lifters collapse down before they start pushing up the push rod and there by oepning the valve, so the popping sound appears to be due to the intake valve opening a little late with the combustion chamber now under a vacuum. i am picking up an oil priming tool tomorrow and will be putting oil pressure to the lifters while turning the engine over. i put one of my old locked solid lifters in and there was no popping until it decided to free up and started plunging down.
