Low compression
#1
01-23-2009, 04:53 PM
Low compression
I have 1986 K10 Blazer with the 305 4bbl, with 147,000 miles. Just had it in for a check and pressure at idle is low, according to mechanic, mid 50's psi. I'm told it should be in the mid 60's and in the 40's probably won't run at all. A bleed-down test indicates %15 loss. The engine has never been overhauled to my knowledge. I'm thinking of running a cc grade oil to help limp it along until i decide to either get it rebuilt or get a reman.
I've heard horror stories of some remans out there and am looking for advice on reputable factory. I'm not against oem but can't seem to find one older than 20 yrs. Also if I go with the reman does anybody know if it is EASY to swap in a 350. The 305 vin code is H with a single rear seal.
I've heard horror stories of some remans out there and am looking for advice on reputable factory. I'm not against oem but can't seem to find one older than 20 yrs. Also if I go with the reman does anybody know if it is EASY to swap in a 350. The 305 vin code is H with a single rear seal.
#2
01-23-2009, 07:07 PM
RE: Low compression
Wait, were you checking compression with the engine running? Even 60psi is way low...
Cylinder compression should be in excess of 100psi in a properly running engine. My 74 k5 is running in the 120psi average. All 8 cylinders are within 10% of this average as well.
Cylinder compression should be in excess of 100psi in a properly running engine. My 74 k5 is running in the 120psi average. All 8 cylinders are within 10% of this average as well.
#3
01-26-2009, 03:38 PM
RE: Low compression
The mechanic was telling me running pressure should be in the 60's. I wasn't tere when he did the test. He did put it on a leakdown tester and he said it was losing %15. I try and get some more info from him. I have access to a compression tester and will check it myself and see.
Maybe he just likes the truck and is trying to scare me into selling it..lol.
It had several minor problems in the exhaust and vacuum system which i have rectified and it runs much better than it did.
Maybe he just likes the truck and is trying to scare me into selling it..lol.
It had several minor problems in the exhaust and vacuum system which i have rectified and it runs much better than it did.
#4
01-27-2009, 10:32 AM
RE: Low compression
15% does not sound too awful bad... 60psi is still way too low. Should easily be above 100psi, but every engine is different.
Heck, my B&S V-Twin in my lawn mower had 185psi on both cylinders! Glad that thing is electric start.
*EDIT* - Found an article for you (just one of many available through a quick Google search):
Heck, my B&S V-Twin in my lawn mower had 185psi on both cylinders! Glad that thing is electric start.
*EDIT* - Found an article for you (just one of many available through a quick Google search):
Cylinder Leakdown Testing - CarCraft.com
Results
Everyone wants to have a hero engine, but don't believe all the hype you hear about 2 percent leakage engines. No cylinder seals perfectly, especially if the engine is nitroused, supercharged, or turbocharged where the top and second rings are set with wide ring end gaps. Even for normally aspirated engines, respectable leakage numbers would be anywhere from 8 to 12 percent with a variation between cylinders of 4 to 5 percent, but it's possible the variation could be as high as 10 percent.
So if you test an engine and see leakage numbers even in the 15 to 20 percent range, this is not cause for alarm. Much of your concern around an engine that needs freshening should revolve around exactly where the leakage originates.
Results
Everyone wants to have a hero engine, but don't believe all the hype you hear about 2 percent leakage engines. No cylinder seals perfectly, especially if the engine is nitroused, supercharged, or turbocharged where the top and second rings are set with wide ring end gaps. Even for normally aspirated engines, respectable leakage numbers would be anywhere from 8 to 12 percent with a variation between cylinders of 4 to 5 percent, but it's possible the variation could be as high as 10 percent.
So if you test an engine and see leakage numbers even in the 15 to 20 percent range, this is not cause for alarm. Much of your concern around an engine that needs freshening should revolve around exactly where the leakage originates.
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