GMC 350V8 LOW COMP

[TEXDOG]

GREETINGS TO THE FORUM
just got the truck back from the mech. said no. 7 cyl had 10lb
comp. blowing white shoke and using alot of gas. 185k miles. I am asking for help as a last effort. I am looking at another truck asap unless I can pull this one out. no emmis test around here to worry about.I am considering adding a pint of LC20. no time for arx. thanks for your time.

 

[Scienceman]

Got this from another automotive fourm. See If this may help:

mhenry4811-18-2006, 06:15 PM
Can anyone help? I have a 1994 GMC Suburban with 350 engine and 140k miles. I have been having problems with it idling rough. I did a tune up but it did not help. Checked the cylinder compression and the #3 and 5 cylinder cylinders are at 30-40 lbs all other at 140-150. I have pretty much determined it is not a head gasket however these cylinders are next to each other. No antifreeze coming out exhaust, and none in oil. I get a intermittent knock when the engine is under stress at acceleration. Any idea what is next? Look forward to some advise.

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maxwedge11-18-2006, 07:38 PM
Welcome to AF. Do a leak down test of these cylinders, remove the adjacent plug when doing the test, air coming out of the non tested cylinder would indicatte a head gasket issue, if air escapes thru the intake or exhaust this is a valve problem.

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biggbabysweetz111-23-2006, 05:44 AM
start out with what max said. if no luck and nothing appears to be out of the ordinary, chances are you've got some bad rings. check the oil for gasoline contamination, if that is the case you are better off just replacing all of the rings. there is also the chance you have some scored cylinders in which case depending on the severity you may need to bore it over and get larger pistons and rings. as for the knock.... you may be tempted to automaticly either go for the cheapest fuel, or the best of the best plus some octane boost. this can sometimes cause a knocking situation, bad timing can cause this as well. use the fuel that is rated for the vehicle. most 350 engines ask for good ole regular unleaded. if thats what your using right this second step up a notch and try out a couple tanks of super unleaded. while it will not harm the vehicle irrepairably to just keep using super unleaded, you will notice over time the mileage can drop.
hope this helps out!

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mhenry4811-23-2006, 07:51 AM
Thanks for the info all,
Fairly sure it is not the head gasket no air leaking from cylinder. Could not determine if it was a valve issue for sure think it may be a ring or both.
I have a local well respected mechanic that will remove re-install and rebuild the engine for $1829, as long as the block is OK. Will find out what was broke when he tears down motor. Since it has 140,000 mi and many oil leaks it is ready to be rebuilt and drive for another 140,000. Hope you all have a great Thanksgiving

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[Bluestream]

Originally Posted By: TEXDOG
GREETINGS TO THE FORUM
just got the truck back from the mech. said no. 7 cyl had 10lb
comp. blowing white shoke and using alot of gas. 185k miles. I am asking for help as a last effort. I am looking at another truck asap unless I can pull this one out. no emmis test around here to worry about.I am considering adding a pint of LC20. no time for arx. thanks for your time.


No additive is going to help a cylinder with 10 PSI. You got good miles out of the engine, if the truck is still good find a used motor and put in it. $400 should get a good used low mile motor.

 

[madibob]

You could try adding Automotive Restore to the oil. I've read that it works and a guy I used to work with swore by it. It is supposed to bring compression back up. The only negative is you have to add it to every oil change. I'd be interested to know if you try it and it works.

 

[Scienceman]

did the mechanic say if he could fix it??????

 

[ARCOgraphite]

NO ADDITIVES! Jeepers! See if its a bad valve then do a quick and dirty valve job - usually 10psi on a comp would be a burnt or stuck exhaust valve. Since you did the comp and leakdown, First teardown step: look for something broken in the valvetrain on that cylinder under the VC. If worse come to worse a targetmaster GM longblock should be under $1000. Std service Chevy smallblock V8's are cheap and plentiful.

 

[TEXDOG]

troy built
the mech said a valve job at least with possible injector issues ect. he did not think I should chase it. my only thought is that i have a short highway commute (20mi day total)and would like to keep using the truck.
thanks

 

[Trav]

Thats very little information to go on.Does the coolant level drop?Any coolant in the oil?Does the coolant blow out of the radiator?Any popping from either the intake or exhaust?Did he do a wet compression test?It maybe a bad head gasket or burnt valve.
The gasket or valve can be done fairly easily on this engine.

Coolant in the oil or piston/ring damage is not worth repairing on this one,replacement engines are cheap enough.

Still at 185K i would probably go for a used low mi engine anyway if the rest of the truck is in decent shape.

As posted no whiz bang stuff in the oil is fixing this.

 

[TEXDOG]

Trav
thanks, this is a quick fix or no fix, other systems are too old to invest in and Im no mech so its a paid for job. retail 2k plus.

 

[ARCOgraphite]

Should be able to put a targetmaster longblock in for under 1500. But throw in belts and hoses you may bbe at your 2000-

 

[TEXDOG]

Thanks to all for the feedback.