The center cylinders4,6,3,5 have no compression. the others have good compression. I did a quick leak down test on cyl. 3 and found intake valve leaking. Engine is in a gmc 3500 with only 60k miles.What would cause this? And will a valve job repair it.
2006-11-26 04:49:45 · 8 answers · asked by jason h 2 in Cars & Transportation ➔ Maintenance & Repairs
sounds like a bad cam or a jumped timing chain :(
2006-11-26 05:15:57 · answer #1 · answered by Anonymous · 0⤊ 0⤋
I own a shop, and build many 350 small block engines. If you got compression coming back through the carburetor, you have either a lobe worn on the cam, a bent push rod, a loose/worn out rocker, or a burnt valve. Run a vacuum test to see what it is doing. I'll bet the needle is all over the place, and if you read the chart for the vacuum gage you will see what I mean. I have seen the lifter get a pin hole worn in it from a worn out cam & lifter set, thus causing this problem. Either way a vacuum test should help you locate the problem. A timing problem such as the rotor button, or cap causing a crossfire will do this too. A worn out distributor can do this. See if you can move the shaft from side to side in the distributor. Put a timing light on the engine to see if the timing is way off, or moving around. It could be the timing chain & gear set going out. 150 pounds of compression is good, and I don't think you would have that much with a burnt valve. This may sound crazy, but I have saw an engine do this with a bad spark plug as in the ceramic broken causing fire to jump. Fire jumping to the center post through the rotor button will do this. Look for a carbon trail that looks like a pencil has been drawing inside the distributor cap. I really don't think the octane booster is doing this, unless you got a bad bottle of it for some reason. Make a line that will come off the fuel pump into a jug of gas. This will tell you if you have bad fuel if it stops doing it. The vacuum gage is your best bet. Honda Guy is very correct in what he is saying. I agree 100% with him. I would rule out the firing thing with the cylinder, before I pulled the valve cover. With it running you will be able to see a valve that is not opening up, as it will look different. You can make a cardboard shield to keep the oil from going everywhere, and don't rev the engine up, or you will have hot oil all over you and the car. If nothing is broke, bent, or worn out, then you must for some reason have a plug firing at the wrong time forcing the exhaust out the intake. Glad to help out, Good Luck!!!
2016-05-23 04:36:10 · answer #2 · answered by ? 4 · 0⤊ 0⤋
Just my two cents on the cam thing... If the cams were worn, the valves wouldn't open as much, which wouldn't cause lack of compression.
What's a 350 doing in a 3500 truck? I thought they only used big blocks in those things. Oh well, learn something new every day.
In regards to easterwabbit's answer, I said "you learn something new every day". At least I don't have trouble finding the correct letters to type on my keyboard, and I still don't think it's a worn cam issue.
His genius did show up when he stated "or something else is wrong"; that pretty much covers all the bases.
Cams not turning sounds plausible. Hard to believe the chain broke at 60k, but if so, maybe it's because the engine was overtaxed due to large payloads being hauled around.
Hey wabbit, if you see Goober tell him I said duh-huh, he'll know what you mean.
2006-11-26 06:23:54 · answer #3 · answered by Anonymous · 0⤊ 0⤋
ignore bitburger he obviously doesnt know much about trucks,, just because its a 1 ton does not automatically mean it got a large engine,
heck there were 6 cylinder engines in 1 tons all the time such as the 4.4L 250ci and the 4.9L 292ci
and yes 350 engines too, very common, alot more common than the 454 engines,
ppl should know stuff before tey try to answer regarding subjects they know nothing about,
anyways, sounds like a cam issue to me, make sure the cam is even turning, pull the distributor cap off and see if the rotor turns with the engine
if it does then the cam is bad or rocker adjutment is way too tight on some of them,
or something else is wrong
2006-11-26 09:28:58 · answer #4 · answered by mr wabbit 5 · 0⤊ 0⤋
Before you jump into a valve job, check the cam shaft. If the engine was a rebuilt, or a "Target-master" the cam is most likely the issue.
Remove the plugs (All of them) and remove the valve covers. Crank the engine and observe the lift on the rockers as compared with the rest.
Good Luck
2006-11-26 04:59:28 · answer #5 · answered by Ironhand 6 · 1⤊ 0⤋
it does sound like a valve problem with it,but i have never see one do that many cylinders a t one time unless it blown a head gasket,and still it would have some compression on it,id pull a head off,and check to see what has happened to it there's obviously something bad wrong with it,and pulling the heads may be the only way you can find it,you may also find it has a bad timing chain it also,,Chevy,s are well known for this problem,,good luck,i hope this help,s.
2006-11-26 04:58:59 · answer #6 · answered by dodge man 7 · 0⤊ 0⤋
Chev 350's are notorious for bad camshafts.
Rotate the engine with valve covers off and measure the valve lift on the bad cylinders?
If the lobes are badly worn on the camshaft you will have to replace it and may as well grind the valves while it is open.
2006-11-26 05:02:27 · answer #7 · answered by michelle_greene_ragio 2 · 0⤊ 0⤋
release the rocker arm nuts and recheck your leakdown test. those cylinders are next to each other and next to the egr port. you might have blown both head gaskets or cracked your heads. did it overheat ??? also you can remove the valve covers and make sure you dont have stuck open valves
2006-11-26 05:00:14 · answer #8 · answered by Christian 7 · 0⤊ 0⤋