My 99 silverado 5.3L wont start or run

10

Asked by Feb 14, 2016 at 01:02 PM about the 1999 Chevrolet Silverado 1500 3 Dr LS 4WD Extended Cab LB

Question type: Maintenance & Repair

I took the rails off and witnessed injectors spraying, I took every plug and watched it spark. I checked compression cold (I have no choice), and I have 30 PSI each cylinder! I thought the only way all cylinders would loose compression at the same time is if I lost timing...opened up timing case and the gears marks are aligned. Is it possible that my gears could have jumped 180 degrees out of time? would I even get compression cold?

10 Answers

88,665

Cold compression will be somewhat different but in your case with 30 lbs it would not matter, and yes for all cylinders to loose compression is either timing, complete head failures, or burned through pistons but that would be caused by sustained preignition or pinging, there is another possiblity, is the valve train, you may want to check the rocker arm throw, there is such a thing as a broken camshaft. HTH

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10

Just to make sure as I check timing...I should have compression building on #1 cylinder as the timing mark align, right?

10

Would compression be low on all cylinders if the lifters weren't pressed up? If the camshaft was broke wouldn't I have different compression pressures on each of the cylinders? Some with none, some with some...

88,665

To answer your the compression question, by the time the piston has reached TDC the maximum pressure will have been reached. If the cam broke just before the number 1 cylinder and was in the correct spot that would put the exact same pressure on each lifter to hold each valve open exactly the same amount, it could be that way but normally you would be right some cylinders will be high and others low. I have heard in a few instances that the oil pressure can get high enough that it pumps up the lifters and hold the valves open slightly, so that might be something else to check.

62,895

Ignition timing and valve timing are different. With only 30 PSI in all cylinders it's either jumped timing (most likely) or the valves are stuck open on all cylinders simultaneously (unlikely). HTH. -Jim

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10

Thank you so much for the help...I changed to different compression gage, believing that mine may be wrong. The pressure was higher (80psi) the new gage is even older but has way higher reading. I put oil in the cylinder as well, and there was no pressure difference. When I rotate the engine by hand, I begin Building pressure at about 180 degrees, on the camshaft, before the timing marks align. This doesn't seem right to me but maybe it is. When I pass trough where the marks align Ive well passed compression and power (where I'm drawing a little vacuum).

88,665

That is about right, you will start building pressure just as the piston starts to move up on the compression stroke. However even 80 lbs is still very low. When the piston reaches TDC that is as high as the compression will get then you are in the power stroke and the piston heads down then back up for the exhaust stroke then just a few degrees before TDC on the exhaust stroke the intake valve will start to open just as the exhaust valve starts to close and you then are on the intake stroke and it starts all over again. So there is a spot where both of the valves are open at the same time, and this is what they call valve overlap, now there is what they call valve duration, and that is how long each valve is open for, and then there is valve lift and that is how deep or wide the valve is open at full rocker depress. Now your totally confused, right?

10

Yes I am, because it's seems my timing marks are aligned with the #1 cylinder at the top of the exhaust stroke. Perhaps for my engine 99 silverado 5.3 liter that is correct...but I don't know. Regardless the timing marks are aligned correctly, Camshaft timing mark at 6 o'clock and crankshaft mark at 12 o'clock so that the marks point at eachother. Thank you for your help, I'm walking away for a while because my head is spinning :)

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88,665

Maybe this will help you understand GM's wired way of timing the cam. See pic.

62,895

Older Cadillac engines were like that as well. #1 is at top dead center (TDC) with the cam dimple and the crank dimple both at 12 o'clock. HTH. -Jim

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