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Discussion Starter #1
HELP!!!!! I have a 96 Suburban 7.4. I was driving a month ago and stopped at the light. When I pushed the gas it lunged and quit. I have changed the timing chain and gears, Plugs, Wires, Cap, Rotor, Cam Sensor, Crank Sensor, Fuel Filter, Fuel Pump and have no change. It acts like it wants to start but it will not fire. I have 56 psi at the plenum with the new pump. I am not getting any codes when a plug it into a code reader. Any suggestions as I am stumped.
 

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Are you getting fuel to the engine? Are you getting spark at the spark plugs?
 

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Next question would be since youve got spark at the cylinder and fuel at the cylinder are you getting spark and fuel at the right time.
I see where you replaced parts, were any of these parts known bad and identifiable as contributing to the original problem or is the present no start condition a continuation of the issue?
 

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We know you replaced theses parts, but did you make sure all of them are plugged in all the way or the harnesses may be damaged. just having a crank sensor unplugged will cause the truck no to start.
 

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Discussion Starter #6
The only part I know for sure was bad is the timing chain. It had jumped a couple teeth and is now correct. The other parts have just been a quest to try and get it to start.
 

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Ok jumping time was the original issue.
Did you check the valvetrain when you assembled the new parts?
The 7.4 is an interference engine, to not get valve damage after jumping time would be lucky.
Is it possible your 180 out?
 

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Discussion Starter #8
I did not check the valve train. I set the timing marks on the crank and cam sprockets both at 12 o'clock as indicated in the repair manual.
 

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X2 on valve damage.how does it crank, fast, slow, normal,lopeing? given the fact the chain jumped, a compression test is in order. i'm getting the chain jumped, you replaced it & still no start, so you started throwing parts at it to no avail ? is that correct? start with compression test & let us know.
 

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It cranks normal. If I spray starter fluid in the intake it will backfire out the intake. I will have to rent a compression tester and work on that tomorrow.
 

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Back fires out the intake?
Check to make sure your not 180 out on the timing.
Easy to do even if you followed the manual closely.
 

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Backfires through intake. that made me reread previous posts, oops i missed it. you said you put crank & cam sprockets @ 12 o'clock, is that correct? crank & cam gears should have a dot on them. dot on crank points to 12 dot on cam points to 6 0'clock. dots together : if not, valve timing will be off. incorrect valve timing/ bent valve can cause backfire. on a lighter note, so can plug wires not in correct firing order. recheck everything you did.
 

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is your timing chain correct, as i said in my last post? low compression across the board indicates incorrect valve timing, ( timing marks on cam and crank not aligned correctly) or valve damage. since all cyls are low i suspect you did not get timing marks correct when installing chain. it is unusual to bend a valve on every cyl.
 

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Discussion Starter #15
The timing is correct. The books show TDC at 12 o'clock and the exhaust at 6 o'clock which I where I lined every thing at.
 

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I've never heard it described that way, but if by tdc you mean the crank and by exhaust you mean the cam then it's correct. if there is no chance you missed it by a tooth or two and your sure the cam & crank gears are not spinning out of position (off their keyways") you have valve train damage.
 

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Discussion Starter #17
With #1 at TDC the crank and cam marks are at 12 o'clock. On the exhaust stroke with the crank at 12 o'clock the cam is at 6 o'clock. As far as I can tell everytihng is in position.
 

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I don't know what manual you are reading but if you google chevy timing marks i'm sure you could find a picture. if i am under standing you correctly you are 180 degrees out. that would explain low compression and backfire. spark is firing when a valve is open. dots on crank & cam should line up. crank at 12 cam at 6. at this point #1 should be on COMPRESSION. " both valves closed" rotor pointing almost directly to #1 coil tower. #1 plug wire may not be in the same position as the tower so look closely at the cap. just curious, do you have timing chain exposed and valve covers off ?
 
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