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Old 07-15-2012, 08:44 PM   #1
kodiakisland
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How to tell if motor has jumped timing

I have a 91 TBI 350 that started running really rough while driving down the road at about 35mph. It will not idle or run in the higher rpms, but can be kept running in the mid range. The computer gives no error codes. It is trying to back fire through the intake as well as exhaust.

My first thought was a faulty ICM/distributor as it was one rusted piece of junk. A new distributor didn't change anything. The engine is still running rough timed from 0-4 degrees BTDC.

Is there any way to tell if the timing chain has slipped without removing the cover?
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Old 07-15-2012, 09:53 PM   #2
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Backfiring through both the exhaust sounds like it might be cross fire. Make sure your plug wires are not too funky.

Probably the quickest way, is to pull the cap off, turn the engine until the indicator is pointing at TDC, and see where the rotor is pointing.

Half the time it should be pointing at no. 1.
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Old 07-15-2012, 09:56 PM
Posted in reply to Fordum's post starting "Backfiring through both the exhaust..."
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Yep, already verified that the rotor is in the correct position when the #1 cylinder is on its compression stroke.

Should have done that prior to removing the distributor, of course, but didn't.
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Old 07-15-2012, 10:05 PM   #4
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Quote:
Originally Posted by kodiakisland View Post
Yep, already verified that the rotor is in the correct position when the #1 cylinder is on its compression stroke.

Should have done that prior to removing the distributor, of course, but didn't.
Well, that would tend to eliminate a jumped timing chain. But maybe not a stretched one or a bad tooth.

If you have an easy way to turn the engine, you can pull the cap, turn the engine slowly until the rotor starts to turn.
Then turn it back until the rotor starts to turn that way.
There will be some slop, but should not be too much. I think there is a degree number that indicates too much stretch, but I have no idea what it is.
If the wires are not crossfiring, I would pull the plugs and examine them plus do a compression check.

It almost sounds like you have a valve or valves hanging open or burned.
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Old 07-15-2012, 10:28 PM
Posted in reply to Fordum's post starting "Well, that would tend to eliminate a..."
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To be clear, the rotor is in the correct position now. I have no idea if it was before or not. I have replaced the distibutor, so its in time with the #1 cylinder now. If the timing chain jumped prior to replacing the distributor, the #1 cylinder and distibutor could be timed correctly, but the cam is not. The only way I know to check that is to see the timing marks on the sprockets, and I don't want to go through that much trouble just on a hunch.

Tomorrow I plan to remove the plugs and turn it by hand. Of course, I have no idea how much slack is supposed to be there so I don't know if that will tell me much.
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Old 07-15-2012, 10:29 PM
Posted in reply to kodiakisland's post starting "To be clear, the rotor is in the..."
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Check your fuel pressure...
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Old 07-16-2012, 08:15 AM
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A vacuum gauge can be used to help determmine if the timing chain jumped...and bad valves too...it will have a low unsteady "jerky" needle reading if it jumped time,and a sudden drop in vacuum that seems "regular" indicates a valve not seating fully............................................. .................................................. ....I agree though,that fuel pressure is important,even as little as 5 pounds under spec can make it run like this...my friend has a '99 chevy pickup with a 5.7 and it refuses to start cold sometimes,but give it some ether and it'll fire right off--and run like the truck described here does-barely driveable,it popsd and backfires,etc...fuel pressure was 6 lbs under spec...he just replaced the fuel pump about a year ago too...AIRTEX seems to make junk lately???.used to be one of the better brand names,now I hear a lot of garages wont use them now...
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Old 07-16-2012, 09:19 AM
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Have you checked the EGR valve?
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Old 07-16-2012, 09:25 AM   #9
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Just so you know, if a Chev "jumps time", the engine will fail to start and run until it's replaced.

Nearly 50% of the teeth on each gear are engaged at all times, if there is enough slop in the chain, or wear on the teeth for the chain to "jump", it will continue to jump. Even if there were a couple of missing teeth, there are still too many teeth engaged.

Any picture (like this one http://www.superchevy.com/technical/.../photo_05.html) illustrates how much wear or damage would have to exist for a chain to actually jump.
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Old 07-17-2012, 09:25 AM
Posted in reply to dyeager535's post starting "Just so you know, if a Chev "jumps..."
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I've had a few small blocks that had the nylon coated cam gear with aluminum "teeth" have all the nylon fall off,and the aluminum get so worn down,that the chain skipped a few teeth and it still ran,but very poorly..one I got home with by moving all the plug wires one hole over on the distributor cap and turning the distributor while a friend cranked it over until I got it to run (poorly!) and we limped home in it...next day when I tried starting it to pull it into the garage,it shot a flame out of the carb and refused to fire at all,the chain had jumped again....I was surprised I didn't bend any valves in that engine............................................ .................................................. ..Usually a jumped chain will make the engine bind up when you crank it over,like its way too far advanced timing...or you'll see and hear compression escaping thru the carb or exhaust while cranking it over.....I've taken many Chevy's apart and was able to emove the timing chain without even taking the cam gear bolts off,the chains were so sloppy!..a few had rubbed a nice hole in the timing cover too!..but they still ran right up to the day I took the timing cover off,so you need a LOT of slack or gear tooth damage before one will "skip" and jump time..
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THANKS RHONDA!
(SOLD IT ..11/30/13..)

Latest addition--1985 K-10 Suburban,6.2 diesel,700R4,3.73's,12 bolt rear..still awaiting repairs and to be registered..
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