93 Chevrlet 1/2 ton work truck with 4.3 liter

Tiny
LILBLUTRUK
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  • 1993 ALL OTHER MAKES ALL OTHER MODELS
I changed out the intake manifold (chasing an oil leak) in my sisters 93 1/2 ton truck. I have it all back together but when I tryy to start it the trouble begins. I turn the key and the truck will crank then back fire through the throttlebody and start up. I set the initial base timing to TDC as indicated and have checked and re-checked the firing order. When the back fire occures it blows the vacuum lne apart that serves the pcv valve. When I attach the hose while it is running it idles fine but when I give it throttle it will blow the hose off again and run. Any suggestions. I am leaning towards a stuck egr valve or that I have a vacuum leak under the throttle body?
Monday, July 31st, 2006 AT 11:13 AM

6 Replies

Tiny
DEMEARSJR
  • MECHANIC
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Souds like a vacuum leak, either around the intake or throttle body.
Make sure all hoses are routed right.
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Monday, July 31st, 2006 AT 1:13 PM
Tiny
LILBLUTRUK
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[quote="demearsjr"]Souds like a vacuum leak, either around the intake or throttle body.
Make sure all hoses are routed right.[/Quote]

Ok. I have replaced the gaskets around the throttle body and double checked the placement of the hoses. All seem good. I started it last night with some success but when I let it stand for a few minutes and came back to start it again the backfiring returned. Any thoughts?
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Friday, August 4th, 2006 AT 12:59 PM
Tiny
DEMEARSJR
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Do you have a reading on the oil pressure.
If it's reading high check the pcv hose first to see if it's clogged.
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Friday, August 4th, 2006 AT 11:39 PM
Tiny
LILBLUTRUK
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The PCV valve and hose are all ok. I read in the manual that the distributor must not even be off by one gear and I suspect it is. Would this cause this type of backfiring due to crankshaft or camshaft positions sensors?
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Monday, August 14th, 2006 AT 11:10 AM
Tiny
DEMEARSJR
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That could cause it.
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Wednesday, August 16th, 2006 AT 9:30 PM
Tiny
LILBLUTRUK
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Ok! Here is what solved the problem. The book calls for the alignment of the indent on the bottom of the distributor gear to be in alignment with the arrow and a 6 that is cast into the casing. This does not exist but it does have a 23 over a large X so I gambled and aligned up these marks. The book also shows that the number one spark plug must face toward the front of the vehicle which is what I did. Woo Hoo! It started great and runs perfect.

Thanks for the help. I know now that to be exact is most important.
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Thursday, August 24th, 2006 AT 5:23 PM

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