1993 Chevrolet Truck Backfires on Start

1993 ALL OTHER MAKES ALL OTHER MODELS
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LILBLUTRUK
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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??
Jul 31, 2006 at 4:13 AM
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DEMEARSJR
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Souds like a vacuum leak, either around the intake or throttle body.
Make sure all hoses are routed right.
Jul 31, 2006 at 6:13 AM
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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?
Aug 4, 2006 at 5:59 AM
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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.
Aug 4, 2006 at 4:39 PM
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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?
Aug 14, 2006 at 4:10 AM
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DEMEARSJR
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That could cause it.
Aug 16, 2006 at 2:30 PM
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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.
Aug 24, 2006 at 10:23 AM

Suggested Repair

Check for a vacuum leak at the intake or throttle body and confirm all vacuum hoses, including the PCV hose, are routed correctly and not clogged. Also verify distributor installation: the gear alignment marks and number-one plug-wire position must match the engine, because being off even one tooth can cause throttle-body backfire.

Conversation Summary

After replacing the intake manifold on a 1993 Chevrolet half-ton truck with a 4.3L, the engine cranked, backfired through the throttle body, and blew off the PCV vacuum hose under throttle. An expert suggested a vacuum leak around the intake or throttle body, incorrect hose routing, and a clogged PCV hose if oil pressure was high. The owner confirmed the gaskets and hoses were good, then found the distributor was misaligned. Aligning the distributor gear with the available case marks and pointing the number-one plug wire toward the front of the vehicle eliminated the backfire and restored normal running.

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