We changed spark plugs & wires on our 1991 GMC & seem to have the order wrong on the distributor cap because truck won't start. need diagram

You can google the engine size and "firing order" to get a drawing or photo of the firing order online.
Example - - 1990 GMC 350 firing order diagram

It can be faulty or dirty spark plug itselfd.Also for reference get the fuel level checked.Is the fuel tank is empty or near to empty.
Try cleaning the spark plugs and then see if that is helpin.On some cases the black carbon layer gets collected on the spark plugs and that causes the problem,so cleaning the plugs help.But if its not helping,then spark plug has to be replaced.Spark plugs get power from ignition coil to ignite sparks.If ignitioncoils are faulty then also there will no spark.--------------

To troubleshot and confirm the issue,i suggest you to go through the help links provided below,check the procedure and it will help you to confirm the issue in your case.
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Click the link below:------

Car cranks, has spark but will not ?

http://howtobyme.blogspot.com/2011/12/car-crankshas-spark-but-will-not-start.html

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The spark plug firing order for car?

http://schematicsdiagram.blogspot.com/2011/06/spark-plug-firing-order-for-car.html

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How to Replace Spark Plugs?

http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-spark-plugs.html

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How to Perform a Basic Engine Tune Up-Spark Plug and Wire Change

http://schematicsdiagram.blogspot.com/2011/08/how-to-perform-basic-engine-tune-up_01.html

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Car has no spark?

http://schematicsdiagram.blogspot.com/2011/06/car-has-no-spark.html

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How does the spark plug wire go on the distributor cap in 1991 Mitsubishi Mighty?

http://schematicsdiagram.blogspot.com/2011/12/how-does-spark-plug-wires-go-on.html

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No spark on 2008 Chevy Avalanche?

http://schematicsdiagram.blogspot.in/2012/04/no-spark-on-2008-chevy-avalanche.html

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How does the spark plug wire go on the distributor cap in 1985 Pontiac sunbird?

http://schematicsdiagram.blogspot.com/2011/12/how-does-spark-plug-wire-go-on.html

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Ford Plug Wires Not Coming out?

http://whoisbyme.blogspot.in/2012/06/ford-plug-wires-not-coming-out.html

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Ford explorer has no spark?

http://schematicsdiagram.blogspot.com/2011/12/ford-explorer-has-no-spark.html

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Chevy impala spark plug firing order diagrams?

http://technoanswers.blogspot.in/2012/02/chevy-impala-spark-plug-firing-order.html

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How to gap spark plug on Chevy Impala?

http://technoanswers.blogspot.in/2012/02/how-to-gap-spark-plug-on-chevy-impala.html

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Spark plug Replacing for chevy impala?

http://technoanswers.blogspot.in/2012/02/spark-plug-replacing-for-chevy-impala.html

Chevy s-10 blazer 4.3l Spark Plug Firing Order?

http://howtobyme.blogspot.com/2011/12/chevy-s-10-blazer-43l-spark-plug-firing.html

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1982 dodge rampage Spark plugs Firing Order Diagram?

http://schematicsdiagram.blogspot.com/2011/12/1982-dodge-rampage-spark-plug-firing.html

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2006 Suzuki Forenza Spark plug Firing Order Diagram?

http://schematicsdiagram.blogspot.com/2011/12/2006-suzuki-forenza-spark-plug-firing.html

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1983 Ford Ranger Spark plug firing order?

http://schematicsdiagram.blogspot.com/2011/12/1983-ford-ranger-spark-plug-firing.html

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Firing Order for 1991 Lexus ES 250?

http://schematicsdiagram.blogspot.com/2011/12/firing-order-for-1991-lexus-es-250.html

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Firing order for 1992 Chevrolet 5.0 engine?

http://schematicsdiagram.blogspot.com/2011/12/firing-order-for-1992-chevrolet-50.html

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How to identify number one spark plug on the distributor cap?

http://schematicsdiagram.blogspot.com/2011/12/how-to-identify-number-one-spark-plug.html

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How to remove rear spark plugs on 1991 Buick Regal?

http://howtobyme.blogspot.in/2012/02/how-to-remove-rear-spark-plugs-on-1991.html

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These details will help.

Thanks.

I had the same problem. Google your engine I had a 4.3. Click images for fiiring order you will find it. You can also click video for setting timing. I got mine running with this information. Just remember that there are two marks on the harmonic wheel one for TDC & BDC. Also you need 3 things air/fuel/spark. Make sure you have these before replacing parts it gets spendy. I got my whole distributer and spark module for 40.00.

Fire order
1-3-4-2

Cylinder #1 will be the one closes to the fan belts and so on 1-2-3-4, on the distributor cap there should be a # 1 stamped on it, put cylinder # 1 spark plug wire on that one, install spark plug wire in firing order clockwise on the cap, the cap some time's might even have the firing order on it.

firing order 1342, distributor, clockwise rotation.

ok number 1 piston should be a top dead centre & mabe on the top of the cap is a mark or number
if not then there should be a mark on the lip of the dizzy!
if not then just choose one near the front & set the points at that point on the point of sparking! [ready to open points] make sure you have spark or you may have left ou the insulator on the points locating pin!!!
then its a matter of the firing order!
i hope this helps
cheers Rob

3.9L V6 Engine. Engine Firing Order: 1-6-5-4-3-2. Distributor Rotation: Clockwise.............did i guess the right motor? let me know/rate me

For a 4 the firing order is 1234 front to back and the distributor fires clockwise. Find one and run wires from there.

For a 6 its 123456 front to back and is also clockwise.

Joel

the firing order is
1-6-5-4-3-2