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Dr. Hamman
Dr. Hamman, Technician
Category: Classic Car
Satisfied Customers: 6286
Experience:  I have 30 years experience repairing, restoring, and customizing cars, and pickup trucks.
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1987 GMC truck: cylinders..the Dis cap, Rotor button, plugs..module

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I have a 1987 GMC truck with a 350 fuel injection, I am getting 125 pounds on all cylinders, But it will not hit on cylinder 8, I have replaced the Dis cap, Rotor button, plugs, wires, Distributer itself, electronic spark module, Could the ignition coil cause it not to hit on just cylinder 8, I have not changed the coil yet.. Thanks Bob

Hi, I will do my best to assist you. When you are finished please click on accept. I don't get paid unless you do. I don't know if you are a pro or a novice so we may have to fine tune the answer. I only know as much about your problem as you have told me in your post, so my answer will be based on what you have posted. Feel free to add any additional info you feel is needed.


The Bob the coil cant be the problem. It doesn't know what cylinder it is firing, so it would erratically miss on random cylinders if it was bad. The problem is going to be a little strange. For an example you can have a sticky valve that just moves slow. It can pass a compression test but it cant operate fast enough to operate at engine operating speeds. I have only seen this twice in 35 years, so it is very rare. When you crank the engine to do a compression test the engine is turning over slow. When it starts it turns four or five times faster than it does while cranking. This is a tough to diagnose problem. I spotted both of the slow sticky valves by pulling off the valve cover and watching the valves closely with the engine running.

The next thing that comes to mind is a big vacuum leak on the number eight intake runner. Possibly where the intake manifold bolts to the head, or it could be from a vacuum line cracked or off. The last thing that comes to mind is a super dirty injector spraying in a pattern that starves number eight. Like I said this one will be a little strange, because you have covered all the bases.

The best way to hunt for a vacuum leak is with a can of WD40, you spray it around where you suspect a leak with the engine idling. If you hit a vacuum leak the engine will speed up for a second or two. You can have a vacuum leak under the intake ports where it is sucking air out of the crankcase, and you can only spot that one by taking the intake off. If you have a sticky valve, you can use some oil flush, or seafoam in the oil. If you use these products strictly follow the label directions. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.


I hope this helps, 100% satisfaction is my goal. If the answer is not clear, please reply and I will assist you more. When you're satisfied, click on accept. I don't get paid unless you do. A bonus and positive feedback are always appreciated, good luck and Thanks.
Customer: replied 7 years ago.
Hello agian, This is a 350 with just the 2 injectors instead of the carb,I will try the other things you told me Thanks, XXXXX XXXXX told me to try replacing the ignition coil, Could the ignition coil be controled by the PCM, & if the ignition coil is sending the signal to the distributer,& since it is only missing on cylinder 8, I think you would be right,........... I think it would be in the distributer itself, but just making sure I can rule out the ignition coil. Thanks for your help... I will pay after you reply to this question.. Thanks for your time.. Bob

Hi, I will do my best to assist you. When you are finished please click on accept. I don't get paid unless you do. I don't know if you are a pro or a novice so we may have to fine tune the answer. I only know as much about your problem as you have told me in your post, so my answer will be based on what you have posted. Feel free to add any additional info you feel is needed.


The ign module just fires the coil and doesn't keep track of which cylinder it is firing, because it doesn't need to. The distributor takes care of all the distribution of the spark, so none of the other electronics get involved with which cylinder is being fired. You will find that the average mechanic-Tech is very weak in electrical-electronics, so you will get all types of replies about what they think. Electrical-electronics is one of my specialties, so I have worked with these systems quite a bit. Your problem is going to be a little out of the ordinary, because you have replaced all the items that will normally cause a problem like this. Check out the things I went over in the last post, and keep your eyes open for anything abnormal. Also watch the spray pattern of the injectors with the engine running. See if the one nearest the #8 intake runner has an uneven spray pattern. If you have any more questions I am here to help. Good luck, and Thanks for using Just Answer.

I hope this helps, 100% satisfaction is my goal. If the answer is not clear, please reply and I will assist you more. When you're satisfied, click on accept. I don't get paid unless you do. A bonus and positive feedback are always appreciated, good luck and Thanks.
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