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Old 07-12-2016, 08:04 PM   #1
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1986 P30 Chevy 454 Misfire

Hey everyone,

I just took out my 86 Executive Diplomat this past weekend to Lake Tahoe and it ran great going up the pass from Sonoma County. Did fine all weekend, but on the way home, the motorhome would loose power, then pick up again, sort of a rocking motion that was worse going up hill. I was able to get home by letting it idle for 10 minutes or so before getting back on the highway and staying at or under 50 mph. Today, I found the #8 spark plug wire boot was melted. I replaced it with an older one and changed the plug too. No good. I also changed the ignition coil and ignition control module. I recently replaced the water pump and radiator (what a job), alternator, batteries, belts, plug wires, plugs, fuel pumps/filters, and cleaned/resurfaced the gas tank. My vacuum gauge sits right at about 16 in/Hg. I'm stuck on this one. Any suggestions? It typically looses power/rocks when it is warm and at highway speeds.

Oh and another thing it does is hesitate and then backfires when I punch the accelerator from idle (800 rpm). I'm thinking these are probably related. Thanks.

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Old 07-12-2016, 09:07 PM   #2
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It sounds like you have a plug wire that is grounding out. IF you are going to replace them, you might want to consider ones that are high temp with ceramic boots. Also check to make sure that you don't have an exhaust manifold that is leaking(either cracked, or gasket leak). To see what is available in the way of plug wires you want to look at what the racers(drag and circle track) are using, they use very high temperature wires and boots.

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Old 07-12-2016, 09:29 PM   #3
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Surging engine is usually fuel related. Put a fuel pressure gauge on it.

After all of the fuel tank work, you should re-check the filters and lines. Could have stirred up some crud and blocked them.
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Old 07-13-2016, 02:26 PM   #4
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Definitely sounds fuel related. It would be worth replacing the fuel filter again. Since it is an '86 it still has a carburetor and those old QuadraJets have a VERY small fuel bowl. A proper rebuild and making sure the float level is correct might fix you right up.
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Old 07-13-2016, 02:49 PM   #5
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If you still have a carburator, have you changed the filter in it, behing the gas line fitting ?
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Old 07-13-2016, 06:15 PM   #6
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When I lost my pressure switch to the intank fuel pump it would cause hesitation, backfiring. As long as I kept the rig under 55mph on the flat I was ok if I tried to accelerate game over. On mine many years ago the manifold was cracked, causing the rear passenger side plug wire to melt. I put headers on it and problem solved. I'm thinking the two problems are not related.
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Old 07-14-2016, 08:02 AM   #7
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Key here is backfire.

Often that is indication of bad timing and maybe hot plug causing fire when intake valve opens.

Check timing first per manual including checking all of the bs smog valves associated with it.

This era often greatly retarded the timing to reduce emissions at a great cost to power but that timing increases energy exposure to cylinder walls instead of piston so engine runs hot.

To fight that they have temp controlled valve that advances timing when hot.

If these systems have been messed with and timing not set properly you will have what you got.

Carb should have distributor port that can be connected direct to distributor and timing adjusted accordingly but a smog inspector may catch that so leave rest in place so it can be made stock every 2 years if a ca vehicle.
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Old 08-02-2016, 10:14 AM   #8
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Ok I'm going to check all of these things. I have a couple of questions though.

The motorhome ran great for a few hundred miles. Wouldn't bad timing present itself earlier if that were the case?

Also when I first got the motorhome and changed the original sparkplug wires, I noticed that the number 8 wire was worn and it broke very easily when I removed it. It seems like whatever is the issue has been around since before I got it.

And lastly, what's the best way to check for leaks in the exhaust manifold other than visual inspection?

Thanks for all of the input, I'm going to replace all of the fuel filters again including the one just before the carb this week. I'll keep ya posted.
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Old 08-02-2016, 10:49 AM   #9
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Did you use premium wires? In the early 90,s I spent half my time changing spark plug wires on 454's. Are the heatshields on the wires? The wires with the ceramic ends worked much better. Champion made a good set. The quadrajet carb's were also bad about the plastic floats going bad and absorbing fuel. This causes a rich condition.
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Old 08-02-2016, 05:32 PM   #10
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Spray heavy soapy water ( tide) on all of the exhaust on cold engine then start up up and watch.
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Old 08-03-2016, 06:52 PM   #11
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Miss fire

This issue is most likely not fuel related if only #8 cylinder is affected. If you hanen't replaced the distributor cap and roter, it would be a good idea . Check for carbon tracking inside the distributor cap. Also at this point a wet and dry compression test on all 8 cylinders should be done. A wet compression test means to squirt a small amount of oil into each cylinder and compare the readings while cranking with a dry test. There should be no more than a 20% difference between the highest and lowest cylinder compression readings. My best guess is an ignition issue
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Old 08-03-2016, 08:43 PM   #12
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I would check the plug wires for leakage. Take the engine cover off and start the engine after dark. Being careful, look down around the wires and wou will probably see some arcing. This is bad plug wires...a common problem on the 454 engine. Buy a set of premium wires with the ceramic boots. Change the wires one at a time and use the silicone grease on each plug boot. Be careful that the wires do not contact the headers.


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