Did this damage engine? Pluq wires wrong, backfiring

andyr

Member
Feb 14, 2010
182
Connecticut
Boat Info
1989 30' Sundancer
Engines
5.7 with alpha 1 drives
Boat yard installed a factory Mercruiser long block 5.7 roller lifters, non vortec. On the test run it backfired occasionally, required more throttle than the starboard engine, seemed to vibrate, not feel right. Put a carb kit in. Ran engine 12 hours various rpms up to 3200. Eventually, couldn't run over 1800 rpm without backfiring repeatedly but would rev up fine in neutral. I found all the the plug wires are reversed. Left side on right etc. Is this new engine damaged as a result? Since the backfiring got worse, was something getting damaged. Valves? What would you do?
 
I would say probably not.install the wires correctly and see how it works out.cross your fingers
 
Unless they ran it really hard, I doubt that they damaged anything. I've accidentally screwed up the firing order on car engines more times than I care to admit in public. It never damaged the engine. I've seen it done by others more times than I have done it. I've never seen anyone else's motor damaged by it either. When it backfires, the explosion either goes out the exhaust valve and through the exhaust manifold, or out of the intake valve and up the intake runner to the throttle body or carberator.

If you have a carburetor, there is a (very) slight possibility that you damaged something in it. The engine will run, but it won't run right. It should be an easy fix once you find it.

You skated on the major threat... a fire. I have seen a couple of those caused by improper firing order.

The worst thing that ever happened to me was it blew out a diaphram on a Holley vacuum secondary carburator. It was either the accelerator pump diaphram, or the diaphram that opens up the secondaries. I forgot which one it was as it happened over 20 years ago.
 
My concern is that the backfiring went from a few times an hour to one day it wouldn't run above 1800 rpm without backfiring repeatedly. That would seem to indicate something got worse.
I swapped carbs with other engine.
 
The first step is to install the spark plug wires correctly and test run the boat. 12 hours is quite a while to run with the wrong firing order especially under a load. However Mercruiser cast iron blocks and cylinder heads take a lot of abuse before any real damage happens, so don't lose hope yet.

At a minimum I would definetly replace the spark plugs, check the ignition timing, and change the engine oil. If you're worried about mechanical damage it wouldn't be a bad idea to have a reputable shop perform a compression test and a leak down test.

Now for the real question/problem. (Please take all this with a grain of salt, you may have a simple case of a couple plug wires in the wrong spot.) You described the plug wires being completely reversed? To the best of my knowledge if you completely reverse or swap the wires side to side the engine won't run. The most likely explaination for swaping the wires side to side is the mechanic who intalled the distributor got it lined up 180 degrees out, in other words the distibutor rotor was pointing at the #1 spark plug wire but the engine was in firing position for #6 (rather easy to to do if your not carefull since #1 and #6 are both at the top at the same time, one exhausting the other compressing/firing) the quick fix is to switch the plug wires at the distributor. However that always leads to problems down the road. The real fix is to remove and correctly reinstall the distribtor, reset the ignition timing and, then install the wires in the correct positions.
 
Before going any further, I would take a look at the distributor. Did they reuse your old one? If so, check to see if it has mechanical timing advance. There will be 2 roughly C shaped metal weights just under the rotor. Make sure that both springs are still present and that the weights move freely. They may be getting stuck or opening up too early. This can give you too much advance on the timing, which can cause backfiring.

I agree with Carrington on changing the spark plugs as well. The ceramic insulation can be damaged by improper combustion. I'd replace the cap and rotor as well. I've have had what appeared to be a perfectly fine distributor cap cause all kinds of strange problems. Cheap insurance that you aren't chasing something really stupid. Getting a compression test once the mechanic gets it running properly should give you a good idea as to whether or not you have any other problems.

Can you switch the fuel lines between the 2 engines? Might be something there.

One last question. What was wrong with the old engine? Any possibility that one of the bolt on parts that you moved may have been causing the original problem?
 
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Boat yard installed a factory Mercruiser long block 5.7 roller lifters, non vortec. On the test run it backfired occasionally, required more throttle than the starboard engine, seemed to vibrate, not feel right. Put a carb kit in. Ran engine 12 hours various rpms up to 3200. Eventually, couldn't run over 1800 rpm without backfiring repeatedly but would rev up fine in neutral. I found all the the plug wires are reversed. Left side on right etc. Is this new engine damaged as a result? Since the backfiring got worse, was something getting damaged. Valves? What would you do?

Before I touched a thing it should go back to the installer...should have gone back long before hour 12...good luck.
 
UPDATE: Turns out only two of the wires were swapped. I checked one from the left side, it was wrong, and randomly one from the right side of the engine and it was wrong, coincidentally the other one. I thought I found a 3rd one wrong and assumed they were all reversed. (By now I was pretty frustrated, dark and cold and not too happy with the yard). Connected correctly next day and it ran strong during a 30 min. test.

Contacted Mercury and they responded immediately: "If the engine has only been run 12 hours then we would not expect that it was run long enough to do any damage to the engine. The fact that this was done would not void the warranty, however, if there was any damage found and it was determined to be caused by this then the warranty may not cover that repair. We would recommend continuing to use the engine and certainly if you begin to notice any problems with the engine you should have it checked out."

Mechanic had swapped wires while trying to diagnose a presumed carb problem. So it ran with the miss-connected wires for an hour rather than 12. Probably okay but I will watch it.

I didn't wait 12 hrs. I told the yard in the first hour of occasional backfires. Spent the next few hours trying to solve the problem from a carb angle which it may have been at that time.

Thunderbolt ignition so no points or mechanical advance.

By the way, no one mentioned cross fire as a potential problem. Certain plug wires should not be installed next to certain others.
 
Did correcting the wires fix your issue? I am having what sounds like the same issue, in neutral it runs a miss but I can tell it up to 5000 rpm if I wanted but under load it struggles to get to 3000 rpm spitting, sputtering and back fires but after awhile it will get on plane and I can trim out all while still back firing. I bought the boat running poorly work a overheating issue so when I fixed that I may have swapped a couple wires not sure. Doesn't overheat though.
 
Did correcting the wires fix your issue? I am having what sounds like the same issue, in neutral it runs a miss but I can tell it up to 5000 rpm if I wanted but under load it struggles to get to 3000 rpm spitting, sputtering and back fires but after awhile it will get on plane and I can trim out all while still back firing. I bought the boat running poorly work a overheating issue so when I fixed that I may have swapped a couple wires not sure. Doesn't overheat though.
Nick, that post was from eight years ago, Andy might not even have that boat anymore.
 

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