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Mercruiser 4.3LX backfire, cough and stall.

D

Dougalsdad

New member
Engine 4.3LX, V6, 205hp
Serial number 00804444
Build 1992
1992 Searay 220 OV

That's the engine data, it's serviced every year and runs really well, usually.
Last year it had new headers, water pump and thermostat.
It starts easily and runs nicely straight off the trailer.
At first it runs really well and sounds good.
After running at 3,200 rpm for about 25 minutes it misses for a short while, coughs and then backfires. I think the backfire is back through the carb as it is very loud, quite a bang.
The engine loses power and then runs very roughly and won't achieve normal rev's.
It then continues to cough and backfire.
The engineer took it for a test run this week, ran it at 4,000 rpm to put it under stress and could not get it to misfire at all.
We took it out again today and the same problem was there.
My thoughts are that as the coil, distributor cap, rotor arm and HT leads are the original items then it's probably an electrical problem. My best guess is that either the distributor cap is breaking down after running for a while or the leads are breaking down so the HT is taking the path of least resistance causing the backfire.
I have cleaned the distributor cap and sprayed it with water repellant but it has made no difference.
I doubt it's a fuel problem as the boat starts, runs and accelerates fine for the first 25 minutes.
I'm planning to replace the cap and arm first as they have the lowest impact on my wallet, everything else just starts to get expensive.
Any suggestions would be very much appreciated.​
 
Ayuh,... She's goin' Lean, under load....

Check the fuel system, from the tank vent, to the carb...
 
Hmmm, but if she's going lean under load then why for the first 25 minutes can we accelerate hard, run flat out, go from tickover to on the plane with no problems? Why is it only after about 25 minutes that the problem manifests itself?
 
Ayuh,... If the tank vent is plugged, it'll take that long for the vacuum to build enough, so's the fuel pump, can no longer pull fuel...
 
Thank you for information. I'll be working on her over the weekend and taking her for several runs. I'll check the vent and report back next week. I presume if it's blocked it'll be where it exits the hull and not further down the pipework but I'll check it all the way through. I suppose I could even disconnect it at the hull fitting and extend the pipe so any fumes are above the deck though a vent.

I've also ordered a new distrutor cap and arm for the Thunderbolt ignition because the boat is 2.5 hours away, each time I work on the boat it is another weekend she won't be used this year. That gives me 2 things to try this weekend, ignition as well as fuel.

Once she starts to cough and splutter I can undo the filler cap slightly (so no water splashes can get in) and see if there is instant relief.

Many thanks.

bondo said:
Ayuh,... If the tank vent is plugged, it'll take that long for the vacuum to build enough, so's the fuel pump, can no longer pull fuel...
Click to expand...
 
work backwards anti syphon valve, plugged vent, bad gas, fuel filter,carb. I had a plugged vent on my 4.3 did the same thing after about 15 min. Granted backfire usually means electronics...but it sounds like your on the right track.
 
Last edited:
Thank you for your help everyone.

At the weekend we did a lot of jobs on the boat but couldn't get her on the water to test the engine. I replaced the distributor cap and there was what looked like a tracking mark along the outside of the cap. One problem we did have was in putting the new rotor arm on, the old one came off easily, the new one was so tight it wouldn't go on more than a quarter of the way, so the old one went back on.

Hoping to get her into the water this weekend to test out the engine and to see if there is a fuel starvation problem.

Fingers crossed and I'll keep you informed.
 
Once again thank you, I'll do that at the weekend. The replacement rotor arrived yesterday, they replaced it free of charge so at least it's agro and not money.
 
The factory loctites them on...don't know why...it fits tight as it is...I never used loctite on a rotor ever.
 
I've never used Loctite on a car's distributor, I can't see why they would use it at the Mercruiser factory. The old rotor came off easily enough so I'm assuming that at some point in the engine's 20 year history the rotor was replaced without Loctite.

I'll not be using any this weekend.
 
What an interesting wekeend that was.

I replaced the distributor cap the other weekend but didn't manage to get the boat on the water, this weeke the tides were perfect and the weather good.

To test the bit about the breather being blocked I removed the fuel filler cap and put gaffer tape over the filler hole but left a quarter inch gap at one side, that way air could get in but spray was unlikely to find it's way into the fuel.

30 minutes into the test run, constant 3,200 rpm, and the misfire reappeared, no backfire through the carb though. This time it wasn't as bad as it has been.

I told the mechanic and asked him to give the carb a full stripdown, check for dirt in the bowl, check the jets and test the fuel pump. He refused, he actually refused a £350+ job. He said that he was not convinced it was a carb fault and that he valued my custom (been with them a long time) more than a job which in his opinion would not cure the fault. He had a look at the old distributor cap and said there was a huge tracking mark and that as the components were old (up to 20 years old but only 318 hours use) that I should replace more ingition components, and then if it still misfired then he would strip the carb down. He's a decent guy.

So, my next job is to replace the coil, leads and plugs, then it's everything on the ignition side replaced. Unfortunately I can't get to the boat until July 7th due to work committments.
 
If you have exhausted all fuel possibilities, my next move would be look for a broken insulator on spark plugs,ignition wires,check ignition timing, you need a voltmeter to check ignition sensor and ignition module on your thunderbolt 4 distributor and right now purchase Mercruiser Manual #18
 
Thank you for the advice.

I've already got a manual and voltmeter, so I'll have a look before going to the boat about how to check the ignition sensor and ignition module.

It's such a long trip to the boat, minimum 2 hours 40 minutes each way that I've ordered new plugs, HT leads and coil, they should be here this week. Each weekend I spend working on the boat is another weekend she's not being used for pleasure.

Originally I was going to be busy this weekend scuba diving, but a cold has changed my plans, so I might be up there after all if the UK weather is okay. The weather is a problem here in the UK.
 
At the weekend all the ignition bits were replaced, so it's now got new coil, leads, distributor cap, HT leads and plugs.

Mercruiser didn't design the engine for ease of access to the plugs, especially number 2.

A 60 minute shekedown run, no problems at all, no misfire and she's running smoothly again. I didn't do anything about the fuel vent, I forgot about it, so it was an ignition problem, something was breaking down after 30 minutes, my guess is the coil but I'll never know for sure.
 
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