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Merc 57 no power after 20 minutes of running Pops and backfires

T

tomcal

New member
"Hi. Have a 1997 engine Mercru

"Hi. Have a 1997 engine Mercruiser 5.7 with Thunderbolt V Ignition, 2 barrel Mercarb. Problem is the boat run perfectly for about 20-30 minutes and then starts to loss power, backfires and while not run over approx. 1200 RPM. This is very repeatable. After it heats up for 20-30 minutes runing at 32000 RPM in gear. Shifting to nuetral after problems starts and reving engine, all is fine. Put back in gear and under load and it immediatly loses power and backfires and pops from carb. Let it cool down for 30 minutes and all is fine until it heads back up after 20-30 minutes under load. It appears to be heat related. Some folks have told me it maybe ignition problem with Coil, optical sensor in distributor or ignition module. Module is very expensive. I don't think its fuel related. Can a Merc dealer test the ignition module? Problem is very repeatable but baffling to find solution.

Any thoughts on what is causing the problem?."
 
Do you have known good fuel in

Do you have known good fuel in it?

I could be a module. I'm not sure how to test one other than using an ociliscope on the ignition. Sometimes you can see an irregular ignition signal by watching the pulse of a timing light.
 
"-- 32000 RPM in gear --
????

"-- 32000 RPM in gear --
???????????? Some turbine!

As it only happens after it warms up, it even could be warped valve situation. As engine warms up and valve stem expands, it could be holding partially open allowing the backfire.

"
 
Easy and cheap to change out t

Easy and cheap to change out the ignition coil. They are known to develop this type of problem.
Rod
 
"What is your engine temp. run

"What is your engine temp. run? Fuel may be vapor locking. I would try a different coil 1st. if I know for sure that the cap, rotor and plugs & wires are good. The optic sensor will usually just quit but can fluctuate its signal if it's covered in rust--older models only. The amplifier module can run wacky.


Optic trigger is under the rotor which won't come off since it is loctited in place; that's why it's the last thing.


Tests for Thunderbolt Ignition: I think this will test your TBI V.

W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced."
 
"Thanks to all for their sugge

"Thanks to all for their suggestions on resolving this problem. I'm installing a new coil this weekend and will let you know how it turns out( easy and cheap first).I'm ruling out value heat problem since when putting engine in neutral,it will rev up fine. Something is breaking down when under load and heat. Don't think its a vapor lock problem for same reason. Thanks to Guy for the very detailed igntion trouble shooting guide.
I'll let everyone now the outcome of coil replacement. Thanks again."
 
I had this problem at the star

I had this problem at the start of last summer. After I put the block plugs back in the problem went away.
 
"Huh? <[img]"http://www.marine

"Huh?
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"
 
"After 20-30 minutes runin

"After 20-30 minutes runing" it begins backfiring-
When it does this look at the voltmeter - is it showing 15-17 volts? 20 mins is about the time needed for the alternator to replace the cranking current - if the voltage regulator is going bad high voltage will cause cross-firing. Happened to me on my old 1979 with standard ignition. No idea how yours might react but it's a simple thing to check. I ended up replacing the factory $400 v reg with a belt-driven alternator with built-in reg for $110. No more problem. Good luck. JW In Dixie
 
"I agree wih Rod. Sounds like

"I agree wih Rod. Sounds like a coil problem to me, this is common symptoms. The coil gets hot after 20mins running and starts breaking down. Replace it and see what happens."
 
Tom
I am having this same exa

Tom
I am having this same exact problem with the same engine and year. Mine is a 4 barrel. I have changed coils until I am crazy.But I can use the coil I just took out at a latter date and it works. The problem seems to come and go so it doesn't always duplicate. And since it runs fine in neutral the shop mech can't help.
It happened at the ramp this morning which is why I am hear now.
I do know you must use a coil rated for the tbi V because of extra heat.
 
They are exhaust shutters betw

They are exhaust shutters between the "Y" pipe and the risers which prevent backwash of lake water into the engine. If they are not functioning properly the hot exhaust can't get out of the engine fast enough.
 
"Thanx guy.[img][/img]
Could

"Thanx guy.

Could be exhaust flappers getting stuck once hot.
Seeing how old the boat is, I would change them in any event. Easy to do and also would save pulling engine should they come apart and lodge down in the Y pipe.
Kurt"
 
"EVERYONE HERE... I have a 91

"EVERYONE HERE... I have a 91 mercruiser (chevy) 4.3 with Thunderbolt IV and mines doing the same thing. My parents own a garage and I am very experienced with cars. I am 99% sure its the ignition module or its high volts from the alternator. My voltmeter was reading around 17v at the coil but I blamed that on my voltmeter battery which was almost dead. Ill check tomorrow and see what it is for sure. It ran fine then I but new haeds, coil. wires, plugs, risers, and started it up to find that it wont rev past 2000 then starts backfiring out the exaust and carb. but idles beautifully! Ill let you no as soon as I find out, or if you do first please post to save me some aggravation!!"
 
"George:

Welcome. Now repo

"George:

Welcome. Now repost as a separate thread for the most responses. Click on TOPICS above, then MERCRUISER STERNDRIVE and finally in the dark blue area START NEW DISCUSSION.

17 V will cause what is happening."
 
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