Intermittent Fuel Starved 5.7
 

I guess the old addage that you can't fix it if it ain't broke holds true. Changed spark plugs, rotor and distributor cap. Also changed out the fuel/water separator cartridge. Fired right off on the muffs so we went to the lake. Ran like a top and pulled the grandkids on skis. Then pulled nephew out on slalom and the thing started cutting and coughing like it was out of fuel even though I had over 1/4 tank.
Pulled him over to the marina and added 12 gallons but no help. I let the thing sit for a while and got up on plane and then it took off like a banshee. After about 5 minutes same coughing and cutting out. It's like it's running rough but seems like fuel starved to me. Pretty sure it's not vapor lock and engine temp and oil pressure stay consistent.
I took it back out today and ran fine for about 15 minutes. Started same scenario all over again. The only thing I did related to fuel flow is change the filter. I primed it and it obviously is working. Could some crud have gotten knocked loose inside the line when I changed the filter? One thing that I did notice was the new filter (mercury) was shorter than the one that was on the boat (quicksilver). The diameter was identical so I can't imagine this being an issue. I'm pretty stumped here since it runs so well for a while then craps out. Thanks for any ideas or guidance.

Re: Intermittent Fuel Starved 5.7
 

Take the filter off and empty contents into a clear container. Let it sit to see if you have water in the fuel. The water will sink to the bottom.

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Re: Intermittent Fuel Starved 5.7
 

OK the good news is the gas out of the filter looks crystal clear. Guess I didn't realize how much the dyes in gasoline have changed over the years. For those of us who grew up around tractors, the amber color of gasoline or the dark green of diesel were visible in the glass bowls at the bottom of the shutoff valve. Looks like a very light yellow color now.

The bad news is I still don't know what's causing the fuel delivery problem.

Re: Intermittent Fuel Starved 5.7
 

Pull the coil off and ohm it out across the primaries and then from the primaries to the output, secondary. The primaries (+ and -) should be zero but can have a little resistance. The secondary side will be in the 20k and lower range. Your problem sounds like a coil going bad. Hudson Steve can prolly give you the exact readings on your coil. He be smart like that.

Re: Intermittent Fuel Starved 5.7
 

OK, that's a great idea. I'll give that a shot and see what I find. Thank you for the info Brian!

Re: Intermittent Fuel Starved 5.7
 

Hey Scottie. Did the coil fix your problem? I have the same boat with the same problem.

Re: Intermittent Fuel Starved 5.7
 

I'll know more tonight when I can pull the coil off. I'll keep you posted on what I find.

Re: Intermittent Fuel Starved 5.7
 

I assume you have a Thunderbolt IV ignition system with the standard Mercruiser coil (392-7803A4). If so then the primary resistance should be 0.60-0.80 Ohms and the secondary resistance should be 9,400-11,700 Ohms.

Re: Intermittent Fuel Starved 5.7
 

Thanks for the info Hudson River Steve!

Re: Intermittent Fuel Starved 5.7
 

I toltd't y'all Steve would have the answer !!!

Re: Intermittent Fuel Starved 5.7
 

Tested the coil and everything checked out. Primaries were .60 and secondary was 10000 and change. I did notice the ceramic insulator around the output was dark brown on one side. Not sure if that's normal or indicates some arcing or other heat related issue. Coil wire and plug wires all look to be in great shape.

Re: Intermittent Fuel Starved 5.7
 

If there is any doubt about the coil I would just change it out, they are fairly inexpensive. It seems to behave like a good coil when cold but it could be overheating and shorting when it warms up.

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Re: Intermittent Fuel Starved 5.7
 

I had the same thought. For $75 or whatever the coil costs it's worth it to make sure. Thanks for the help Steve!

Re: Intermittent Fuel Starved 5.7
 

I installed the new coil and a larger fuel filter. Took the boat and it ran like a champ for a couple of runs down the lake. After about 45 minutes started the missing symptoms again. I stopped the boat and opened up the engine compartment. Sat around for about 10 or 15 and took off with no problem but about a mile down the lake it started again.
I have two theories about what is happening. Either the fuel pump is weak and is struggling to keep the fuel bowl full or heat is causing intermittent failure in an ignition component. I've got a fuel pressure tester but there's not room to get the fitting in front of the fuel line at the carb or the pump. I may need to build a temp fuel line with rubber hose to splice in the pressure gauge. The fuel pump doesn't make sense though because it runs so well starting out. As for ignition I guess I can run through the ICM diagnostics. I hope it's the fuel pump as the distributor looks like a pain to get out.

Re: Intermittent Fuel Starved 5.7
 

Thanks Scottie. I am dead in the water until I can get this resolved. Eagerly awaiting your reply. Thanks again.

Re: Intermittent Fuel Starved 5.7
 

Well, the coil didn't fix it?
The next thing is the ignition module. I don't think it's mechanical cause heat doesn't generally affect these components. (sans vapor lock!)
I believe you have the Thunderbolt V. Do you have a buddy you could borrow his to troubleshoot?
I'll loan you mine if you'll pay for shipping.
HudsonSteve. Will mine run his?

Re: Intermittent Fuel Starved 5.7
 

I'm not really that familiar with these boats, but if you have a mechanical fuel pump, I think you have a sight tube to indicate a leaking or ruptured diaphragm in the pump. As I under stand it, if you see fuel in the sight tube, the fuel pump needs service. Keep in mind that as the engine heats up, the fuel will tend to vaporize and will need more pressure to keep that from happening. What pressure works when the engine is cold may not work when it's hot.

Re: Intermittent Fuel Starved 5.7
 

Thank you Brian! I might take you up on that offer. The motor is a 95 so does that mean it's a V or is there another way to determine? I'm going to hook up the fuel pressure gauge tonight and see what I get. If the fuel pump looks good then I think the ICM is the next thing to tackle. I'll let you know once I get the pump tested. Thanks again!

Re: Intermittent Fuel Starved 5.7
 

OK, I've been thinking through this issue and making sure I'm accounting for all the anomalies when the engine starts running rough. It seems ironic that the engine starts running rough around 2600 RPM's pretty consistently. This is the start of the range, I believe, when the ICM provides full advance to the ignition. I have also noticed a low idle speed at times which I can't directly correlate with the other issue but it happened on Saturday after I was supposedly letting the engine cool down.

Would either of these other bits of data lean toward the ICM or still possibly point to fuel delivery? Does the ICM control idle speed by altering the spark? If so then those two things seem possibly related. I suppose the Ignition Sensor could also cause some sort of issue but it seems that would be more erratic in nature. I'm still going to test the fuel pump tonight but wanted to get thoughts on these other symptoms. Or, I'll just say to heck with the whole stinking mess and replace the distributor with one of the electronic replacements and call it done:)

Re: Intermittent Fuel Starved 5.7
 

just finished a bunch of research on yours today. Seems that your ICM is prone to filling up with water. I don't know how but there were enough out there that Mercury totally changed the replacement module. I read where folks would pull the module off and would just shake it. They could hear the water inside.
One fella got a couple oz's out of his.
Bad news is, if it's that, it's $450.
I was originally looking at the knock sensor. You may want to double check that. You'll need a descent volt meter. You can test it with a timing light and a hammer.

Re: Intermittent Fuel Starved 5.7
 

Makes you go Hmmmm. How in the world would that much water even be close enough to the module? I wonder if it's condensation that has built up in the modules over the years. I might pull mine off to see if I can hear anything. I"m still not sure if this is a IV or V module so that would confirm that as well. It's definitely mounted on the distributor but I thought there was a IV version that mounted that way. Haven't had time to do the fuel pressure check yet.

Re: Intermittent Fuel Starved 5.7
 

Fuel pump worked like a champ. Cranked for 15 seconds and got just under a pint of gas. Looks like it's the ICM. I pulled it just now and it says ThunderBolt IV on the module. That means it's the older module without knock sensor? Now I'm at that question that I've seen others stress over. Should I replace the whole ignition system with a newer HEI or try to find an ICM?

Re: Intermittent Fuel Starved 5.7
 

I don't think the V will work on my ignition if it has the IV module will it? I might try to find someone that has a working module just to confirm. Thanks Brian for the offer.

Re: Intermittent Fuel Starved 5.7
 

OK, call me crazy but I think this engine was set at 0 degrees advance timing at idle. Just for fun I threw the timing light on and it showed advance at zero. Just to confirm since there are no timing marks, the deepest v is 0 and then BTDC goes 4, 8 and 10 represented by each point on the timing mark plate. I adjusted to 8 degrees BTDC and the thing started really purring. However, I want to make sure this is not false feedback. Can anyone confirm the marks on the timing plate? Not sure this explains everything I'm experiencing but missing 8 degrees of advance could be significant. It looks like original merc paint so did they paint over the markings or are they just not there?

Re: Intermittent Fuel Starved 5.7
 

Did you have to ground the purple (?) wire before looking at timing?
Wish I had the answer you're looking for but I have the '97 454.

Re: Intermittent Fuel Starved 5.7
 

According to the manual, the Thunderbolt IV does not require you to do anything special to time the system. The Thunderbolt V system requires that you ground the PUR/WHT wire to place it in base timing. You also need to ground this wire to adjust the idle speed or mixture. https://www.google.com/url?sa=t&rct=...24jmmNvYWTFpmQ

Re: Intermittent Fuel Starved 5.7
 

Awesome link Steve ! I didn't ground while doing carb adjusts. Thanks bud!!

Re: Intermittent Fuel Starved 5.7
 

Reporting back on the issue. I have the problem resolved and will review everything I've done which may help someone in the future with a similar problem.

This all started with a tune-up including spark plugs, rotor, dist. cap and fuel filter. The engine would "diesel" after a hard run so I suspected the plugs may have carbon buildup. After the tune-up all seemed well with the world until about 40 minutes in and I started experiencing engine cutting, coughing and backfires over about 2500 RPM's.

I initially thought fuel supply since I had changed the fuel/water separator filter. Turns out the fuel pump was fine and now has another brand new filter to keep him happy. After some suggestions from Hudson Steve and Brian Belcher I began looking at the ignition system. The coil tested out OK, but I replaced it since it had some pretty significant carbon tracks on the insulator. Still had the same symptoms so many of us felt the ICM was suspect so I pulled that off to just take a look. It looked OK with no signs of heat or water intrusion.

At the same time I checked the timing and was surprised to see the advance was 0 degrees at idle. I adjusted the advance to 8 degrees and it seemed like the motor was running well. I also pulled the Ignition sensor and it was corroded which probably didn't help. Replaced the sensor and alakazam it seemed like that fixed the problem. HOWEVER, when running the boat I discovered intermittent backfiring and still running rough but not as bad. Threw the timing light on and number one hole showed internittent firing. Pulled the wire from the distributor and no change in engine idle speed. I also confirmed there was plenty of fire through that distributor plug as my hand got a dose of 40000 volts. I also noticed the number 8 wire which is right next to number 1 was showing firing at a very fast rate. I noticed when I checked the timing the first time the number 1 wire was not firing regularly. I thought my timing light (very old) wasn't picking up the pulses. (Another clue that I ignored)

It all began to make sense. The number one wire must have a crack in the shielding and was jumping over to the 8 wire. All that to say I went to O-Reilly's this evening in a small town about thirty minutes from the lake. Bought a cheap set of chevy plug wires and swapped out 1 and 8.
Got back to the lake at sunset and then pure joy at hearing the small block purr like a cat in a dairy farm. Left the trailer on the ramp and had time for two good runs down the lake. Ran and sounded better than I've heard it since I bought the boat.

The plug wires on the boat looked fairly new and were Sierra Performance wires. I "assumed" they were fine and didn't bother changing them due to laziness. I still think the sensor was part of the issue but the wires should have been replaced when I did the tune-up. Thanks for all the help guys! My grandpappy said we should learn from mistakes but they don't have to be our own!

Re: Intermittent Fuel Starved 5.7
 

wow, I haven't here for a few years, early on this forum answered a lot of the questions I had when I first got my boat. The first posting I see today in forum describes symptoms of what I was researching.

Background, boat was idle for over a year, due to "life getting in the way". Took boat out, ran great for about 15-20 minutes, then chugged along at around 2700 RPM max. This has happened, water in fuel. So, I changed filter, and since The gas was old and I wasn't sure if I had stabilized it, I drained the tank. Back on water this weekend, now I am getting the exact same symptoms described here. Engine starts right up, and flies for first 10 minutes, then struggles to keep at 3K rpm. shut engine off, to check fuel line. restart engine after 20+ minutes runs great for another 10 minutes...

I have the tiny 4.3 engine, think I might have the V ignition, Are the coils the smae, i.e ohm specs... It's cheap enough I might just replace it for peace of mind. Hoping for fairly cheap solution...

Jim D
1996 Shabah 4.3LX

Re: Intermittent Fuel Starved 5.7
 

Hey Jim, if you know there was water in the fuel system I would confirm that is resolved before checking other systems. When you said you checked the fuel line did you take the line off or check the filter? You might dump the fuel filter into a clear jar and see if any water settles to the bottom.

If the fuel is clear then a good place to start would be to test the coil, I think they are the same resistance specs, and pull the distributor cap. I wonder if condensation could have built up somewhere that could cause intermittent problems. I've found a timing light is invaluable for checking on whether you're getting good fire from the ignition.

Be methodical and don't move on to the next thing until you're certain albeit plug wires etc:)