'88 5.7 inboard dies after running smooth, restarts after few minutes, smooth again

[kip2001]

Hi all, I've got a new to me last year Carver Mariner 3297 with dual inboard v drive engines. Starboard engine is the one acting up, serial number 0B956094. Some background on the engine, last summer it ran great though the last time we had it on the water it experienced roughly the same problem I'm about to describe albeit it started back up within 30 seconds. Over winter I replaced the distributor cap, rotor, plugs, plug wires, changed oil & filter, and fuel/separator filter (no water). While replacing those I did note that the coil looked pretty old, definitely older than the port side.

Here's what's happening, except for one time that I possibly flooded the engine pumping the throttle (these engines seem to like to just turn the key, not pump the throttle 2-3 times like my older carbed merc on cold start), it starts great cold and on normal restarts where I turned off the engine. After a period of time of normal cruising, around 1200 rpm for 30ish minutes, the engine will just cut out. It doesn't appear to be starved of gas, I have a fuel flow sensor that detected gas flowing until the engine shuts off, I also removed the cap from the tank fill with no difference, and the rpm gauge doesn't do the classic out of gas rpm flutter before death. Once the engine cuts out, (and this same thing happened that time I mentioned above at a cold start), it will just crank, doesn't even seem to try to catch. One possible tell is that while cranking the tach seems to go crazy, fluttering around up to 3000-4000 and just swinging around. After a period of time relatively short, no more than 5-10 minutes, the engine starts up like nothing ever happened and will run fun until back in its slip (so far that's been another 30-40 minutes). I have fun brought the rpms much above 1500 yet and it doesn't seem to have any problems during idle maneuvering. Unfortunately this problem always seems to happen while on the river so diagnosing with my multimeter hasn't been possible.

So my question is, this appears to me to be electrical. I know I need to check wire connections wherever possible, do these symptoms suggest to anyone a specific component at fault? Coil, ignition sensor in distributor? The tach is the weird part to me, under "normal" cranking it definitely doesn't go crazy, it'll read a few hundred and then display correctly, so maybe that's a tell?

Thanks all!

 

[Scott Danforth]

Check the ignition using the troubleshooting tree in the stickies at top of the forum

You should have TB V ignition

 

[alldodge]

With it just cutting out it most likely is electrical, most likely a loose/bad connection

Serial number shows TB4 but troubleshooting guide for TB5 is the same
https://www.mercruiserparts.com/bam/subassembly/31631/954/60

Look under the Dist cap at the pickup. The original pickup used a 2 wire style and ground use obtained at the base of it. The ground issue caused Merc to redesign the pickup with a 3 wire style which one was a ground
https://www.mercruiserparts.com/892150q02-sensor-assy-ign

 

[kip2001]

Okay, thanks for the tips! I'll give those a check

 

[cyclops222]

I would bet your life on it being electrical part. The long run / heating up and same cooling down time fit perfectly. While it could be a wire connection. I lean a lot to a electrical part heating up and cooling down.
Quick check to do when the engine does that heated up shutdown. Pull the easiest sparkplug boot off of a plug. Put a CLEAN GOOD plug into the rubber boot. WEARING a thick winter glove. HOLD the sparkplug against a UNPAINTED BARE METAL bolt head. have someone else crank the engine. NO SPARK ? Very helpfull data. Is the engine CARBURETOR or fuel injected ?

 

[kip2001]

Went to the boat this morning and took the attached pictures. The pickup sensor in the distributor was replaced at some point, it has a ground wire attached to a bolt on the engine so it has a good ground. The other two wires have the slices you see. The coil I think may be original based on the paint? Seems unlikely but definitely it's older. Engine started right up, I tried messing with the wire connections and it never missed a beat at idle or 1500rpm (neutral). Unfortunately too windy to take the boat out today. Coil didn't seem to be getting hot though

 

[alldodge]

To see if your loosing power to the coil, could run a couple wires from ground and + side of coil to a voltmeter or small 12V bulb.

 

[kip2001]

Oops, here's the pictures

 

[cyclops222]

I can not see clearly enough.

 

[alldodge]

Might clean the connections

The hard walled hose laying across the coil wires might cause some wear issues with the insulated terminals. I would remove the connections and see if they are clean and insulation is intact

Wonder why there is hard wall hose there?

 

[kip2001]

Might clean the connections

The hard walled hose laying across the coil wires might cause some wear issues with the insulated terminals. I would remove the connections and see if they are clean and insulation is intact

Wonder why there is hard wall hose there?

I'm not sure either, it's connected to the raw water side so idk if someone put it in to aid with winterization. Doesn't make too much sense, the other engine has it too, it's on my list of things to take care of, I hadn't even thought about it rubbing against the wires on the coil though. I will say, the wires for the sensor shown above the dist cap were tucked in below the distributor, I pulled them up for the picture and chased the ground wire to make sure it was connected.

Edit: I need to chase that hose, it actually may be connected to the coolant side, maybe it was used for changing the coolant since the fill cap is blocked by the bilge cover. That cover is removable, I did it this spring but certainly not something you'd pull up frequently

 

[dubs283]

I'd say that hose is definitely for the coolant/af side of the system. Im guessing raw water enters/exits at the end caps on the exchanger

The running issue most likely has to do with ignition. With an older (guessing tb4 with the large, square module mounted on the riser?) thunderbolt ignition having an intermittent issue like yours is frustrating. I've seen every component fail/have issues and it really depends on age/quality of the components.

FYI I have seen more than one ignition module of 1988 vintage fail with the exact symptoms of yours. Not saying that's what it is but could be along with any other component

Sadly the t bolt modules are no longer supported/offered from mercury. The update is a voyager/hei complete system

 

[kip2001]

I'd say that hose is definitely for the coolant/af side of the system. Im guessing raw water enters/exits at the end caps on the exchanger

The running issue most likely has to do with ignition. With an older (guessing tb4 with the large, square module mounted on the riser?) thunderbolt ignition having an intermittent issue like yours is frustrating. I've seen every component fail/have issues and it really depends on age/quality of the components.

FYI I have seen more than one ignition module of 1988 vintage fail with the exact symptoms of yours. Not saying that's what it is but could be along with any other component

Sadly the t bolt modules are no longer supported/offered from mercury. The update is a voyager/hei complete system

I believe it is thunderbolt IV, I'll have to snap a picture of it to confirm. It is a newer module design than my last boat, much slimmer and more V shape.

Hoping it doesn't come to that, I'm going to check for voltage loss and sand all connections next time I'm at the boat. I also ordered a replacement sensor and coil, I'll try to hold off replacing until I can actually diagnose either at fault. With two engines probably good to have replacements on hand either way. Hoping it's not the module itself but if it is I'll probably go that replacement route rather than try a used one.

 

[kip2001]

One more thing to add, I have an old synchronizer unit, it seems to work mostly correctly, it's solid green when synced, red lights for port/starboard too low. Would that complicate the wiring at all/something else to inspect?

 

[dubs283]

Being a slimmer, v shape means the module is of newer vintage than the large square one but wouldn't necessarily rule out failure. It's definitely the last portion of the ignition system that would fail ime

Reading through your first post again and seeing the most recent one mentioning an erratic tach and signal synchronizer either of those may be an issue. Easy test is to remove the tach lead from the coil and leads from the synchro unit that connect to the tachometers. Run the boat as you normally would, see if the issue returns

Hopefully the sensor you ordered for replacement is oem, have seen issues with aftermarket ones. Coil not so much, any replacement ought to be suitable as long as its the correct 3.0 ohm

Unfortunately the troubleshooting guide is basically for a no spark situation, not intermittent. Need to perform the tests when the engine won't run otherwise you're just guessing. Bring appropriate tools with on a test run and leave the passengers aside unless they are willing to help troubleshoot. Sometimes a helper is handy

 

[kip2001]

Being a slimmer, v shape means the module is of newer vintage than the large square one but wouldn't necessarily rule out failure. It's definitely the last portion of the ignition system that would fail ime

Reading through your first post again and seeing the most recent one mentioning an erratic tach and signal synchronizer either of those may be an issue. Easy test is to remove the tach lead from the coil and leads from the synchro unit that connect to the tachometers. Run the boat as you normally would, see if the issue returns

Hopefully the sensor you ordered for replacement is oem, have seen issues with aftermarket ones. Coil not so much, any replacement ought to be suitable as long as its the correct 3.0 ohm

Unfortunately the troubleshooting guide is basically for a no spark situation, not intermittent. Need to perform the tests when the engine won't run otherwise you're just guessing. Bring appropriate tools with on a test run and leave the passengers aside unless they are willing to help troubleshoot. Sometimes a helper is handy

Thanks for the tips! Out of curiosity, is there any problem with just swapping the gray signal wires from the engines to the tachs at the helm? My thought is swap tachs and see if the other engine starts acting up. I'll also investigate how my synchro is wired in. Either way I'll have tools with me to investigate.

The parts are sierra, I've always had good luck with them, I don't trust any other after market brands. They haven't bitten me *yet*.

 

[dubs283]

Just leave the known good running engine setup alone aside from removing the connection to the synchro along with the in question tachometer. You only want to try one process at a time otherwise you lose track of what you've done

I can't recall ever having an issue with the latest style oem ignition sensors. I have had issues with new sierra brand sensors but that was 5+ years ago, no idea of their current quality, ymmv

 

[kip2001]

Just leave the known good running engine setup alone aside from removing the connection to the synchro along with the in question tachometer. You only want to try one process at a time otherwise you lose track of what you've done

I can't recall ever having an issue with the latest style oem ignition sensors. I have had issues with new sierra brand sensors but that was 5+ years ago, no idea of their current quality, ymmv

Okay, gotcha. I'll definitely keep that in mind if I end up replacing the sensor.

 

[nola mike]

Before going through all of these gymnastics, confirm that you spark when it's acting up. I throw a timing light on a lead as a prelim test, though of course a spark tester is better.

 

[kip2001]

I only had a little time to stop by the boat today, I prioritized cleaning contacts to the starter battery and coil as well as checking over contacts to the engine for obvious corrosion. Only one post at the starter slave solenoid looked corroded, everything else looked good. I do believe there is some voltage loss occurring somewhere, I measure 12.3v at the positive side of the coil, and 12.6v directly on that battery. I want to sand the contacts at the battery selector switch next and then I'll take the multimeter and try probing around to isolate where that voltage loss is occurring. I was also able to confirm the module is a stock thunberbolt IV.

Both engines share a battery (new this season) and there's a ground between engines as well as separate grounds to the battery. I'll get the voltages from the other engine too next time for comparisons sake.

Before going through all of these gymnastics, confirm that you spark when it's acting up. I throw a timing light on a lead as a prelim test, though of course a spark tester is better.

I agree, other than the above maintenance items, I will have the timing light hooked up and ready to test next time it acts up.

Just want to check procedures for testing when/if I get this problem again, let me know if there's anything wrong with it, basically just taken from the official testing guide but I'll likely need to move quickly.

To start, I'll have the timing light already hooked up to the cable going to the coil to the distributor (does it matter if I have it here or on an individual plug?). I'll also have the multimeter ready to go as well as remote starter trigger.
When engine dies, leave key in run position.
Use timing light and remote starter trigger to test for spark.
If no spark, follow the trouble shooting guide. Couple questions, does the WHT/RED wire always have 12v even when key isn't in run? Guide doesn't mention turning the key to run for that check. Also, the original guide says to check the wire and the terminal on the sensor. That terminal is fully sealed now but there could still be an issue with that, right?
I suppose my only problem with me substituting the coil if I get to that, based on my limited access, it seems likely the engine will restart regardless at that point which means only time will tell if that was the problem or the TBIV.

Okay, and then if the timing light does show spark when cranking at first, then this somehow is actually a fuel issue or a weak coil? (I'll also feel to see if it's burning hot)