Rough Idle & Timing Light Problems

2002 Mercruiser 5.0 Litre V8, Alpha drive, Thunderbolt V ignition, 2 bbl carb, fresh-water-cooled with heatx

1. Repaired no-spark condition by replacing corroded distributor ignition sensor module. Had to drill out seized distributor cap screws to get cap off & then drill/tap them to repair distributor.

2 Now engine runs:
Starts OK, rough/erratic idle hot & cold (choke working OK)... sometimes nearly dies, other times runs OK for 30 seconds
Tried 2 different good timing lights. Both are intermittent (sometimes no flash on #1 cylinder) and marks jump around - did manage to set timing at marks [10 degrees BTDC] (jumper lead used to lock timing at full retard)
New spark plugs come out cold/dry fouled with black soot after about 1 hour total running in neutral trouble-shooting problem (Mercruiser manual says that's a sign of weak ignition)


Carb removed and disassembled - very clean, no water or dirt, float level checked OK
Replaced fuel filter - no water or dirt in old filter
Filled fuel tank with new marine gasoline
Flame arrester clean - engne runs the same when on or off
PCV valve clean & not stuck open
Changed oil & oil filter (Sierra marine 25-40 semi-synthetic) - old oil was clear & looked OK
Compression checked - 190 to 200 psi all 8 cylinders
Tachometer and alarm horn working
Battery tests OK & cranks engine quickly
Alternator charging
Engine temperature normal (160 F) & risers not hot (100 F). Heatx full of coolant & no leaks.
Engine wiring harness plugs and terminals shiny & clean (except at 35 amp circuit breaker near carb, which was replaced)
Spark plugs: old ones replaced had light brown insulators and shiny black noses all 8 cylinders - OK
Replaced distributor ignition sensor, rotor, cap, HT leads (spark plug wires), ignition coil, spark plugs, ECU (electronic control unit/module)
Distributor shaft not worn & rotor not wobbling
Inside of cap dry & clean
HT leads (spark plug wires) run in wire guides & crossed as little as possible
Replaced main engine 35 amp circuit breaker near carb (corroded terminal)
Cleaned/tightened all battery terminals and cable lugs including on battery main switch & starter solenoid & engine block ground stud
Carb accelerator pump spraying fuel when throttle lever moved forward
Engine will rev to over 3,000 rpm (wants to go higher) in neutral without missing or backfiring

Thinking worn timing chain, low voltage/voltage drop/bad connection in engine harness wiring, ?

How do you get the plastic guard off the carb mixture screw so you can adjust the idle mixture?

OK, thanks for that. I'll do it tomorrow.

To add to my original diagnosis information: I found Mercruiser Service Bulletin #2003-06 - "V6 & V8 GMEFI With No Start, Engine Mis-fire, or Engine Shut-off Conditions" which states "Whenever looking at a HVS [? High Voltage System] related problem make sure the rotor (under the distributor cap) is tight on the distributor shaft. This is a pressed fit and you should not be able to rotate or remove the rotor/metal plate off of the shaft."

Then I found Mercruiser Installation Instructions #89283 (February 2004) for the Thunderbolt V ignition system rotor which states that "...the distributor rotor should be installed on the distributor shaft with Loctite 271 threadlocker so it cannot move". The new rotor I installed was an aftermarket part, not a Mercruiser OEM part, and did not come with the Mercruiser instructions. I installed the new rotor with grease on the shaft, so it can be pulled off easily, and so it might be moving around contrary to the Mercruiser instructions - an possibly making the timing erratic and the engine misfire at idle.

When I check the timing chain tomorrow I'll also degrease the shaft/rotor and install the rotor with Loctite.

Check the measurement on the contact at the top is the rotor to be sure it is 1/4 of an inch. The service manual says to check that also. That bit me once

Replace the cap..........

Thanks to all who made suggestions.

Checked timing chain - a couple of degrees: OK
Loctited distributor rotor - next day customer sea trialed boat (I wasn't there): says engine ran fine

I will check tomorrow to see if engine still has misfire/rough running at idle... maybe customer didn't lift engine hatch and didn't feel it.

Timing chain OK
Installed distributor rotor with Loctite

Engine still runs rough at idle
Tried brand new Snapon Tools strobe timing light... only flashes once in a while on No. 1 cylinder plug lead, flashes OK when hooked to coil- distributor lead

Did you check the shift interrupt switch? Disconnect it and recheck.

You do know it normally fires every OTHER turn of the crank on #1. It is a 4- cycle engine, it takes 2 turns of the crank to fire once, in between this it is firing on #6 alternately. Your crank comes to TDC on each revolution, either #1, & #6, but your Cam, & Dist. turn at 1/2 speed of the crank.

Rob8888 said:

2002 Mercruiser 5.0 Litre V8, Alpha drive, Thunderbolt V ignition, 2 bbl carb, fresh-water-cooled with heatx

How do you get the plastic guard off the carb mixture screw so you can adjust the idle mixture?

Click to expand...

Pull them off, or break them off,,, whatever it takes.

Chris: Good point. I did remove the shift interrupt wires and connect them with a jumper - no change. I forgot to mention it in my first post. Thanks

To answer my own question somewhat... the spark plug wire voltage must be too low to fire a timing light regularly (and to fire spark plugs regularly - there's the misfire at idle) either because the voltage from the coil is too low, or because the new distributor cap or rotor/timing ring for the ignition sensor are defective. Perhaps the rotor doesn't point directly at the cap inner spark plug wire terminals when the timing ring triggers the ECU to make the coil spark? I'll check the coil wire voltage, the spark plug wire voltage, and compare the cap and rotor to other new ones.

Rob8888 said:

Chris: Good point. I did remove the shift interrupt wires and connect them with a jumper - no change. I forgot to mention it in my first post. Thanks

To answer my own question somewhat... the spark plug wire voltage must be too low to fire a timing light regularly (and to fire spark plugs regularly - there's the misfire at idle) either because the voltage from the coil is too low, or because the new distributor cap or rotor/timing ring for the ignition sensor are defective. Perhaps the rotor doesn't point directly at the cap inner spark plug wire terminals when the timing ring triggers the ECU to make the coil spark? I'll check the coil wire voltage, the spark plug wire voltage, and compare the cap and rotor to other new ones.

Click to expand...

Actually connecting them with a jumper wire will activate the shift switch circuit. Just disconnect the switch and retest.

If you connected the shift switch wires with a jumper and the problem still remains then you may have found your problem.

Docksidemarineservices said:

Actually connecting them with a jumper wire will activate the shift switch circuit. Just disconnect the switch and retest.

If you connected the shift switch wires with a jumper and the problem still remains then you may have found your problem.

Click to expand...

That's what I was thinking as well, connecting it with a jumper is normally grounding it, but wasn't sure.

kimcrwbr1 said:

Shifting slow into gear is not good for the clutch dog. The shift interrupt switch causes the engine to stumble to allow the dog to seat fully into the gear. Make sure the engine is running close to 100% as possible, test the interrupt switch and confirm it is aligned properly with the shift cam.

Click to expand...

The shift interrupt switch ONLY activates when shifting OUT of gear, Not into gear.

WHen shifting into gear one should shift quickly and firmly so only one clunck is heard/felt. This allows the clutch dog teeth and gear teeth (corrosponding dog teeth) to engage quickly and completely.

When shifting out of gear the syntrifical forces on the gear and clutch dog make it almost impossible to pull them apart therefor causing resistance on the short shift cable thus causing the V notch to move and actuating the shift interrupt switch. This takes a split second of ignition interrupt and the clutch dog will seperate from the gear.

kimcrwbr1 said:

1..... Shifting slow into gear is not good for the clutch dog.
2..... The shift interrupt switch causes the engine to stumble to allow the dog to seat fully into the gear.

Click to expand...

1.... Correct! You'll want to make the shift into gear quickly and decisively.... and likewise when returning to Neutral!

2.... All due respect..... that is not the intent behind the SI (shift interrupt) or SA (shift assist)!

kghost said:

The shift interrupt switch ONLY activates when shifting OUT of gear, Not into gear.

WHen shifting into gear one should shift quickly and firmly so only one clunck is heard/felt. This allows the clutch dog teeth and gear teeth (corrosponding dog teeth) to engage quickly and completely.

When shifting out of gear the syntrifical forces on the gear and clutch dog make it almost impossible to pull them apart therefor causing resistance on the short shift cable thus causing the V notch to move and actuating the shift interrupt switch. This takes a split second of ignition interrupt and the clutch dog will seperate from the gear.

Click to expand...

Agree!
Only when/if the shifting system has an issue (such as a worn/stiff cable, a bent shift shaft or a sticky spool or slidding sleeve) will we see problems while going into gear!

While coming from Gear back into Neutral, the Dog Clutch teeth are under prop thrust load due to the prop's speed being greater than the hull's speed. This prop thrust load wants to keep the doved tailed Dog Clutch teeth engaged (the nature of Dog Teeth).

The disengagement friction (against the shift cable's outer jacket) causes the SI/SA system to momentarily lower engine RPM until the prop thrust is reduced enough to allow for dog teeth disengagemet.


And BTW... final SI/SA adjustment must be made with the boat in the water.




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Yes, all well, & good, but the OP said he's not getting any spark. What I'm trying to figure out, or remember here, should you disconnect the shift interrupt switch, or jump it? I would think one way, you'd have no spark, and the other way you would? Just not sure which? The same thought applies with the shift assist switch? Should it be disconnected, or jumped?

The shift interrupt shorts the ignition to ground when the switch is activated. One of the first things mentioned in the service manuals is to check the interrupter switch when a no spark condition exists.

To remove it for testing simply disconnect the wire from the ignition that connects to the switch/terminal block. Tape it off so it does not contact any ground.

Some ignitions the wire connects to the neg side of the coil this would be a grey wire and some, it is in the harness and connects to the white with green stripe wire going to the ignition sensor in the distributor.

Depends on the year/ignition design.

Many Thanks kghost, for clearing that up for me, & others!

I didn't report correctly what I did to test the shift interrupt... I just disconnected the 2 wires to it. The engine ran the same, with a random/erratic misfire at idle.

The customer used the boat on the weekend and apparently it ran OK. I think running at speed cleaned the plugs, anyway now the Snap-on timing light flashes most of the time on all spark plug wires including number 1, and the idle is not as erratic. Whenever the timing light skips a flash or 2 the engine also misses. I set the timing on number 1 cylinder with a jumper to ground connected to the purple/white timing advance lock-out lead. There is no degree plate, just a "V" in a small casting on the timing cover and a line on the edge of the harmonic balancer... presumably this is 10 degrees BTDC. With the jumper still in place to lock the ignition I revved the engine to 2,500 rpm... the ignition advanced about 5 degrees even with the jumper wire in place [yes, the jumper wire was connected properly]??

I moved the strobe timing light pickup from number 1 wire to number 6 wire. Cylinders 1 and 6 fire alternately every other crankshaft rotation (1843 - 6572] and therefore should use the same timing marks... I found number 6 was firing about 10 degrees advanced from where number 1 was firing (which was right at the timing marks where I set it). According to an old tuneup book I've got that means "either the distributor cam which opens and closes the points is worn out so the points do not open exactly every 45 degrees of distributor shaft rotation" - with a Thunderbolt V electronic ignition without points that means the Hall effect ring gaps are not spaced exactly the same - "OR the distributor shaft is slightly bent so that the rotor is wobbling and not turning in a perfect circle inside the cap" OR "the distributor shaft bushings are worn so the points cam and/or rotor are moving around and changing the timing" OR "the timing chain is worn out". I know the distributor shaft bushings are not loose and the timing chain is not worn out, but I will remove the distributor cap and see if the rotor wobbles slightly when I rotate it by cranking the engine, and change the rotor and cap just in case it makes a difference.

MikDee said:

Pull them off, or break them off,,, whatever it takes.

Click to expand...

Or you can buy the tool. On some carbs, breaking off the guard will leave you with having to turn the mixture screw with a small needle nose pliers since end of mixture screw not slotted.

sandkicker said:

Or you can buy the tool. On some carbs, breaking off the guard will leave you with having to turn the mixture screw with a small needle nose pliers since end of mixture screw not slotted.

Click to expand...

Okay, sorry I spoke in haste, not break, but if you can pull the plastic caps off, and cut off the stops, then put them back on, that works. Otherwise, yes there is a tool for this it's reasonably priced, I have old one around here somewhere? Haven't had to use it in years.

Even though distributor had all parts replaced with new: sensor, "reluctor" wheel, rotor (loctited on - it's tight), and cap... I replaced it with a brand new OEM Mercruiser part. Problem solved.. suddenly strobe timing light started flashing regularly and I could set ignition timing... and engine runs very smoothly with no misfire. I have the old distributor on the bench and trying to determine problem.