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4.3 V6 Sparks, fuel, compression --> cracking but not starting

maracucho

New member
My faithful port-side v6 4.3 carburated in not starting but it is turning (cranking). I had to say that I replaced the distributor cap , distributor sensor (internal), coil, new sparkplugs cables and rotor .. sparkplugs are 1 year old. Like I said never had problem with the engine just replaced the parts because I did the same maintenance to my starboard one so I want to keep both engines with the same maintenance schedule.


I have BRAVO 2 setup so my neutral safety switch is located at the control box level at the helm.. It is not like the Alpha 1 that the neutral safety switch is located at the engine area linkage

Boat was on the trailer when I did the maintenance and the engine started right up after the parts were installed.. so I said , great I ready for spring . Well as soon as the boat hit the water at the lake .. the engine ran for 5 seconds and then die. Here is a list of all the parts and troubleshooting complete so far.

1) Check all the cables , voltage from the ignition switch to the coil = Coil is getting 12 V (purple cable)
2) Replaced the kill switch ( because I had a new one sitting in my garage) .. Did not want to take any chance with an old one..
3) check spark from coil to Distributor.. = STRONG spark
4) Check spark from Distributor to sparkplugs = Strong spark
5) Check timing .. verify that timing chain did not jumped.. everything i s fine. timing is correct on TDC the new rotor is pointing to #1 at the distributor
6) swap the ignition module from my working starboard to the now failing port engine == no difference ..still no running
7) Connect directly to the + pn the coil the positive of the battery... no difference
8) replaced my oil sensors pressure units at the oil filter area. ( good pressure)
9) pull out the distributor to check if the bottom gear was missing a piece. Distributor was en excellent condition
10) checked the inside of the camshaft with a endoscopy camera to check for broken parts .. nothing found
11) Swap the ignition switch from my working starboard engine -->did not make a difference
12) Starting fluid on carburetor .. I have plenty of fuel , but I want to try with the starting fluid.. well the name of the can got my attention at this point ---no running
13) disconnect the ignition module for the just in case that need to be reset in order to get the new signal input from the new oils sensors unit.. I think that maybe one could send a signal to the ignition module because there is one that shutoff the fuel pump for safety ..ayway .. did not make a difference.

14) Going to swap again the ignition module with the new oil sensor to see what happened

remember this engine never never gave me any problem , timing , carburetor, everything was just fine ... actually , after the new parts were installed it ran fine at the trailer , and just for 5 seconds on the water and that was it.

15) I also checked on the batteries..(1 year old battery) new batteries
16) Fuel is fresh .. my starboard engine does not complain and its getting fuel from the same tank as the portside one
18) Of course that I tried several coils.. same result.
19) I swap the distributor pick up sensor (inside de distributor)
20) It is using brand new sparkplugs cables.
21) sparkplugs are dry and clean
22) Pressure on piston #1 is 120..


So I have fuel (even with starting fluid), Spark , compression and the engine does run... What I may missing here ..? please help..
 
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1. neutral safety switch on alpha or bravo is in the or should be in the shift control box. This if actuated would not allow the engine to turn over (crank) at all.

what you are referring to is the shift interrupter switch on the alpha. Not your issue so forget about it.

if you have spark, fuel and compression then it should start. 120 psi is low but it should still start. You should have 150-160 psi on a normal engine.

Here is the Mercruiser ignition test procedure. Try this and report back vs swapping out the kitchen sink approach.
I understand some of this you have already reported to have done but redo it.

Tests for Thunderbolt Ignition:



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. (battery voltage)

#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. amplifier.

#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.

Ignition amplifier is the (black control box) out side the distributor.
Ignition sensor is in the distributor
 

#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. (battery voltage) =>>> L
ets be cleared. If I connect the + test lead from the voltmeter to the cable coming from the distributor the I have .3 volt.. nothing but If I connect to the end from the cable coming from the ignition amplifier (black box outside of the distributor) then I am able to see the 12 V when the ignition key is turned on.


Following instruction
#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.

Indicate that my ignition amplifier needs to be replaced... well I tested my IA swapping to my starboard engine and it worked just fine so my Ignition amplifier is good..

I have good spark from coil to distributor and from distributor to sparkplugs.. I also tried my Ignition amplifier from my good working engine on my portside (problem one ) without luck.
,
Lets not forget that The engine started when it was on the trailer after I replaced : rotaror, coil, new sparkplug cables, distributor cap, igintion sensor inside the distributor, and the sparkplugs are 1 year old which were working just fine .. I have never seem an sparplugs gap changed... The engine started at the trailer, tow weeks later I got the boat to the lake and the engine worked for 5 seconds after the boat got to the water , then just die and here I am. Sparks, fuel in carburetor , compression..and a engine not running. today I am rechecking the gad on the sparkplugs .. I do not know what else to do and I am going to change the sparkplugs cables for the old ones ... oh well .. I am out of options now .. " the engines worked after all the new parts were installed at the trailer"
just die 5 seconds after the boat is on the water
 
The amplifier is the "black box"

The ignition sensor is inside the distributor.

Did you swap this between engines?

I have had functioning sensors fail minutes after starting and engine.

But you say spark exsists on non starting engine.
Does it exists at end of spark plug wires at spark plugs?

If so then maybe distribitor turned and timing is way off.

Need to confirm what base timing is.
In other words you need to find top dead center on compression stroke and make sure rotor is pointing at number 1 spark plug wire/terminal on distributor cap.

let us know if the makes any difference.
 
Latest update from yesterday,
I do have 12 V from the red/wht cable coming from the amplifier "black box" like I said before, but it does not hurt to check it again right :) . Anyway, I replaced all the spark plugs with brand new ones set to .045 as manual and top of the engine label stated. The old sparkplugs were in excellent condition , but hey I alrady have the new sparkplugs so I install them, Checked on the brand new sparkplugs cable which by the way are 1K ohm compare to 12 ohm or 5k ohm from the old ones.. Made in USA cables no the Chinese ones ( I used the same cables on another set on my starboard engine without problem). I checked on the spark from the coil to distributor.. strong spark and also checked on the spark at the end of the spark plugs .. I also have sparks. I do have videos , but I do not know how to upload videos on the forum so far.. Well, I can smell the gas getting to the carburetor (which I have never mess with it because it works perfect from day one) remember this engine worked fine after all the parts were installed at the trailer , ran for 5 second on water and then died..

I know that fuel is in the carburetor , but I put some starting fluid (which I hated specially in a bilge area) anyway just to remove the missing fuel variable and nothing happened. Engine cranked just fine, but not starting.

The timing was already checked and made sure that it was TDC and the rotor on the distributor was on location 1 , that was to check if the timing chain jumped and thank GOD it did not. after the TDC was found and the rotor was on 1 , the engine was turned several time to check if the rotor got out of location , but always came back on TDC to location 1. Timing chain did not jump.

Distributor got pulled out to check if the gears at the bottom were mess up .. nothing wrong there. I even checked inside the camshaft with the camera. Nothing either.
the coil was swapped with the old coil which still good , but just for testing .. no change. I used the old two wire ignition sensor to try ..nothing . Installed the new 3 wires ignition sensor back to the distributor. I was planning to swap the ignition sensor from the working engine ( like I read that they could go bad after few minutes) but to tell you the true I am afraid to mess up the only working engine I have left so I rather buy a new one.

I also swapped the black box (ignition amplifier) and work just fine on my stb engine..
thinking to put the old rotor ,,, but c'mon there is nothing to it .. it is just a brand new piece of metal.. Also the distributor cap is brand new ..

I checked for spark on each terminal of the distributor going to the spark plugs and Also at the end of the cables before they get to the spark plugs .. I know is duplicated work , but I was just checking how much less spark I get due to the resistance of the cable before they go to the spark plugs... which is minimal 1K ohm.
I know that At this moment the engine is on time , but need adjustment after the distributor was pulled out for checking .. I need to get it back to optimal condition when it finally start , but at least I should be able to start with some miss fire , but start.... for an engine to work just need fuel, spark and compression. with I have all of them ..

Help please...no fun to see everybody passing by and I am in the office (the bilge)
Thanks
 
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We don't see twin inboard , just outboards so I'm just asking . Do motors 1 CW , 1 CCW ? Do they have different firing orders? Could motor have jumped 1 tooth in time?
 
Compression test the whole engine, If you are hanging an intake valve, it will never start, no vacuum.

You can have some one crank the engine while you choke the carb with rags (forcing a vacuum in the intake) and it will eventually fire if an intake valve is stuck open/burned or stretched.
 
Back from the boat today.. I had another pickup sensor (new) replaced inside the distributor, installed new gapped to 0.045 sparkplugs last weekend.. did not make any difference. Today i checked the pressure on each cylinder..here are the values..

Piston 1 = 127 close to 130
Piston 3 = 130
Piston 5 = 140
Piston 2= 125
Piston 4= 140
Piston 6 = 150

I also tried using starting fluid with full throttle while cranking (i hate starting fluid specially in a bilge). No starting. I also checked spark on every single sparkplug.. strong spark. And My TDC on piston #1 is on mark with the rotor position on location #1..

Spark, fuel and compression.. and the f!@##% engine does not run... two months already..
I forgot the new sparkplugs were dry but i can smell gas so gas it getting to them
Any more ideas...
 
Try replacing the rotor or put the old rotor back in. They have been known to short down to the dist shaft

Back from the boat today.. replaced the rotor withr the old one.. nothing chaged. Stoll no running. I checked on the voltage at the coil while crancking .. from 13 vol down to 10 volts
 
If you have spark at the right time and you have good compression numbers then that leaves just the fuel. You say it ran on land ok and when at the lake it died almost immediatly.
So , either your just about out of fuel or you sucked up a load of water . Check the filter/sep by pouring the contents in a clear container, Then place a paper napkin down the carb, pump the throttle , remove the napkin and see if it burns
 
If you have spark at the right time and you have good compression numbers then that leaves just the fuel. You say it ran on land ok and when at the lake it died almost immediatly.
So , either your just about out of fuel or you sucked up a load of water . Check the filter/sep by pouring the contents in a clear container, Then place a paper napkin down the carb, pump the throttle , remove the napkin and see if it burns



First, I do not know anything, but as an old farmer, I would by pass the carb., and pour a thimble of gas into one or two cylinders, replace the plugs, and if you have spark, something should happen. Worth a try, because plugs should be wet if your getting fuel.
 
First, I do not know anything, but as an old farmer, I would by pass the carb., and pour a thimble of gas into one or two cylinders, replace the plugs, and if you have spark, something should happen. Worth a try, because plugs should be wet if your getting fuel.

When I checked the piston pressure I checked the one week old sparkplugs , they were clean and I was able to smell fuel on them. If they were wet then I would said that the engine is getting drown by fuel, but they were dry and fuel smell was strong . So fuel is getting there
What I did with open throttle while cranking the engine use starting fluid directly to the carburetor ... nothing happened.

I filled up the fuel tank with fresh non-methanol gasoline before getting to the lake. The same tank for both engines and the starboard it running just fine.. I also replace water/fuel filter .. I forgot to add that to earlier comments


Am I need to bring some holy water and go for an exorcist ... :D
 
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Hi I read your thread this morning if you put starting fluid into the carb and it didnt fire then you got no spark might show when in the open but under pressure you got none starting fluid will burn with no compression but it does need a spark you should have a blue spark that will jump around 1/4 to 3/8 inch from the end of the plug lead all you have done points to not hot enough spark Mike
 
A normal functioning Thunderbolt IV or V should have a bright blue spark that will jump a 1/2 to 9/16" gap with no issues.

If you have a yellowish spark it is weak.

The only other issue i can suggest is your timing/firing order is wrong.
Like you are either 180* out or you have the firing order in reverse.


Please post your wiring firing order.

standard rotation engine.
A V8 would be (1--8-4-3-6-5-7-2)
Your V6 should be (1-6-5-4-3-2)
Both V8 and V6 should be in the direction of rotor rotation.
 
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A normal functioning Thunderbolt IV or V should have a bright blue spark that will jump a 1/2 to 9/16" gap with no issues.

If you have a yellowish spark it is weak.

The only other issue i can suggest is your timing/firing order is wrong.
Like you are either 180* out or you have the firing order in reverse.


Please post your wiring firing order.

standard rotation engine.
A V8 would be (1--8-4-3-6-5-7-2)
Your V6 should be (1-6-5-4-3-2)
Both V8 and V6 should be in the direction of rotor rotation.



If all else has failed, pull the entire ignition system from the running motor and swap them. Not only putting the running system into the non running engine, also put the system from the non runner into the running motor and see if that engine runs to eliminate that system in its entirety. Pull the fist, coul, amp and shirt harness that connects the 3
 
If it ran for 5 seconds and stopped, firing order not 180 out.
Thats right engine ran before on the trailer fine , then it ran for 5 or 6 seconds on the water.

I checked on the voltage at the coil while cranking the engine to check if by any chance the coil was not getting enough juice .. but looks to me that I am on the range for for the coil to work .. it fluctuated from 12v to 10 volts... Anyway , I got a marine mechanic today to check the engine ... I explained everything I have done to the engine and he checked all the cables and stated that everything is " connected right".. but he mentioned if my battery was old becasue it sound weak ... one year old battery and he tested and I was right it was putting 13Volts ,he said that the starter is my problem .. I know that I had to replace my starboard engine starter last year ... so the port one is not new < BUT >.. but it does not even sound as bad as the starboard engine starter when I replaced ... Here is a video of the test I ran and you can hear the engine cranking ... to be honest I just want to convince myself that the starter is the root cause of my problem ...and no just go get another part to see if that fix the problem thing ... Here is the Video..

https://youtu.be/bo7NS0Vd2B4
 
If all else has failed, pull the entire ignition system from the running motor and swap them. Not only putting the running system into the non running engine, also put the system from the non runner into the running motor and see if that engine runs to eliminate that system in its entirety. Pull the fist, coul, amp and shirt harness that connects the 3


well ,,thats what happens when you post from your cell phone, let me correct my atrocious spelling:
Pull the distributor, coil, amplifier and short harness that connects the 3 and swap them. of course, make sure both engines are @ TDC for ease of installation.
 
I can tell you that now I have a fast cranking engine that is not working :) .. crank nice and fast , but still is not working. Having fuel, spark on every single sparkplug, compression everything that is needed for an engine to run .. well mine is still not running. Here is the link of the video with the new starter https://youtu.be/9NuxW5LC-JM


You can see That by the time I recorded the video I forgot to install back the water hose connected to the manifolds... water on the bilge.. oh well .. I can tell the water pump its working :) . Water hose was installed after the video.

next step .. holy water and exorcist .
 
Need to ask a question without rereading everything.

Has the intake manifold been off and reinstalled?

if your shure your getting fuel, have you pulled a plug and confirm they wet with fuel?

did u answer the firing order for that engine?
 
Need to ask a question without rereading everything.

Has the intake manifold been off and reinstalled?
No . Engine ran fine 2 weeks before going to the lake, ran for 5 or 6 seconds on the water. ( you may think that it used the fuel left on the carburator and then just died due to a bad fuel pump, but it does not run even with starting fluid..

if your shure your getting fuel, have you pulled a plug and confirm they wet with fuel?
Yes I am sure that I the fuel got fuel.. smelled it on the brand new sparkplugs.

did u answer the firing order for that engine?
, Engine ran on the trailer fine before it was taken to the lake and I had checked the sparkplug cable location several times. ( actually me and my brother on separated time to avoid collusion ;) )




I checked if my engine had a fuse at the back of it ( I believe it call TKS fuse or something like that ) , but my engine does not have .. that why I could not find it on the service manual.

I read somewhere that I could remove the bundle of cable socket at the engine and bypass all the sensors and try to crank it as bare metal engine.. I know that I have to provide 12 V directly to the coil , but I need to find more information about it .. because I may need to provide 12v to the fuel pump... I still looking for the thread where it mention that procedure..

thank you for the help ..keep the suggestion
 
Get two toggle switches.
One should be on/off
Other should be momentary.

Three 16 ga wires, i would suggest 4 ft long each with alligator clips on one end of each wire.
Also use fork or ring terminals ends for switch connections. Dont just strip wires and wrap the around terminals....do it right!!
Red, yellow, purple wire colors.

connect short red wire to one terminal on each switch.
Also coonect long red wire to ONE of the same switch connection.

Connect purple to other terminal on On/Off switch.

Connect yellow to other terminal on Momentary switch.


On/Off switch in the OFF position.

Connect red to battery.

Connect yellow to slave solenoid small yellow with red, or starter solenoid same wire, small yellow with red stripe.

Connect Purple to coil +

With all wires connected, unplug if you want, the large wire harness connector.
(Make sure key is out of ignition switch)
Disconnect throttle cable from carb and you will work this by hand.


To use this rig correctly,

Turn on/off switch on. This applies power to coil.
Now activate Momentary switch. This will engage starter.
in your case, if engine starts, release Momentary.

If engine starts, to shut down, turn On/Off switch OFF.
 
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