Thunderbolt IV Problems

[Erschen]

Overview: I have an 89 Mercruiser 4.3l with a Thunderbolt IV Ignition. This engine is a long-block replacement last year. The only mod I have done to the long-block was swap out the flat-tappet cam/silent chain and gears sent with it for the original roller cam/chain and gear set this year. When I started the engine after the cam swap it would not run above 2500rpm. I set the timing to 8deg btdc below 800rpm. It runs fairly smoothly around 1200rpm but has an intermittent miss but when the throttle is advanced the engine backfires through the carb.

What was new last year: New long-block, battery, and fuel pump. This thing ran well once we had it dialed in to spec.

Due to an oil leak from the intake manifold valley, I pulled the manifold and decided to swap the cam while I was in there.
After putting it all back together and adjusting back to spec I started it up. Then the problems began.

What's I've done so far: Checked and re-checked the valve timing, valve lash, ignition timing, plug gap, plug condition(all fouled with gas), and compression on all cylinders. I've rebuilt the carb and replaced the following: Spark plugs, wires, distributor cap, rotor and sensor wheel, distributor sensor, coil, and ignition amplifier module.

I'm at my wit's end. I've read all I can find here about this problem, and some have replaced the whole system with a Mallory or MSD with good results. I would prefer to keep my original system if possible. What am I missing?

 

[joewithaboat]

Re: Thunderbolt IV Problems

Try grounding the distributor body.
Double and triple check the ignition wires for proper order.

 

[81 Checkmate]

Re: Thunderbolt IV Problems

What kind of ignition module did ya replace with?

The same spec as the ORIGANAL V-6?

 

[Erschen]

Re: Thunderbolt IV Problems

Thanks for the suggestions. I checked the grounds and they were good. I'll check them again. I have verified the correct firing order many times but I'll do it again, too.
I replaced all the ignition components except the coil with new, never-installed before mercruiser thunderbolt iv ignition parts with matching part numbers. The ignition amplifier module stamp number is 6v-18.
The exception is the coil which was from a mercruiser dealer that had a napa parts store integrated with it. Maybe they messed up because it's a napa coil.
It probably is good to mention I checked the spark quality during my run through the diagnostic flow chart and found that I have spark but it seems weak.

 

[Failproof]

Re: Thunderbolt IV Problems

Cam swap. Is it possible cam timed incorectly? Push rod lenght incorrect? Lifters not set correctly?

 

[Aatcman]

Re: Thunderbolt IV Problems

Cam swap. Is it possible cam timed incorectly? Push rod lenght incorrect? Lifters not set correctly?

I think Failproof is on to something here.....

You took the timing set and cam from an older engine and put it in a newer engine....
HOW MUCH older engine?
If your new longblock is a Vortec for instance, and the old one was not, the CAM GRINDS ARE DIFFERENT and you can't just swap the cams even though they fit....

I also would not reuse a used timing chain... But that's just me.......

Hope this helps

 

[Erschen]

Re: Thunderbolt IV Problems

I see where you're coming from. I suspected the same things at first. I tore the engine apart twice to check the cam timing. The pushrods and lifters are the originals with the cam. I've reset the valve lash 3 times while it was running to verify it was correct. I can check it again. As far as the age of the engine, it's the correct year block, but the company that built it put an older-style flat tappet cam in it.
One problem I can visually watch happen is the spark missing intermittently while setting the timing with a timing light. Correct me if I'm wrong, but this would not be affected by the cam, right?

 

[joewithaboat]

Re: Thunderbolt IV Problems

I see where you're coming from. I suspected the same things at first. I tore the engine apart twice to check the cam timing. The pushrods and lifters are the originals with the cam. I've reset the valve lash 3 times while it was running to verify it was correct. I can check it again. As far as the age of the engine, it's the correct year block, but the company that built it put an older-style flat tappet cam in it.
One problem I can visually watch happen is the spark missing intermittently while setting the timing with a timing light. Correct me if I'm wrong, but this would not be affected by the cam, right?

Did you try grounding the dist body yet?
There are two type of coils internal and external resistor. You sure you have the proper one.
Swap the old cap/rotor back on.
Check the ignition wires, i had new ones that were bad before.

 

[Erschen]

Re: Thunderbolt IV Problems

I cleaned and tightened all the grounds in the ignition (distributor body included), checked the firing order again, ohm tested all the wires (.76 max), and found the coil wire to be leaking voltage (it shot an arc from between the dist boot and wire)- I'll pick up a new one of those.

I readjusted the timing to 8* btdc and test ran the engine. No change in symptoms.

I did not save the old distributor cap and rotor because they were the originals and needed to be replaced.

Just for the nuts of it I swapped in the old coil. The problem seemed to get slightly worse. Put the new coil back in and it improved slightly. (The original is a mercruiser thunderbolt iv, the new coil is a NAPA ECHLIN 901 - napa part #ic88?)
Could this be my problem?

Also for the nuts of it I swapped back in my old amplifier module. No change in engine performance. May the old one still be good?

As far as the engine goes, this is not a vortec engine. An aftermarket cam manufacture's website indicated that older model cams from this engine are compatible with this model. Except, please note: the block is an 89 era and these are the original cam, lifters, pushrods, timing sprockets, and chain from the original 1989 engine -which was in excellent shape except for the freeze-cracked block from impropper winterization. Also the heads are factory rebuilt for the longblock. IMHO this problem does not exhibit a consistent misfire or backfire which would be common for a valvetrain malfunction. Will nota idle yet but the engine runs nearly flawlessly- albeit a few sporadic misses and occasional drifting in the spark timing- between 1000-1500rpm. Once it reaches 2500rpm it backfires through the carb and runs extremely rough.

There was another thread that had a similar problem. I think it was called something like "three coils in two days". He described my engine operating symptoms nearly to a tee except I haven't taken this ol girl out pn the water yet (I don't like being stranded!)

Thanks for the help so far! Keep it coming, we're bound to figure ot out sooner or later.

 

[achris]

Re: Thunderbolt IV Problems

Sounds very much like an ignition problem... Because it's not points (and requiring a resistor or resistive wire) you need a coil with the words "Use WITH external resistor" on it... This means the coil has NO internal resistor, which is what you need for electronic ignition.

I've looked at that coil on the NAPA website and there just isn't enough information there to be able to say if it's the right one...
What I have found on the website are these...

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=BK_7357268_0006537056
http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=BK_7357280_0006537056

Either one of these will work with Thunderbolt IV...

Chris......

 

[Bt Doctur]

Re: Thunderbolt IV Problems

So, you went from a flat tappet to roller with girdles and retainers .You say it originally ran good till you changed the cam again? I`d be looking at the cam install
How are you checking cam timing, and did you use a degree wheel to confirm cam grind specs?

 

[Erschen]

Re: Thunderbolt IV Problems

Thanks for thethe input BT Doctur. I made the huge mistake of not starting the engine before I tore it down. Therefore I can't be sure it ran perfectly before the cam swap. That's a mistake I'll never repeat... it has made finding this problem much more difficult.

Unless they

 

[Erschen]

Re: Thunderbolt IV Problems

Stinking smartphone... I'll post from my laptop!

 

[Erschen]

Re: Thunderbolt IV Problems

Thanks for thethe input BT Doctur. I made the huge mistake of not starting the engine before I tore it down. Therefore I can't be sure it ran perfectly before the cam swap. That's a mistake I'll never repeat... it has made finding this problem much more difficult.

Unless they

To finish the thoughts...

With all humility... unless someone can show me that they (mercruiser) redesigned the crank, heads, and valves during the years they used the same block as my 89, I want to assume the internals are identical to the originals. The block DOES have bosses for the lifter retainer and a cam thrust plate mount. The block does NOT have a bore for a balancing bar as found in the vortec models. I could be totally wrong here. Please help me if I'm wrong. Here's what I have to support my hunch: The compression on all cylinders was within 2psi and at the top end of the acceptable range. The last time I had the timing cover off I advanced the cam 1 tooth and test ran it - it caused a consistent afterfire condition. I reinstalled the timing sprockets with timing marks to their proper position and the afterfiring stopped. Also, one condition I can physically witness happening with a spark tester or timing light is the spark intermittently failing to fire and the timing advancing and retarding during about 10-20 pulses through the #1 plug wire. The spark condition is the same through all the wires, I just can't verify timing on any of the other ones with the exception of #4. IMHO it seems more logical that the ignition system is having fits. Also a bonus of fixing the ignition first is not having to tear the engine down. I'm not in any way dismissing your point. If we can't track it back to the ignition, I will tear the engine down and check it all. If it really gets bad, I will completely rebuild it with the original parts (I still have the block, crank, heads, etc.) I'll save that for last. Much thanks.

That leads me on to Chris. Thanks man. The Napa distributor does say "For use with Electronic Ignition". But as you said, it really doesn't give us a whole lot to what's inside. My mercruiser manual was very specific about not being able to substitute the coil for an ordinary one. I'm with you on checking one of those out. Also, one of the local Mercruiser dealers just ordered some Sierra coils they said would work with the thunderbolt iv for $45ish. Do you think it would be worth picking one of them up instead? If so, what part number should they be?

 

[achris]

Re: Thunderbolt IV Problems

The previous engine to the one I have in the boat now, was a '94 4.3LX, with exactly the same ignition system as you... I had a problem about a year before I sold it (to a friend who is still 'over the moon' with the deal)... Long story short, a hole had corroded in the bottom of the coil. That allowed the oil to leak out and the coil was overheating... I replaced it with an 'off the shelf' coil... None of this 'Mercruiser special' BS.... It's a standard automotive coil, without an internal resistor. Nothing special about it... The one I used is a Bosch GT40...

From what you have said it definitely sounds like you have a spark problem... From the year I assume it has the amplifier mounted to the port elbow and a small harness going to the sensor module in the distributor. A few things to look at (you might have already looked at them, by hey...) Check the connections are all clean and tight. Make sure there is a ground wire from the dissy body to the engine block and that the ground wire (black) on the amplifier is also on a good clean ground (CRITICAL!!!)... Check the solder connections on the sensor module for corrosion.

Do you have the Merc service manual for the engine? If so, run though the Thunderbolt IV troubleshooting chart on page 4B-18... It might show something up, it might not... But I suspect finding your problem is going to be as much about ruling things out as finding the culprit...

Good luck and keep posting and letting us know how it's going...

Chris........

EDIT: Just had a look at the circus diagrams, and noticed a couple of things worth trying. You will have 2 grey wires on the '-' of the coil. One is from the amplifier, the other, the tacho... Disconnect the tacho wire and check it... The other thing is that you'll have a wire to the shift interrupt switch, a wht/grn. Disconnect that one and see if it helps... Remember, ruling out also is helpful... Scientific principle: Rule out everything it can't be, and whatever is left, no matter how bizarre, must be the answer... OR something like that...

 

[Erschen]

Re: Thunderbolt IV Problems

First, on the list for elimination in the Ignition System:

The original coil looks to be in fine shape and the new coil says for use with electronic ignition, so I'm assuming it works correctly on this system. We'll eliminate that for now... I'll explain later.
The amplifier module harness connections are clean and tight.
The amplifier module ground connection is clean and tight.
The distributor body ground wire connection is clean and tight.
The ignition sensor module is brand-spanking new with bullet connections to the harness.
The grey tach wire and white/green shift interrupter switch wire disconnected.

Up to this point the problem remains.

What has been confirmed in the Ignition System:

There definitely is spark! (To be explained later ) This is true for both the old coil and new coil. (This also causes the diagnostic flow chart to be less helpful because it is designed to diagnose a no spark condition.)
The spark plugs are brand-new NGK BR6FS.
The spark plug wires are brand-new quicksilver brand from the local Mercruiser dealer (Mercruiser Suppression 1Q12).
The distributor cap is brand-new from the local Mercruiser dealer.
The rotor and sensor wheel are brand-new from the local Mercruiser dealer.
The ignition sensor module is brand-new from the Mercruiser dealer.
The ignition amplifier module is a brand-new unit that was never installed. (I purchased this from a private seller who found his ignition problem to be a faulty coil before receiving his replacement ignition amplifier module. So... he sold it to me, new in the package.)
The base timing is set to 8* btdc. (But, it fluctuates from advanced to retarded and misses sometimes.)


So, while I was listening to the engine run I noticed a buzzing noise coming from the back of the engine. Turned out it was the distributor making all the racket. I listened closer and I could hear arcing inside the cap. From my experience, a distributor should run nearly silently, correct? Anyhoo, I started checking the wires (new NAPA Belden units) to see if the boots were loose or something. POW! An arc shot out from between the coil wire and it's boot that connects it to the dist cap post (from between the wire insulation and boot.) To verify the exact location of the spark origin I held a grounded screwdriver near the location where I received the shock. The location was confirmed. That got me thinking to myself, "Self... If there is that much resistance between the coil wire and the plug, something must be seriously wrong with the dist, rotor, or wires." Well, the wires were the only part non-mercruiser spec. So, I replaced them today with the quicksilver units. That did not solve the problem, and the noisy distributor continued.
Then, I went to the next step back up the spark path to the distributor cap itself. This proved to be interesting. Looking inside the cap at the pickups for the plug towers I noticed what appears to be burn trails on the after side of the pickup for plug #2 and the before side of plug #6. The trail is especially heavy near the #6 pickup. This leads me to believe the spark is firing out of time from the rotor after it passes plug #2 and before it passes #6. The "trails" are localized to those two plug pickups - I could not see any on any of the other pickups. Maybe if it's run longer others will develop, too? (I will try to post a pic if what I am describing is hard to grasp. Let me know.) Have you ever seen a condition like this before? If so, what caused it?

Potential causes or to-test list:

Ignition sensor module not reading sensor wheel correctly?
Ignition sensor module to ignition amplifier module harness faulty?
Ignition amplifier module failure?

Is there a scientific way to test these components? Also, the golden question... Is there anything else in the ignition system that could be the source of these symptoms?

 

[achris]

Re: Thunderbolt IV Problems

Sounds like the dissy is out of sync ... That is, the point when the coil fires it NOT when the rotor is pointing directly at the tower on the cap....

Not sure how to investigate that one... I'll have a think about it...

Chris...

EDIT: I'm going to ask a really silly question... But I have to ask... It is a 6 cylinder rotor you have in, not an 8....

 

[Reel Poor]

Re: Thunderbolt IV Problems

Do you have the part number for the module you installed and the engine serial number. It's been a while since I messed with V6 TB-iv but I was thinking the ID mark was V6 14, I could be wrong. The last two numbers correspond with the maximum advance not including the base timing. So a V6-18 would total advance at 26 degrees and a V6-14 would total advance at 22 degrees. If I'm correct 4 degrees shouldn't cause a backfire but could cause premature detonation/spark knock in the engine.

 

[Erschen]

Re: Thunderbolt IV Problems

Chris, you've got the right idea, the spark seems to be firing when the rotor is not in line with the plug tower. But the thing that doesn't make sense is that it's firing ahead of one tower and after another. But, in answer to your silly question, I am running a 6cyl distributor cap on my 4.3 v6 (Mercury Marine 815407). I wish it would be something that simple. No worries man.

Speaking of part numbers... good catch Reel Poor. Must've been a typo. I have a V6-14 stamped ignition amplifier module. The factory tag on the valve cover doesn't contain a serial number for the engine, but there are 2 tags with serial numbers for the drive unit (those are probably not helpful in this instance), sorry. Is there another location for the engine serial number?

So here's the plan of attack:
Disconnect Tachometer and Shift Interrupt Switch. Test run. Pull the Distributor and give it another good inspection. Reinstall, set base timing to 8*btdc and test run. Pull the Ignition Harness and Ohm test all the leads. Reinstall and check grounding. Test run. If problem not solved, get gun, shoot ignition module. Try again.

Nah, seriously, if the problem isn't solved after the last test run I'll bust out the digital timing light and record the timing curve at 500 rpm intervals, also the max advance and retard during the misfires at each interval, and post the results.

Later.

 

[achris]

Re: Thunderbolt IV Problems

...But, in answer to your silly question, I am running a 6cyl distributor cap on my 4.3 v6 (Mercury Marine 815407). I wish it would be something that simple. No worries man.....

I asked rotor, not cap.... Cap would be obvious, like 2 spare holes. ....

Chris.......