1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Good Afternoon,

I am seeking information as per the below questions for the engines installed in a 1990 Wellcraft Gran Sport. The engines (and the boat in general) run great and I am experiencing no problems what-so-ever. However, I am really huge on preventative maintenance which is consistent with many of your recommendations.

Since purchasing the boat, I put about 10 hours on it and would like to change the impellers, give the engines a tune up, change the oil, and generally prep the boat for spring/summer use. I realize I am asking for a ton of information but I would be very grateful for any and all you are willing to share. I'm very mechanically inclined and a fairly good "motor head" but new to "marine" engines so am a bit confused by some of the information obtained thus far.

For example:

* The engine spark arrestors over the air cleaners say "Mercruiser 7.4L Blue Water Inboard Thunderbolt IV Ignition".
Photo attached

Q1. Would you share any information you may know about these engines, what the Thunderbolt IV ignition system is, and how I find the parts and tune-up specifications (plug gap, points gap, dwell meter readings, and timing info) needed?

* The engine serial numbers (per survey documentation - I have not physically looked) are 624786 and 624933. This is confusing as I look at various sites for Mercruiser engine serial number ranges and see serial numbers with "OB" and "OD" and then the digits listed. I don't see the serial number ranges applying to my engines.

Q2. Is the survey info incorrect or by chance are there alphabetical letters before the numbers that are not specified in the survey report?

* Based on what I have been reading, changing out the impellers fairly often is recommended. I'm grateful for this advice and would like to change out the impellers for the raw water cooling system and the fresh water system as well.

Q3. Is my research indicating that the raw water pumps are located at the left side of the front of the engine, down toward the bottom of the engine correct (when viewed facing the front of the engines)?

Q4. Do you recommend changing both the raw water and fresh water system impellers since I have no idea when these were last serviced or changed?

* I saw two different videos (both were well done) which showed changing the impellers. One was for the fresh water system (on the front of the engine requiring removal of the belts, pulley assembly, etc) and the other video showed changing the impeller in the raw water housing.

* Again, any and all information would be most welcome or if you wish to direct me to web sites for my own research, this would be greatly appreciated.

VB

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Do you have a straight inboard or do you have an outdrive?

Are there any numbers on the decal on the flame arrestor cover? There is usually engine and drive/transmission numbers there.

If not, look on the metal tag just above the starter. It will have the serial number of the engine. (a camera will help since you can't get in there to look)

No points, the distributor is all electronic.

Raw water pump should be on the starboard side of the engine, belt driven, down low. That is the only impeller you have to replace.

This should be the manual for your engine, it covers all 89 to 92 engines. The serial number will verify.
http://boatinfo.no/lib/mercruiser/manuals/mercruiser15.html#/0

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Hi Don,
I have an inboard...or I should say...a v-drive system. Engines into transmissions going forward then direction reversal to aft coming out of the transmission to 1 inch prop shafts coming out of the hull to the props under the boat (with rudders, etc). From the literature that came with the boat; I believe it has a Borg-Warner transmission system (?).
I will check the flame arrestor...come to think of it...I do believe I saw some sort of tag there.
OK, will have camera in hand next time I am down there in the engine area hanging upside down and trying to read numbers...camera will be very helpful!
No points in the distributor...great information again! If don't mind another question; would a tune up simply involve changing spark plugs, perhaps plug wires only?
Glad to hear that belt driven pump on the starboard side of the engines are the impellers to be replaced. I saw a video showing five bolts holding the housing on but then in some schematics I've viewed, I see only a bolt or two. I'll look at the site you provided for additional information on this point as well.
I will check the web site you provided and see if I can find additional information overall!
Thank you VERY much for your prompt and informative reply!
This is a GREAT website/forum and I've learned a lot just reading over questions and answers....thanks to you and others!
I try to support iBoats by purchasing from recommended dealers, etc as I want to promote and perpetuate this information flow.
Thanks again...
VB

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

That manual I linked you to is the OEM service manual, and should have everything you need to know about replacing the impeller, tuneups etc. Everything.
Tuneups usually include spark plugs, cap and rotor, plug wires if they are worn,cracked, or corroded on the engine. Along with a timing check, compression check, and carb adjustment.
I have always found it best to remove the pump and bracket as an assembly, then replace the impeller, gaskets, and any other worn out parts.

You can also look in the manual for other maintenance info. There is a schedule in there.

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Since purchasing the boat, I put about 10 hours on it and would like to change the impellers, give the engines a tune up, change the oil, and generally prep the boat for spring/summer use. I realize I am asking for a ton of information but I would be very grateful for any and all you are willing to share. I'm very mechanically inclined and a fairly good "motor head" but new to "marine" engines so am a bit confused by some of the information obtained thus far.

Click to expand...

Ayuh,.... Go up 'n do some readin' in the Aduts Only section at the top of the forum,...

Lotsa learnin' to be had in there....

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Will do Sir...thanks for the direction to that area and I see the specifics on how to ask a question and such...I'll follow that format and other recommendations in the future...thank you! VB

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

VB,

The serial numbers you posted are short of the alpha characters at the beginning, but through elimination I have determined one of them to be 0D624933, a 1992 7.4l MIE carb engine. (MIE = Mercruiser Inboard Engine). The other number doesn't match anything correct, maybe you need to re-check it. (It shows up as a 150hp Hino diesel)...

The fresh water (engine circulating) pump should last many many years and requires no 'routine maintenance' apart from just visually checking the weep hole for a leak. My past experience with the Thunderbolt ignition system is that once the timing is set (unless the distributor is moved for some reason) it will stay set for many years. My previous V6 engine has TB-IV, (now in a friends boat) and I check the timing every few years, it hasn't moved in 18 years. The plugs get changed about every 200 hours, the leads were changed from factory to silicone about 15 years ago and are still good. The cap and rotor are now about 10 years old and still doing fine. About the only real thing of significance that's been changed in the last 10 years are the belts on the front of the engine and the ignition coil (it got a corrosion hole in the bottom, the oil leaked out, it overheated and caused severe mis-firing)... And a carbie clean out about 7 years ago.

Good luck.

Chris.........

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

A word to the OP I-boats has some of the best marine mech's in the business,,now with that said they like to see maintance done at a schedule that would rival aviaton level's. Car tech is very similar but marine tech is far more fragile..in term's of failure rate. So if you stand back a bit that is normal. One thing to note a marine failure is very expensive in realtion to automotive failure...hence all the caution.

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Hi Chris,
Thank you for your very informative posting. I'm not sure how you did the "magic" with the serial numbers but I appreciate your finding the information for one of the engines. I intend to head down to the marina and check those numbers! I don't quite understand the purpose of the "OD" in front of the numbers and why some engines have different letter codes but again, I'll check the engine numbers as soon as I can. I've learned a lot from you about the Thunderbolt Ignition system and it is, in-deed, a pleasure to hear it is such a trouble free system! Again Chris, thanks VERY much for taking the time to post here in the forum and I hope I can repay the favor. By the way, I have visited your Country and it is truly beautiful! I didn't get a chance to visit Perth as I was in Sydney, Brisbane and then I went to Port Douglas for a week of relaxation. It was a great trip and everyone I met was fantastic. Thanks again...and Happy New Year...you're already well into the new year while its only 10:00 PM here in Texas.
VB

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Yes, I really enjoy the advice provided here. I know exactly what you are saying about aviation maintenance (A & P's aren't pay for what they know, they are paid for what they prevent from happening!) and it would be nice to see the bar raised on marine maintenance. I find preventative maintenance to be very important and have learned many lessons from the aviation world in that regard. I've noticed that any part stamped with "FAA" or "USCG" approval has a substantial price increase, just due to the reliability of the component! The same applies to the labor costs if one chooses to pay to have work done in the marine or aviation industry. VERY expensive. Thanks for sharing your comments here in the postings and I appreciate your words of advice and caution! VB

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

VerBoatin said:

.. I'm not sure how you did the "magic" with the serial numbers but I appreciate your finding the information for one of the engines. ...

Click to expand...

Not much 'magic' involved... I noticed .... First a bit of history on Merc engine numbers.

It all started about 1963. Merc realised they needed a serial number system that was 'future proof', and came up with a number involving 7 digits. Sounds simple and reliable enough, especially when you consider that, at the time, they were churning out only about 100,000 units a year (that includes outboards as well). The numbering system should have lasted about 100 years! But then the demand increased and serial numbers were being used up at a far greater rate than anticipated, and they reached 6999999 around 1984 (higher numbers were being used in Australia and Belgium). The next step was to change the first number to a number and a letter, then increment the letter every 100000 numbers. The 0Axxxxxx were done with by the end of 1986, and they moved on to 0Bxxxxxx.... and so on... (we are currently in 1Dxxxx type numbers, I think)

So, back to my 'magic'. I noticed you posted 6 digit numbers, and that you mentioned that the boat was 1990. From that it was easy to deduce that the numbers should begin with 0C or 0D... I put those into the parts look-up program (I used to be a dealer) and saw what engine it came back with... The 0D came up with model 37431B4ES. Then just cross-referenced that with the years and came up with a 1992 7.4l MIE ... Easy. I also tried the other number with all combinations but kept get a Hino diesel... Hence my belief it might be incorrect .

Chris.......

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Hi Chris,
Wow...that is amazing...are you sure you don't work for the Australian Secret Intelligence Service? My goodness, that is some investigative research and information you derived from some a mere six digits! VERY impressive bit of research! So, the "OA, OB," etc, etc was simply a way to gain more numbers without adding additional numbers....? Being the "novice" I am, I thought perhaps the "O" meant "Outboard" or something of that nature and here I was trying to figure out how I find a serial number with an IB. Then you provided the information on MIE (Mercruiser Inboard Engine) and the various information in the Mercruiser on-line manuals (to which I was graciously referred by Don S) and other on-line sources is making some sense to me. When you provided "model 37431B4ES"; would this be the engine model number? I create files on these sorts of things so I can refer back to it later. Dang Chris, if you could bottle what you have forgotten about all this, it would be worth millions! I cannot thank you enough! Dan (aka: VB)

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

I kind of have to disagree with Chris that it's a 92. Since we are guessing what the letters are in front of the numbers, both your above numbers are for a 7.4 from 89 when using a C. If I try a D I also get Hino diesels.

So he gets 92 and I get 89. You still need to get the real numbers off the engine to find out for sure.

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

One other note. you posted two numbers, 624786 and 624933, both are for 7.4L carbed inboards from 89. The difference is the 786 number is for a Borg Warner transmission and the 933 number is for a Hurth transmission.
The Borg Warner also makes the V-drive. So why both numbers on the survey? That's the question at this point and why we need the actual serial number of your engine, and not just the survey. Who knows where he got the numbers.
The other thing, is the 933 is a RH rotation engine. Does this boat have twins by some chance? The picture looks like a single engine in a compartment.

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Don S said:

... Does this boat have twins by some chance? The picture looks like a single engine in a compartment.

Click to expand...

In the original post he referred to engines, a couple of times. That's why I assumed both numbers were meant to be engine numbers. The reason I suggest that they are 0D is also in the first post, where 0B and 0D are also mentioned. 0C is possible, let's see what the full number is when Dan gets back to us....

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Hi Don and Chris,

Yes, it is a twin engine inboard (sorry if I didn't make that clearer in my original post). I showed only one engine in the photograph just to show the air cleaner information when I was referring to the ignition system and such.

Along those lines, with twin 7.4L's installed, does one engine turn the opposite direction of the other in an attempt to eliminate the "Prop" factor (wanting to swing the boat to starboard as I understand it and torque, etc)? Or, do both engines rotate the same direction and the transmission reverses one of the props? I'm very confused on this as I've read all sorts of postings and articles that discuss this sort of thing. Or, perhaps I'm just "dazed and confused"....

The boat is a 1990 so it would be more consistent that it would have something like 1989 engines, yes? I'm not absolutely sure the boat has Borg-Warner transmissions but I recall seeing some documentation indicating such in the publications I have on the boat itself.

I'm going down tomorrow to get the numbers and frankly, I will admit, I was very disappointed in the survey for a variety of reasons I won't get in to here. I didn't check the serial numbers and as you indicated, no telling where those numbers originated. I provided the marine surveyor some documents from previous service on the vessel and the technician may have written down the number(s) wrong and the surveyor simply copied them.

Perhaps you could verify that I should find serial numbers with two letters (example: "OC" then XXXXXX numbers...yes)?

I appreciate your assistance in all of this and I'll provide you some accurate and reliable data on which to base further analysis. I still find it absolutely incredible you were both able to obtain the information you did with only the engine serial numbers (even perhaps inaccurate ones). Again, thank you much...

I'll get in tomorrow and get back to you...

Dan (Verboatin)...

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Instead of doing a bunch of "what if" questions, get the numbers and let's see what you really have.
Counter rotating engines were common back in that era for twins, and yes it was to counter act prop torque.

I still find it absolutely incredible you were both able to obtain the information you did with only the engine serial numbers (even perhaps inaccurate ones). Again, thank you much...

Click to expand...

We both have access to dealer programs. Actually, Merc has a site that does about the same thing, if you understand the system and know to stick a letter in front of your numbers.
https://webapps.brunswick.com/literature/literatureSearch.do

Just use your serial numbers and add the C in front and see what you get Try the D and a B as well.

Re: 1990 7.4L BlueWaterInboard (454 CID) with Thunderbolt IV Ignition

Hi Don and Chris,

Forgive the delay in getting back to you...work is interfering with my life here dang-nabit!

Well, after crawling all over the engines in cramped quarters, I verified that the information is, indeed, on the air cleaner cover and that information is correct.

The engines are (as you both expertly attested):

The starboard engine information states:
Model # MIE-7.4L-OPP
Left Hand Rotation (LH)
Serial Number: OC 624786.

The port engine information states:
Model # MIE-7.4L-OPP
Right Hand Rotation (RH)
Serial Number: OC 624933.

I pulled the sea water pumps and removed the impellers. They both need replacement. (I also pulled the impeller out of the 6.5 Kw Westerbeke Generator and it too needs replacement.)

Back to the engines, may I pose additional question(s):

I don't know why there is information published regarding a "Hurth" transmission. Perhaps they installed one in some models, I dunno.

I saw both transmission data plates and they specify "VD" (Velvet Drive) and I believe these are made by Borg-Warner, yes? (Please let me know if I am in error on this point?)

In searching for the impellers, I "thought" (a dangerous thing!) there might be two different part numbers.

This was not the case.

For the Starboard Engine:
Through Performance Parts Technologies (PPT) site:
Part #: 47-59362T 1

For the Port Engine:
Through Performance Parts Technologies (PPT) site:
Part #: 47-59362T 1

My research and I believe your previous advice was also to replace the "Quad Ring" (circular "O" Ring) as well, yes?

The housing, gasket water wear plate, and the other components looked pretty good.

Need I also order the Impeller "Plug" to go with the impellers? I did not see one when I removed the old impellers so I'm thinking this is not needed?

Any other parts I should replace while I have it dismantled?

Lastly, I have read one should use soapy water solution to lubricate the impellers, others say grease while some say that deteriorates the rubber (any petroleum product does), and I've also read a light spray of a non-petroleum product such as a silicone spray is recommended. Would you mind rendering your expertise once again on this point?

I also would appreciate your advice on how one might prevent running the impellers "dry"? Is there a way to "prime" them with water before starting the engines and waiting for water to be sucked into the system? The Westerbeke Generator specifically states "Do Not Run Dry" but I'm unsure if the starting process is included or if they can use the lubrication you recommend until the water starts flowing through the system. Again, any advice more than welcome!

Again Gents, I appreciate the assistance!

Dan (VB)...
P.S. Included a photograph of the Port Engine air cleaner tag. The Port and Starboard Engine tags are, of course, identical except for the serial number and the rotation of the engine...