[chriscorbett]

Hi,

I have been having an ongoing saga with the mechanic I am using to repair my 1997 7.4l Mercruiser.

I'd appreciate any help or advice any one can offer as he doesn't seem to be getting to teh bottom of he issue.

To explain I probably need to list in odrer what has happened so far:

1) Engine was running but spluttered under load and limped back to port. This was in May!
2) Engine ran fine not under load but missed and backfired slightly at high revs whilst in neutral.
3) Engine suddenly stopped startung entirely - tuned over but no spark.
4) Mechanic gets involved and changes 'micro switch' in distributor.
5) Engine then started but blew a hole in the exhansut manifold which was very rusty.
6) Boat had to come out of water and engine out of boat as there was no room to get the manifioled off whilst in teh baot and teh bolt heads were too rusted.
7) New manifolds/risers fitted and boat put back in water. Engine started and ran perfectly in Neutral.
8) Pulled away and after 50 yearsd engine spluttered and stop kind of like it had got choked up.
9) Afer that no spark.
10) Coil changed - engine started.
11) Next it's lost spark at the spark plug again and just turns over.
12) Mechanic changed microswicth again and still teh same problem except he said that ther was a spark at the spark plug for a couple of revolutions and then nothing.
13) He know doesn't seem to knoe what to do!
14) Theer are acouple of other faults that he says aren't related but I'm not so sure. Firtsly the fuel gauge doesn't work and secondly the depth gauge showing the 'FT' sign but no reading. All seems electrical to me but he says the engine and those faults can't be related.

I have a 1997 211 Mariah Talari if that makes any difference or helps anyone suggest what I do next!!

Help ...please.


P.S Alarmingly he also seem sto be suggesting that the oil when it was drained appeared to have a couple of litres of petrol in it but can't explain how it might get there? Am I being had over by thsi guy?

Any help much appreciated.

Chris

[SEMIJim]

Originally Posted by chriscorbett

1) Engine was running but spluttered under load and limped back to port. This was in May!
2) Engine ran fine not under load but missed and backfired slightly at high revs whilst in neutral.
3) Engine suddenly stopped startung entirely - tuned over but no spark.
4) Mechanic gets involved and changes 'micro switch' in distributor.

That "micro switch" isn't a micro-switch at all, but points. Given your description of the symptoms, points would've been my first guess, too.

Originally Posted by chriscorbett

5) Engine then started but blew a hole in the exhansut manifold which was very rusty.
6) Boat had to come out of water and engine out of boat as there was no room to get the manifioled off whilst in teh baot and teh bolt heads were too rusted.

All of which is unrelated to the original problem, or, near as I can tell, the subsequent problems.

Originally Posted by chriscorbett

7) New manifolds/risers fitted and boat put back in water. Engine started and ran perfectly in Neutral.
8) Pulled away and after 50 yearsd engine spluttered and stop kind of like it had got choked up.
9) Afer that no spark.
10) Coil changed - engine started.
11) Next it's lost spark at the spark plug again and just turns over.
12) Mechanic changed microswicth again and still teh same problem except he said that ther was a spark at the spark plug for a couple of revolutions and then nothing.
13) He know doesn't seem to knoe what to do!

Then he's not a very good mechanic. These mechanical ignition systems are really relatively easy to trouble-shoot and repair.

Originally Posted by chriscorbett

14) Theer are acouple of other faults that he says aren't related but I'm not so sure. Firtsly the fuel gauge doesn't work and secondly the depth gauge showing the 'FT' sign but no reading. All seems electrical to me but he says the engine and those faults can't be related.

I would hesitate to say "can't," just because I've learned, over the years, to think to think twice before making pronouncements of an absolute nature , but I would tend to agree.

Originally Posted by chriscorbett

P.S Alarmingly he also seem sto be suggesting that the oil when it was drained appeared to have a couple of litres of petrol in it but can't explain how it might get there?

Unburned gasoline in one-or-more cylinders washing down the cylinder walls and into the crankcase?

Originally Posted by chriscorbett

Am I being had over by thsi guy?

I don't know as you're being "had over," but it sounds like he's not a very competent mechanic. Most of what you describe is pretty basic mechanical ignition system internal combustion engine stuff. Nothing really all that advanced or complicated, really.

Jim

[chriscorbett]

Quote
"Unburned gasoline in one-or-more cylinders washing down the cylinder walls and into the crankcase?"


Is that probably just where it's been turned over an not started or could it be something more serious?

Any ideas on what I should be doing next (short of getting a new mechanic)!!

Regards
Chris

[SEMIJim]

Originally Posted by chriscorbett

Quote
"Unburned gasoline in one-or-more cylinders washing down the cylinder walls and into the crankcase?"

Is that probably just where it's been turned over an not started or could it be something more serious?

If there was enough gasoline in the oil for the mechanic to make note of it, I'd think it'd be more than just from turning it over w/o it firing-up. But, then again, I don't know how much cold cranking was done, how much y'all were working the accelerator pump while doing it, what the mechanic meant by "a couple litres" (two litres seems to me to be a lot of gasoline in the oil), etc. And given that he seems unable to figure out what's wrong with a simple mechanical ignition system, well...

Originally Posted by chriscorbett

Any ideas on what I should be doing next (short of getting a new mechanic)!!

If your mechanic is stumped by the ignition problem, I'd suggest you need a new mechanic. Sorry.

Jim

[Phlorida]

+1 I agree with Jim's advice, find a new mechanic. Putting a 7.4l together from almost any condition isn't that difficult if you have have the necessary competence. Your mechanic is just guessing on your nickel.

[chriscorbett]

Well the mechanic said he'd fixed it, somthing about relocating an earth. Anyway I went down started it up, took it round the marina, all fine.

Went down there today it started up at first and then won't start again, back to just turning over but seems like no spark. Stinks of fuel but that' probably just were it's flooding.

Getting really fed up now as summer is amost over here and I haven't used it once.

[SEMIJim]

Originally Posted by chriscorbett

Well the mechanic said he'd fixed it, somthing about relocating an earth.

Relocating an earth? As in ground? Just how old is this guy, anyway? Heck, I'm about as old as dirt, myself, and even I don't use "earth" to mean "ground." I may use "earth ground" to distinguish a true earth ground from a non-earthed ground, but I'd never refer to the common ground on a boat as "earth."

Originally Posted by chriscorbett

Anyway I went down started it up, took it round the marina, all fine.

Went down there today it started up at first and then won't start again, back to just turning over but seems like no spark.

Hmph. Intermittent problem. Those are the hardest to trouble-shoot. But it sounds like it's out more than not, so the real cause of the problem should be traceable.

Originally Posted by chriscorbett

Stinks of fuel but that' probably just were it's flooding.

Yeah. I'm hoping you're running the blower full-time when all this is happening.

Originally Posted by chriscorbett

Getting really fed up now as summer is amost over here and I haven't used it once.

Then find a better mechanic.

Or...

Learn about how to do some of this for yourself. It's not magic. There's nothing mysterious about it. It just takes knowledge. (Tho experience helps .)

Here's how simply a standard "points & condenser" ignition system works:

• A rotating lobe on the distributor shaft opens and closes a set of points
• When the points close, 12VDC is supplied to the "primary" of an ignition coil, building a magnetic field in it
• As the points "fall off" the lobe, they open, the 12VDC is removed from the coil's primary, and the magnetic field begins to collapse rapidly
• As the magnetic field in the coil's primary collapses, a high voltage is induced in the coil's secondary.
• Depending upon which cylinder is to be fired, a rotating "switch," formed by the distributor cap and rotor, distributes that high voltage, called "spark," to the appropriate spark plug.

That's all there is to it!

Now, in order for all this to work correctly, there are a few things that have to be right. The points have to open and close at the right time and for the right amount of time. (These are called "timing" and "dwell," respectively.) The rotor in the distributor has to be pointing to the right plug when the points open. The condenser, which is there to save the points from arcing and destroying themselves when they open, has to be of the correct value and in good condition. Obviously the coil has to be working. (They do fail.)

But, all-in-all, it's a relatively simple system. Hell, I built-up a "spark-gap" transmitter out of old TV yolk windings, a 6VDC lantern battery, a manual (Morse code) key and some copper wire (for an antenna), that operated on a not dissimilar principle at the age of about 12 or 13 years old.

Jim

[chriscorbett]

Thanks for the help.

I understand the principles involved in general terms but it gets more complicated when other factors come into play like:

a) The engine has electronic points in the distributor (Thunderbolt). They've been changed although not with new, presumably as it does start sometimes then these are working (as wrer the ones that were originally in there)
b) I have no idea what solenoids and salve solenoids do nor whether they could be part of my problem here.
c) Grounding seems to play a major factor here but I don't know wher the ground points are or how many wires go to ground from the engine to check.
d) Might I be better just replacing all the spark plug leads, distributor cap, rotor arm and anything else?
e) When it runs it runs fine so it must be something simple.

Regards

[SEMIJim]

Originally Posted by chriscorbett

a) The engine has electronic points in the distributor (Thunderbolt). They've been changed although not with new, presumably as it does start sometimes then these are working (as wrer the ones that were originally in there)

That changes everything. You said he replaced a "micro switch" in the distributor. There's no switch. From Mercruiser Thunderbolt Ignition Tune-Up Kits at Sterndrives.com:

The Thunderbolt system does not use points. Instead, it has an electronic "Sensor" in the distributor which sends a low voltage pulse to the Ignition Module each time a Sensor Wheel tab passes through the Sensors gap.

The Sensor Wheel is attached to the Rotor. When you buy a rotor, you get the sensor wheel. They should always be replaced together.

I'd start out be replacing the parts suggested on that page.

Originally Posted by chriscorbett

b) I have no idea what solenoids and salve solenoids do nor whether they could be part of my problem here.

If the engine's turning over: No.

Originally Posted by chriscorbett

c) Grounding seems to play a major factor here but I don't know wher the ground points are or how many wires go to ground from the engine to check.

On a boat, the engine block is "ground," along with everything else connected to the negative battery terminal. If the engine's turning over reliably, the "ground connection" is not your problem. (It's a big, thick black cable that goes from the engine block, directly to the battery's negative terminal.)

Originally Posted by chriscorbett

d) Might I be better just replacing all the spark plug leads, distributor cap, rotor arm and anything else?

I would just start with the kit noted on the page I referenced above.

You have what sounds like an intermittent ignition problem. Sounds like electronics, to me.

Originally Posted by chriscorbett

e) When it runs it runs fine so it must be something simple.

Bad assumption

Jim

[chriscorbett]

Hi,

Just tried ringing the mechanic for the 5th time today and as he hasn't rung me back or been down to the boat I'm now going to start afresh with a new mechanic OR try and fix myself.

I've already identified something that may not be helping but I'm not sure it would cause the problems I've been having.

Here we go...it's got the wrong spark plugs in it! My research shows a 1996-1997 454 should have NGK BR6FS but mine has got BPR6FS.

Now it's been running for the last couplel of years on those plugs so I don't think that is my problem but perhaps someon could enlighten me on whether this would cause the engine too not start all of a sudden.

Thanks