Head-scratcher Ignition problem on 4.3L... Help please?

[erikth]

Hi guys.. newbie here. I picked up a 170 BR last summer and have been enjoying it greatly. Ran like a dream for the first season. Did the winterization myself with help from youtube. Starting from spring fire-up I've had nothing but problems.. At first it was hydrolocked which was puzzling because I flushed everything with RV antifreeze, and then drained the whole cooling system for the winter. Cylinders full of water (both sides), oil looked like chocolate milk, etc. Everyone I talked to said cracked block but I was optimistic it would be salvageable based on my flush and drain. Long story short I did a top end rebuild (intake and exhaust - did not do head gaskets) and a couple oil changes later I have beautiful oil and it ran.. I was thrilled.

However, the first time I put it in the water I had issues going beyond 1800 RPM. Would lose power and backfire. Re-checked timing, tried to tune the mix screw on the carb, but no improvement. I rebuilt the carb and it briefly ran better (got it to 3000 RPM once at no load). Since then the performance has steadily degraded. I first suspected fuel supply, so changed all fuel filters, flushed the fuel tank and filled with fresh gas, and even put a primer bulb in the fuel line so I could give it a bit of extra pressure with a squeeze.. nothing helped.

After ruling out fuel supply I started looking at ignition. I put in new cap, rotor, and plug wires and checked with a timing light to make sure the Thunderbolt IV module was advancing the timing. It was. I noticed that when using the timing light the flashes were not consistent. At first I thought that it was the timing light because I borrowed it from my neighbour and it was made in the 70s. (It was a non-inductive one where you clip the light on a spring that you insert right on the spark plug.. old school!) After giving myself a shock with this (literally) and being annoyed with the missing flashes I decided to get a new inductive timing light.

This is where I start narrowing in on the problem... Even with the new timing light, I would clearly see breaks in the sequence of flashes - this was plainly visible by pointing the light at the white fiberglass hull. It seemed like it was missing 3 or 4 out of every 10 flashes. I re-checked and it was the same with both the oldschool and inductive timing lights. This corresponded to the sound of the misfiring engine and so I am convinced this is the root cause of the problem. I started replacing ignition components. First the ignition sensor inside the distributor, then the Thunderbolt IV module itself, and finally the coil. None of these made any difference. Worse.. as I was doing all this, the performace was slowly deteriorating.. as measured in the idle getting rougher and rougher and missing more and more flashes as seen on the timing light. At present it's missing more than half the flashes and still barely manages to run.. but very rough... shakes the boat.

I have cleaned all the connections, tested the ignition and grounds and tested all the wires in the harness that clips to the Thunderbolt IV module. Everything checks out. The missing sparks can be seen on all cylinders, not just cylinder 1. I removed the grey wire that goes to the tach and the white/green wire that goes to the shift interrupt switch to rule those out.

I've been searching through forums on thunderbolt ignition troubleshooting, but all the problems seem to relate to no spark at all. My problem is that I have sparks sometimes, but they are not consistent.

I thought I would throw this out and see if anyone has had similar issues. I've reached the point where I've run out of ideas to try. I've replaced every component there is in the igntion system and the problem persists, and is even getting worse as time goes on.

Any ideas on what would cause inconsistent spark firing? I'm open to any musings.. thanks guys!

 

[erikth]

One obvious thing I forget to mention.. I've also changed the plugs twice.. once at spring commissioning (Champion RV8C), and then again last week when I did the wires/cap/rotor (NGK BFR6S). Gap was set to spec of 0.035

 

[Espos4]

Is there any side to side play in the distributor shaft?

If it wobbles, that will give you an inconsistent gap at the magnetic pick up. If this is a point type distributor, you would have erratic point gaps as the distributor spins.

Also (you’ll be hearing this until you do it) you need to do a compression test and post the results. You thinks it’s an ignition problem, but this is a good test and indication of any other potential issues.

 

[BillK2632]

1. Are you sure you got the 6cyl rotor? It should have 5 "teeth" on it. Maybe try the old rotor, I got a bad one from Sierra once - I could not see anything wrong with it, but putting the old back in things were fine.
2. Spark plug wires in the right order? Sounds simple, but we have all screwed that one up.
3. You said you have a Thunderbolt IV which should not use base timing mode - but the differences between Thunderbolt IV and V can be confusing. Make sure you don't have a purple/white wire hanging loose from the wiring harness. If you do that is how the engine is put in base timing mode to set timing - ground it to set timing, other wise it should be disconnected - not grounded.
4. When you installed the new ignition module, did you run a ground wire for it? The old module is open, and is grounded by the screws holding it in, the new one is sealed and there is a separate ground wire. When I replaced mine I ran the ground wire to one of the mounting bolts on the ignition module mounted to the riser.

like was mentioned, you had some issues with hydrolock and did some work on the motor, do a compression check to establish a baseline.

 

[erikth]

Thanks for the ideas guys! Here is some further info on all the points mentioned. (in order). I'm busy today but I will be back at the boat tommorow to check on all the items that need more investigation.

Espos4
1) I did not noticed any sideways play in the shaft while I was adjusting timing (which was quite a bit), but I will try to wiggle it and specifically feel for this tomorrow. From my understanding this is NOT a point type distributor - there is no switching mechanism or cam inside.. only the rotor and an electronic sensor.
2) I did a compression test early on shortly after clearing the hydrolock. Results: 1=194, 2=117, 3=202, 4=178, 5=215, 6=208. Cylinder 2 was clearly bad. I was told that it would probably be OK once I got the water sorted out and oil cleaned up, but I have not rechecked it. I will do this tomorrow and post current results!

BillK2632
1) Unfortunately the old rotor was destroyed on removal (it was really stuck in there) so I can't try putting that back. However, I can visually inspect it and count the teeth on old vs. new. I did not notice any difference after changing the rotor so I think it's probably the same but will re-check.
2) I have rechecked the wire order countless times. Will do once more for fun.
3) The wiring harness has grey and purple to the coil, white/green and white/red to the sensor in distributor, and black to ground. No additional wires. I'm re-using the same harness for both the old and new ignition modules. The module has an extra pin and the harness has an extra hole, but there is no wire in the harness. Both the old and new modules say Thunderbolt IV on them.
4) The existing harness had the ground wire that was grounded to the bolt connecting it to the exhaust riser, and then a wire from there that goes to a ground rail. I have verified this has 0 ohm resistance to the negative battery terminal so it should be a good ground.
5) As mentioned above I will update with the current compression values once I re-check them tomorrow!

Thanks for the ideas everyone.. I'll get the additional details required tomorrow and post back. At this point I still strongly suspect ignition but I have ruled out pretty much everything I can think of there, and I'm certainly open to the fact that it could be something much worse in the engine. I've priced out a new base engine from MichiganMotorz at $3000 and I'm ready to pull the trigger on that if I can be sure it would fix things. But I certainly don't want to do that and have the same problem after...

 

[Ferris]

Wondering if you solved this...
If not I'd suggest looking at the hall effect pick up sensor in the distributor housing.

 

[erikth]

Hi All.. sorry for delay in getting back to this.. the Fiance gave me a speech about how much time I was spending "fussing" with the boat....

I have taken it out a couple times in the last week and just putted around at idle (just far enough into the lake to open the cooler and crank the stereo) doesn't run smooth but it clunks along slowly.

Here are some updates:
- I rechecked the compression and it looks very good. {1:155, 2:153, 3:146, 4:152, 5:147, 6:155}. Test was done at about 80 degrees ambient temperature but not with the engine hot.
- I checked the old rotor and it was identical to the new one. Both have 6 teeth which makes sense for a V6 .
- I changed the spark plugs for a third time.. this time went with AC Delco MR43T as I have heard they are the best for Mercruiser. Gapped to 0.035 as per the thunderbolt IV manual spec .No improvement.
- Rechecked the spark plug wires one more time to make sure they were connected properly to the right terminals on the cap. (I am using the big numbers as a guide - I understand the small numbers are for reverse rotation and I should be using the big ones)

Note to Ferris - I have already replaced the sensor inside the distributor. It's brand new (along with the coil, wires, and ignition module/amplifier)

Unfortunately the situation is still the same. Missing sparks as can be seen with the timing light and can only run at idle. If I give it any throttle at all it just starts backfiring. Also, I can see unburned fuel in the exhaust and the spark plugs are black when I take them out, so I am pretty sure I still have an ignition problem.

Any further ideas would be much appreciated... Thanks in advance.

 

[BillK2632]

I think I would start with the spark plug wires and MAKE SURE they are connecting in the right order. Don't rely on the numbers on the distributor, do this:

• Bump the key until you get the timing mark on the flywheel at -0- or very close to it. This puts the #1 Cylinder at Top Dead Center (TDC).
• Pop the distributor cap and see which spark plug wire the rotor is pointing to - make sure it is connected to cylinder #1. The one that is front port side.
• Then going around the distributor clockwise, connect the wires in this order 1-6-5-4-3-2
• Everything you describe points to the spark plug wires being out of order.

Like this diagram: https://www.google.com/search?q=mer...AhVll1QKHXGZAtwQ9QEILDAA#imgrc=6t14j6oMMWsINM:

 

[Ferris]

I just noticed you mentioned the pick up in the distributor. Given everything you replaced and you see gaps in the timing light. I'd start by checking all the connections, connectors and wires.

 

[erikth]

Hi Bill -

Thanks for the idea. I took the cap off and used a ratchet to crank the engine so the rotor moves all the way along. I noticed the timing mark lines up on the flywheel at 2 different points in the rotor cycle which are 180 degrees opposite. The wires were correctly arranged 165432 clockwise based on one of the points. I decided to try the other one and so I rotated the cap 180 degrees and then connected the wires clockwise 165432 from the other side. When I did this, the engine wouldn't start. So, I guess I will go back to 165432 clockwise from the original position.

Any other wiring configuration I could try?

 

[erikth]

Ferris - I have already checked all cinnections and cleaned up all contact points. I'm going to try replacing some of the wires and see if that helps. It's possible the wire from the sensor to the ignition module or ignition module to coil is corroded

 

[BillK2632]

No sounds like it is right - the time it was 180 deg opposite on the distributor was the exhaust stroke, each cylinder should only fire on the compression stroke.
Have you replaced the ignition sensor like Ferris mentioned? That a common problem also. The old style were open with soldered wire and tended to cause problems - new ones are sealed. This is the equivalent of the points in an old style system.
Yours is probably the old style that is open, mounts inside the distributed, the "teeth" on the rotor run through it and that is what distributes the "spark" to each cylinder. I replaced my old one a few years ago.

New one Looks like this: https://www.google.com/search?q=mer...AUICygC&biw=1600&bih=769#imgrc=XPvgTs7ld_QEpM:

 

[erikth]

Yep, I have replaced the ignition sensor (inside the distributor). The old one was rusty and I was really hopeful that was the problem. The new one is sealed as you mention and looks like a much better design. However it did not improve things at all .

In summary, I have replaced the coil, ignition sensor, ignition module, ignition wires (coil to distributor and distributor to plugs) and plugs.

I have ordered some 2.8mm female Spade terminals and I will custom make a replacement for the ignition module wiring harness and try that when they arrive. Seems a long shot but I'm totally out of ideas at this point.

Any other thoughts are always welcome no matter how obscure.

 

[BillK2632]

Did you ground the new ignition sensor? The old one grounds by the screws, but the new one you must run the ground wire to a good ground, most people ground it to one of the ignition module bolts on the risers.

Replacing wires is not a bad idea, could very well be a problem. I was not having problems when I replaced mine a few years ago, but the original wires were starting to crack and there was corrosion on some of the connections. Technically you should do your best not to allow any of the wires to cross over each other as best you can.

 

[erikth]

Yep.. I grounded it to a ground rail that is directly connected to the main engine ground. I cleaned all the connections at the time.

EDIT: Slight correction - The sensor is grounded directly to the engine block ground (as per the instructions). The ignition module is grounded to a ground rail that is connected back to the same engine block ground.

 

[erikth]

Here is a video of the boat in the water, taken about a month ago. At this point it was idling smooth and we got it to about 1500RPM before the backfiring started.

The current status is worse. At present idle is rough and it starts backfiring with any throttle at all.

 

[Bt Doctur]

Cylinders full of water (both sides), oil looked like chocolate milk, etc. Everyone I talked to said cracked block but I was optimist

You have bad valves, valve seats, or sticking valves giving you the backfire thru the carb

 

[erikth]

You have bad valves, valve seats, or sticking valves giving you the backfire thru the carb

I have not taken off the valve covers and inspected the valves since the hydrolock. Good idea to check this. Will do that next. Just curious.. would bad valves explain the missing pulses seen with the timing light?

 

[BillK2632]

Starting to think you might have bigger issues - have you done a compression test?

 

[fc3]

I have not taken off the valve covers and inspected the valves since the hydrolock. Good idea to check this. Will do that next. Just curious.. would bad valves explain the missing pulses seen with the timing light?

I do not understand how you performed a "top end rebuild" without changing the head gaskets! I really don't understand that you did a top end rebuild without removing valve covers. What do you consider a "top end rebuild?" If your engine didn't look like this, you didn't do a top end rebuild.




Right off, I'd pull the heads, inspect and replace the head gaskets. Could be leaking from the cooling jacket to the cylinders. What does the engine oil look like?

Other than that, assuming the head is okay, it sounds like the timing chain slipped a couple of teeth. Stretching a chain because of contaminated oil is very easy. There is a procedure for checking the valve timing. I don't remember the procedure for GM engines, and my old service manual doesn't reference V6s and the new one only covers the big blocks.

The process I recall is something like, turn the engine until cylinder #6's intake is opening and exhaust valve is closing. Insert a spacer between the #1 cylinder's intake valve stem and rocker arm and let the lifter bleed down until it's in effect a solid lifter. Then turn the engine until #1's intake valve shows that it's open a certain amount as shown on a dial indicator. The engine timing mark should be between X and Y degrees. Other than that, you have to pull the water pump and front timing cover to align the timing marks, which should be easy enough to do except if your engine has that stupid GM design that requires the oil pan to be dropped to remove the timing case cover. It's possible to just drop the front of the oil pan a little by removing the oil pan bolts in front and loosening the ones in back.