Intermittent Spark on #6 Cylinder

mbrown · 07-05-2007, 08:54 AM

Guys...

I have a problem that I can't seem to get to the bottom of. I have just rebuilt a 1980 911 SC 3.0 engine and turned it into a 3.3L turbo with 3.3L turbo pistons, cylinders, rods and crank. I have also removed the CIS and ignition and replaced with fuel rails/injectors/3.3L inlet manifold and electronic ignition (Ford EDIS 3-coil system), with a Megasquirt MS-II V3.0 computer.

My problem is this: I am getting an intermittent spark at cylinder #6. I know this because I connected a timing light to the #6 lead and got an erratic strobe light flashing. The other spark plug leads flash consistently.

So far I have:
1. Replaced the #6 spark plug-no change
2. Replaced the #6 lead-no change
3. Swapped #3 & #6 leads on the coil pack-no change
4. Replaced the #6 fuel injector-no change

I have found that if I connect the #6 spark plug lead to a spare spark plug and ground it to the engine while it is running, I get a consistent spark, but when I connect the #6 lead back onto the #6 spark plug in the engine, the intermittent spark appears again.

Next, I'm going to replace both #3 & #6 spark plugs and replace the ignition coil.

A compression test revealed:
#1: 130 psi
#2: 130 psi
#3: 135 psi
#4: 135 psi
#5: 130 psi
#6: 130 psi

Is this a little low? Compression ratio is 7:1.

I'm running out of options here...any thoughts?

Regards

Mike

911pcars · 07-05-2007, 10:55 AM

How does the engine run? Does it misfire? Have you run a cylinder load test (does each cylinder contribute)?

Compression numbers seem fine in light of the turbo CR.

Sherwood

mbrown · 07-05-2007, 11:51 AM

The engine runs, but misfires. I can hear an intermittent popping in the exhaust system.

How do I run a cylinder load test?

Regards

Mike

cstreit · cstreit's Garage

So if I read correctly, the whole #6 ignition assembly runs fine. Have you looked at the temp of the exhaust?

if you've determined that #6 is sparking correctly then I'd start looking for something strange with the injector driver.

911pcars · 07-05-2007, 12:30 PM

Quote:

Originally posted by mbrown
The engine runs, but misfires. I can hear an intermittent popping in the exhaust system.

How do I run a cylinder load test?

Regards
Mike

Mike,
I should have named it a "cylinder balance test".

Open each spark plug path in turn and observe if the engine speed drops equally. If all equal, each cylinder is contributing. The cylinder with no rpm change is the weak one. Probably confirms it's no. 6.

Can perform a similar test by momentarily opening the signal path to each injector. That would confirm Chris' suggestion.

Sherwood

mbrown · 07-05-2007, 01:01 PM

G'day Chris...

The whole ignition system for #6 seems to run correctly, if the #6 spark plug in not inside the cylinder head, but just grounded to the side of the engine (as seen by consistent timing light flashing).

Once I connect the #6 spark plug lead to the spark plug in #6 cylinder head, I get an intermittent spark.

I haven't looked at the exhaust temp. Do you mean the #6 exhaust temp. only? What should I be looking for? A difference compared to the other cylinders?

The fuel injectors are connected in two banks of three (#1-#3 & #4-#6). So a problem with one would be seen as a problem with three of them. I assume I need an oscilloscope to check the waveform?

Sherwood...

I'll try the 'cylinder balance test' both with removing spark plug leads and with fuel injector connectors.

Thanks for your help gentlemen...keep it coming!

Regards

Mike

sww914 · 07-05-2007, 02:48 PM

Did you put anti-seize on the spark plug threads?
Sometimes it will interrupt the ground circuit.

cstreit · cstreit's Garage

I see, so you're sure then that you have a weak or intermittent spark to #6 then... A few random thoughts...

1. A bad ground (while it seems unlikely) is possible I suppose...
2. A weak coil would maifest itself in this way. it's harder for a spark-plug to work in a compressed cylinder vs. open air.
3. You say you're running EDIS right? Does that use a wasted spark? How's the other cylinder running.?

mbrown · 07-06-2007, 05:31 AM

G'day sww914...

No anti-seize on the spark plugs.

Chris...

1. Shouldn't be a bad ground...everything is grounded to the same place...everything.
2. I'll replace the coil this weekend
3. EDIS uses wasted spark. The other cylinder (#3) is just fine. I even swapped the #3 & #6 leads over on the coil and still #6 spark was intermittent, even though it was getting its voltage from the #3 post on the coil.

Regards

Mike

cstreit · cstreit's Garage

Hmmm.., That really only leaves the plug and the wire then doesn't it. Since you've swapped the leads that really rules out the coil and coil-trigger.

mbrown · 07-06-2007, 09:13 AM

G'day Chris...

I have also replaced the complete #6 spark plug lead...

Regards

Mike

kirkf · 07-06-2007, 03:59 PM

I would change the spark plug first.

A bad plug will fire outside of the cylinder, but may not fire consistently in the cylinder under pressure.

Kirk

Laut 1 · 07-11-2007, 11:43 AM

I recently struggled with my 74 911 missing and was told Bosch plug quality has gone down. I tried a set of Denso Power plugs and the miss is gone. The car starts easier and generally runs better. The plugs are pretty pricey at $12 a copy.

304065 · 07-11-2007, 01:47 PM

Does it follow the plug? Does it follow the wire? What's the gap.

Laut 1 · 07-11-2007, 01:51 PM

What are you referring to by following the plug and wire?
There is no gap setting required with the Denso plugs.

304065 · 07-11-2007, 04:12 PM

I mean, does the problem follow the plug? If you remove the plug and swap it with the plug in #1, does, the misfire occur in #1, or does it remain with #6? Ditto the wire.

"No gap setting required with the plugs," every plug has a spark gap or it's not a spark plug. If it gets too wide, the amount of voltage required to fire it in higher than normal atmospheric pressure may exceed the capacity of the ignition system. I would try all new NGK's.

Laut 1 · 07-12-2007, 06:12 AM

I didn't get into troubleshooting exactly which cylinder was missing I just put the Denso plugs in and problem solved.

mbrown · 07-12-2007, 08:01 AM

Gentlemen...

The latest:

1. Replaced the EDIS coil
2. Replaced #3 & #6 spark plugs

No change, the spark was still intermittent.
But after going for a drive around the block a few times, it seemed to clean itself out. So, finally I have a good spark on all 6 cylinders! Relief!

Have also replaced the fuel injectors...will take it out this weekend for a long ride and see what happens. I'll let the Autotune system within the MegaSquirt computer do its thing and tune the engine.

Thanks for all your help.

Regards

Mike