determining if a spark is delivered
 

If I attach an inductive timing light to each spark plug wire and the light goes off, does this necessarily mean that the plug is sparking?

I adjusted valves, installed a new distributor, and installed new wires and plugs on my 911 and when I drive the car it definitely feels as if one of the cylinders isn't firing. I suspected that something wasn't healthy with one of the wires/plugs. However, upon attaching my timing light to each spark plug wire the light goes off.

In any case, appreciate any feedback. I went to emissions last week and failed. I am guessing that the reason is that gas is being dumped into one of my cylinders without being given the encouragement necessary to burn. It's not all bad though since a trip to emissions always warrants a Guinness!

Christian--78 911SC

No, it doesn't. What you can do is get a spare plug, pull a plug wire of a plug, put it on the spare, put an aligator clip on the threads of the plug, ground it to the engine somewhere, and crank it. You should see a bright blue spark.

I was in the garage and setup a mirror so I could crank the engine, look back, and see the plug fire in the reflection. ..obviously, it may take a couple of mirrors to see it fire if it's not near the rear of the bay.
but that won't help you if you're having trouble getting the plug wires on the plugs themselves.
If a plug does fall off, you'll know. The car will definitely run very lumpy...it's much worse than just running poor. It will not be right. You will want to pull over.

Many thanks. The car runs very lumpy as you stated....made me want to pull over. I'll check the plug wires again...they were a pain in the hiney to put on so it wouldn't surprise me if I missed one.

long blade screwdriver and another person to turn the key works wonders to see if you are getting spark

Did you have the same problem before replacing all these items? Have you checked you injectors for spray pattern, and the fuel distributor?

Marius

There is also spark testers.

http://www.thetoolwarehouse.net/shop/TTW409.html#989

I like the in line design. Works for me, mainly when resurrecting old heaps that have sat for a decade. I think they are cheap and handy enough that everyone should have one in their tool chest. They can save you from being shocked, potentially leading to heart problems or even death.

George

One of the oldest tests for determining if a particular cylinder is getting a spark is to remove the primary wire one at a time from each cylinder. If a particular cylinder is indeed not firing, any detected miss will not change when the non firing plug is not connected. To avoid shock stop the engine when connecting or disconnecting wires.
You should feel some resistance when pushing the connector onto the sparkplug. It will kind of click when it goes past center. A slight pull will then verify that the plug is connected. Try installing the connector onto a plug in your hand to get the proper feel.
If all the plugs are indeed connected and firing, then look at fuel or compression for the source of your problem. That's my 2 cents. Good Luck and be sure to let us know how it comes out.
Doug

Doug,

Regarding: "You should feel some resistance when pushing the connector onto the sparkplug. It will kind of click when it goes past center." I get this click on all the plug wires except for one (poss. 2) of them. So, I realize that the obvious answer is that this one plug wire isn't on the plug. However, in spite of the greatest effort (less the effect of a Guinness or two), I cannot get the plug wire to go any further. When I pull on it it seems like it is on there but not like the ones that clicked. The plugs and plug wires are new. I believe the wires are of the type that were originally put on this vintage car. Comments?

Marius....I didn't have this problem before replacing the items, however, I am inclined to believe that regardless of this fact that a problem with the injectors is causing a different problem. When I pulled the original plugs the passenger side plugs were all very black. I got some good advice on this from another thread.

Thanks for all the tips. Upon resolving this problem I'll report back.

Christian

A spark plug outside the combustion chamber has different characteristics than a plug inside the chamber. You can get firing outside, and not inside, if the plug is fouled.

Yes, check the resistance of each wire. Make sure the wire from coil to distributor is in good shape. Clean all connections, including the nut-and-post connections at the coil, the all-important coil ground, the CDI connections, etc. Ground strap between tranny and tub.

Change spark plugs. Change cap and rotor (most folks do not know what hard-working parts these are, and the tendency for them to get carbon-tracked. If caps were clear (I wish they were), then when they get old we would see the electrical storm inside. My personal opinion (actually, I'd call it knowledge) is that these parts are done at about 10K miles.

Here are some important cautions to observe ... from a document on CDI troubleshooting/repair I am working on:

http://forums.pelicanparts.com/uploa...1106939102.jpg

http://forums.pelicanparts.com/uploa...1106939139.jpg

Pull a plug wire and hold it to your tongue while your wife turns the key. Get a camera ready to see if anything happens.

Ironically, my wife already suggested that one. Given that I used the "if I work on it myself it really won't cost that much" line to support my buying the car I suspect that her suggestion was probably rooted in my buying the car in the first place. She overlooked the camera part though so I'll be sure and pass that recommendation along. :)

Christian

My wife helps me when needed. She can never figure out why she the one that keeps getting shocked.

Update....to try and eliminate the spark plug wires as the source of the problem, in addition to Doug's suggestion (I don't have the KD tool yet), I replaced the new wires with the old ones. As noted above, I didn't have the troubles with the old ones. Turns out that I still have the same problem. To eliminate the plugs I also swapped those out...still the same.

So, given that the distributor, cap, and rotor are all new (distributor is actually a rebuild), I am now suspecting that -- as noted by Marius -- the problem lies with the injectors.

The car sat for probably 2 months while I was waiting on the distributor to be rebuilt. Maybe in sitting so long, things that weren't problems have become problems?

In any case, I'll check the injectors this week and clean all the connections as suggested above and report back.

Christian

How about the coil and the ignition box? Can you do a swap on those, with a friend's material? I had a friend spend serious cash, getting his ignition diagnosed on a 3.2. Even had a spare DME put in. Turned out to be the coil. :D

The coil at least could be intemittent? I am not sure about the ignition box. I just think both items are easier to swap out than fiddling with the injectors. The injectors on the SC also are very basic. I can't see them gum up in 2 months.

George

George...turns out that I have a MSD box and coil waiting to be installed. I bought it because I had this problem where after about 30 minutes of driving and usually as the temperature crept near the 210 mark the car would just stop. I'd turn the key and she fire right back up. Although I realize that there are many other reasons that these symptoms can be exhibited (WUR, vacuum, etc.) given that the original box and coil were on the car I figured it was a good thing to swap out.

In any case, I've held off on installing the MSD system because, with my February emissions deadline approaching, I wanted to reduce entropy in my car and leave things alone. Alas, given your suggesion, it might be time to dive in.

Christian

"The coil at least could be intemittent? I am not sure about the ignition box. I just think both items are easier to swap out than fiddling with the injectors. " - aigel -

"Although I realize that there are many other reasons that these symptoms can be exhibited (WUR, vacuum, etc.) given that the original box and coil were on the car I figured it was a good thing to swap out." - Minardi -

Why waste time & money focusing on the ignition unit & coil when the main problem is a weak/bad cylinder?

Check out this web site (www.systemsc.com) on the Troubleshoot (basics) & Waveforms (CDI & distributor signals) pages,
if you really need to analyze the CDI ignition system.

Back to the emissions. Maybe you can post your results, for the folks here to read them. I am no expert at all if it comes to emissions, but there are people that can tell you pretty well what's going on, looking at the numbers.

The car idling may not do the same than under load...

George

George...I'm going to head back to emissions later this week. I will post my findings.

Lorenfb....Regarding the bad cylinder, although I hope to eventually do a compression test, wouldn't such a condition present itself on regular intervals with an amplitude that is of a consistent magnitude? In my case the pulses seem to occur at nonregular intervals and of varying amplitudes. I did check out the web site. Pretty cool.

Huh? compression test?

If you believe you have actual ignition issues, still, take the car to a shop that has a Sun analyzer ignition scope ... that will tell you if ignition is the issue, and which cylinder is weak!

It sounds to me like you have CIS injection problems, not ignition issues ...