Need help diagnosing a miss after tune-up

Maxhouse97 · 09-12-2011, 06:52 PM

Just finished my first tune-up on an '87. Replaced plugs, distributor cap and rotor. Was diligent about plugging in wires to where they came from. As soon as I started it up it sounded "different." Then I drove it and it vibrated more than usual so I think I have a miss (but have never heard one before). Can you help me figure out what wire/plug is not firing? Through multiple searches I have heard of folks:

- Using a timing light? (How does that help - you can't see the plug fire with it can you?)
- Unplugging a wire and seeing a spark at its tip?
- Using some induction light?
- Testing resistance and voltage over each wire?

How should I troubleshoot this?

BTW I also read about some plug shrouds/protectors. When I pulled the wires there was a separate cover by the plugs. How far into the plug recess should this go? All the way to the plug or closer to the top? Don't know if this could be the cause of my miss??

Thanks.

DRACO A5OG · DRACO A5OG's Garage

Hey Tony,

Sounds like a cylinder is not firing, ergo no spark

Easy, Check that each plug on the spark plugs are seated properly, you should hear and feel a "CLICK", if not it is not seated.

ON the Dizzy Cap, places some di-electric grease on the nipples and press them is and listen/feel for a "CLICK". Sometimes due to the tightness of the rubber to dizzy cap there is a bit of vacuum pressure not allowing the plugs to seat all the way down on the the contacts.

You may need to use a "pin tool" and let the air out as you push down, but with di-electric grease it will slip right in. I used the BOSCH DIZZY GREASE sold by our host, great stuff and will not melt.

Also check the rotor is seat properly as well, I am sure it is but why not check and also when replacing the cap check for proper alignment, flush.

When I did my first "tune up" my #6 was not fully seated onto the sparg plugs and she stumbled on 5 cylinders

How old are the wires, if OEM you may want to consider swapping to Magnacors, plus they look *****en, I recommend using the Beru (OEM) dust shields, they hold up better than the Magnecors.

Jim

j911brick · 09-12-2011, 08:51 PM

Check the firing order again. Failing that, put the old parts back on and see what happens.

DRACO A5OG · DRACO A5OG's Garage

The Dizzy Cap are marked, at least they are on mine.

stlrj · 09-13-2011, 04:53 AM

Quote:

Originally Posted by Maxhouse97

BTW I also read about some plug shrouds/protectors. When I pulled the wires there was a separate cover by the plugs. How far into the plug recess should this go? All the way to the plug or closer to the top? Don't know if this could be the cause of my miss??

Thanks.

The stock Beru ignition cables are extremely unforgiving and intolerant of the slightest miscalculation/misinterpretation in installation. This is one area where being obsessive compulsive is a plus.

Cheers,

Joe

MBruns · 09-13-2011, 05:35 AM

Remember just because its a new plug in the box does not mean its good, I have seen many plugs over the years that don't work as well as what you just removed. Another thing to remember if you drop a plug on the floor drop it again in the trash.

Mike Bruns JBRacing.com

kidrock · 09-13-2011, 07:54 AM

1) improperly connected plug wires....check each wire from dizzy to plug.

2) improperly seated rotor, or rotor is defective. IIRC, another Pelican recently had this issue.

3) improperly seated dizzy cap.

I'm inclined to believe that it's going to be a plug wire issue. This happens frequently.

schumicat · 09-13-2011, 08:14 AM

A timing light can tell you whether plug is firing. the light has a clip that goes around the plug wire which will flash the light when the plug fires. by moving it to the various wires you confirm they are each firing. you can get a cheap light from harbor freight for $8. re: the protectective rubber thing, it should sit about flush with the hole in the valve cover to tightly seal it against water etc. I agree with the others that one wiis probably not seated at one of the plugs or on the cap.

Maxhouse97 · 09-13-2011, 08:45 AM

Great responses guys.

- Thanks for the timing light thing - I will get one of those.

- As for wires - they are Magnacores that were new 30k miles ago so I think they should be good.

- I will get some dielectric grease and try the dizzy cap again. As I recall the rotor has a notch in it so it is hard to misalign.

Will do all of this tonight. Should I put dielectric grease down by where the wire connects to the plug?

dshepp806 · dshepp806's Garage

I use a bit of that grease on the plug tip,..then mout the plug wire,..couple of twist, L-R...remove,..then SNAP back on,..just habit.

Doyle

kidrock · 09-13-2011, 01:56 PM

the Magnacores are a little notorious for not completely snapping at the plug, and not completely sealing at the dizzy cap.

matstermag · 09-13-2011, 04:32 PM

Hi Tony,

Did you gap the plugs or at least eyeball them? I have seen the electrode squashed straight from the box. If you think its misfiring then you can shoot the exhaust, at the head, with a temp gun. A cylinder that is down will have a cooler pipe due to lack of combustion. I have used my hand before,not recommended, but you got to be quick!

Cheers

Maxhouse97 · 09-13-2011, 04:50 PM

Heat gun or hand - that's a good idea. I did visually look at the plug gaps - none were collapsed. Going back to work.

Maxhouse97 · 09-13-2011, 07:12 PM

No luck w/ the grease - I pulled and re-seated each plug after a dab of grease. Still a miss. Will try tomorrow to see if I can tell which cylinder is not firing w/ the heat gun/hand trick. Guess plugs are next.

I tried testing the wires but my volt-meter is crap (I think).

matstermag · 09-13-2011, 08:41 PM

Hey Tony,

Just a thought but you might check the connectors at the injectors and make sure they are all still plugged in also. Anyway,keep working on it you will figure it out, its usually something simple that you wouldn't normally think of and after hours of double checking you find it and it takes 1 minute to fix. Thats my luck anyway , besides it's a good way to learn about your car and it will help build confidence for future repairs.

Cheers

Lorenfb · 09-13-2011, 10:43 PM

The easiest way to determine a dead cylinder and/or a mis-firing one on a 911 3.2,
is to one-by-one disconnect each injector connector. The one that has the least effect
on the idle when disconnected is the problematic cylinder.

JohnJL · 03:53 AM

Loren has mentioned what might be your next step. With the car idling, disconnect each plug in turn. When you disconnect #1, if nothing changes in the sound of the motor you know the problem is with #1. If the engine changes sound then the issue is not likely (or solely) with #1.

You dont need to completely pull the plug wire out, just pull until you get a "click."

Be careful with pulling a plug wire all the way out and messing with it when the car is running. You will get a very nasty shock if you become the conductor between that connector and the motor, and you only need to be close to it. Remember, that electric charge is designed to jump gaps and set things on fire ;-)

EDIT: I re-read Loren's post, though Loren was referring to disconnecting the injector and I was referring to spark plugs, either should work. Do whats easier for you (or do both in succession to confirm.)

Maxhouse97 · 09-14-2011, 03:49 AM

OK great will do tonight - thx.

Maxhouse97 · 09-14-2011, 03:27 PM

Ha I found my issue but it had nothing to do with the plugs or wires:

The injector was disconnected! Don't know how this happened - maybe I bumped it. But it looks broken off clean. Question is how hard is it to replace. Can I buy a single electrical connector and splice it in or do I need the whole harness? I am looking up the parts now. Crazy!

Maxhouse97 · 09-14-2011, 03:41 PM

OK I can't find this part - any help?