Intermittant mis

Steve76Targa · 09-14-2009, 09:30 AM

Been fighting this for a while. 79 3.0 that started missing occasionally then would smooth out and run normal. That went on for a couple of weeks, I thought I got some bad gas and changed stations. Then it morphed into a full time mis as if there was a rev limiter cutting in. I was due for new plugs, wires, cap and rotor anyway so I figured that one of those was the culprit but it still misses. I can put my timing light on any plug wire and see the missing happening. I've been checking this board for info and have had several "ah-ha" moments and would go check something out, but everything looks good so far. Dizzy's in great shape, vacuum advance and retard working normally, tried to make the green wire fail by wiggling it around while checking continuity, checked for arcing coming from the plug wires, tried a different coil, no luck so far. I'm wondering what the symptom of a Permatune going bad would be. Do they just quit, or start acting up first, or something in-between? Any and all suggestions are welcome. Has anyone put together a comprehensive list of all the little things that show up on this board? That would be an invaluable tool....

Thanks, Steve.

rwest · rwest's Garage

Hi Steve,

It sure could be the Perma-tune unit, mine was doing all kinds of intermitent failings, it was real hard to trouble shoot too, finally I was able to verify that it was shorting out when it warmed up. I was carrying around a timing light and multi-tester for weeks! I changed mine out for a MSD unit, because of price and because the Perma-tune caused me so much grief!

Good luck, Rutager

T77911S · 09-15-2009, 07:25 AM

get rid of the permatune and put in an MSD CD and coil.

Steve76Targa · 09-16-2009, 08:02 AM

Thanks guys. I was leaning towards the MSD already, just wanted to make sure there wasn't something simple I was overlooking.

Steve.

T77911S · 09-16-2009, 08:43 AM

it could be the wires between the coil/dizzy and the CD, but the MSD will replace all that.

Steve76Targa · 10-08-2009, 02:16 AM

Okay, so MSD unit and coil are in and the problem is still there. Any other suggestions? I can't find anything wrong. I have the dizzy out again, checking the "green wire from Hell", as others have called it. It's still green, nothing visibly wrong, and have zero resistance on both wires. The only symptom that might be telling for somebody more in the know than me is that it smoothes out when I remove the vacuum advance hose and plug it. I drove it like that for a few days before the MSD install, but would rather cure the problem if I can find it.

Thanks,
Steve.

azasadny · azasadny's Garage

I've had a similar problem and I changed out the plug wires and the points are bring replaced right now.

Paulporsche · 05:37 AM

If you still have points, check or change your points and wires, as ArtZ said. Oops, I see you have changed the wires. Did you check them for continuity and resistance? I had a new one recently w/ a bad end. Make sure they are firmly seated @ both ends.

Do you still have the 76 dizzy or have you swapped in one from a 79? If you still have the older style, as I do, you just have a vac retard, not an advance.

If you have an advance, maybe there is something wrong w/ the advance unit itself, or there is a vacuum leak. Also check the operation of the diz. You say it is in great shape. Does it rotate freely by hand and then spring back? Do you see any gunk or broken springs? Gunter has a great thread on dizzy overhaul.

Checking ignition first, as you are doing, is a good idea. Make sure the fuel pump, and relay, fuse and wiring are good. Then move on to vac leaks and control pressures (WUR).

Steve76Targa · 10-08-2009, 08:05 AM

Thanks, yes the dizzy is spotless (I was actually disappointed when I got to the bottom, it looked like brand new, everything moves freely and springs are good). Yes the P.O. swapped in the whole rear end from a 79 after a rear end collision, so it does have vac. advance which works fine both advance and retard. I haven't checked the plug wires resistance, the problem was there before the wires, but might be worth a look anyway. I'm pretty sure it's ignition because of the mis you can see with a timing light.

Keep em coming, one of these ideas will be it...

Steve.

KingSalmon911SC · 10-08-2009, 09:23 AM

Steve;
I'll Bet You Already Have, Check the Distributor Cap for Carbon Tracking.

Richard

Steve76Targa · 10-08-2009, 02:47 PM

Hey Richard, what exactly do you mean by "carbon tracking"? There are some suspicious looking burn marks next to each post in the cap. Maybe I have a mismatched cap and rotor?

Steve.

stlrj · 10-09-2009, 05:24 AM

Steve,

Sometimes a bad injector with a poor spray pattern could cause your intermittent miss.

Cheers,

Joe

Paulporsche · 10-09-2009, 06:10 AM

I was getting a high speed miss which turned out to be caused by a bad connection on one of the wires to the coil. They were factory crimped by MSD. About 3 yrs later, I had a no start situation, also caused by a bad MSD coil wire which had failed internally.

KingSalmon911SC · 10-09-2009, 07:51 AM

Steve;

Carbon, burn marks, can build up on the electrodes inside the distributor cap. This is caused by the high voltage"arc" between the rotor and dist. cap. Additionally, in many cases, electrical dust gets deposited inside the dist. cap between electrodes - likes to deposit in minute cracks in the cap. Anyway, when there is a buildup of carbon on an dist cap electrodes, that increases resistance to fire that corresponding cylinder so the "arc" following the path of least resistance, occasionally arcs to a nearby cylinder instead; hence, an intermittent miss.

Make sure your cap is clean inside and dry. Same for the rotor and that the rotor leading edge is clean & uniform in electrode curvature, and is making good clean contact with the coil electrode. When in doubt, replace as they are the cheapest and most abused parts of the system.

Lastly, you mentioned you replaced the coil. Sometimes a coil can have a short or open in the secondary windings - showing up only after the coil warms up from engine heat; thus, expanding the windings; hence a intermittent short or open occurs.

If you have access to a engine analyzer (scope), it would point out either/both of the conditions above; as well as, spark plug wire arcing, coil wire arcing.

Good Luck, you'll find it!

Richard

79 911 targa · 10-09-2009, 04:43 PM

did u change ur plugs????? try a little smaller plug gap.....i know on every other car i`ve owned (turbo or NA) over time the gap will open up and cause a miss at higher revs .. just something to think about..good luck and i feel ur pain

stlrj · 06:44 AM

At least you know it's not an ignition problem since you've already replaced everything. Maybe it's time to step back and take a different approach?

Paulporsche · 10-10-2009, 05:50 PM

Could the miss be seen via a timing light if it is caused by something other than ignition (say eg WUR)?

James Brown · James Brown's Garage

check the engine ground strap and make sure that the engine is well bonded to ground.

Steve76Targa · 11-12-2009, 09:24 AM

Okay so here's where I'm at now...

I replaced the cap and rotor again because of half inch long arc marks on the leading side of each post, with no change in how it runs. Because the marks are on the leading side it seemed logical that my timing was way advanced, but is dead on 5btdc at idle. I went back in the dizzy to see if I missed something, and thought maybe one of the springs was maybe getting weak, one had a little play in it, so I doubled up on it to see what would happen, no change. Then I decided to check the timing at high rpms, which I hadn't done yet, and wow, with the vacuum advance hooked up it was going well over 30, more like 40btdc. With the vacuum advance disconnected it went right to 28, but well before 6000 rpms. I'm thinking I have a vacuum problem, like the retard is switching to advance to soon, but what would make that happen? It switches back and forth fine when throttling up and down, but what rpm should it happen at? I haven't seen that info anywhere.

For now I'm just running with the advance disconnected hoping someone will have an "ah-ha" moment. It doesn't have the power it used to and has a slight hiccup at low rpms, but it's better than not driving it at all!

Steve.

Walter_Middie · Walter_Middie's Garage

Steve,

Yes, you will get a lot of advance with the vacuum hooked up - that's why you check it with the vacuum plugged.

Your total timing at 28 degrees sounds fine. It will quit advancing before 6,000 RPM - that too is fine.