911 won't run properly when hot: Help!

Hi Everyone,
Despite my earlier thread ( http://www.pelicanparts.com/ultimate/Forum3/HTML/005123.html ), I thought my car's poor running was caused by an Alternator fault that had gone wrong again (blown diode repair). However, it was not that. Basically, the car starts OK from cold, after about 5 mins of running it starts to misfire slightly then ultimately will get worse and worse until it will not idle and misfires severely over about 1,500 revs. making it undrivable. I changed the points, corrected a slight tracking fault on the coil. The coil is about 1 year old btw. The plugs seem fine too. I had it to a fuel injection specialist look over the CIS who said everything was OK but 2 HT leads are showing poor resistance. But, would this cause such a huge problem - that happened just a week ago all at once??
I'm thinking of swapping the coil over tomorrow - borrowing a friend's '72 911E one, but not sure if it's the same though, will check part #. Has anyone any ideas or had a similar problem?
Thanks in advance....
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Saffs;

OK, if you have two bad ignition wires, that needs to be addressed FIRST, before swapping coils! Get a multimeter (buy or borrow) and test all six wires, for end-to-end resistance ... it should be in the 3-5000 Ohm range -- the actual value is not important, but it must be a steady, consistent value! Also test the rev-limiting rotor -- it should measure approximately 5000 Ohms (+/- 10 %) from the center contact to the outside contact that the spark jumps from.

Most faults are because of an 'open' resistor in the Beru sparkplug connector, usually a permanent open, but they can be intermittent. It 'unscrews' from the wire, counterclockwise, and replacements can be bought, either at a dealer or independent supplier. I assume your CIS engine has or had the wires with the stainless braid, but the earlier non-shielded wires from your friend's '72 911E can be used to substitute for bad wires, and I'm sure you will see immediate improvement.

The CDI-system coil is Bosch # 0.221.121.001, and the primary should measure from 0.4 Ohms to 0.6 Ohms, and the secondary from 650 to 790 Ohms. The CDI-system coil is unique and has a turns ratio of 1:100, intended to convert 350-460 Volt pulses to an output of 35,000 to 46,000 Volts to the distributor lead. A conventional coil should not be substituted for the CDI-coil 'pulse transformer' because the CDI box can be ruined by the impedance mismatch of the windings!!!

This should give you a starting point for troubleshooting. Good luck!

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Warren Hall
1973 911S Targa

Thanks Warren,
I will do this tomorrow starting with the HT leads first. So just one spark plug connectors could cause, i.e, not the wire, the connector, running problems this bad??
I'll let you know how it goes.
Thanks Again,
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

I can't speak for Warren, who can most certainly speak for himself, but yes, this problem (Beru connectors) can cause your symptoms. The wire could be bad, but my experience has been three bad Beru connectors, no bad wires.

Here's a tip I hope most of you will forgive me for repeating. A car needs just three things to run properly.

1. Compression
2. Spark
3. Fuel/Air mixture

Unless the engine has just been rebuilt or is VERY, VERY, VERY old it has compression. The first thing to look at in troubleshooting cars, generall is the ignition system. This is where most of the faults are, traditionally. And, restoring proper operation of the ignition system is pretty straightforward. the right spark, at the right time. After your testing and Beru replacements, ignition timing should be checked. that being done, you know the problem is fuel related.


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'83 SC

Update on my frustrating problem!
Very little time today but I confirmed that the rotor arm is OK and so is the dis. cap. But, I did swap coils with my friends 911E (the same part # one) and the car ran fine! I drove it for 30 mins and no trace of the problem when normally it would be manifest after 5 mins. Put my tools away. Felt happy, to find that when I started it again the misfiring was back as bad as ever - but not worse....does this suggest anything? Can coils be burnt by the CD box or is it just pure coincidence?
I couldn't swap the HT wires over as my friends 911E didn't have the ground shields on one of the wires that my car has, and time was running out too....
Anyone any ideas - I'll change the HT leads on Mon. with new ones.....
Thanks Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Anthony,

You do not have to use the ignition wires with the braided shields -- early, less expensive wires will work just fine!

No, the CDI-unit can't burn out the coil! That can can happen from running with open-circuit condition without a load on the coil, however, because it CAN cause arcing between secondary windings!

What I suggest is that you take the car to a shop with an ignition analyzer scope, to see if the output is 'normal' when the car is misfiring or running rough. I'm not suggesting that the shop fix your problem ... just get an answer as to whether or not the ignition output Voltage is 'normal' when the misfiring is happening!

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Warren Hall
1973 911S Targa

Thanks Warren,
Funnily, I found the LT lead tracking to the HT lead on the coil in my earlier investigations of this problem. What could cause the "open circuit " btw?
Thanks again,
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

The open in the secondary circuit may be from the resistor in the plug connector at the plug being broken. If there is excessive resistance in the secondary circuit, it will find another path to ground through the primary circuit, hence the arcing at the coil. What probably happened was one or more plug wires went bad causing the arcing at the coil. This killed the coil. So you should replace the bad leads and the coil if this is the case.

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Tyson Schmidt
72 911 Cabriolet

Anthony,

You REALLY NEED TO CORRECT THE IGNITION WIRES PROBLEM BEFORE DRIVING THE CAR ANY MORE!!! That is why I put the wires FIRST on the list!

I'm not totally convinced your coil is fried ... if it was, it would most likely not start at all. But, on the odd chance that it is dying and 'wounded' ... that is why I suggested the check on an engine analyzer scope. If damaged, the coil's output would be low and marginal, in the 15-25KV output range!

If the problem is just multiple bad Beru connectors, the plugs are probably marginally fouled, but a cld CDI-system will put out 45-50 KV when cold, first thing in the morning, and the spark will jump across the open Beru connector, and even blast a dirty sparkplug ... for a while, until the CDI-system warms up and the output Voltage falls to the 35-40 KV range, then the miss develops again! That is probably why you experienced the reappearing miss after you swapped coils!

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Warren Hall
1973 911S Targa

Thanks Again!
The wires are ordered for Monday. hopefully, I haven't blown my friend's 911E coil! I'm just working on it at home and not using other than test drives, I'll wait for the wires now....
Cheers,
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Another, happier, update!
Today I swapped the HT wire with poor resistance with one of my friend's 911E (a restoration project, thankfully, for me borrowing bits) and the fault appears, so far, to of gone! The new wires are on order and I'll change them all. Noticing on my car verse the 911E (on carbs now) the one of my HT leads has grounding straps on it to the coil mount. Is this totally necessary? Just in case my ordered set doesn't have this....and it's essential.
Thanks again Warren and everyone!
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html

Another, happier, update!
Today I swapped the HT wire with poor resistance with one of my friend's 911E (a restoration project, thankfully, for me borrowing bits) and the fault appears, so far, to of gone! The new wires are on order and I'll change them all. Noticing on my car verse the 911E (on carbs now) the one of my HT leads has grounding straps on it to the coil mount. Is this totally necessary? Just in case my ordered set doesn't have this....and it's essential.
Thanks again Warren and everyone!
Anthony

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'75 911S Targa
http://member.aol.com/asaffery/my911s.html