HELP!! 911 SC hesitating/stalling and refusing to restart until cooled

[Kris G]

Hi guys, I'm looking for some help with an ongoing issue with my 78 911sc,

Symptom.

About three months ago the car refused to start while warm after a 20 minute pause, exhibiting the symptoms of a fuel vapourlock. A mild backfire through the airbox ( no damage due to pop-off valve), then it finally fired up after about ten minutes of on and off cranking.

Car ran fine for a month, then on the way to a club event started exhibiting an intermittent misfire that lasted till the car stopped.The car was going up a hill during this episode and continued until the car was stopped by me switching it off
The car cooled and I returned home home with no misfire.This episode did not show the Fuel vapourlock symptoms though.

A month later the car was started after sitting for an hour, firing up fine before dying a few seconds later from what felt like fuel vapourlock again. After ten minutes of cranking I got it going; however through the trip home the car exhibited an intermittent misfire again, stalled and was quickly restarted, then ran fine before misfiring badly a few minutes later to the point that the car backfired, popped and finally stalled. The most obvious thing I observed outside of the misfiring and hesitation was the Tacho needle surging wildly. After letting the car sit till the temp needle touched the top of the white zone the car fired up as if nothing happened.

Thinking I had two separate issues fuel and electrical, I replaced the fuel pump and check valve, the fuel accumulator and fuel filter. All components replaced were the originals so I thought an entire change over would fix the problem and any other future issues.

I had the CIS rebuilt a couple of years ago, and when I conducted the simple vacuum test of taking the oil tank lid off during engine running, she demonstrated all the symptoms associated with the loss of vacuum until a put it back on - so I know it is fine.

For the electrical issue I changed the plugs, leads and the coil is a Brazillian replacement; however I kept my black original. The CDI box made the familar hi pitched whine at startup so all was good,

The initial result was quite promising with the car performing nicely as it had before the start of the problems.

After about week the car demonstrated the bad misfiring, then backfired, popped and stalled refusing to start. Sure enough once cooled to the top of the white zone on the temp gauge the car started without fault.

I tried checking the CDI box for the familiar hi pitched whine during the period when it wouldn't start; however I couldn't make it out ( doesn't help that I had heavy traffic around me at the time and my hearing is stuffed after two tours of Iraq and three to Afghanistan - but whatever!!) so I deduced it was the CDI box latching up.

Out the box came and I sent it to pelican parts for a rebuild, Bob who does the rebuild said the thing was behaving fine when he received it, but he chose to do the overhaul anyway which I'm grateful for. With the CDI box back in the car I took the car for a run and it ran beautifully - until I arrived in the garage half an hour later. The car was idling with the needle just under the 1000 rpm mark, when engine had a mild hesitation that registered for a split second with the Tacho needle dipping to 750-800rpm before near instantly returning to just on the 1000rpm mark, Two seconds later the engine shut down clean by itself as if I'd switched off the ignition-which I hadn't. There was no prolonged coughing popping or upward spiking of the Tacho needle this time.

The car would turn over but not start. I heard the clear hi pitched whine of the CDI box. Thinking it might be the brazillian coil I pulled it off and replaced it with the old one; however there was no change, with the car turning over but not starting.

I felt the CDI box and it was hot, but not quite enough to burn yourself on.

I again let the car cool to the top of the temperature gauge's white mark and it yet again started up as if nothing had ever happened - and this was on the Brazillian coil.
The green wire to distributor is also brand new, so I'm sure it isn't it.

I can also say the alternator was replaced about 12,000 miles ago with a new unit, so I'm pretty sure it isn't causing a overcharge as it would have blown everything as it did when it failed.

The car had an entire engine rebuilt 5 years ago and I haven't had any issues.

Guys I am at now at a loss and open to any help. I am eager to get some sort of remedy for this as it is driving me nuts!!!

Photo of the car below:

[boyt911sc]

Kris,

Do you have an inductive timing light? From what I could deduced from your information, the problem seems to be temperature dependent and I suspect it to be an electrical anomaly in the system. Hook up an inductive timing light and check if you get a strong ignition signal while cranking the starter (engine warmed up). You also need to determine your residual, control (cold & warm), and system fuel pressures. Lastly, have your alternator bench tested. The Brazilian made ignition coil is notorious for intermittent failure. Avoid using this coil. Keep us posted.

Tony

[DanielDudley]

Check your warm up regulator, especially if you feel the problem could be caused by an excessively rich mixture. It always runs fine when cool. IIRC, there is a 12 volt connection to that as well. Check that first.

[Kris G]

Thank you boyt911sc and Daniel Dudley for your responses. This is the sort of info I've been after.

I'm going to the local auto shop tomorrow to get an inductive timing light. it has been on my list and stupidly I should have got one earlier. The temperature issue is the only constant I've noted with all of this I knowing my luck it will be some minor electrical thing which costs bugger-all to fix or will be a simple terminal clean!! (spent $3000 dollars once on trying to sort a high running temperature issue to discover it was a faulty temperature sensor on the engine which cost a few bucks!!)

As for the fuel system pressure, I can confirm Porsche Centre Brisbane checked the fuel system pressure cold when they did the fuel pump replacement and found it was fine, but warm however...I don't know if they checked that -I'll follow it up.

I'll definitely check the warm up regulator - I have had suspicions about this component

I think an excessively rich mixture may be highly likely too. The car is consistently stobborn to start at every cold start up. It behaves initially like its got wet plugs with the cylinders uneven in their firing. The motor then begins to hunt wildly (has never stalled during this). It then eventually settles after two minutes to a warm up idle of 1100 rpm before slowly settling further to about 800-850 RPM. After the car was warm it ran fine and never missed a beat- particularly with warm starting until recently. The hunting started years ago when I took it to a regular mechanic in Sydney. When I got it back from him after one service it was doing this. As he was a Porsche trained mechanic I figured he knew something I didn't (he also advised me to push the accelerator to the floor when doing a cold start - which I thought was
rather airbox-deadly behavior so I ignored it!) More disappointing is that authorised Porsche Centres in four cities have never done anything about the wobbly starting and hunting despite it being bleeding obvious.

I have a new mechanic in Brisbane who is highly regarded with the official club, I will also pass this info onto him as he is beginning to know my car and it will certainly help him narrow the issue if I can't sort it myself in the meantime. I'll keep you posted.

Cheers Kris

[Jonny H]

A common fault on a Bosch CDI is for the trigger input (from distributor) it to become 'sensitive' to battery voltage. E.g. the box fails to fire a spark if voltage is too high or too low. Of course, battery voltage changes with temperature. Failure at high temperatures is common too.

[Kris G]

Originally Posted by DanielDudley

Check your warm up regulator, especially if you feel the problem could be caused by an excessively rich mixture. It always runs fine when cool. IIRC, there is a 12 volt connection to that as well. Check that first.

Daniel that is an interesting point regarding an excessively rich mixture and it is something I forgot to mention as I've lived with it for so long. As I started trying to problem solve the current issue on this forum, I have discovered that the car is showing the hallmarks of running too rich. Cold start is difficult with the engine behaving like it had wet plugs for the first 10 seconds before hunting for about 2 minutes. The engine then calms at about 1100rpm, before slowly settling to what looks like 800-850 rpm. As the engine warms up under driving this settles further at about 900 rpm. Between the settling of revs to the point which the hesitation/misfire/nonrestart occurs it will turn on an off with great ease and fire up like a trooper.

Most annoyingly this excessively rich mixture was set up by a Porsche trained mechanic who I saw for years. I should be surprised but this the same person that told me I should put the accelerator flat to the floor when doing a cold startup (something I ignored for the sake of my airbox!!!) I have found a new mechanic in Brisbane ( co club member) now and I have to show him this.

[boyt911sc]

Kris,

Do you have an '81SC? ROW/Euro or US/CAN spec? What is the Bosch ID number on the WUR? Do you have a CIS pressure gauge kit? Keep us posted. Thanks.

Tony

[Kris G]

Originally Posted by boyt911sc

Kris,

Do you have an inductive timing light? From what I could deduced from your information, the problem seems to be temperature dependent and I suspect it to be an electrical anomaly in the system. Hook up an inductive timing light and check if you get a strong ignition signal while cranking the starter (engine warmed up). You also need to determine your residual, control (cold & warm), and system fuel pressures. Lastly, have your alternator bench tested. The Brazilian made ignition coil is notorious for intermittent failure. Avoid using this coil. Keep us posted.

Tony

Thanks Tony. Your message has prompted me to go and but an inductive timing light this morning. You are right - temperature seems to be the only constant in this entire episode and I'm convinced it has to be in the ignition. When Porsche Centre Brisbane did the fuel pump change they conducted a pressure test on the system and it came out fine. With good CIS system pressure and all the known culprits (checkvalve/pump, accumulator and filter) changed it should by rights be fine. The Brazillian coil is the one that troubles me. At least with the inductive timing light I'll start to narrow down the likely causes. With the CDI repaired and passing a pretty rigorous post-rebuild test and the fact it was whining away during the refusal to start, makes me more suspicious of the coil. As stated in my original post, After changing back to the original coil during the non start and turning the car over it still failed to fire. Both coils may be faulty which means I'm going to have to dig up a reliable third. Jonny H in this thread produces a highly regarded coil which I think I will be investing in.

[Kris G]

Originally Posted by boyt911sc

Kris,

Do you have an '81SC? ROW/Euro or US/CAN spec? What is the Bosch ID number on the WUR? Do you have a CIS pressure gauge kit? Keep us posted. Thanks.

Tony

Mine is a 78SC US/Can Spec car.

I'll get the ID number off the WUR. I think I will be buying a CIS pressure gauge kit as well.

[Kris G]

Originally Posted by Jonny H

A common fault on a Bosch CDI is for the trigger input (from distributor) it to become 'sensitive' to battery voltage. E.g. the box fails to fire a spark if voltage is too high or too low. Of course, battery voltage changes with temperature. Failure at high temperatures is common too.

The box went through a post rebuild heat/load test for seven hours solid and apparently didn't miss a beat - would the issue with the trigger input show-up during that test or is this issue something that can show up under real engine loading?

[Kris G]

So, I went out to the car and conducted the following things:

First I removed the Brazillian coil and replaced it with the black coil. I noticed that their was a missing compression washer for the positive terminal. I do know this was missing prior to first changeover from the original coil. The lack of compression washer meant the wire was not snug against the terminal base with the nut tightened down. I added another washer and succeeded in tightening it properly.

Next I checked the pug into the CDI. I removed it and put it back ensuring it was in properly. I then removed the pug to the WUR and inspected it. There was bit of grim but a gave it a light clean ( read- blew on it) and replaced it ensuring it was tight. I checked the connector to the green wire.

I then started the car. The engine felt slightly ruff for about 3-5 seconds, then gently hunted around 200rpm before settling at 1100rpm for warm up, this slowly descended to 850-900 rpm. The idle was pretty smooth. I allowed the car to war up and proceeded to raise the engine to 2000 -3000 rpm. I fluctuated this with a few sustained bust at 4000 rpm. I did this for about half an hour (which is far longer than the time on the road before I was having the (breakdowns). The engine was at full running temperature with the engine hood down the whole time. During this period I experienced not hesitations or even a hint of misfire. Idle at running temperature was 900rpm.

I shut the engine down, paused for 5 seconds and started it again - no problems, starting without problem and settling quickly to 900rpm. a ran it a few minutes longer, again fluctuating the revs to put the engine under load.

Now for the big test. I shut the car down for 5-6 minutes, easily the amount of time it took for the engine to sit before not restarting. I turning the key and she sprang to life smoothly and again settled into a 900rpm idle.

While I'm not sure it is fixed it definitely the most promising result I have had in a while.

The takeaways I have at the moment is that I'm thinking the Brazillian coil has malfunctioned and has caused the CDI to latch-up.

Added to this is a positive wire to the coil which may not have been secure and had a poor contact which got worse under temperature. I also think I may have had a poor contact the main CDI plug.

The engine bay is toasty, so I'm leaving it for a further 20 minutes to induce some more heatsoak and will give it a restart test.

Stay tuned.

[Kris G]

After twenty minutes with all ignition components subjected to a heat soaking everything fired up correctly.

I will continue to monitor the situation over the next week with a few local drives. One clear takeaway is that I need to sort the rich mixture issue. That is definitely something which has to be rectified. I think the WUR will have to be overhauled as well as it looks a bit average on the outside, so lord knows what it is like inside.

[mysocal911]

Originally Posted by Kris G

The takeaways I have at the moment is that I'm thinking the Brazillian coil has malfunctioned and has caused the CDI to latch-up.

1.It's unlikely that the coil will cause the CDI to "latch-up". That's usually the result of a bad
component internal to the CDI.
2. It's likely that you localized the problem to the coil itself. So now leave the original black coil
connected so at least you can eliminate a coil as being the problem, i.e. the black coil very rarely
if ever fails.
3. Before assuming that the CDI was bad and needed a "rebuild", the issue of a bad coil should
have been eliminated first.
4. Next time the engine dies, immediately check for a spark by pulling the coil wire.

[Kris G]

Originally Posted by mysocal911

1.It's unlikely that the coil will cause the CDI to "latch-up". That's usually the result of a bad
component internal to the CDI.
2. It's likely that you localized the problem to the coil itself. So now leave the original black coil
connected so at least you can eliminate a coil as being the problem, i.e. the black coil very rarely
if ever fails.
3. Before assuming that the CDI was bad and needed a "rebuild", the issue of a bad coil should
have been eliminated first.
4. Next time the engine dies, immediately check for a spark by pulling the coil wire.

Thanks mysocal911.

The black coil will be staying on the car. The decision to rebuild the CDI was also based on the thing being subjected to an overcharge many years ago - it was about getting piece of mind and being able to say hand on heart that the internals were 100 percent reliable to eliminate that as a possible issue - The capacitor inside was a bit nasty and was beginning to show intermittent failure, hence the sometimes it ran fine - sometimes it misfired scenario. Plus it means I have 100 faith in the component. I don't mind spending the money

[Vereeken]

You need to do a search for silver bullit.
The Brazilian coil is notorious.
The only good thing to come out of Brazil is a wax job.
It also says a lot on bosch quality control. Not that they are intrested but hey.

Classic retrofit in the uk sells a black coil that works Well and looks original.

[mysocal911]

Originally Posted by Kris G

The capacitor inside was a bit nasty and was beginning to show intermittent failure, hence the sometimes it ran fine - sometimes it misfired scenario.

You were able to test the capacitor to verify that, and the CDI was the actual cause of the mis-fire, right?

[Kris G]

Originally Posted by mysocal911

You were able to test the capacitor to verify that, and the CDI was the actual cause of the mis-fire, right?

Yes that is correct. That capacitor was tested and was found to be unreliable.. The CDI was then bench tested for 7 hours at high RPM and heat and it passed with flying colours. Even the repairer was concerned it was being plugged into a Brazilian grenade

[Kris G]

Originally Posted by Vereeken

You need to do a search for silver bullit.
The Brazilian coil is notorious.
The only good thing to come out of Brazil is a wax job.
It also says a lot on bosch quality control. Not that they are intrested but hey.

Classic retrofit in the uk sells a black coil that works Well and looks original.

Wax Job - Brilliant!

Oh well like everyone you think the Brazilian grenade won't go off for you....and then you're proven wrong. I've seen plenty of dud coils straight out of the packaging from other manufacturers - but Bosch?? its damn disappointing.

I will say the classic retrofit coil has caught my eye as has their CDI box.

[Kris G]

Guys I took the car for a solid hour long run following the successful garage test last weekend.

I think I can say confidently that the problem is solved. It looks like it was two issues.

1. The Brazillian Coil was checked and was a dud - so Bosch Australia will be hearing from me.

2. The follow up issue after I refitted the original black coil was related to a wire to the coil's positive terminal not being tight enough. Evidently the poor connection was fine under cooler temperatures, but once the heat soak came on the weak connection couldn't handle it. A compression washer solved the problem.

The car is also confirmed as running too rich, so adjustments will be made.

Thanks to the united Brains trust out there for plugging away on this one - your efforts were greatly appreciated, - happy Porsching!!

[boyt911sc]

Kris,

This would a good time to get your control fuel pressure data. BTW, you have not shared with us the Bosch ID number of the WUR. From a cold engine, measure your cold control fuel pressure until the warm control pressure stabilized. The duration of the test should last about 5 mins. plus. Stop the test once the WCP is steady and not changing. Keep us posted.

Tony