CIS Engine Cut Out Help

[Mothy]

After trying to track down my problem for a few days and doing various searches here I need some help. ts been a while since I have posted.

I have a 1983 911SC which has been used for many years and has previously run well. It is quite modified, so it make the troubleshooting somewhat difficult. For instance it does NOT have any of the system under the passenger seat (OXS Control unit), so for all intents and purposes it is running as per an earlier SC.

About a month ago, the car started to play up. Started fine and would be ok driven for about 30 minutes and then start to misfire/hesitate when accelerating, almost like it had an ignition problem. Cruize would still be ok but any time I pressed down on the accelerator it would feel like it was cutting out - and occasionally backfire (sounded like it was in the intake - I have the pop valve fitted). Eventually it would get to the point where the car just would not run at all without pops and farts. It almost feels like the fuel pump is cutting in and out.

If I stopped the car and checked connections to coil and also the fuse board, it would start up again OK and run fine for about 5 mins then start the same issue, getting progressively worse with time until I needed to stop again - effectively to let things cool down I assume.

Interestingly, I was on my way to a Motorkhana meet and provided I didn't have the car idling while waiting in the queue for my turn, it would run fine. If I had the car idling for any length of time it would misbehave badly.

It has now added a further problem. I blows the fuel pump fuse 16 (25A) before I can even get it started. If I disconnect the electrical lead to the WUR then it starts fine and runs for a bit before complaining. If I plug the connection back onto the WUR, it blows the fuse.

Swapped Fuel Pump Relay - no change
Checked wires to the coil - no change

Recent changes to the car (well before the problems started)
- New Fuel tank
- New Fuel pump
- New fuel filter
- New coil

Checked the WUR part number and looks like mine is an earlier one - 0-433-140-033 from a 76-77 model??? Have no idea why that's the case. Its been fine for the past 18 years that I have owned the car.

Have checked a few things:
resistance across the terminals on the WUR is 24ohms which I understand to be OK
voltage to the WUR is 14v only when car is running.
WUR connection points are open to ground (not earthed which I understand is correct
I do not have the Supplementary Air Value
I do not have the OXS relay
I do not have RPM Limiter Switch
I do not have an alarm system
All these are normally associated with he Fuel Pump circuit.

I have the Classic Retrofit CDI fitted - has worked fine for the past 4 years.

The CIS is 40 years old so who knows what might be the issue. I am suspitious its going to the the WUR but thought it best to ask some of the wise minds on this board before I start down a track that might be a dead end.

Thanks

Tim

[Schulisco]

Tim,

very hard to help here. I suppose there is more than one issue.
The initial issue could be imho a dying ignition coil. The fact that it starts bogging after half an hour and then running on idle almost normal afterwards could point to this due to a thermal problem of the ignition coil.. Replace it and test it on a ride.

After that we can proceed. Troubleshooting a CIS has to be done step by step and logically! Otherwise you get in trouble because you never remind again how the engine behaved after making this and that...

Thomas

[allaircooled]

I had a similar issue once when I replaced my coil with one of those silver ones. You mentioned you replaced the coil at some point, if it’s a silver one I would try borrowing a black original one and see what happens. Just a thought.

[PeteKz]

Ditto comments on coil. If you have the old one, swap it back in. Why did you replace it in the first place? Report your results. If you still have the problem, we'll offer some more analysis. It does sound to me like an ignition problem rather than a CIS problem.

The 033 WUR is not from an OXS car, as you noted, it's from an earlier car. So, your CIS should work like an earlier CIS system. 24 Ohms is in the correct range. It should get 14-ish volts when the electrical system is charging, and battery voltage before the system is charging.

If you haven't tested for vacuum leaks, do that too. Old CIS systems often have vacuum leaks, which will complicate your troubleshooting.

[Mothy]

Thanks for the feedback. Gives me some homework for the next weekend! Appreciate the responses.

[Mothy]

Did a bit more discovery stuff last night and found that insulation on the wires into the warm-up regulator had lost their insulation and were likely shorting out.

Disconnected the engine wiring harness from the car (big 14 pin plug) and did some resistance tests for the connection between pin 10 and the WUR connector plug - was getting closed circuit on both connection in the WUR plug, although one was higher resistance than the other.

This led me to check the Auxiliary WUR on the right side (turns out I do have one) - disconnected the plug from that and my resistance measure issue disappeared it was picking up the resistance in the Aux WUR = OK.

So now I have isolated the short in the WUR plug lead and are checking for anything else on that circuit.

In the wiring diagram it shows a Supplementary Air Valve in parallel electrical connection to the WUR. Are the Supplementary Air Valve and Auxiliary WUR the same item or am I still looking for something else?

[83_Silberpfeil]

Hi Tim - I read your opening post carefully about 4 or 5 times. I experienced similar symptoms on my 83 SC (USA spec) a few weeks ago, particularly the bits that I've highlighted below. Like you, I also thought the FP (that I had just replaced w new unit) either went out or got clogged from fuel tank rust.

Eventually the car stalled out completely, and would no longer start. I towed to my local air-cooled tech/specially in here in LA. He diagnosed/tested and determined that the CDI had hailed. It was my original factory Bosch unit. He replaced it with a PermaTune unit, and also replaced the coil (Perma as well).

Just wanted to share a similar experience and data point with you. I saw you mentioned you had a fairly new CR CDI unit. If your other diagnosis does not reveal the culprit, then consider looking into the CDI.

Good luck, and look forward to your update.

Originally Posted by Mothy

After trying to track down my problem for a few days and doing various searches here I need some help. ts been a while since I have posted.

I have a 1983 911SC which has been used for many years and has previously run well. It is quite modified, so it make the troubleshooting somewhat difficult. For instance it does NOT have any of the system under the passenger seat (OXS Control unit), so for all intents and purposes it is running as per an earlier SC.

About a month ago, the car started to play up. Started fine and would be ok driven for about 30 minutes and then start to misfire/hesitate when accelerating, almost like it had an ignition problem. Cruize would still be ok but any time I pressed down on the accelerator it would feel like it was cutting out - and occasionally backfire (sounded like it was in the intake - I have the pop valve fitted). Eventually it would get to the point where the car just would not run at all without pops and farts. It almost feels like the fuel pump is cutting in and out.

If I stopped the car and checked connections to coil and also the fuse board, it would start up again OK and run fine for about 5 mins then start the same issue, getting progressively worse with time until I needed to stop again - effectively to let things cool down I assume.

Interestingly, I was on my way to a Motorkhana meet and provided I didn't have the car idling while waiting in the queue for my turn, it would run fine. If I had the car idling for any length of time it would misbehave badly.

It has now added a further problem. I blows the fuel pump fuse 16 (25A) before I can even get it started. If I disconnect the electrical lead to the WUR then it starts fine and runs for a bit before complaining. If I plug the connection back onto the WUR, it blows the fuse.

Swapped Fuel Pump Relay - no change
Checked wires to the coil - no change

Recent changes to the car (well before the problems started)
- New Fuel tank
- New Fuel pump
- New fuel filter
- New coil

Checked the WUR part number and looks like mine is an earlier one - 0-433-140-033 from a 76-77 model??? Have no idea why that's the case. Its been fine for the past 18 years that I have owned the car.

Have checked a few things:
resistance across the terminals on the WUR is 24ohms which I understand to be OK
voltage to the WUR is 14v only when car is running.
WUR connection points are open to ground (not earthed which I understand is correct
I do not have the Supplementary Air Value
I do not have the OXS relay
I do not have RPM Limiter Switch
I do not have an alarm system
All these are normally associated with he Fuel Pump circuit.

I have the Classic Retrofit CDI fitted - has worked fine for the past 4 years.

The CIS is 40 years old so who knows what might be the issue. I am suspitious its going to the the WUR but thought it best to ask some of the wise minds on this board before I start down a track that might be a dead end.

Thanks

Tim

[mikedsilva]

Originally Posted by Mothy

After trying to track down my problem for a few days and doing various searches here I need some help. ts been a while since I have posted.

I have a 1983 911SC which has been used for many years and has previously run well. It is quite modified, so it make the troubleshooting somewhat difficult. For instance it does NOT have any of the system under the passenger seat (OXS Control unit), so for all intents and purposes it is running as per an earlier SC.

About a month ago, the car started to play up. Started fine and would be ok driven for about 30 minutes and then start to misfire/hesitate when accelerating, almost like it had an ignition problem. Cruize would still be ok but any time I pressed down on the accelerator it would feel like it was cutting out - and occasionally backfire (sounded like it was in the intake - I have the pop valve fitted). Eventually it would get to the point where the car just would not run at all without pops and farts. It almost feels like the fuel pump is cutting in and out.

If I stopped the car and checked connections to coil and also the fuse board, it would start up again OK and run fine for about 5 mins then start the same issue, getting progressively worse with time until I needed to stop again - effectively to let things cool down I assume.

Interestingly, I was on my way to a Motorkhana meet and provided I didn't have the car idling while waiting in the queue for my turn, it would run fine. If I had the car idling for any length of time it would misbehave badly.

It has now added a further problem. I blows the fuel pump fuse 16 (25A) before I can even get it started. If I disconnect the electrical lead to the WUR then it starts fine and runs for a bit before complaining. If I plug the connection back onto the WUR, it blows the fuse.

Swapped Fuel Pump Relay - no change
Checked wires to the coil - no change

Recent changes to the car (well before the problems started)
- New Fuel tank
- New Fuel pump
- New fuel filter
- New coil

Checked the WUR part number and looks like mine is an earlier one - 0-433-140-033 from a 76-77 model??? Have no idea why that's the case. Its been fine for the past 18 years that I have owned the car.

Have checked a few things:
resistance across the terminals on the WUR is 24ohms which I understand to be OK
voltage to the WUR is 14v only when car is running.
WUR connection points are open to ground (not earthed which I understand is correct
I do not have the Supplementary Air Value
I do not have the OXS relay
I do not have RPM Limiter Switch
I do not have an alarm system
All these are normally associated with he Fuel Pump circuit.

I have the Classic Retrofit CDI fitted - has worked fine for the past 4 years.

The CIS is 40 years old so who knows what might be the issue. I am suspitious its going to the the WUR but thought it best to ask some of the wise minds on this board before I start down a track that might be a dead end.

Thanks

Tim

Worth checking your signal wire from the distributor, to the CDI... the "green wire"..

[PeteKz]

Tim: Did you swap back to your old coil yet? If not, do so before further troubleshooting.

The wiring to the WUR, AAR, etc. runs heating elements in those components during start and warm up. After the engine is fully warm, the electrical parts of those components don't do anything. You can unplug them with no difference. Therefore, even if you have a problem with the wiring to those parts, it will not cause the engine to stall after the engine has warmed up.

Since you are blowing the FP fuse it does appear you have a short in the wiring to the WUR or other component, so you will need to find and fix that, or just disable the whole circuit for now. If you disable the circuit, you will need to keep your foot on the gas pedal for a minute or so until the engine warms up. But that will allow your to continue troubleshooting. FYI, I removed the components and pipes and hoses from my SC engine to get rid of that crap in the triangle of death, but my car has a hand throttle, so I just set that during start and warmup. Now I can actually see and get my hands back there!

Try the coil first, because it's the easiest thing to swap, and you already have the old coil. If you are still having problems, then you can rule out a bad coil. Then check the green wire and CDI and distributor.

[Mothy]

Ok. More work done on tests and the shorting definitely was the wires to the WUR. Fixed that and now the fuse no longer blows. I have assumed the Supplementary Air Valve and Auxiliary WUR are the same thing. Looks like this


I’ll take it for a long local run next and see if the shorting was causing the original issue. If not then I’ll start the swaps as suggested. Yes I do have the old coil still.

[Schulisco]

Mothy,

good that you figured out and fixed the short on the WUR.
Indeed I wouldn't expect that this was the only culprit.

As said before - when you go on your next ride carry a proved working replacement coil and some tools with you, like additional spark plug for testing.

I'm a bit confused on your wording on the valves. I suppose you mean AAV (addidtional air valve) and the auxiliary valve (not WUR)?
See jimsbasementworkshop website for the correct part names to prevent misunderstandings:
https://jimsbasementworkshop.com/CIS/pages/comp_page.html Clicking the links opens the pictures as below:

AAR: It regulates indeed the additional air necessary to burn richer mixture controlled by the WUR.


AAV: Gives extra air to start the engine while cranking. After engine runs AAV gets closed due to vacuum on the output line and remains closed as long as the engine runs.


Thomas

[harv666]

All the symptoms are the same for so many problems but I 100% had the exact same thing happen and finally deduced after days of testing all the systems that it was the wur that has been causing all of this as it slowly died Thankfully for one amazing pelican member, I'll have a rebuild in my hands asap.

[Mothy]

Took it for a longish drive and the original problem is still there. Drives fine for a while then starts cutting out under acceleration.
Stop to check things like electrical connections and it starts right up ok and runs fine for a few minutes, then cut out starts again.
Will now work my way through all your suggestions.

[HarryD]

As a quick test, take an ice chest with ice cubes with you for your next drive. When the car starts to fail, see if the coil or CDI box is hot. Put some ice on them and see if it temporarily fixed the problem.

[Schulisco]

Originally Posted by HarryD

As a quick test, take an ice chest with ice cubes with you for your next drive. When the car starts to fail, see if the coil or CDI box is hot. Put some ice on them and see if it temporarily fixed the problem.

Principially no bad idea, but technically. Electricity and humidity are no good friends....especially on high voltage ignitions...

I recommend to use a can of ice spray instead. Much easier to carry and to handle...and even safer. And furthermore several times usable to proof that the problem is a temperature problem (cool down ignition coil and box with the spray, go on and - if issue returns, repeat)

Thomas

[HarryD]

Originally Posted by Schulisco

Principially no bad idea, but technically. Electricity and humidity are no good friends....especially on high voltage ignitions...

I recommend to use a can of ice spray instead. Much easier to carry and to handle...and even safer. And furthermore several times usable to proof that the problem is a temperature problem (cool down ignition coil and box with the spray, go on and - if issue returns, repeat)

Thomas

Good plan.

[mysocal911]

Originally Posted by 83_Silberpfeil

Hi Tim - I read your opening post carefully about 4 or 5 times. I experienced similar symptoms on my 83 SC (USA spec) a few weeks ago, particularly the bits that I've highlighted below. Like you, I also thought the FP (that I had just replaced w new unit) either went out or got clogged from fuel tank rust.

Eventually the car stalled out completely, and would no longer start. I towed to my local air-cooled tech/specially in here in LA. He diagnosed/tested and determined that the CDI had hailed. It was my original factory Bosch unit. He replaced it with a PermaTune unit, and also replaced the coil (Perma as well).

Just wanted to share a similar experience and data point with you. I saw you mentioned you had a fairly new CR CDI unit. If your other diagnosis does not reveal the culprit, then consider looking into the CDI.

Good luck, and look forward to your update.

Unfortunately you now have the CDI unit with the highest failure rate of any! Get a spare to carry on long trips.

[Mothy]

Ok - its been such a long gap - very busy doing other things and only recently got back to trying to fix this up.

Step 1 - Got a replacement light switch and that has fixed the unrelated but troublesome electric issues.

Step 2 - got a replacement WUR and the problem has not changed - so that is not the problem. Still works OK for a 10 minute period then plays up once everything is in warmed up status. If I stop and let the engine cool down, the problem goes away for about 3 minutes then comes back.

Symtoms include the engine cutting out - like the electrical supply has been cut off for about a second or two, but only when the throttle is opened - idle is fine.

Looking at the wiring diagram, it's possible that the relay is activating as if t thinks it's on starting mode - but since the key is not in start position the line connection for relay in start is a dead connection. I'm also going to put in a test light to see if the cutout is caused by the relay activating

Next check after that is going to be the AAR to see if that is misbehaving once the engine warms up.

Next after that will be the Air Flow sensor switch in the intake manifold.

Step by step - we will get there!

[Mothy]

Well - after almost 5 months I've finally getting back to fixing this issue.
In the mean time I'd been diagnosed with major brain tumour - surgery to remove last Oct, recovery, 6 weeks of daily radiation treatment (taking me up to xmas) and now in the middle of 6 months of chemo treatment. All going well and feeling fine - Dr is very positive things are tracking well.

Ok - getting back to getting the car back on the road. I took it for it's first drive in 5 months last night and things have not changed (as expected) - works fine for about 10 mins then will not behave above 2,500 revs. Able to drive home from the local roads OK but if I try to hard accelerate at all it just pops and engine cuts out till back to low revs. Gets worse as I continue to drive (things getting hotter?)

One thing I did notice once home last night was that the CDI+ unit was VERY hot - could not leave a finger resting on it. Am planning to swap over back to an old standard CDI unit and see if the problem disappears.

[HarryD]

Originally Posted by Mothy

Well - after almost 5 months I've finally getting back to fixing this issue.
In the mean time I'd been diagnosed with major brain tumour - surgery to remove last Oct, recovery, 6 weeks of daily radiation treatment (taking me up to xmas) and now in the middle of 6 months of chemo treatment. All going well and feeling fine - Dr is very positive things are tracking well.

Ok - getting back to getting the car back on the road. I took it for it's first drive in 5 months last night and things have not changed (as expected) - works fine for about 10 mins then will not behave above 2,500 revs. Able to drive home from the local roads OK but if I try to hard accelerate at all it just pops and engine cuts out till back to low revs. Gets worse as I continue to drive (things getting hotter?)

One thing I did notice once home last night was that the CDI+ unit was VERY hot - could not leave a finger resting on it. Am planning to swap over back to an old standard CDI unit and see if the problem disappears.

Hope your recovery is 100%


Easier, Take it out for a short ride with a filled ice chest, when it fails and the box is hot, put some ice on the unit to see if it starts working properly. If so, you have confirmed your problem.