Request for help with CIS problem....

[Army 352P CWO]

I have a '79 SC with the original CIS equipment, US spec. I've put about 20K miles on it in the last 6 yrs of ownership without any CIS problems.

Recently it started to intermittently (about 25% of the time) give me problems starting - it could happen either cold or hot. When the problem occurred it would either catch and die, or not catch at all.

In my trouble shooting, I discovered that if I raised the air flow sensor plate lever with the ignition 'On' (in the key #2 position) that I would sometime just hear a 'click' from the front of the vehicle with none of the fuel injector 'squeal' but that other times the fuel injectors were squealing appropriately. If I didn't hear the squeal, by applying a little forward or rearward pressure to the sensor plate level as I raised it I could often get the squeal to occur. But sometimes even that wouldn't do it.

I thought that the fuel distributor plunger might be getting hung up, so I removed the mixture control unit and the fuel distributor and reassembled per the instructions in Bentleys. Nothing looked too dirty or gummed up, but I still cleaned the mixture control unit mechanisms and the plunger with some air intake parts cleaner. The plunger moved smoothly and freely in the fuel distributor bore and I was careful to never let it come completely out. I reassembled the unit, including a new fuel distributor O-ring (never saw an old one when I disassembled in the first place), a new air flow housing gasket, and new fuel line sealing rings.

Now, the car runs worse. I still have the intermittent starting problem - only now it's much worse - like the car starts less than 5% of the time. And I still have the intermittent fuel injector squeal when raising the sensor plate lever, even though I always hear the 'click' from the front of the vehicle. Additionally, the engine won't idle. When I do manage to get it started, I can only keep it going if I give it gas and keep the RPM's up over 2,000.

I didn't touch any of my fuel/air adjustments when I did this.

Any ideas what's going on with either of these problems? Either (1) the intermittent fuel injector squeal or (2) the new not idling problem?

I've already spent a couple hours searching the forum and couldn't find anything that I thought was similar. Thanks.

[Joe Bob]

CIS is all about fuel pressure. Borrow or buy a set of gauges and work from there. Do you have a Bentley manual?

[tirwin]

Well, you've done a few things that are good to check. A (hopefully) good read is the "CIS for Dummies" thread I started. It is still a work in progress.

Now on to your problem... First thought is dying fuel pump.

[kerthunk]

As long as you have fuel pressure, the injectors should squeal when you lift the sensor plate. That pressure is produced by the fuel pump.

The click at the front of the car is probably the fuel pump replay. There is a safety cutoff switch attached to the sensor plate. If the plate is closed, then the fuel pump won't run. As soon as it is lifted, it triggers the relay to allow it run.

Do you hear the fuel pump running when you lift the sensor plate?

If you can't hear the fuel pump, there are a few things you can try:

* Plug in a "known good" fuel pump relay.

* Make sure you are getting 12v to the fuel pump relay. Best to test it at fuse #16.

* Make sure you are getting 12v at the fuel pump. Takes getting under the car...

* There is a plug at the back side (looking at the engine) of the mixture control unit, if you unplug it, then it will always allow the fuel pump to run. I would try disconnecting it, turning the ignition to on and listening for the fuel pump.

* The RPM limiter is also attached to the fuel pump relay. You can just unplug it to bypass.

If you CAN hear the fuel pump, then you will need to hook up the CIS gauges and figure out what kind of pressures you have.

[ossiblue]

Originally Posted by kerthunk

As long as you have fuel pressure, the injectors should squeal when you lift the sensor plate. That pressure is produced by the fuel pump.

The click at the front of the car is probably the fuel pump replay. There is a safety cutoff switch attached to the sensor plate. If the plate is closed, then the fuel pump won't run. As soon as it is lifted, it triggers the relay to allow it run.

Do you hear the fuel pump running when you lift the sensor plate?

If you can't hear the fuel pump, there are a few things you can try:

* Plug in a "known good" fuel pump relay.

* Make sure you are getting 12v to the fuel pump relay. Best to test it at fuse #16.

* Make sure you are getting 12v at the fuel pump. Takes getting under the car...

* There is a plug at the back side (looking at the engine) of the mixture control unit, if you unplug it, then it will always allow the fuel pump to run. I would try disconnecting it, turning the ignition to on and listening for the fuel pump.

* The RPM limiter is also attached to the fuel pump relay. You can just unplug it to bypass.

If you CAN hear the fuel pump, then you will need to hook up the CIS gauges and figure out what kind of pressures you have.

Good advice.

I suspect the fuel pump also. I agree that the click you hear is the fuel pump relay and the fact that you don't always get the injectors to squeal is because the pump is not running. Tweaking the plunger arm will have no effect on the fuel pump once the plunger is barely lifted and you hear the click--it either runs or it doesn't. I think your manipulation of the plunger and subsequent spraying of fuel was coincidental and had nothing to do with the fuel distributor.

My suggestion would be to completely check out the fuel pump first to insure it consistently runs when it's powered, and then follow up with pressure and volume readings.

[Army 352P CWO]

Thanks for all the responses so far. I do have the Bentleys. I did replace the fuel pump with a new Bosch unit within the last 15K, so I hope that's not the culprit. I will so some further checking while under the car (hopefully tomorrow), though, just to be sure. I'll also check the relay and at the fuse panel for appropriate voltages. Can I swap out one of the other relays with the fuel pump relay, or do I have to get another fuel pump specific relay?

I've read in this forum that you ARE supposed to hear the fuel pump running, but I've also read other posts where people say they never hear theirs running. What's true, or can it vary from model to model? I'm guessing that when people say you can hear it running, they mean even when you're not under the car with your ear up to it - that you should be able to hear it while standing beside the car, correct?

Tim, I have read through most of your "CIS for Dummies". That's nicely done. You've done a great job of pulling the pertinent info together. Thank you very much - a great service to all.

I do know I may have to break down and buy the fuel pressure gauges, but was hoping to avoid it just now as this problem initially seemed to be an all-or-nothing on/off kind of thing happening .... until my 'fix' resulted in the car not wanting to idle now, that is...

Just FYI, I've also replaced the injectors with new ones approx. 10K ago, and I also replaced the sleeves and O-rings at that time. The RPMs always drop when I remove my oil filler cap, so I don't think I've got any vacuum leaks going on, or at least didn't until my 'fix'.

thanks again.

[tirwin]

I cannot say conclusively that you shouldn't hear the fuel pump on all years with the key in the on position, but that is my understanding. Mine is an '83 so I have all the bells and whistles they could do with the CIS system.

If you reach around with you left arm and feel around on the back side of the engine a little bit down and to the left of side of the air box, there is a plug that plugs into the pressure plate sensor. It is a bluish green color. That should be connected. If it is disconnected, the fuel pump will run with the key in the on position.

When my fuel pump runs without the engine being started it is a distinct sound. A loud hum is the best way I can describe it. Here is an old video I did. When I switch on the fuel pump (jumping the connection), you can hear the sound.

Porsche 911 CIS Cold and Warm Control Pressure Test - YouTube

If you don't have a known good relay, just try jumping the pins with a fused circuit for a quick test. I'm not sure if another relay will work. I think the others have a different capacitor.

Like the other guys said, check the pump and relay. If those check out, I think it would be time to get the gauges and work your way through.

[T77911S]

start with the easy stuff.

relay then fuse. the fuse may not be making good contact.
test for power at the relay pin 30. then jump 30 to 87a (?) to make the fuel pump run. much easier than crawling under the car.

[47silver]

My 75 runs when the key is on. I think sc's have the switch on the sensor plate. This is a safety item if the car is damaged, key on, motor operation, then stop the fuel pump. I think it is the fuel pump circuit. Bad ground for the fuel pump.

[ossiblue]

Let's not confuse the issue. You have a 79 SC. The fuel pump is not supposed to run with the key in the "on" position. Period.

CIS cars from 73-75 did not have the pump relay nor the air flow sensor switch so the pump will run on those year cars with the key "on." Yours should not.

With that in mind, go ahead an begin your troubleshooting. You should first establish that your power to the pump, via the relay, is correct. Secondly, you should establish that the relay is transferring that power to the pump. Third, you need to confirm that the pump runs as soon as power is applied, every single time--either by removing the relay and jumping socket 30 and 87a and turning on the key, or by unplugging the air flow sensor and turning on the key, or by lifting the plunger. It's your decision on how you want to test the reliability of the pump turning on.

If all the above is done and checks out, then you will need to test flow and volume of the pump and a gauge set will be necessary.

[boyt911sc]

Army,

If you like to find and fix this problem before the winter weather arrives, you need to hook up a fuel pressure gauge. The FP maybe running and that does not mean it is delivering the correct fuel flow!!! The FP maybe not the problem but you have to eliminate the most common culprits in CIS troubleshooting. By knowing the control and system fuel pressures, you'll have an idea if you'll be able to start the engine or not. Don't over looked the ignition too.

Someone just posted 'it is all about fuel pressure' and I'll add this, 'plus correct air flow'. These are the two (2) most common culprits in CIS and yet often over-looked.

Tony

[Army 352P CWO]

Ok, the fuel pump is running. I disconnected the plug on the back of the mixture control unit and heard it with the key 'On'. I even jacked up the car, crawled under, and took off the belly pan/guard to feel it vibrating. Now I know what it sounds like. Granted, though, I haven't confirmed it's pressure yet.

I've got 12 volts at fuse #16. And, I've got 12 volts at relay ports 86 and 87A, which I guess is consistent with the fuel pump operating as advertised.

Now I believe at least part of the issue is an intermittent connection between the fuel pump relay and it's port connections. With the key 'On' and the sensor plate propped up and me not hearing any fuel injector 'squeal' I can touch the relay and suddenly get the squeal. Also, if there's no squeal with the touch, I can start to pull out the relay and suddenly get the squeal. I removed the relay and opened up the cross-cut hatches on the prongs of the relay with a knife blade to make the prongs larger in diameter and that appears to have made the connection much better. However, I'm going to buy a new relay just to make sure there's not something internal going on as well.

Seems like L.J. was right about the intermittent squeal just being coincidental with me manipulating the pressure plate. Jeez, that was confusing me.

So, I guess I'm to the point of needing to buy some fuel pressure gauges to figure out the rough idle. The thing that kills me, though, is that the rough idle didn't start until after removing the FD and the mixture control unit, so I think it must be that I've created a vacuum leak somewhere by moving parts around. I've double checked all the fuel connections that I disconnected to make sure they're back on appropriately.

thanks for everyone's suggestions!

[Vereeken]

With the fuel pump running check the amperage it is pulling (you can do that on the relay or at the pump itself.

The higher the amperage the closer it is to checking out and the louder it becomes.

I think about 9-10 amps is normal.

The relays are known to go bad. I have one that fails when it gets hot.

Are you sure the wiring from the AFS to the relay is still 100%.

[Army 352P CWO]

Ok, so I went back out to the garage after lunch and finally figured out what was causing the rough idle. Time for a little humility on my part.

I again disassembled everything down to, and including, removing the fuel distributor and was double checking everything and every connection I could have possibly disturbed in my initial work. Well, wouldn't you know it, I found that there really had been an old rubber fuel distributor o-ring - squashed flat as a piece of paper and stuck to the bottom of the fuel distributor. In my initial post I had commented on how there hadn't been one when, in fact, I had put the new one right over it without seeing it even though I had been looking for it on my initial disassembly.

I scraped it off and continued my search for anything else that looked out of place. I didn't find anything else, but when it was all put back together the engine started and idled just like it used to.

Thanks to everyone for their suggestions on the initial starting problem. Everyone on this forum has been great!

At this point it looks like the male starter relay 'prongs' just weren't making good contact with the female port connections, but I'll have to drive it a little while yet just to make sure that was it. Fingers are crossed, and thanks again to everyone for there time! It's like T77911S said, "Start with the easy stuff."