CIS Odd Rough Running (at least to me)

[Jay Laifman]

My 1973.5 911T is running beautifully. Perhaps better than it has for a decade or more. But it has this one oddity. If I drive it hard, when it comes back to idle, the idle speed is lower and rougher than normal. Then it clears up again after driving it less hard. Once it clears, it idles as smoothly as ever, and pegged at a single rpm.

I don't know what to suspect. My first guess is that the plunger may not be sliding as smooth as it should. But that's just a big guess.

Anyone have any thoughts?

[Paulporsche]

With CIS issues, it's always good to first, make sure all your ignition components are functioning correctly.

Then ensure you have no vacuum leaks.

After that, you should check your fuel and control pressures with a CIS fuel pressure gauge and compare your findings with the factory chart for your model.

There are a huge number of threads on this forum regarding CIS running conditions so you could research them for other's problems and solutions, and you may find a shortcut that solves your problem, but you will find that the above procedure is really the best way to go about diagnosing them.

[Jay Laifman]

Thanks Paul. Yes, I've owned this car for 35 years and have been involved in many CIS threads on Pelican and elsewhere. I do have the various pressure testers and have run it through the ringer on that. Because this was an oddity I haven't seen before, I thought I'd ask.

As I said, it's not like it has an ongoing problem. It is working great. Idling great. Great power. Smooth through the whole range. I don't think the regular pressure tests would be off if all those were good.

[SkiVT]

Would it be something related to heat like a vacuum leak from a hose that gets hotter and expends a bit during the hard running then cools off? Seems like my engine compartment has more heat after a spirited drive than after my commute. However a hot summer day should be similar so does it happen under that condition as well? Does that heat change the wur pressure just a bit? Something else tied to heat?

[Paulporsche]

What brand of CDI box do you have? There has been a lot of talk regarding Permatunes when they get hot, although any other brand that is 30+ years old could have issues.

Have you checked for vacuum leaks, incl the injector O rings?

Are all your ignition components good? How old are your wires?

Have you checked the height of the sensor plate when this problem occurs?

I'm wondering if your WUR is a little off when hot. Obviously this can only be verified with a control pressure or mixture test.

[Jay Laifman]

Thanks for all the thoughts. Everything on it is pure stock Porsche.

I did do a smoke test a few months ago. The only smoke came out of a couple injectors. I replaced all the injector seals! I got all the seals from Porsche directly.

The wires are also factory, braided covered CIS wires. I got them in the last 10 years. Not sure of the exact date.

I'm going to try to narrow down the symptoms. As I said, it is driving 100% beautifully. I just drove somewhere and noticed it was great, even when I got off the freeway. But then a few miles on surface streets, I did some aggressive acceleration. And once again, the idle dropped about 200 rpm and gets a little gruff. After a few more stops, I held the rpm up with the pedal, and then at the next stop, it seemed to be back to its normal idle speed and smooth.

On my next drive, I'll get it into the same condition and lightly move the throttle plate to see which direction has which result.

THANKS for the thoughts!

[Jay Laifman]

Oh, I should add that when I did the smoke test and got the new o-ring seals, I also replaced the large o-ring between the throttle box and the air box. I did it "just because." I think the idle issue was happening before I did this change. But right now I can't swear to it. I would think that if I created a leak there, I'd have other symptoms. But maybe this is one of them.

[Dave Kost]

Jay,

Weird. I've had 2 of these engines and ended up pulling the CIS system off and replaced all the rubber and gaskets. Found a couple of the intake gaskets were deteriorated

Check your fuel pressures-
System
Cold control
Warm Idle
Warm part throttle
Warm Full throttle

(The last three tests, are testing the Throttle valve.)

Check Ignition distributor for smoothness with a timing light- Clean up. Maybe the weights are locking up.

In the engine compartment should be a decal on RPM and CO settings at Warm Idle. You need a CO meter or AF ratio meter to do this properly. I have a Innovate LM2. You can't guess or do this by ear.

I would hook up a continuous monitoring system and see what changes when you go into low idle mode.
Fuel pressures - Fuel pressure gauge
CO /Air-fuel ratios - Innovate LM2
Battery voltage - The one the goes in the cigarette lighter.

I assume that you have the proper FD, Wur and Throttle valve in place.

Ill dig up the spec sheet I have on the 1973.5 engine.

[Jay Laifman]

I think I figured out what may be contributing to it. I am going to do some tests - and yes do the full suite of pressure tests. I'll report back once I see.

[Jay Laifman]

Well, first thing I found. The Throttle Position Valve doesn't seem to be doing anything. No fluid is going through at any position.

I have two others I must have collected over the years. So I tried to "test" them by blowing into one of the ports. But nothing seems to go through, no matter the position of the throttle. Is that just a bad test? I used my air compressor, but wasn't sure how high to turn up the pressure.

[boyt911sc]

Originally Posted by Jay Laifman

Well, first thing I found. The Throttle Position Valve doesn't seem to be doing anything. No fluid is going through at any position.

I have two others I must have collected over the years. So I tried to "test" them by blowing into one of the ports. But nothing seems to go through, no matter the position of the throttle. Is that just a bad test? I used my air compressor, but wasn't sure how high to turn up the pressure.

Connect a pressure gauge and check the pressure readings @ 3 different cam positions. That will give you an idea if the TPV is good or defective. Blowing pressurized air is a meaningless test.

Tony

[Jay Laifman]

Thanks. But the one on the car is showing no change. I can put on the others, but no sense doing it if they are bad too.

[Dave Kost]

Here is a flow chart of your CIS system.

It shows how the Throttle valve and WUR work together changing the Control Pressure.

I would clean both devices and blow out all the fuel lines, especially the return fuel lines before condemning the 3 throttle valves. I think its odd that all three are bad.

Here is the pressure you are shooting for

[Jay Laifman]

Thanks. I guess I'm asking that I understood the failure mode of the TPS is that they let gas through at all times, not just at WOT and idle. So I would think that if I couldn't blow any air through in any position, they are bad - or at least not good. I put some carb cleaner down the small pipe for the hose and have it soaking upright. Maybe if it is just blocked by old gas, it will clean it up.

I have also emailed two vendors to see if they happen to have one. Probably not.

[Dave Kost]

Here is one....
https://www.ebay.com/itm/361690684358?fits=Model%3A911%7CMake%3APorsche&epid=1157301872&hash=item543671dbc6:g:uKgAAOSwZVlXtJQf

[Jay Laifman]

Thanks. I ordered it. But Porsche itself still sells them. If this one doesn't work, I'll probably just buy one from Porsche.

The thing is that I don't know how to know if these are good or bad. Like I said, the failure mode I believe is usually letting too much gas by - not no gas. I sprayed the inside of the two I have off the car, and let them soak overnight with the carb cleaner fluid in them. Nothing changed. No matter how hard I blow into it, no air comes out the other side. I have to think that a proper working one would at least have some air getting by when rotated to either end. Unless I'm misunderstanding something here.

I will try some penetrating oil next to see if the plunger is just stuck and the carb cleaner isn't getting through.

[Jay Laifman]

In case anyone cares, I cut open one of the throttle position valves that I had to see what was going on and what I could do to fix it.

Here are some observations and comments:
First, the cam is not equal. There is a longer ramp up on the idle side, with a shorter one on the WOT side. This of course makes some sense. But from all the drawings I've seen, I assumed it was the same at both sides.

Label 1: the ball rides on the cam, pushing up the spring. On the top of the spring is that little piece with the pointed top.

Label 2, 3, 4 and 5 all screw together in that order.

Label 3 pokes down through the hole in 2. And the pointed top of the spring pushes into it.

Label 4 are two thin metal discs.

Label 5 has the 0 ring which metal discs go up against. When the cam is pushing the spring up, it pushes up the metal discs and sealing the hole in the center of 5 so that gas cannot go out. In theory, when the cam loosens the spring, the discs get flexible enough for gas to push through the center.

The feeder/intake pipe goes to the bigger hole on the side. The exit gas goes through the center.

I see threads about people soaking them in cleaner to make them work again. However, after seeing this all apart, I can't see how that would ever help.

[Jay Laifman]

Ok I must admit total foolhardiness. And apologize to Paul, Dave and everyone. I so firmly and wrongly believed my pressures were testing correctly.

I retested and they are wrong. My TPV was not doing anything because there was not enough pressure on it. There is not enough pressure on it because the WUR appears to be letting out too much gas.

I currently have an 009 on there. I had a series of 029s that were bad. But I threw one of the 029s on there and the pressure did go up, and closer to the correct range. And then the TPV started working. Is it possible that the 009 just lets out more volume than the 01 and 029?

I have no explanation as to why it is running so nicely with the 009 on there, other than that idle issue. But I am going to start all over and see where I can get.