Jay Laifman

I just wanted to say that my 73.5 911 is now singing nicely. I think I can still dial it in some. But, I'm pretty confident that it's fixed. I'd like to thank most enthusiastically Art Zasadny, Jim Williams and Pete Zimmerman. I wouldn't have fixed it without them. I've seen so many guys come through here with CIS problems, showing their desperation. I felt just like them. I'm glad I did not give up hope. I'm amazed how much patience and help Jim and Pete provide. I look back at my posts and see how silly I looked. I think, "wow, that guy (me) should give up and take it to a shop that knows CIS." But, I'm so glad that these guys came through. I learned so much about my CIS and I'm so empowered to know that I fixed it (or THEY fixed it with my hands doing their bidding!)

Here are some details of what I had and what I learned for those of you who may be in similar situations. My car was running rough. It was hard to tell if it was fuel, electric or an off balance engine. It started fine, hot or cold (though previously the $12 return valve contributed to a decade of hard starts). It couldn't both idle and run well, it was really one or the other. Also, it really pretty much maxed out to a slow acceleration at about 5,000 rpms, though it used to happily spin past redline. I took it to a smog place and got a CO reading which was way too high.

I bought a CIS fuel pressure pump from Performance Products. It was about $200. I later learned that JC Whitney had one for $60, which seems just as good. One thing to know is that the Performance Products one is made by Baum and is an excellent tool. But, you have to hit the pressure release valve between readings, much like an tire pressure gauge. The JC Whitney one apparently drops with the pressure.

Many of you probably already know this. But, I learned that the system builds up a certain amount of pressure. It is then *reduced* when the Warm Up Regulator and Throttle Position Regulator releases some gas out of the system, which drops the pressure. The released gas then circulates back to the tank.

I initially misdiagnosed my Throttle Position Regulator as the problem because it did not allow enough change when I rotated the throttle. But, I later learned that it was fine. It was that the WUR was letting out too much gas at all times. So, even if the TPR in it's closed position at partial throttle, the pressure was too low. The TPR releases more fuel at idle and WOT.

I found that my system pressure was too high. I think this is odd because as far as I know, the pressure only drops when the spring in the valve in the fuel distributor weakens. I took out two of the discs in the valve and lowered the pressure to the exact right spot. This still didn't resolve anything.

I installed a new WUR and the car was better, though still not right. I kept driving around town adjusting the richness, an 1/8 of a time. But, I couldn't find a sweet spot. On the other hand, the 5,000 rpm max was gone. It kept pulling and even got a kick about then. So, obviously, with the WUR leaking out too much gas, keeping pressure down, it just wasn't working.

Finally, per Pete's insistence, I checked the air flow sensor. To my knowledge, this has never been touched by hands since the factory, like the pressure setting. But, the plate was about 1mm below the line it is supposed to be. After a lot of trial and error in bending the spring, I got it right on. The side gaps were all good. I thought it would be difficult to take of the top to get at it. But, it was easy.

A little more adjusting of the fuel ratio after that, and it's running very nicely. I'm still going to go to the smog place to get a CO reading and get it perfect. But, that's it.

Thanks again to Art, Jim and Pete.

Brett San Diego

Jay,

1 mm below where it should be when the engine is off? So you're talking about just changing the resting point of the air sensor? Wondering why that would affect the system when running since air flow will raise the sensor plate into the cone above the rest position when it's running.

Or maybe I'm not clear on the spring that you're adjusting. Where is it exaclty? Does the spring change the position of the plate relative to the rest of the lever arm assembly (or change the position of the lever arm relative to the piston in the fuel distributor) so that you essentially change the "fuel map" across the working range of the sensor plate movement.

Brett

Jay Laifman

Well, my advice is only as good as what I've learned from Pete and Jim. But, yes, you are correct that this is only the resting spot. I don't know where it sits at idle. Perhaps right on the resting spot. If so, that would be critical as that is where the base line is for setting the fuel mixture. I guess you'd be right that my adjustments that I made while I was driving should have been able to find a spot where it ran just as good above idle as it is now. I don't have an answer to that. All I know is that I didn't do anything different in trying to find the right spot before and after the adjustment, and it didn't work before and did work after. Hmmmm.

Brett San Diego

Now, that I think of it, I think you're right. The system has a throttle bypass air flow passage for when the car is at idle, so I believe the sensor plate sits at its resting position at idle. Since you adjust the fuel mixture at idle (sensor plate at resting position), you'd have a fuel map that was off throughout the sensor plate movement range if the plate was low to begin with. Good information. Thanks for posting the resolution.

Brett

Peter Zimmermann

Brett: Sensor plate height adjustment is done with the engine off, and the pressure released. The "spring" that Jay mentions was no longer used on your car, thankfully, you should have an adjustment screw. Jay is right when he says that the sensor plate adjustment provides a baseline, and that adjustment is crucial. The system is designed for the plate to rest at a specific point in the parabola, and also to move smoothly through the opening without touching. It is one of those often overlooked adjustments, and if the plate is a little off it can play heck with the diagnosis process.

LaughaC

You guys rock! I look forward to your advice the next time my CIS acts up.



When the weather gets cool it surges at idle, but I won't highjack this thread and will post when the problem returns this fall.