Houpty GT

The car did have the incorrect CSV flange. This has fixed the cold idle speed but still leaves me with a lean mixture off of idle. I will update more when I figure a few more things out.

Houpty GT

This is a screen capture of a graph showing my air fuel ratio from a cold start on a hot summer day. What you can see is the
1.initial air fuel ratio is just about that of air
2.AFR drops below 13-1. Most likely caused by a working cold start valve.
3.AFR goes to over 16 and works it's way down to 14. I do not know why.
4.AFR works its way up from 14.1 to 14.9. WUR takes this long (30 sec.) to lean out the mixture in the summer.

My fuel pressures are all in check and the fuel pressure accumulator hold pressure to spec. The problem gets worse in the winter and also after semi-hot (1 or 2 hour) restarts. The car also backfires when the engine is cold and you try to accelerate even for the first few minutes in the summer.

31 deg.C air temp
2.45 bar control pressure (spec 2.45 rich end - 3.4 lean end)
67 psi system pressure (spec 65-75)
30 seconds for electric heater to raise control pressure to 3.6 bar (spec 3.4-3.6)

2.6 bar with heater disconnected and smooth idle gives 13.3 AFR
3.65 bar after heater connected gives 14.1 AFR.

Specified duty cycles for frequency valve warm-up all work also. This and engine temp variations also explain some of the variation in AFR. No oxygen sensor was connect to the ecu for any of these test.

So does anybody have any ideas what is causing this?

Houpty GT

I would like to share a couple updates and bump this thread back up. I recently checked my fuel distributor to make sure it was the correct part # and it was. The fuel distributor was replaced with an overhauled unit by the last short term owner. He was just replacing parts because he did not know what he was doing. Is there anyway he could have installed it incorrectly to cause a problem?
When I got the car, the control pressure regulator was clogged with rust and I had to clean it out to get it to work again. I also cleaned the fuel tank with acid and have replaced the filter many times. One of the last things I did before letting the car sit most of this last year was pull the injectors and test them of a bench rig with Techron. They all sprayed but a couple of them improved after a few pumps. Not great but acceptable the porsche mechanic said. I do not think the fuel distributor is clogged since it moves smooth. The next big test I am going to to is an injection quantity balance comparison by spraying the 6 injectors in test tubes while still hooked up to the fuel distributor.
I am also planning to buy a 0-100 psi sensor to record system and control pressure with the LM-1 AFR meter and see if the pressure is holding under load.

Houpty GT

Yesterday was the first time I have driven my car in 5 months. I have installed the fuel pressure sensor for my data logger but have not uploaded the data to my computer yet. I have not checked my injector balance but I have 2 checks I am going to perform first. I am going to recheck my fuel pump delivery which I may have checked at an incorrect location in the past. I know the pump and the filter were good but there could be a clogged line somewhere. I am also going to check my control-pressure through flow. I had never heard of this before but I read that it can cause mixtures that are too lean or too rich. Typical is listed as 160-240cc in 60 seconds.

Does anyone have an exact spec for the 1981 911SC for control-pressure through flow?

Houpty GT

I should have some fuel pressure logs from my Innovate data logger once I get my computer to cooperate.

I ran the 2 fuel delivery test and took a picture of the results. The green 2 liter bottle that was slightly melted as I dried it in the microwave, has the results from the fuel pump delivery. This measurement was taken from the bottom of the accumulator. In 30 seconds it delivered what appears to be slightly less than 1 liter. The delivery rate is supposed to be 1.0 liters minimum. Fuel filter has been changed several times and the pump has been changed twice. Rust from the fuel tank was an issue when I got the car. I am not quite sure where to check for a clogged line?

The label-less clear 20 oz. plastic bottle contains the results from the control-pressure through flow. According to the Bosch Fuel Injection and Engine Management book, a typical fuel delivery should be 160-240 cc (about 5-8 oz.) Judging by just eye this appears to be within this tolerance. I do not have the exact delivery amount that should be expected. The book states the fuel pump delivery is typically 0.75 liters but the Bentley states 1.0 liters. Do we also expect a 1/3 increase for the control-pressure- through flow?

jimwms

Typical control pressure flow is 225 cc per minute. This is the same spec for all the 911 and 930 Fuel distributors. The 160-240 number also comes from the Porsche Workshop Manual. This flow is determined in part by the 70 psi regulated by the Fuel Distributor, and by the throttle hole size internal to the FD in the metal diaphragm, about .015". The control pressure flow is not dependent on the Fuel Pump flow, that is only a number that says the pump should flow at least that number at the system pressure.

Houpty GT

Have you ever seen these bad before? What was the cause?

Today we noticed the car shuts off when the oil cap is removed. I will investigate this but I doubt it will fix my mixture variation that happens coming off of idle.

jimwms

If you mean an out of spec flow rate, yes. Most usually caused by a rebuilt fuel distributor when a new diaphragm was used with an oversized restriction. This creates too much flow, and the WUR can't regulate the pressure which = too rich a mixture.

There should be a cone shaped restrictor in the vent line leading from the oil tank back to the rubber airflow boot. The restrictor will let the tank fumes vent but not let enough false air into the boot to lean the mixture to the point that the engine dies when the oil cap is removed.

Houpty GT

I bought a complete 911SC fuel injection system last October. I changed out the fuel distributor with one I got and the car runs much better. Obviously the fuel distributor rebuilt by Jaytan and installed by the last owner created this final problem. I thought I would contact Jaytan Inc. in Orange, CA about the lousy product they sold the previous owner for $370 but it looks like they have changed names and there is nothing but bad reviews about them out there. Jaytans claim that in, "99% of the instances it is not the Jaytan fuel distributor," has no merit. Beware your rebuilds!

I would have some fuel mixture logs and report back but I have been having some problems after it has been sitting. Dead battery, no start, flooding. I thought I lost my ignition system but it just ended up being flooded. Horizontally opposed motors do not drain themselves back into the crank case. I'll get the car out this weekend for some fun.

Houpty GT

I still have another problem that started showing up a few years ago. Occasionally the duty cycle will slowly go to max lean when it should be in open loop. It would do this even with the 02 sensor disconnected and I figured the wire was picking up stray voltage somehow. I disconnect the wire for the 02 sensor at the ECU and it still went to max lean once this last Sunday. After a restart the car went back to open loop and acted normal again. I have another ECU and I am going to try to see if it happens again. It is had to track down a problem if it is sporadic.