Winter Projects 2019/20: part 1- Intake removal

 December 27, 2019. There was a lot of work to do to get Gretchen ready for spring. I planned on replacing the timing and balance shaft belts, any of the rollers that needed it (determined by bearing feel), the water pump if necessary, the accessory belts, front of engine oil seals, plus anything else that could be taken care of easily, or was necessary for safety, reliability, or driveability.

The air cleaner box and air flow meter were the first to come off, followed by the the fuel rail with injectors and fuel pressure damper attached. Even though the 4 injectors are only held into the manifold by single o-rings, they had been in there for a while and it was quite difficult to pull all 4 out together while attached to the fuel rail. WD-40 was helpful to lubricate the dry o-rings. You must take care not to damage the injectors in any way, they are pretty expensive, precision parts. The fuel pressure regulator stays attached to the fuel rail. The fuel pressure damper, mounted to the front of the intake manifold, is attached to the fuel rail by short hoses. It is best to just unbolt it from the manifold and leave it connected to the hoses.

 

 The intake duct, throttle body, and intake manifold all came off as one assembly.  This was a lot easier than removing the fuel rail, as the intake is made of magnesium and is very light. There are a couple of vacuum lines to disconnect underneath, a few bolts, the throttle linkage, and out it comes.

With the intake assembly on the bench, you can see the idle control valve and its hoses leading from the intake tube and to the plenum, and the venturi plumbing underneath. This is a prime area for air leaks.

In a "perfect" 944S engine, all of the idle air goes through the idle control valve. The throttle body is completely closed during idle; there's no throttle bypass air adjustment. The computer (DME) has 100% control of the idle airflow, fuel/air mixture, and ignition timing in its mission to set the target idle speed, which is not adjustable. Bosch calls this "Adaptive Idle Speed Volumetric Efficiency Regulation". I believe the 944S was the first to have it. The interesting thing about this is that it makes the engine really tolerant of air and vacuum leaks, because it just adjusts the fuel, air, and timing to compensate. That can make finding leaks a bit difficult. Smoke testing or pressurizing the intake with air seems to be the most effective way to do it.

There were leaks all over the place as you will see, contributing to a rough, lean idle, especially when the AC compressor was running. Normally, adaptive idle will compensate for a few leaks, but I believe what was happening was that there were so many big leaks that the system couldn't adjust itself that far. It was really prone to the "stall returning to idle" problem, especially warmed up with the AC on. This is where the engine stalls as you are rolling to a stop, just as you take your foot off the gas pedal. You read about this issue a lot in the 944 forums. There have been a lot of out-there explanations and fixes suggested, but in my (admittedly limited) experience, I've found that the problem gets progressively less of a problem as you fix the air leaks.

The throttle body area in the 944S is very cramped. You can't get to anything or even see very much. This hose with the blue silicone all over it is the air oil separator hose connection back to the intake. The gap faces the firewall and was totally impossible to see with everything installed. I knew the nipple was separating from the intake tube; I could feel it and see it moving in a way it shouldn't have been able to, so I tried to silicone it up as a temporary fix until I could get to it. Even though I had put a ton of silicone back there I still left a huge gap!

 This is a great example of a "False Air" source. False Air is a Bosch term used to describe sources of intake air which don't pass through the air flow meter, and therefore don't get accounted for in the fueling calculations. Since this leak is on the atmospheric side of the throttle plate, it really shouldn't be considered a vacuum leak, but it's equally problematic in a Bosch Motronic controlled engine. I refer to these atmospheric leaks simply as "air leaks" or "false air".

This deteriorating hose was a big false air suspect for me, but in fact it held vacuum when I tested it. These hoses aren't available any more, so for the time being I filled the cracks with silicone while putting the hose under vacuum. Any cracks that went all the way through would pull the silicone into it. A very temporary fix, but hopefully I can find a good NOS or used hose somewhere and replace this one before too long.

There was a huge air gap here at the inlet of the throttle body, where the boot and the clamp were installed incorrectly.

Next time, I will begin cleaning the engine. Stay tuned!