73 911T MFI pre heat hoses

[Jjason3131]

Trying to figure out where to connect inner pre heat hose off of MFI thermostat. I have new outer paper/aluminum/plastic hose on order. Current one is damaged on ends. (See Oval area) Planned to cut it down for use as inner hose. Also, any issue using same type hose for Inner version? As I was disassembling, I discovered that prior owner had simply taped the inner duct of the thermostat closed (see red arrow). According to post of 06-10-11 reference is made to 'engine tin' as place to connect inner hose, but these eyes cannot see where the connection is supposed to be made. Any help appreciated. Car runs very rich and trying to solve that problem.

[manbridge 74]

Start at the heat exchanger on the left side. The big thick rubber type hose goes from forward area of heat exchanger fitting to engine tin by cylinder 2. Other side of tin from this is where your smaller double hose connects, other end going to t-stat. Also present in 73 are the hoses from intake pre-heater one underneath at cyl 3 tin area. The other comes up from rear of exchanger through tin in area just in front of left side chain housing.

Porsche put some thought into the thermostat system. One of the parameters is the rate of heating of the air hoses to the thermostat. Changing the type of hose changes the heat-up characteristic of the thermostat.

The best place to start is stock. Most changes involve getting the thermostat to turn off sooner. This can be done with an aluminum hose inside the stock rubber hose. There is a VW hose that can fit over the double concentric hoses from the tin to the thermostat.

Big issues are that there aren’t any air leaks at the heat exchanger or hose to the heater control box.

[Jjason3131]

All good with the hoses that you mention. The one that has me confused is the placement of the "inner" hose that exits the thermostat at the point of the red arrow. It is shown on some of the detailed drawings as a much shorter hose than the "outer" hose that enters the thermostat directly above and within the oval. Once I connect one end of it to the exit point shown by the red arrow, where does the other end of the "inner" hose connect? Thanks, J

[Jjason3131]

Additional photo posted of exit point for inner hose. It is my assumption that the fan is drawing air through the thermostat from the heat exchanger in order to shut off the cold start mechanism in the MFI. As the prior owner had the inner hose exit point blocked with tape, and not ducted to anywhere, the thermostat could not do its job and the engine was always running rich, as if it was cold. I just need to know where to attach the inner pre heat hose to the fan shroud. There is a nipple almost directly below the thermostat but there is a line attached to it that exits the engine compartment at the same place that the oil breather hose from the oil filler tube exits the engine compartment. Is it supposed to go there? should there be a splitter on the nipple? thanks.

[Jjason3131]

After having read several additional posts on this subject, Im beginning to understand that the words "inner" and "outer" as they refer to the pre heater hoses relate to one hose with multiple parts, not to their location. In at least one photo I have seen of a similar engine, it shows no hose attached to the exit point that I identified in the photo below. I assumed that there was a necessary suction effect from the fan to draw air from the heater box through the thermostat to turn off the cold start mechanism, but perhaps that exit point just needs to be open?? In any event I removed the tape that was sealing it shut. Again, your thoughts are welcome.

[kwikt 911]

The inner/outer hose is referring to the 2 piece hose only connecting to the top portion. The outlet area on the pump is always open...no hose!

[manbridge 74]

In cold Colorado I’ve ran another paper/foil hose from exit to either fan shroud nipple if available or to the area right next to the fan to help pull air across t-stat. This really helps turn it off in below freezing temps.

[RSTarga]

Nothing goes there, it is open.

[dicklague]

not sure what climate you live in, but here in So Cal. I removed the thermostat and made an aluminum plate with a 6MM bolt and lock nut. I use my AFR intstrument permanently installed on my dash to monitor AFR when warm.

I start the car with a dash mounted push button hooked to the solenoid valve on top of the fuel filter that squirts raw fuel into the trumpets.

This gives me full control over the start and warm up. I ALWAYS had problems with the thermostat making the MFI too rich.

I have been running this setup of 15 years with no problems.

[dicklague]

Here is some more information. Do some searching on this site, lots of great, informative MFI discusssions.
MFI cold start problems

[Jjason3131]

Thanks to all for quick replies. Really appreciate the info.