Carpokes Porsche Forum

I could have added this to another thread, but thought it might also stand on it’s own as a reference.

Most of the fuel hose/line size alternatives for the Porsche 944 have been noted in many places. While I’ve seen replacement fuel hoses in 3/8”, 5/16”, 7mm, -6an (and more), it leads me to wonder which one is closest to factory sizes? Since I’m DIY for the fuel feed/return lines in the engine compartment, it’s a good opportunity to take some measurements and document them. This is based on my 1988 944 S with original fuel hoses. Due to uneven hose shapes, the measurements could be off a 10th of a mm, give or take. First the measurements, then what I did, with a parts list at the end.

Factory hose/pipe sizes

Following the fuel … the metal fuel feed line is a 10mm outside diameter (OD) metal line, with an 8mm opening which feeds the fuel hose going to the fuel rail.


The nut connecting the fuel feed metal line to the fuel feed hose is an M16x1.5 fitting. The fuel feed hose, above the exhaust headers, has a 7.8mm (call it 8mm) interior diameter (ID) polyamide (nylon) hose, surrounded by 2mm of rubber, for an overall 14mm OD. The rubber-surrounded polyamide hose was phased into production during 1988. Looking at the current Porsche replacement hose, it appears to be the same construction, however, the replacement hose I looked at was an inch or so shorter than the original hose, which pulls it out a bit more than the original hose over the exhaust headers. The hose-end fitting of the fuel feed hose that goes into the fuel rail has a 6.2mm opening and uses a M16x1.5mm nut.

Any fuel not consumed by the fuel injectors leaves the pressure regulator through a 5.1mm opening, into the fuel return rubber hose with a 7.5mm ID and a 13mm OD. This rubber hose connects to the metal return line with a 6mm opening and an 8mm OD. The nut at that fitting is M14x1.5mm.


My DIY replacement fuel hoses

So with those sizes in mind, here’s what I did (standard caution applies … this is intended for education and to share what I did, any use is at your own risk). First, thanks to Carpokes user icb for his post on his DIY solution, which provided good direction and was a big help.

I wanted to reuse both the M16 and M14 fittings between the metal fuel feed and return lines and the rubber hoses. The M16 fuel feed fitting has a polyamide interior hose which is challenging, as the polyamide hose fits tightly around the fitting barb. If you’re DIY, and it’s a Porsche fuel feed hose with the polyamide interior, the rubber outer layer can be sliced off, however, proceed carefully taking the polyamide hose off the fitting. If you score the fitting, like with a Dremel or saw, it can’t be used, as shown below (thankfully not mine).


The original rubber fuel return hose is easier coming off the M14 fitting, since it’s all rubber and not polyamide/rubber like the fuel feed hose.

Since hose barb fittings are slightly different for polyamide and rubber hoses, I used two 15.5mm Oetiker clamps, at 90º to each other, for mounting the new rubber fuel hose on the fuel feed polyamide fitting at the metal line (probably overkill). At the fuel rail side connection, I replaced the fuel feed stock polyamide cone nipple with one made for a rubber hose, and used a single 15.5mm Oetiker clamp.


On the fuel return rubber hose, since the fuel flow is lower pressure than the feed, a single 14.5mm Oetiker clamp was used on both ends.

For the fuel feed rubber hose, Gates 8mm ID Barricade MPI hose was used, which is rated to 100% ethanol, good to 275º, and a working pressure up to 225 PSI, more than enough for the 944 S 52-58 PSI fuel feed. For the fuel return hose, like icb, the Cohline 7.3mm ID high pressure hose was used, which is also rated to 100% ethanol, good to 257º, and a working pressure up to 145 PSI, easily handling the lower fuel return pressure. The Gates hose uses 15.5mm Oetiker clamps and the Cohline hose uses 14.5mm Oetiker clamps.

With the hoses complete, there was still the question of how to protect them from heat. The original rubber covers could be reused, however, I didn’t know if the 37 year old (now) hard rubber covers would be as effective in 2025 as they were in 1988. Depending on the type of hose being used, there are a number of options … I went with black Thermo-Tec Heat Sleeve, with a braided fiberglass interior and silicone rubber covered exterior, which has a 500º operational rating. The hoses are also positioned by the firewall, to lower exposure to exhaust header heat.


In the last step, the DME relay was removed and a jumper installed to power the fuel pump and check for leaks.

Alternatively to DIY, you can buy the Porsche hoses, which for a 944 S lists at $190 (fuel feed hose) and $201 (fuel return hose). There are other non-Porsche hose options too.

Parts mentioned and sources:

14.5mm and 15.5mm Oetiker Series 154 high pressure clamps (Belmetric CHP14.5YLWSS and CHP15.5YLWSS)
Cone nipple for 7-8mm rubber hose and M16.5 fitting (Belmetric CNPL7-8/16X1.5)
Swivel nut for cone nipple M16.5 (Belmetric CNPL16X1.5NUT)
Gates 8mm ID Barricade MPI hose (Pegasus Auto Racing Supplies GTS‑2734‑05)
Cohline 7.3mm ID high pressure hose (Belmetric RH7.3HP)
Thermo-Tec Heat Sleeve 3/4” x 3’ (Summit Racing THE-18075)

Nice write up....Thank you!

Nice job!
When I did this, I checked every joint for leaks, looking for any traces of moisture or fuel odor on my fingertips, every time I fired Gretchen up, and also when I returned her to her garage afterwards, for about a month, wanting to make absolutely sure there were no leaks at all. Luckily there have been none. I still check this way every once in a while since fires suck so badly.

This is great information, thank you for documenting things so carefully and sharing the results.

One thing to note is that for the polyamide lined hose, it is generally not considered good practice to use hose clamps. This is part of why it's so important that the barbs on those fittings remain fully intact. You can see this on the OEM hoses, where there is no clamping force from the beauty cover things right below the fittings. I don't have sources off the top of my head, but my memory from reading about this is that certain folks (and maybe even Cohline) consider the use of clamps to be deleterious to the seal.

One other note for folks that want to go down this route is that heat is your friend to remove the polyamide hoses from the fittings. Since you don't need to worry about the old hose, you can heat the ends up pretty hot, and then pull the old hoses off.

Cheers

One thing to note is that for the polyamide lined hose, it is generally not considered good practice to use hose clamps.

Good point and reminder if anyone tries DIY replacement with a polyamide hose. New hoses used above are rubber hoses.

One other note for folks that want to go down this route is that heat is your friend to remove the polyamide hoses from the fittings. Since you don't need to worry about the old hose, you can heat the ends up pretty hot, and then pull the old hoses off.

Dang, new news! Will file that one away ... my 944 S has uncovered polyamide lines between the fuel rail and fuel damper, which will need attention at some point. Did some searches and couldn't find information on a range of service life for polyamide fuel hoses.

Thanks!

I rebuild all of the fuel lines on my German cars after learning that everything is barb fittings on the DIN hydraulic stuff.

I use Cohline 2240, which is 100% ethanol proof, and is the S tier stuff. Bel Metric has it by the foot for a good price. Far better quality than anything you'd find locally somewhere.