924Board.org

[gml]

Looking for some help with my fixing my control pressures.

My 924 has been running with system pressure = control pressure (4.6 bar) for a long time. It never ran well, but it ran. I've rebuilt the WUR but it did not fix my issue.

This week I took apart the WUR again. The screen looked quite clean, but I removed the back of the WUR, so no spring/pin pressure on the diaphragm, and this "baseline" control pressure was still equal to system pressure (4.6 bar)

After a few rounds of soaking the inlet screen with throttle body cleaner and blasting it from the inside out, I started getting lower and lower baseline pressures. I'm at 0.6–0.8 bar now.

My issue now is that with the WUR re-assembled, cold control pressure is at my baseline (~0.8 bar) and it doesn't rise with the heater/engine temp. My car wasn't able to idle with control pressure. It had the best throttle response I've ever experienced with this car, but it wanted to die unless I gave it throttle. Turning my tester gauge to force system pressure and the car ran exactly as it had before (stable idle when warm, but poor throttle response).

Cold control pressure is too low, but the warm not rising is odd. The strip heater assembly is measuring 20.1 Ohms at the pins (right on spec). The car wiring to the strip heater is putting out 10 V. When I plugged the strip heater assembly in on its own the green part was getting hot to the touch. I tested by letting the strip heat up for about 4 minutes and during that time the control pressure was around 0.6 bar and not moving at all — after that test I got the WUR off and disassembled pretty quick, and I felt the heater and strip were still pretty warm.

I then tested "pseudo-warm" pressure by assembling the WUR without the bimetallic strip + heater, so it was full spring pressure on the pin. This "pseudo-warm" pressure was 2.9 bar, which is also low.

To summarize:
- System pressure is 4.6 bar (lowest within spec)
- Baseline (no diaphragm pressure) control pressure is 0.8 bar
- Cold control pressure is 0.8 bar (low; expecting around 2 bar at this temp)
- Warm control pressure isn't registering even though heater is working
- "Pseudo-warm" (max diaphragm pressure) control pressure is 2.9 bar (lower than what I need warm control pressure to be (3.4–3.8 bar)

I'm thinking things are just out of spec, perhaps due to my rebuild, and I need to recalibrate the strip and the diaphragm.

1. Increase pseudo-warm pressure to target ~3.6 bar
I believe that gently tapping in on the fuel cell (the dark metal face where the fuel lines bolt into) will increase pressure on the diaphragm when the pin is pushing down on it.

2. Increase cold control pressure to target ~2 bar
I believe I need to tap down on the pin where the bimetal strip bolts on, making the pin closer to being flush with the front of the WUR. The aim is to ensure the bimetal strip is moved closer to the diaphragm, allowing more spring force to act on the pin. I suppose right now the bimetal strip is not allowing the spring to act on the diaphragm at all, to the point where even when the bimetal strip heats up it's still too far away to let the spring act on it.

Before I go hammering on my WUR what do we think about this? Does my plan make sense? Does my theory of why control pressure isn't rising when warm make sense? Maybe I messed up these tests?
_________________
George

1982 924 NA 5-speed

[924RACR]

It entirely makes sense that you may need to readjust the plug to set the control pressure right; there was actually an article in Up-Fixin decades ago describing how to make it more adjustable, so you can dial it in... basically would thread a small screw into the plug, tap it in too far with a hammer, then unscrew to dial back the pressure to a desired value in a controlled fashion.

I actually did this for my racecar, with it still installed (just unbolted from the intake), didn't take long at all. Then I converted to EFI and got 100% control of fueling.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[gml]

Wonderful, thanks! Yes I've seen lots of people drill into that pin and tap threads for a bolt where they can pull it in and out. I'll see how frustrated I get with tapping on the pin before I pull out the drill and taps
_________________
George

1982 924 NA 5-speed

[Raize]

I would just leave the fuel cell as-is if you can achieve 2.9 as a hot pressure and consider it a performance upgrade as you’ll now be running 12-14 AFR instead of 13-15 as it should be. Based on my experience with the NA k-jet and its AFR curve. Do you have to pass an emissions test?

Do the drill and tap thing or you’ll be there forever going back and forth with disassembling to get to the heater strip. It’s really easy.

[gml]

Thanks Raize, that's encouraging. I wasn't really looking forward to messing around with the fuel cell. And all signs are pointing to making the WUR adjustable, I was pretty intimidated to drill into it, but I think I'll give it a shot.

For those interested, Pelican Parts has the text of the Up-Fixin article that Vaughan mentioned: https://www.pelicanparts.com/techarticles/911_warmup_reg/911_warmup_reg.htm
_________________
George

1982 924 NA 5-speed

[Riggard]

1 and 2 are true. I recommend you first set the warm control pressure, you do this by tapping in (or pushing out) the fuel cell. After that set the cold control pressure, this can indeed be done by making the pin adjustable with a bolt.

It's important that you do it in this sequence, because the warm setting, influences the cold setting, the cold setting doesn't influence the warm setting much

What I typically do is remove the WUR from the manifold, unclip the fuel lines and remove them from the clips above the cilinder head. That way you can position the WUR somewhere convenient. I happen to have a big pipe-kind-of-wrench that I then use to push in the fuel cell with the pressure meter hooiked up. That way it's really easy to set the warm pressure. (before I start, I usually open the WUR and push it out using a vise, so that I start with really low pressure and can increase it by pushing in.)
My cold pressure is adjustably by bolt.

[gml]

I had quite a bit of good luck with hammering the pin down today. The hammering moved it down approx 1mm, then I decided to retest. Cold control pressure is now about 2 bar and warm pressure reached the pseudo-maximum of 2.9 bar within just a couple of minutes of the heater being plugged in.

Just took one round of hammering down with a punch, and things seem a lot better. I didn't bother making my WUR adjustable at this point, I'm just happy that the car seems to start and idle okay (after giving it a bit more air from the throttle bypass screw). Throttle response is way better. Once I get my coolant tank replaced I'll see how my car performs on a spirited drive. I realize the control pressures are still 0.5-1 bar out of spec so I'll look into the adjustability mod if I feel strongly that it still isn't running to its potential.

For anyone attempting this at home I have a few tips:
1. Make sure the fuel cell (the dark circle with the fuel inlet/outlet) doesn't get hammered down as well, as this alone would cause a general control pressure increase. I set up my WUR on some wood blocks so that I could hammer the pin leaving the fuel cell suspended.
2. Hammering from the outside once or twice to unfreeze the pin seemed to help.
3. Thread in the 10mm nut on top of the stud for the bimetal strip before you start hammering. I did not do this, and flattened a couple of threads with my punch 😬
_________________
George

1982 924 NA 5-speed

[Riggard]

Generally speaking I'd say stay away from the hammer if you can, and use a vise instead But in the end... what works, shall do.

What also helps is to heat the strip for like 40 minutes, that way the whole WUR housing expands, as the housing is alu and the pin/cell are metal and alu expands more than metal, things loosen up a bit.