CIS fuel pressure test problems

thomastee · 08-22-2012, 04:45 PM

Hi folks,
K jet experts please can you help? I'm trying to do the fuel pressure tests on my 74 2.7 (regular 150hp) engined car and I'm confused.
I hooked up the gauge between fuel dist and control pressure regulator as recommended and got a 40 psi system pressure plus a very low fuel flow rate. I then read that for 74-75 cars you have to isolate the throttle pressure regulator(?) to get the right pressure readings. I couldn't easily do this as access was limited, so first I tried just hooking up the gauge to the fuel feed line right after the fuel filter, so before the fuel distributor. This showed that I have a pretty good system pressure of just under 5 bar and a good flow rate (1000cc in 30 seconds).
The problem is that when I try to connect the gauge again between fuel dist and CPR and I take out the banjo connection that feeds the throttle pressure regulator, I would expect to get the system pressure again, but I don't - I now get only 30 psi and a trickle of fuel.
Does anybody know what this means? Is the fuel distributor blocked or am I still not doing the test properly? The fuel dist is a reconditioned one installed less than 2 years ago by a very reputable specialist here in the UK.
I'd like to do the test properly so I can check the control pressure and warm control pressure etc. but I don't understand why no pressure is coming through the system.
Could it be a sticky fd piston?
Anyway, thanks for reading and for any suggestions you may have in advance!
Thomas

ossiblue · 08-23-2012, 06:03 AM

On cars with the throttle pressure regulator, the banjo fitting should be removed from the fuel distributor and connected to the gauge set on the outlet side--between the outlet and the hose to the cpr. If you still get low system pressure after correctly hooking up the gauge set, it's possible the pressure relief valve, near the inlet of the fuel distributor, is not functioning properly and allowing fuel to return to the tank at a lower pressure than spec.

thomastee · 08-23-2012, 06:45 AM

Many thanks for the reply L.J.
So if I connect the banjo downstream of the gauge and downstream of the shut off valve, why would my system pressure reading be any different? Do you mean that the banjo should be after the gauge T but before the shut off valve? If so, I don't quite get why my pressure would be different then compared with leaving the banjo connected at the FD. (Sorry if I'm being dumb!)
Also, is the pressure relief valve the little spring-loaded one to the side of the FD? If this isn't functioning properly, does that mean I have to take the FD off to replace or is there a less heartbreaking solution?
Thanks again,
Thomas

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by thomastee

Many thanks for the reply L.J.
So if I connect the banjo downstream of the gauge and downstream of the shut off valve, why would my system pressure reading be any different?

Good thinking. It would not be different with the valve shut off. However, when you open the valve, fuel will flow through the control system and you will have a different reading which is control pressure.

When the system is "open", you only read control pressure. I can't exactly explain all the dynamics but fuel not used by the injectors is sent back to the fuel tank. For a layman like me that helps explain why control pressure is less than system pressure - some of the pressure is bled off.

Bob Kontak · Bob Kontak's Garage

Here is the pressure valve. Looks pretty straight forward as far as removal and replacement. Never remember reading where FD has to be removed.

thomastee · 08-23-2012, 11:52 AM

Bob, thanks very much for the reply and the picture of the valve, very helpful indeed!
Here's my update:
I hooked up the gauge just to do the valve closed system pressure test and the pressure was very low (2 bar). I then started playing around with the pressure valve, took it out to see if anything was binding - it wasn't. After playing around, trying to understand what adding shims might do, it turns out that the valve worked better (pressure higher) with fewer shims, i.e. more loading on the valve spring. So I took all the shims off but the pressure still wasn't enough, so I started adding a few tiny spacers between the spring and the bolt so that it would get more loaded. Well, that started sending the pressure to the right region and I got just over 4 bar with as much loading as I could cram onto the spring.
Thinking about this a bit more, it strikes me that my pressure valve doesn't look like your parts diagram Bob, in that the sharp piece the arrow is pointing to is missing. I certainly didn't see anything like that. Actually, the parts diagram for my car doesn't have that pointy piece but it does show a smaller shim in between the stopper and the spring - which is essentially what I'd added with those few tiny spacers.
I think I'm onto something here! Now if only I hadn't lost the line fitting for the banjo bolt I could start the car and give it a whirl...

4sd911 · 08-23-2012, 12:24 PM

I'm going to jump out on a limb and say that you are not hooking the tester up right, can you post picture of it hooked up? If in fact you need to mess with the pressure valve as you describe with the tester hooked up right then you have problems with you fuel distributor. You should be adding shims to increase the pressure, I don't understand that you first remove the shims for more pressure then you add spacers? Whats that. Between what?

thomastee · 08-23-2012, 12:40 PM

Hi there,
I'll post a picture over the weekend as I've packed up for the day but the basic setup is line from fuel dist to T connection, gauge then shut off valve and then line to CPR. This is with the banjo bolt to the throttle position reg removed entirely.
Sorry if I'm not describing this very well. There is a wide shim (maybe it's just a washer) that goes around the hollow bolt so that inserting more of those drops the pressure since the bolt is then further away from the body of the FD and the spring is more relaxed.
Then there are some mini-shims/spacers I've added that fit on the inside of the hollow bolt and serve to compress the spring more. Adding more of these has resulted in higher system pressure as more force is applied to the valve. Is that how it's supposed to work?
With the setup as described, I get a system pressure of 4 bar then when I open the valve (without power to the cpr), the pressure doesn't drop. I'm still not 100% sure what I'm supposed to do with the banjo bolt that feeds the throttle pos reg but the fact that the pressure isn't dropping tells me I'm definitely doing something wrong!

thomastee · 08-23-2012, 12:46 PM

To explain, my pressure control valve doesn't look exactly like the picture Bob posted. I think that's ok for my car as the parts diagram I looked at does match what I have, but I could be wrong. I think the pin-type pressure control valve Bob shows applies from -78 onwards...

4sd911 · 08-23-2012, 02:16 PM

The one that goes around the bolt with a cavity is a copper sealing washer, just use one. The shims that go inside the cavity behind the spring are what regulate the pressure. The banjo fitting that comes from the throttle position regulator needs to be in the right place on the tester or it won't work, I've been there before.

The tester should have 3 lines, first line is attached to the fuel distributor which runs to the double fitting on the tester, one side runs to the gauge the other to the valve, this way when you close the valve you can check the system pressure, then from the valve, that line connects to the banjo fitting from the throttle position regulator and the warm up regulator together, you will need to use the fitting from the fuel dist. to connect the two together.

Hopefully that clear, if I had a picture of mine hooked it would be much easier to see how it works.

thomastee · 08-23-2012, 02:34 PM

That is a great answer, totally clears things up both on the valve and the test setup, thanks! The ONLY problem is that I've lost the fitting from the fuel dist! Aaaaaarg, I've been looking for it for hours, I even went through the garbage just in case...
Anyway, I can't wait to try this out as it may just mean that the car will run well after MONTHS of hesitation and bucking!
Thanks all for your help
TT

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by thomastee

my pressure control valve doesn't look exactly like the picture Bob posted.

I got lazy and posted the 1974 fuel distributor Pelican shows. Here is the diagram for the 1974-77 911 ROW car. I bet it looks a little closer.

thomastee · 08-24-2012, 12:52 AM

Yes, that's it! Only there were no small shims (marked no. 5 in the pic) so I had to find some of those... I'm going to go and look for that fitting one more time now, I'm gonna move everything in the garage out onto the street!

prebordao · 08-24-2012, 03:23 AM

What kind of bucking and hesitation are you having and at what revs ?
Under load ?

thomastee · 08-26-2012, 09:22 AM

Hi there,
The hesitation and bucking is between 2000 and 3000 rpm. I assume it's due to a lean condition caused either by a vacuum leak or low fuel pressure. The ignition system is ok and so is the CO mixture. I had a vacuum leak around the pop-off valve which made things better when fixed, so if the issue doesn't go away after I fix the fuel pressure, I'll go back to looking for vacuum leaks...
Have you had this problem?
Thomas

prebordao · 08-26-2012, 09:51 AM

Yes very similar and for quite sometime. Tried several approaches, nothing solved it completely. Pressures, ignition are ok.
I suspect a hidden vac leak but removing the airbox is such a pain that i've been postponing.
For me it gets worse the hotter the engine is. When cold is barely noticeable.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by thomastee

I had a vacuum leak around the pop-off valve which made things better when fixed,

I would re-visit that repair straight away.

thomastee · 08-26-2012, 01:47 PM

Hi Bob, do you mean that it's likely that the problem is still at the pop-off valve? I wouldn't be surprised if you were right. Is there a best way to seal this? I've used high temp silicone but I could certainly go back in, sand everything down and make sure all surfaces mate perfectly again. Otherwise, checking for leaks with carb cleaner has shown nothing else and I've replaced injector seals...

thomastee · 08-26-2012, 01:54 PM

P.S. Prebordao, now that you mention it, I'm pretty sure my car gets worse when warm too. It's had the bucking for months and is driving me slightly insane. I won't rest until it's gone!

ossiblue · 08-26-2012, 05:06 PM

If your fuel pressures now are in spec, it's quite likely the bucking problem is vacuum leak related, especially if it tends to get worse as the engine warms. Air/vacuum leaks lean out the mixture and as the engine warms, the fuel pressure regulator is also leaning out the mixture making a lean condition even worse. Your pop-off valve is a likely suspect since it was a problem before. I'd suggest you totally remove and re-glue the valve in place with a good epoxy rather than use a silicone sealant, but that's just me. If that doesn't fix the problem, you'll need to do some investigating of the entire intake system and vacuum system to locate possible air leaks--there are dozens of possible suspects--but at least this has been done by many, many of us and there are ample threads available for help.

Keep us up to date.