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Old 01-25-2022, 12:39 AM   #1
adamdrives
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Default k-jet fuel pressure test results

Hey guys, looking for some guidance interpreting my fuel pressure test results. The car is my 83 245 turbo. New pumps, 740 in-tank. JY fuel distributor. Frequency valve dwell has been checked and is within spec, although never from cold.

The main two reasons for the test are the car chugs and is difficult to drive for about a minute or two when started in the morning at around 40-50 F ambient, and also I want to get the most out of my car. I do get fuel from the cold start injector when the cars cold, but not when I follow the greenbook directions, so more on that later.

I followed the steps from k-jet.biz:



converted to 'merican units:

50* = 19-23 psi
68* = 23-29 psi
86 = 29-35 psi
warm = 50-55 psi
WUR vac line off, warm engine = 41-46 psi
typical System Pressure = 70-78 psi

The car had been sitting all day and temp was about 60 ambient when I started.

I started with Cold Control Pressure (tap open, started car) and got 82 psi

after about ten seconds, with the engine running I closed the tap on the gauges to read System Pressure. my reading didn't change from 82 psi.

After this I shut off to begin reading the warm up temps but the car wouldn't re-start. By the time it had cleared up and was running, system pressure was between 72-76psi. I let it run for a few minutes to see if it would change. After a few minutes running the t-stat housing read about 160 and there was no change in pressure.

Questions:

Step 5 says re-fit the WUR electrical connector to read warm up pressures, so I ran steps 1-4 with it unplugged. It doesn't actually say to do that, did I mess up? Could that explain my very high cold readings?

Is my cold control pressure way too high?

If my warm reading is on point, is it unlikely I'm leaving power on the table? At full boost (13 psi) I'm hitting 13 afr in first and second. usually I limit first and second to 10 psi or so and it stays under 12. with an npr, b230 intake and 15g, I don't think I'm close to 200 wheel, which is where I'd like to be.

https://youtu.be/zyCSeb7S-Z0

Not sure why afrs are so high in that video, usually lower
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Last edited by adamdrives; 01-25-2022 at 12:44 AM.. Reason: added video
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Old 01-25-2022, 07:10 PM   #2
dl242gt
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Cold start up chugging is normal with these engines if you have the thermal cut out switch connected. It provides a super rich mixture by grounding terminal 7 at start up. Once the engine block gets above about 60 degrees. The thermal switch should open and terminal 7 won't be grounded.

Your very high control pressure is leaning out the engine on cold starting. You want to leave the connector on the warmup regulator while testing. That high control pressure can make the engine stall and require several starts. You can try removing the control pressure regulator and cleaning out the holes for the fuel. There is a screen in there that can get blocked. Basically you should be starting up the engine cold with the valve open and see the gradual increase in control pressure till fully warmed up. If you have the steady high control pressure than there is a blockage of some type in the line or the regulator.
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1982 242 turbo. 338k miles. MVP coilovers and 3" exhaust. Flowed 405 with a V15. Cossie turbine housing with upgraded compressor housing. 90+, IPD remote oil filter. Some other goodness, too. Been lots of fun over 25 years. Restored in 2k. Now ready for a 2nd restoration.

1993 245 Classic, 435k miles, enem V15. IPD bars and chassis braces. Simons sport exhaust from Scandix. sbabbs ezk chip. Been a good road warrior. Genuine Volvo rebuilt leaky M47.
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Old 01-26-2022, 12:58 AM   #3
adamdrives
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Thanks for that info! I left the gauge on the car and let it sit all night, and tested at 70 ambient midday today. Control pressure started at 60 psi and steadily rose, so I am definitely too high. I believe I had the WUR out and cleaned it up when I did the harness, but I can't remember for sure. It hasn't ever stalled when cold, but it does take a while to "wake up" so that makes sense.

I've always had the feeling this car drives different every time I get in it, and that was confirmed today after testing when I stayed in 12 afrs through first and second at wot. Maybe the wur is on the fritz?
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Old 01-26-2022, 09:47 AM   #4
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You are, indeed, describing a CPR that is not working correctly. The high-pressure readings are driving the FD to lean out.

The filter in the fuel input to the CPR is the most likely issue. It is multi-layers of very fine wire mesh and is impossible to remove without destroying it. I acquired a rebuilt CPR that had a bronze filter installed. Bronze filter elements are available on ebay but you will probably have to cut to fit.

Here is a link to a disassembly/assembly web page.

http://b2resource.com/PDFs/wurservicing.pdf
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Old 01-26-2022, 12:37 PM   #5
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Thanks for the link, looks like it's more or less straightforward to take apart. I didn't see how to remove the screens, have you ever attempted that before?
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Old 01-26-2022, 03:34 PM   #6
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Taken several apart for cleaning with mixed success. The device is a very simple pressure regulator. There are three things that can go wrong with one.

The filter screens get clogged. Carb cleaner and compressed air has had mixed results. Suggest you dig the screens out with a sharp awe and replace with bronze filter.

Trash on the pressure regulator disk or damage to the disk. Visible issues that respond to cleaning. The pressure disk is the only significant repair part in a rebuild kit.

Heater coil. Measure with a VOM. Just a few ohms. If your pressure changes with temp then it is ok.
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Old 01-26-2022, 11:02 PM   #7
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Thanks for the tips. I talked to a shop down the road that's works on a lot of classic Mercedes and got a good connection for a rebuild service, so I figure might as well just get it taken care of that way and know its solid.
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Old 02-12-2022, 12:39 AM   #8
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Had the wur rebuilt and dropped almost 2 afr at wot, so that was definitely an issue. I haven't re-checked pressures but obviously the wur was leaning me out. No more chugging on cold starts, although I am pegging rich. I used fuel injection service in Tracy for $250, I'm very happy with the result. May do the same in my 242 just because what are the chances the original wur is 100%

I had the afr gauge wired to my unused clock connections, switching to direct fuse power and chassis ground made a big difference, it is responding much quicker. Before it would just hover around 14 until the car got warm, now I can actually see my cold afr.
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Old 02-12-2022, 01:23 PM   #9
dl242gt
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Glad you got that sorted out. The pegged rich on startup is normal. Kjet uses a very rich mixture on startup. Especially if it's cold weather and the thermal cutout switch is grounded.
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Old 03-13-2022, 02:35 PM   #10
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Is it normal to chug for a minute or two? If I don't do a manual cold start and hold rpms at 1500 or so for a minute the car will have no power. On that note, should there a high idle cold? Mine stays at 900-1000 no matter what. My 78 242 will idle high when cold.
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