'79 450 SL CIS Woes (Woahs?).. Could use some expert opinions!

[oldcrowe]

Morning guys,

I am having difficulties with a MBZ we have here;

Vehicle runs rough at all times, mostly when warm/hot. Tested fuel pressure, found no change in cold, warm and system pressure.

Cold control = 72lbs, warm control = 72 lbs, System Pressure = 72lbs.

Checked for blocked return line, plugged WUR (disassembled, cleaned, replaced O-ring), no change. Crack the return lines and pressures wobble a bit but no real drop (1-2psi).

Day #2:

Sys Pres : 72lbs

Cold Con Pres: 65lbs

Fuel Rate: 850ml/30 sec @ 12.4V (NAPA FI book shows 1L)

Did not allow to warm up. Will remove screens in CPR and test again.

Also: If I vent the pressure at the gauge with the release, idle picks up until the pressure returns to its 65 psi.


Day #3:

Poked holes in CPR screen, CCP seems to have risen a pound or two.. could be crappy gauge or connections.

Flushed return line from CPR to dizzy and from dizzy to tank with carb clean and air- No change.

Ran engine with return line emptying into remote container- No change.

Perhaps there is blockage in the CPR return line ingress in the dizzy- Will pull and investigate.


No luck. Cleaned plunger, polished lightly on wheel as it had some considerable scoring.

Reassembled, runs worse than ever.



I am more familiar withe VW set ups, but I would assume this is very similar. On my VW's I would remove WUR, tap iron plug out a bit, tap cold start pin a bit and start setting fuel pressure from base line- However, I am reticent to start doing that on a vehicle that does not belong to me.

ANY help that could be passed along would be greatly appreciated! At this point I am a little lost..

thanks!

russ

[Frank Reiner]

OC:

Consider installing a WUR bypass line equipped with a needle valve, so that you are able to set the control pressure manually.

Cold control pressure should be under 20 psi., rising to ~ 50 psi. when warm. By setting the CCP manually it will be possible to isolate the WUR from the fuel distributor (FD), and perhaps determine if there is a fault in the FD as well as the apparent fault in the WUR.

[ctaylor738]

Are you disconnecting the electrical lead from the WUR when testing control pressure?

It sounds to me like the VW procedure is identical to what you would try do to this car to get it working. Frank is right, there may well be a problem in the FD, but you can't even test for that until you get a decent CP reading.

[oldcrowe]

Frank- Thanks for the suggestion. I will build a bypass regulator tomorrow and test.

CTaylor- Yessir, plug is disconnected while testing. Will test with Frank's suggested regulator and report back.

Thanks!

[pmckechnie]

I don't understand something. You say that you have the plug disconnected when testing. If you disconnect the plug, the control pressure won't change, It will stay constant, but it should be low at 20 to 30 lbs.
Disconnect the line from the center of the dist to the CPR at the CPR. Of course, catch any fuel that comes out in a container. Control pressure should be 0 lbs. If it is, reconnect it and disconnect the other line (return) and check the control pressure. The fuel will be coming out of the CPR so rig up some way to catch it. Control pressure should be 20 to 30 lbs if the engine is cold. If it is still low, then there is a restriction in the return system.

I don't know how to post a link, but here is a copy/paste I found that may help. It is from a post by ctaylor738 on this site.

Control pressure

10°C (50°F) 1.2-1.6 bar (17-23 psi)

15°C (59°F) 1.4-1.8 bar (20-26 psi)

20°C (68°F) 1.6-2.0 bar (23-29 psi)

25°C (77°F) 1.8-2.2 bar (26-32 psi)

30°C (86°F) 2.0-2.4 bar (29-35 psi)

35°C (95°F) 2.2-2.6 bar (32-38 psi)

40°C (104°F) 2.4-2.8 bar (35-41 psi)

If the pressure tested is incorrect, retest with a new control pressure regulator.

WARM UP TEST

Connect the vehicle harness connector to the control pressure regulator.
Energize the fuel pump by bypassing the fuel pump relay; after 5 minutes, the control pressure should be 3.4-3.8 bar.
If these values are not obtained check the power at connector.
If there is current at the plug, replace the control pressure regulator.
If the rise in pressure occurs too rapidly, replace the control pressure regulator.
Shut off the fuel pump.

I hope some of this helps. I have been down this same road several times. If the control pressure is not close a bad CPR or a restriction somewhere is about all it can be.

Paul