Engine backfire - WUR

[Tedjr]

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I have a 1983 911SC 3.0 Euro car. I am having a problem with the car backfiring through the airbox upon rapid acceleration of the engine when it is cold. I recently installed a pop off valve in the air box and the backfire problem seems to have appeared as I can only remember it doing so very infrequently in the past. Maybe it was not as noticeable before as the valve pops open readily now. I have checked for air leaks on the pop off valve and can find none. The car starts fine warm or cold, idles ok, and once warmed up runs great.

In attempting to diagnose the problem I installed a test gauge and recorded the following information.

System pressure – 69#

Control pressure

Engine cold and running – 35# at 80 deg. F (80 deg. F is about as cold as it gets here in the early mornings)

Engine warm and vacuum connected – 56#

Engine warm and vacuum disconnected – 47#

The Warm Up Regulator is Bosch # 0438140089

I have been unable to get any information for my car or a pressure graph for the WUR. The information for the US cars is different as my car more closely matches a 1979 model than an 83’. My car only has a catalytic converter and smog pump. It does not have lambda control or any devices other than the WUR, Aux Air Regulator, Aux Air Valve, Deceleration valve, and Cold start injector and temp. sensor.

Do you have access to pressure/ temp. graph for the WUR?

Do you know if I have the correct WUR for my application?

My WUR has a small screw and jamb nut on the top. Is this to enable it to be adjusted? Did it come that way or was the WUR modified? If it needs to be adjusted with this screw how do you do it? As I understand it the engine backfires under quick throttle acceleration because it is too rich. Is this true?

Do you know where I can get a shop manual for my car? I already have a Bently manual, a Haynes manual, and a Bosch FI manual by Probst. Problem is they do not address some of the unique differences of the Euro cars. An owners manual would be nice to have also.

I know these are a lot of questions. I would like any help you can provide. The nearest Porsche repair facility is a three hour drive from here and they mainly want to mess with race cars. If you cannot help me perhaps you can tell me where to find help.

Thanks

[Paulporsche]

If your car is like a 79 your cold cp should be about 2.3-2.7 bar (33-39PSI) @28C, so that sounds good.

Warm should be 46-53 (attached) 39-45 (discon).so you are a little high.

I think the backfire is from too lean, not too rich. Do you have a vacuum line attached to the side of the WUR for throttle enrichment?

I think the screw and nut are the non factory adjustment system . You should be able to loosen the nut , push the plug in a little and retighten. This will probably lower both cold and warm pressures. If you want to lower just the warm, there is a screw on the bottom behind a cover that you can turn.

You can try richening the mixture about 1/16 to 1/8 turn to see if that does it. If not, how are your plug wires, rotor and cap?

[konish]

Too lean when cold is what causes the backfires. The nut/screw set up indeed sounds like a modified WUR. Simplt tap it down to enrichen (decrease control pressure) and if you go too far you simply use the nut to bring the post (aka plug) back up to a leaner position.
There seems to be a trend of checking the pressure with the engine running but neither the Bentley nor Probsts book outline this as a test procedure...when it states running, I think they both refer to the pump running via the jumpered terminals of the fuel pump relay socket. Your system pressure looks good...65-75 psi is the range. Your cold control pressure looks good too.
Your warm control pressure seems a bit high but vacum line graphs are only provided for the 78-79 model years (which seems to follow your logic of your car being a closer 79 match)...after which the graphs do not have different setting for vacum line readings. (46-53 attached, 39-45 removed)
A remote possibility is the internal heater in the WUR may be "closing" the ports too quickly on your car...causing a higher control pressure before the car is actually warm. My WUR had smaller, secondary element and by adjusting a very tiny screw on the secondary heating element I can control the rate at which the WUR closes (it actually changes the resistance going to the main heating element). Basically I can make the car run on the rich side for a little longer if desired. Your Euro may also have this being an 83, but hard to say.
I however suspect that you may have an air leak...easiest thing to diagnose with a can of WD-40...squirt it at the base of the pop off vlave and see if the idle changes. If not there, open up the WUR and have a look inside...

R/
Dustin

[Paulporsche]

You may also get your WUR to keep the lower cp longer by disconnecting the elec supply. You have to experiment w/ this.

It would be a good idea to check for vacuum leaks inother areas besides the pop valve.

[konish]

Paul,
Unplugging the electrical connection will not allow the WUR to fully close even with the motor at temperature. I found that the interior of the WUR does not get sufficiently hot enough and my car ran RICH with it unplugged. I actually tested the CP after an afternoon of driving and it never fully reached fyll warm CP. Not sure if it is due to the air space inside the WUR, the fact that cooler fuel passes through a portion of it or what....just a thought.
I still suspect a SLIGHT air leak on the pop off valve installation which would explain the cold, lean back fire problems.
Ted,
Does the car run better when warm? If so, it would seem as the engine warms to operating temps, the effect is dampened a bit, but when cold its simply too much "cooler" air for the CIS to deal with and it manifests itself as a lean opo....?

R/
Dustin

[Paulporsche]

Tedjr,

Have you tried richening the mixture. It may be as simple as that.

It could also be your AAR. If you search there are lots of threads on cold start and warmup problems.