911SC will not idle when hot. Exciting graph inside!

[dino_jr]

Hello... I posted in my old thread but it has dropped off the page. So...

My car is a 911SC built July 1980, UK/RHD car. (ROW spec)
* Car run 100% for last 18 months.
* Suddenly wouldn't start 2 monhs ago.
* Replaced Fuel Pump Relay and Fuel Filter.
* Battery, Starter Motor, Spark plugs all good.
* Tested the WUR & AAV are closing when 12V applied. All OK.

I then "plugged" the WUR fuel tank return line, and the car started!
I refitted the return line back into WUR as normal. So now car starts every time. i.e Nothing has been fixed, I assume the plugging exercise "jolted" a mystery valve into action, allowing the car to now run.

Car starts from cold and idles fine But from ~5 mins later, the idle begins to hunt and then engine dies. I tried driving around the block but the car died every time I come off the throttle. Car restarts reliably but will not idle when hot.

Re-did my pressure check:

* System pressure at fuel pump on: 4.6 bar and stays constant.
* I have plotted my control pressures as below:



* Fuel pump on at zero, WUR (Cold) measured for 1st minute: 1.1 bar max
* WUR Electrical connector in after 2 minutes / green arrow: (Warm) 2.7 bar max
* Fuel pump off at 8 minutes / blue arrow.


1. Is the pressure drop off from 8 minutes my residual pressure? Is the rate of dropping correct?
2. If not, is this likely to be a drop off consistent with an air leak, failed FP check valve, FA broken etc?
3. My cold and warm value are approx 0.5 bar too low, in reference to others I have seen. What could be the issue here or are they actually OK?

All help appreciated. I'll attempt to check for air leaks and CSV function this weekend, but wonder if something else is the issue. Thanks.

[boyt911sc]

Dino,

What is the Bosch ID number on your WUR? The residual pressure (fuel) drop has nothing to do with vacuum leak (air). Obviously, the control fuel pressure (cold and warm) is out of spec. but we need to identify it first. Keep us posted.

Tony

[T77911S]

your CP looks low

if you have an ROW then you should have the vacuum controlled WUR.
IF the ROW has the thermo time valve(TTV) it will be connected between the WUR and the throttle body. if this goes bad you will have low CP.

you can bypass it to test it.
if it gets really cold where you are I would fix it. it drops the CP even more for cold starting and for a very short time for cold running. this will help with lean backfiring.

nice graph BTW.

[Vereeken]

What WUR is this? A 089? July 80 could be 089 already.

If it is a 089 with vac enrichment you need to apply vac when measuring CCP.

You also need to tell us the ambient temperature when you measure CCP.

Do not get fooled. The US guys have other WURs so when you suspect your CCP to be low it is simply because you did not apply vac.

Your pressure drop after shut off is fast and brutal and outside of tolerance.

From my comfortable chair this can be a scenario for the dying engine.:

You have a 089 wur. All worked well. Now the Thermal Valve is kaput. It is stuck in the closed position. As a result no leaning out of the fuel supply under normal load conditions meaning a very rich mixture. No problem when engine is cold problem when engine is hot. You ofcourse also have a blocked off decel valve like everyone in Europe, so no extra air when the throtle is closed sharply= overly rich = stall.

BTW this is not an explanation on the sharp pressure drop. No idea what that could be since you already replaced your accumulator.

So first check the number on the wur. If it is with vac enrichment you need to apply 550mbar of vac before measuring CCP.

[dino_jr]

Hi, this is my WUR... Not easy to read but I think the PN is 244 France 0438 140 089.
So a 089 part?

[dino_jr]

OK so I apply a vacuum to the port on the top (the one by the PN) or the port on the end/side?
I need to get a vacuum pump then, something like this I assume?

[dino_jr]

Hi Vereeken what is the Thermal Valve exactly? The Cold Start Valve?
BTW I haven't changed the accumulator, is that a common part causing a pressure drop like I have?

Your Comfortable Chair scenario is EXACTLY right!

[T77911S]

the TTV is an electrically controlled valve that blocks vacuum to the WUR when cold to lower the CCP to help with cold starts and reduce backfiring. it is a very nice feature to have when its really cold.
at 2.7bar it looks like there is no vacuum to your WUR.
I would start with the vacuum enrichment circuit.

the vaccum line runs from the WUR to the TTV(if you have one) then to the throttle body.
it may "T" off with a line to the decal valve.
it connects to manifold vacuum. it should be a port that is always below the throttle plate.

[Vereeken]

Yup a 089. Yes vacuum to the side port not the top. Top port is only a vent.

550mbar max to the side port. Wur should hold that vacuum with no issues.

this will raise your pressure with 0.6 bar roughly.

Next you measure the CCP with pump running but engine off.

Note the ambient when you do that and report back.

if you have a vacuum hose small diameter you can bypass the Thermal valve.

If the car does no longer have the issue you found the source.

Intrested to see what CCp you show in the summer when using the right procedure.

[dino_jr]

Vereeken... thanks, I have ordered a vacuum pump, will report back early next week.

T77911S... can you link me to a cct diagram ? Looked at the common diagrams, no mention of a TTV?

[Vereeken]

The correct diagram for the 089

[Vereeken]

The right way of routing all the small diameter hoses on a ROW SC. Special attention to each hose going to the correct port on the throttle body. Front or aft, below or above plate.

[Vereeken]

The infamous thermal valve. Pierburg brand NLA.

[dino_jr]

Thanks. So what do Porsche call the electrical connector on there? Still can't see it any schematics ...

[Walt Fricke]

Low residual pressure can be a sign that the one way check valve on the fuel pump is leaking fuel backward into the pump and thus the tank.

The fuel accumulator has a spring loaded diaphragm in it. If the diaphragm gets a hole in it, fuel can enter the lower (spring) chamber. At some point, Porsche/Bosch decided to drain the lower chamber into the fuel return system back to the gas tank. I suppose this could cause a premature drop in residual pressure.

Hard to connect this to your idle issue, though. Too fast a drop in residual pressure normally is just a hot start problem.

[T77911S]

I had no residual pressure on my 77s, ran fine.

you can bypass the TTV as long as it starts fine and idles ok in the winter.
mine went bad, failed in the closed mode so no vac which meant lower CP all the time

[dino_jr]

Hi all. So my Vacuum pump arrived and I have retested:



As before:
* Fuel pump on at zero, WUR (Cold) measured for 1st minute.
* WUR Electrical connector "ON" at green arrow.
* Fuel pump off at 8 minutes / blue arrow.

Orange trace: WUR Vacuum line attached (previous test)
Purple trace: Vacuum pump attached to WUR at 400mmHg (~550 mbar): Looks like Orange trace +1 bar.
Grey trace: Engine started, so this is my Control pressure as the WUR warms up.

Touching the throttle after 18 minutes killed the engine (as usual). Grey & Purple do not match... Is this a sign the TTV has not opened?

BTW I checked the connector on the TTV:
* 14 ohms across the pins.
* At FP on, TTV goes to 12V for 30 sec then switches off.

Not sure what I have learned here... Any suggestions what to try next?
Engine is now hot so won't idle, so no more testing tonight annoyingly.

[Vereeken]

Well, this is no joke but I am color blind so your graph really doesn't do it for me.

Anyway, we also need to know the ambient temperature when you did that test.

So with my limited color sight. The highest bar is with your vac pump correct? That looks (ambiet temp aside) very good. The WCP inclusive of vac is what it should be and I believe you need about 3-4 minutes to get to that stage with 12v applied. Which is normal.
The drop in FP also looks good and stays high enough for long enough after engine off.

The lowest graps is ex vac pump? So yes there is no vac assistance. So either your TTV remains closed or you have it connected incorrectly.

Next comes this sentence: At FP on, TTV goes to 12V for 30 sec then switches off.

That is not right. The TTV needs 12 V ALL the time for as long as the engine is running.

But even with only 30 sec it should have openend and we should have seen a temporary hike in control pressure. So I think you have 2 issues. A stuck TTV and an electrical issue.

Tony can give you a link to TTV testing and the opening times of the thing. But 10-20 sec is what it should be I believe even 5 when it is hot outside.

I think (other issues aside) if you fetch a small diameter vac hose;of about 60cm you can bypass the TTV and your issue should be solved. The TTV is nice for cold climates. If you live in Texas not so much.

[SkiVT]

If TTV wasnt opening after 30 seconds, the orange and grey plots would be the same as vacuum would be held off. Those two plots should be similar for 30 seconds until TTV opens and vacuum takes over. Search "TTV testing" as Tony has a good overview. Make sure vacuum line from WUR to TTV is connected to the outlet closest to the edge of TTV and not the outlet in the center of the TTV. The center outlet should go to decel.

[T77911S]

might be easier for me to just tell you how it should work.

when you start the engine cold, the TTV is closed. this lowers the CCP even more. after starting, the TTV should OPEN after about 20-30 seconds. this puts vacuum on the WUR and bumps the CP up about .8bar. WUR is still warming up so actual CP (to me at this point) is not really relevant.
all I would be looking for is starting pressures and ending pressures, with and without vacuum.
actually CCP without vacuum is not an issue at this point because if it works for the WCP then is works for the CCP.

so, pull a vacuunm on the WUR and see if it holds. once warm, connect the vac pump to the hose going to the WUR and check vacuum going to WUR.
check WCP with vacuum and without vacuum.

that hand pump is a nice tool to have around. I am surprised at how much I use mine. just used over the weekend.