Baffling WUR Behavior (Sorry, it's long)

[Paulporsche]

I've been having trouble getting my mixture correct. If I set the mix for best warm running, the engine is too lean for a cold start. If I set it to start, I get that exhaust blatt blatt blatt noise upon lifting the throttle and a hunting idle indicative of a too rich mixture. This tells me the wUR may be out of wack.

My mech and I set up the CIS gauge to the WUR this morning. The ambient temp was 21C w/ a cold (sat overnight) engine. The chart says I should get a ccp of 2 to 2.5 bar. When we adjusted it to that, the engine would die. In fact, anything above about 1.1 bar caused the engine to go lean and die.

We didn't test the system pressure, but the residual pressure was better than min spec.

The engine started beautifully @ 1 bar and idled nicely @ high revs, then eventually settled to about 1000. Once it started warming however, the control pressure stayed @ 1 bar. @ 176F the cp was still 1 bar, which tells me maybe the bimetal strip is kaput. We leaned the mix slightly and got a nice idle and I drove it. At first the pickup and response was very good. Then the engine started to feel choked, sluggish, surged on acceleration and I got some exhaust popping and a hunting idle.

Up to about 180 or so I still got a 1 bar cp reading.

After driving home today (ambient about 30 in some stop and go traffic) I saw 210 which is not unusual for this engine. The engine was still peppy but would still hunt @ idle. On the way I made 2 stops. One was for about 20 min. The engine started on the second short pull and ran like before. The second was about a half hour later for about 15 min. The engine would not restart unless I had someone turn it over while I raised the sensor plate. I am thinking the mixture is now too lean.

This condition suggests to me that the WUR somehow isn't warming up correctly, raising the cp and leaning the mix as it should, since it was @ 1 bar so long. However, once the heat soaked the WUR it must have finally gone too lean for starting. Unfortunately I couldn't put the gauge on it then to see what the hat cp was @ that time.

My mech set the timing. He said that full advance is reached by 4000 rpm. I am running the old points style dizzy.

So what is going on? Why does will engine only start @ 1 bar even in 21F?

Why does the cp stay @ 1 bar for up to 180F?

Why does the engine appear to run rich, yet, after stopping 15 min on a hot day , refuse to start as if lean even though residual pressure is OK?

Anybody have anything on this?

I think I have some WUR disassembly in my future.

[Paulporsche]

As a followup, I have another question.

Has anyone bench tested their WUR? I see someone posted that he got a cold resistance of 9.2 ohms. Can anyone else verify this? Or provide any specs for warm resistance?

Any other way to bench test a WUR?

[kach22i]

The WUR gets power?

I'm going to have to do my homework before messing with mine.

[Porsche_monkey]

I assume this is recent and sudden? i.e. your mixture screw is correct? I know you've had your car a while, or I would suggest a PO corrected WUR issues by adjusting the mixture....

[ossiblue]

Your evaluation of the WUR does seem reasonable--failure of the bimetal strip, subsequent overly rich mixture, later heat sink leaning, and all complicated by the compensation of the mixture screw turned to a lean position.
Clearly from your work already, the WUR is not functioning properly. I'd suggest starting there. I remember reading a thread here about someone who changed the heating speed of the bimetal strip by rewrapping the heating element. He may have posted resistance values, I cannot remember, but maybe a search will turn it up.
I had major issues with my CIS and my mechanic began with step 1, the WUR, when the pressure numbers came back out of spec, much like yours. Once the cold pressure, warm pressure, and residual pressures were fixed, he went on to other factors (which you don't appear to have.)
Bottom line, take care of the WUR, then adjust mixture using a gas analyzer. I'll see if I can locate the thread on the heating element. Good luck and keep us posted.

Edit: One last thought. See if your mechanic can "borrow" a working WUR, install it and set your mixture.
Of course, if all works well with the borrowed WUR, you have solved your problem.

Foun a thread with resistance numbers:
WUR ohms question

[john walker's workshop]

if the thermo-valve passes vacuum to the WUR when cold, they run leaner than if it did not. some thermo-valves have been removed and the vacuum hose run direct to the WUR.

[Bobboloo]

Check to see if the WUR is receiving 12volts. Also check the impedence of the WUR coil to make sure the coil isn't "open". The impedence varies depending on the year model of your motor. The impedence amount is stamped on the side of the coil inside the WUR.

[Alan L]

You can also just put an ohmeter across the WUR terminals - not the power socket end - the WUR end, and see if you get any resistance. If you do, it most likely is OK (the warm up windings). The amount of resistance just varies the warm up time. But a broken wire (can be fixed) will give nil reading - and no warm up pressure - just cold pressure.
While you are there you can check for 12 V on the other end of plug. From memory the engine needs to run - not just turn key on. Think it is wired to the air plate contact switch circuit.
Alan

[Paulporsche]

Thanks for all the good responses. The symptoms are recent, so I can rule out any PO interference. also, the thermostart valve is in position. I'm thinking bad windings so that's where I'm going to go next.

These symptoms are in evidence to varying degrees w/ 3 WURs I have had on the car, but since they are all old and used, they could all be bad!

I'm in FL now w/o the car, helping my mother move, so it will be another 2 wks or so before I can get to this.

[mca]

I thought that you had an Audi WUR in there? Did you change back to stock?

[Alan L]

If you have changed 3 WURs and similar symptoms - how about an air leak.
Richer cold mixture = better start. As it warms up, leans off with the WUR, and the air leak - til too lean to start.
You could check next time is hot and wont start - turn your mixture adjust screw 1/2 turn clockwise. If it fires up nice, you have provided enough extra fuel to compensate. Next job = find the leak. Any air entering that does not go thru the air plate = lean mix.
Alan

[T77911S]

hey paul, i have a 77s, mine had the thermotime valve TTV on it. also, my WUR was just replaced before i got it and i checked presures shortly after as a reference. i am at work so this is from memory.
cold- .8bar
cold after the TTV opened- 1.8 bar. about 15 seconds
warm- about 3.5 bar
i did not check the resistance of the bi-metal strip but i can if you would like.
my TTV is bad so it is bypassed, up there i would think it would be a big help for cold starts. when mine was working it never backfired when starting, now just a little.
PO said replaceing the WUR fixed his hot start problem, although it is starting to do it again, i think it may be the check valve this time. i just trun the key and lift the sensor plate for about 3-4 sec. then go and start it.

[Alan L]

Here is a thought - it may not be the WUR - but your Aux air valve/regulator.
This has a mechanical slot which allows more air for cold start (along with more fuel from your cold pressure in WUR). This combination is your cold start 'choke' system. If the AAR is stuck open, ie not closing on the bi metallic finger (they do fail - broken wire, or disc grunged up) then your warm mix will be too lean and your cold one just right. Changing WURs won't fix this. But repairing the AAR will. You can check by pulling the hose off the AAR when engine warm - torch and mirror to look down the throat at the disc. There should be no slot showing.
If there is - that is your problem.
Alan

[Porsche_monkey]

It's a bit difficult to get at in the car, but when it is warm you can reach in with needle nose vice grips and pinch the hose off. If the idle changes then it is leaking.

[Paulporsche]

Thanks again, everyone.

MCA,

I started getting a lot of "blatt blatting" from the exhaust when I closed the throttle which was telling me my mix was too rich, so I started fooling w/ other WURs. I've never verified the control pressures on the Audi one, so that's due to be done.

Alan L,

You're right. Air leaks are an obvious thing to check for. My mech said he didn't find any.

The AAR is one to look @ as well. I think mine is good since it holds the cold idle up for a few minutes and then returns it to normal as it should.

T77911S,

Thanks for those figs. If it's not too much trouble I would like to hear what you (or anyone else) gets for resistance figs for the WUR strip. Also, does this fig indeed change w/ temp and/or time as Alan L said?

When I get home I think I'll be checking resistances. Since the CP remained steady for so long, I'm still leaning toward the bimetal strip windings.

It may not be for a couple of weeks, but I'll report back.

[Paulporsche]

I just read that the resistance should be 24 ohms for the 045 WUR, which is one I am using. A higher resistance indicates a longer warmup time, say for use in colder climates.

[Alan L]

The resistance alters the current draw (=heat) = warm up time.This is not as critical as having some resistance - compared to infinite = broken wire. So, first check you have some. The warm up time is not that critical - it just varies the synchronicity with the WUR/AAR warm up time - which each work via same mechanism.
I would still physically check the AAR - while your engine shows smptoms of warming up normally - don't forget the WUR is also doing the warm up phase - which restricts fuel flow and drops revs independant of AAR.
Alan

[Alan L]

Not wishing to cast any aspersions on your mechanic - but is he an experienced P mech?
The air leaks can be hard to track down, and it can take some time to be sure there are none. A lot of them are not easy to get at with engine in - intake runner rubbers, injectors seals, not to mention all the ancillary hoses.
Alan

[Alan L]

Not wishing to cast any aspersions on your mechanic - but is he an experienced P mech?
The air leaks can be hard to track down, and it can take some time to be sure there are none. A lot of them are not easy to get at with engine in - intake runner rubbers, injectors seals, not to mention all the ancillary hoses.
Alan

[T77911S]

i went back and re-read your initial post. i would think the heat from the engine would eventually make the pressures change, if not, then the WUR would be bad.
check for 12v at the WUR.
just to be clear, the pressure never changed from the initial 1 bar?
i take it you are moving the pin on the WUR to change the presure.
as you change the pressure, are you changing the mxture screw also?
i have not looked inside a WUR in a looong time, but isnt there a nut that holds the metal strip down? i wonder if that could come lose?