CIS fuel pressure rising too rapidly

smakuch · 11-23-2012, 07:31 AM

I have a puzzling problem with my 83 SC. This car has been my daily driver for 15 years so I'm quite familiar with it.

I had to park the car for the last 3 months so I'm dealing with aged fuel as well. She ran fine in august when the temp was over 90 in FLA. When I got her going again last month the temp was in the 80 & it started fine but wasn't quite right til I got up to temp.

Now the temp has been around 70....She still starts great but goes full lean in 90 seconds. Start up fuel press is 15lb. (1 bar....I need to get a "bar" gauge so we can communicate ) & in 90 secs it goes to over 50lb & is not driveable. It doesn't do this when I pull the wire from the WUR.

I went out early this AM when it was 50 degrees. It started fine with 11lbs & gradually went to 30lbs & stayed there for several minutes. When I blipped the throttle there wasn't any lean backfire issues....reved fine.......stunned me.

I went out 2 hours later when it was 60 degrees & it started fine @ 15 lbs & went to 50 within 90 secs. Backfire when I blip the gas.

I've already checked the accumulator & it seems OK.

Any ideas?? I've stumped the local guru!

Thanks, steve

smakuch · 11-23-2012, 07:32 AM

I forgot to mention that I've put on my spare WUR & nothing has changed!

CountD · 11-23-2012, 07:58 AM

Quote:

Originally Posted by smakuch

I forgot to mention that I've put on my spare WUR & nothing has changed!

For some reason, I still suspect the WUR. Just because you swapped it out with another one doesn't mean that the other WUR is within spec. Both of them might be bad (I have had this happen many, many times over the years). If the WUR is original, then it is 30 years old, and the other if it is used, is probably just as old. You said the accumulator is OK, but how do you know this?

smakuch · 11-23-2012, 08:13 AM

The replacement WUR is the one that was running fine when I took it off. They are pretty simple to clean & adjust. The accumulator is not leaking. My pressures are correct.....just the rate of rise is baffling. I noticed an adjusting screw inside the WUR..........might this change this rate??

Bob Kontak · Bob Kontak's Garage

Check the resistance on the plug leads to the bi-metallic strip in the WUR. There is a value of 25-ish ohms you should see.

Does it eventually over-lean out with the WUR unplugged?

Scott R · Scott R's Garage

Quote:

Originally Posted by Bob Kontak

Check the resistance on the plug leads to the bi-metallic strip in the WUR. There is a value of 25-ish ohms you should see.

Does it eventually over-lean out with the WUR unplugged?

I'm with this idea as well, seems like a dead cylinder to me. 90 seconds is about right, they do heat up quite quickly. If you have a way of testing the CO% you will see this as well in high CO from the unburnt fuel.

Bob Kontak · Bob Kontak's Garage

I was also thinking fuel return line partial clog but then read where it does not happen when WUR unplugged. An obstruction does not care if the WUR is plugged in or not.

I can't find the WUR specs for the later SC's but full warm has to be in the 50psi range. 30psi will provide a nice rich mixture for running and Smakuch stated this works fine. Was this with the wire unplugged? I am thinking the engine warmth will eventually push it up to 50 psi (but not sure if engine heat alone will do this) and this normal warm control pressure will trigger the symptom. The 30 psi is maybe masking another problem as ScottR is thinking.

Agree with Scott on the 90 seconds as ok. Believe the newer SCs warm up quicker than older ones for better emissions.

smakuch · 11-23-2012, 09:38 AM

I only have about 11 ohms on the one that I took out. I'll check the car @ home later for the other one. I've never let it run long enough to lean it out.

Bob Kontak · Bob Kontak's Garage

Suggest doing some research on WUR resistance and correct values. I am pretty sure in the 20's is ok but I am not sure if 11 ohms means the kiss of death.

I am also not absolutely certain if the newer or older SC WUR's are faster or slower on warm up. I think the newer (090) are faster.

WUR Polarity ???

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by smakuch

I only have about 11 ohms on the one that I took out. I'll check the car @ home later for the other one. I've never let it run long enough to lean it out.

smakuch,

When I was reading the early part of your post, I was thinking 'bet he has a 10 Ohms' reading on his WUR!!!! And then on your last post you mentioned that it was 11 Ohms. A couple weeks ago, a member from Missouri needed some assistance and his Porsche mechanic called me. Same year and model as yours.

Generating a 50 plus psi control fuel pressure after 90 seconds from start-up is too quick!!!! And if you have some undetected unmetered air, it would be worse. What's the number on your WUR? Do you have a -089 or -090? US or Row car? PM me and I'll tell you what do next.

Tony

K9Torro · 11-23-2012, 01:12 PM

Hey I live just North of Jville and let my car sit a few months then started it only to find a dead cylinder I posted a thread about my fix pm me if you want to talk I will give you my number. Take a temp reading on each cylinder exhaust and make sure they r all running. If not give me a call.

Todd

Scott R · Scott R's Garage

Warmup to control pressure is between 1.5 and 2 mins from his starting temp of 70 deg ambient. More than two mins will indicate he has a faulty heating element in the WUR.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by boyt911sc

PM me and I'll tell you what do next.

Just let the board know what you do to fix.

I'll bow out since these guys are sharp.

Tony has one or two extra WUR's if you need to obtain one.

smakuch · 11-23-2012, 03:20 PM

Thanks a lot to all of you guys helping out.....It's a learning experience for us all!!

A little history;
The WUR that I replaced last week was an 090....that was older.....perhaps the original that was reading 11 ohms. I'd cleaned & re-adjusted it some time ago & has been trouble free until now.

I put in an 090 that was new about 10 years ago that had worked fine before. It now reads 26 ohms!! This one is rising to 50lbs in 90 secs......I checked it again when I got home earlier! Real "Head Shaker!!"

I had the entire induction off 1 year ago & replaced all of the intake boots & whatever was questionable. While I had the engine out for clutch repair that seemed a natural opportunity to put in the 964 cams that had been on the shelf for a while. Sure helped out the SSI/Monty exhaust.

Thanks, steve

Bob Kontak · Bob Kontak's Garage

Tony mentioned air leaks. Run through a procedure to check for the sucking of false air.

I had fits with one intake runner not being tight enough on the head - i had just had the engine out and re-did all the intake bits. Took me a long time to find it.

smakuch · 11-26-2012, 03:07 AM

I've not had much time to spend on this project, but the bottom liine is that I suspect the accumulator. I've noticed some fuel moisture on the bottom threads so it must be leaking.

Tony, I sent a PM on Sat but it must not have went thru.

I'll post a follow-up when I get an accumulator.

Thanks All,

steve
83 SC Coupe

jeffs9146 · 11-26-2012, 06:36 AM

I would suspect either a blocked return line to the tank, or a vacuume leak!!

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by smakuch

I've not had much time to spend on this project, but the bottom liine is that I suspect the accumulator. I've noticed some fuel moisture on the bottom threads so it must be leaking.

Tony, I sent a PM on Sat but it must not have went thru.

I'll post a follow-up when I get an accumulator.

Thanks All,

steve
83 SC Coupe

Steve,

Got your PM and have not read this post for a few days. Why would you replace your FA (fuel accumulator)? FA has nothing to do with your problem unless you want to spend buying parts. Like what you said, you are having problem with your fast rising control pressure. So I would focus on checking the control fuel pressure first before replacing CIS component/s.

This is what I would do if I'll be doing the investigation:
1). Check and record the cold control fuel pressure with the WUR electrical plug disconnected.
2). Connect the plug and observe the gradual increase of the control fuel pressure versus time (sec.) until the control pressure stabilizes (max.).
NOTE: Do the above tests with the FP running and no need to run the engine.

Another part of the investigation is to determine the integrity of your vacuum system. Other people like to use combustible liquids or gas to check for air leaks. I prefer pressurized air instead. Not until you have tested and confirmed the absence of significant air leak would you tinker or change the mixture setting.

Check your residual fuel pressure. There are several parts in the system that directly affects residual pressure aside from fuel accumulator. Pressure test your FA, if it leaks then it no good. Keep us posted.

Tony

smakuch · 01-26-2013, 04:46 AM

Tha Accumulator cured it.....Thanks All

montauk · 04:14 AM

I'm new to Porsche 911 ownership and have been struggling with a cold start problem on my 1980 SC. One thing I did find regarding the WUR resistance readings is that both are correct.

The 072 in my car has a heating element in it with three electrical connectors. There's also another set of contacts attached to the arm. The resistance changes from high to low as the temperature rises. So depending on the temperature, you may read around 10 ohms or you may read around 26 ohms. You can see the ratings in the third photo. I guess the 16.5 Ohm resistance is connected in series with the contacts. This combination is connected in parallel with the 26 ohms resistance. Around 10 ohms when warm with the two elements in parallel. With 26 ohms, the cold temperature resistance, there's less current so it takes longer to warm up.

I bought a used 090 WUR on ebay last week and took it apart. It's got a similar setup but the adjustment screw for setting the secondary contacts faces the other way. I guess they made it that way so you could just remove the bottom of the WUR and adjust the screw.

Here's some photos of my original 072 and the 090. You can see in the photo of my 072 where a portion of the reinforcing rib has been removed. You can also see the resistance ratings printed on the ceramic element. It's the same on both the 072 and 090.

I've got no idea what the correct temperature is for the resistance to change. Does anyone have any information about it?

Any help would be appreciated.

Thanks

Quote:

Originally Posted by smakuch

Thanks a lot to all of you guys helping out.....It's a learning experience for us all!!

A little history;
The WUR that I replaced last week was an 090....that was older.....perhaps the original that was reading 11 ohms. I'd cleaned & re-adjusted it some time ago & has been trouble free until now.

I put in an 090 that was new about 10 years ago that had worked fine before. It now reads 26 ohms!! This one is rising to 50lbs in 90 secs......I checked it again when I got home earlier! Real "Head Shaker!!"

I had the entire induction off 1 year ago & replaced all of the intake boots & whatever was questionable. While I had the engine out for clutch repair that seemed a natural opportunity to put in the 964 cams that had been on the shelf for a while. Sure helped out the SSI/Monty exhaust.

Thanks, steve