Surprising Fuel Pressures... Time to figure out the poor cold running?

[SkunkWorks]

Thanks to Moozak for letting me borrow his gauges.

Issue: My US 1980 SC has a 2200 RPM stutter/buck at all operating temperatures. Generally seems to run rather lean when cold evidenced through occasional intake backfires and enhanced bucking from above. And has a low cold idle @6-700 RPM.

Been chasing the cause for the past few months, but I didn't want to take the car off the road and the issue was livable before. Would appreciate Pelican's insight and suggestions for things to check!

Things to note: Replaced all intake gaskets, performed a smoke test for vac leaks, adjusted timing to spec, CO adjusted to spec of 0.8, oil cap off results in lowered RPM and rough running.

Known possible issues:WUR (072) has resistance of 48 ohms but wouldn't this just affect how long it takes to warm up? and pulling the O2 relay with car running results in no change to running.

Performed the test with outside temps at 9C/48F

System: 62 psi/4.27 bar Slightly low, but close enough? Spec is 4.5-5.2 bar

Control: 10.5/0.7 Super low. Bentley calls for CP cold of 1.5-2 bar @ 10C
30 sec: 12
1 min: 16
2 min: 22
3 min: 27
4 min: 30
5 min: 32
7 min: 34
9 min: 35
12 min: 36/2.5 Also low, spec is 3.4-3.8 bar

Residual: 62 psi
10 min: 19 psi

Questions:
- Does my system pressure being off by 3 psi a major cause for concern? Will check the FP for leaks today
- Control pressure is very low. But wouldn't this result in very RICH condition? Currently car seems to run lean, not rich...
- The delta for my control pressure is 25.5 (36-10.5) while spec is 28 (50-22). Is this an indication that the WUR needs to be rebuilt and not just a case of readjusting the CP through the knocking of the plug?

I think that's it for now... Plan of attack? What to check first? Have Bentley, and have extra beer to go around. Thanks Pelican!

[SkunkWorks]

Not getting much, er, any traffic in here but I'll keep it updated and use it to collect my thoughts. Also, I hate those troubleshooting threads where OP never updates it after advice is given...

Anyway, crawled under the car today to check the connections at the fuel pump, there was slight weepage so tightened that up but the system pressure didn't increase - still at 62 psi. Hmm.

Also wanted to double check my CO with my brand new wideband sensor and at idle, AFR was around 14.2. Not bad for my tuning 'by ear'!

... Till I checked the AFRs around 2500 RPM, immediately into the 15s and 16s. That would explain the bucking and intake backfires.

Now for what would cause that; Since control pressure is so low, it SHOULD be rich as all hell right now, but it isn't. The only other thing that would out-symptom that and cause a lean condition would be the frequency valve, I believe?

It should be an easy check but since I've no one around with another 80 SC to compare FV buzzing/vibrating, I don't know if what I'm feeling is the vibrating or simply pulsing from the fuel pump. No buzz from what I can hear and the vibration is pretty subtle. Time to check that next!

[Hil911]

Since you have silence - I thought that I'd put some cents in!

Yup - low pressure is rich. That should be very rich.

I think that your control pressure is significantly low. You have to bear in mind that the control pressure is played against the tuning. By which I mean that you can have a low control pressure and still tune it rich but that the profile will probably come out wrong from idle to running under load.

I took my WUR apart recently and expected the bimetal strip to be flexible like an aquarium thermostat. It is actually really hefty and all the other adjustments have to spring from it. This makes me worry about the temperature dependence. You are pretty cold there and as your resistance is rather high (Amps=Volts/Ohms) you may simply not be generating enough heat. I wonder what would happen if you wrapped it up a bit?! You are right that it will heat slower but I would have expected it to end up colder as well.

As for my thoughts about rebuilding the WUR - that strip is pretty hefty! If the other parts (rubber diaphragm, metal diaphragm and "nail") are OK I'm not sure that there is much to gain. Mine was "rebuilt" a few years ago, but looking inside the parts are all original ...

Sorry not to be the expert that you were hoping for but maybe my uninformed comments will inspire further response ...

[SkunkWorks]

Appreciate the response either way!

No doubt - it is entirely possible the entire mixture screw is dialed in way too much to compensate.

Hmm, I'm thinking my metallic strip needs replacement possibly? If not from the high ohms resistance but the fact that the difference between my cold to warm values is quite a bit lower than the gap between the cold to warm values per the Bentley...

On a related note, are WUR rebuild kits even available for sale (new bimetallic strip, o-rings, etc.? Did some searching but seems my only option is to send it in somewhere.

[Hil911]

I wonder if the strip ever wears out? As far as I could see the heating element is just wrapped around the bimetal strip in a band. I'm sure that one could replace that. I've never seen a kit to rebuilt them but oxide coated nichrome or something would probably do as a replacement element.

I didn't look at the metal diaphragm against the petrol but it looks as though it would be a plain piece cut from a sheet.

The rubber diaphragm on mine for the vacuum response is custom: rubber/fabric with a metal disc. But they are probably all different for different years.

[T77911S]

all your pressure are low. this will make the car run rich BUT, you have compensted for it by adjusting the mixture. that does not mean all is well. also, if your mixture is set correctly disabling the O2 sensor should not effect it.

here is what i would do, and have done, but then i dont mind getting myself into a hole, its how i learn.
i would make some shims and bump the system pressure up.
then i would knock the plunger on the WUR that has the fuel lines attached to it down to raise the warm CP. at this point i would assume the WUR is bad and i have nothing to lose by adjusting it. if i did not adjust the system pressure up i would set the WCP on the hi side of the spec. then i would adjust the mixture.
next day check and set cold CP. you will likely have to knock the small pin down to raise the CCP. if you go too far with either one you have to take it apart and knock it back out then start over.

make sure all your intake bolts are tight along with the clamps on the rubber boots on the intake runners.
how are the injector sleevesa nd O rings?

i see a lot of people have the same issue with a stumble that have an SC.

[boyt911sc]

Originally Posted by SkunkWorks

Thanks to Moozak for letting me borrow his gauges.

Issue: My US 1980 SC has a 2200 RPM stutter/buck at all operating temperatures. Generally seems to run rather lean when cold evidenced through occasional intake backfires and enhanced bucking from above. And has a low cold idle @6-700 RPM.

Been chasing the cause for the past few months, but I didn't want to take the car off the road and the issue was livable before. Would appreciate Pelican's insight and suggestions for things to check!

Things to note: Replaced all intake gaskets, performed a smoke test for vac leaks, adjusted timing to spec, CO adjusted to spec of 0.8, oil cap off results in lowered RPM and rough running.

Known possible issues:WUR (072) has resistance of 48 ohms but wouldn't this just affect how long it takes to warm up? and pulling the O2 relay with car running results in no change to running.

Performed the test with outside temps at 9C/48F

System: 62 psi/4.27 bar Slightly low, but close enough? Spec is 4.5-5.2 bar

Control: 10.5/0.7 Super low. Bentley calls for CP cold of 1.5-2 bar @ 10C
30 sec: 12
1 min: 16
2 min: 22
3 min: 27
4 min: 30
5 min: 32
7 min: 34
9 min: 35
12 min: 36/2.5 Also low, spec is 3.4-3.8 bar

Residual: 62 psi
10 min: 19 psi

Questions:
- Does my system pressure being off by 3 psi a major cause for concern? Will check the FP for leaks today
- Control pressure is very low. But wouldn't this result in very RICH condition? Currently car seems to run lean, not rich...
- The delta for my control pressure is 25.5 (36-10.5) while spec is 28 (50-22). Is this an indication that the WUR needs to be rebuilt and not just a case of readjusting the CP through the knocking of the plug?

I think that's it for now... Plan of attack? What to check first? Have Bentley, and have extra beer to go around. Thanks Pelican!

Skunkworks,

If I were doing the troubleshooting of your CIS, I would focus on the following:
1). You have the wrong WUR internal parts. The WUR housing is -072 as you stated. I would bet my money that someone installed the wrong bi-metalic heater either by you or someone before you. Don't expect it to work as well as the real WUR-072.
2). Cold control fuel pressure out of spec.
3). System fuel pressure out of spec.
4). Ideally, it would take between 3 to 4 mins. (or shorter) from CP to WCP depending on the ambient temperature.

With the engine fully warmed-up and running, place your fingers on the FV located behind the engine. It should be vibrating or pulsating when in operation. You can't hear the FP from the rear with the motor running unless you have super sensitive sense of hearing.

In case you need to borrow a calibrated WUR-072 for your tests, I'll be more than happy to accommodate you, provided you have a USA shipping address. You don't need to buy the WUR, just use try it on your car and return when you are done.

BTW, say hello to Moozak (Jon). He called me a few times when he was having problem with his CIS last year. And he's very capable doing systematic troubleshooting. He should lend a hand fixing your problem. Keep us posted.

Tony

[Hil911]

On my unit (1976 2.7) tapping the plug in reduces the control pressure. And because the bimetal strip is so stiff, it has to be done very carefully. I used a digital caliper to measure the depression after each few taps. I aimed for 0.2 mm steps.

[Walt Fricke]

I think a good check when cold control pressure is too high is to open the WUR up and put a very thin washer between the bimetallic strip post and the strip (opposite side from the nut). This has the same effect as tapping the plug, but you don't have to untap it if you tapped too far. I am also told that you can only tap it down, then back up, so often, after which it is no longer tight in its bore.

I made a washer from some thin shim stock because all the washers in my junk box were way too thick for this. When guys say small movements, they mean small.

However, the bimetal strip should have zero influence on hot running. The strip bends up as it heats, which removes it from contact with the spring below it. At that point, only the spring (and any diaphragm pressure below the spring) controls the CP. And even if the electrical part is shot, eventually the engine heat will do the same thing.

I don't know if these bimetals ever wear out. I suspect that their bending is far below their fatigue point, so they can cycle endlessly within their range. But I am just guessing here.

[boyt911sc]

Originally Posted by Walt Fricke

I think a good check when cold control pressure is too high is to open the WUR up and put a very thin washer between the bimetallic strip post and the strip (opposite side from the nut). This has the same effect as tapping the plug, but you don't have to untap it if you tapped too far. I am also told that you can only tap it down, then back up, so often, after which it is no longer tight in its bore.

I made a washer from some thin shim stock because all the washers in my junk box were way too thick for this. When guys say small movements, they mean small.

However, the bimetal strip should have zero influence on hot running. The strip bends up as it heats, which removes it from contact with the spring below it. At that point, only the spring (and any diaphragm pressure below the spring) controls the CP. And even if the electrical part is shot, eventually the engine heat will do the same thing.

I don't know if these bimetals ever wear out. I suspect that their bending is far below their fatigue point, so they can cycle endlessly within their range. But I am just guessing here.

Walt,

Have you observed that the problem with the cold control fuel pressure (if any) is it is too high and not too low. The two most common causes of this out of spec (high) cold control fuel pressure for WUR's are:
a). Micro screen mesh at the delivery port of the WUR gets dirty and restricted in fuel flow.
b). The bi-metallic has not returned or deflected back to its original set point. The coil spring is exerting an upward force while the bi-metallic spring is resisting this force. Heat causes the bi-metallic spring to deflect upward and reduces the resisting downward force. As it cools down, the bi-metallic spring contracts to its cold state condition and pushes the coil spring down.

As this cycle of heating and cooling occurs repeatedly over a course of time (long), the bi-metallic spring fails to return to its original setting point and now very slightly higher than before. Thus resulting to a change in cold control fuel pressure.

Tony

[E Sully]

I would suggest getting the fuel delivery system in range before wasting time trying to adjust around it. Make sure the filter screen in the tank and fuel filter in the engine compartment are clean and not obstructing flow. The injectors are designed to operate at a certain range. Pressure and total fuel flow below the proper range will most likely deliver less fuel than necessary.
Get the System Pressure up to specification before doing anything else. It is lower than it should be, and everything else needs to be checked after the main pressure is in range. Low control pressure would make the mixture richer, but only if the System pressure was in spec. I would think the Bosch engineers had a minimum pressure and flow rate for a good reason.
Check the fuel pumps delivery rate to make sure it is flowing enough fuel, 1,000cc in 30 seconds at the junction of the return lines with System pressure in range.

Only after this is checked would I play with the WUR.

[john walker's workshop]

if nothing changes when you unplug the O2 relay, you need to deal with that issue first as it may be yuor entire running problem. if the frequency valve is not activated, it feels like you're dragging an anchor. no other adjustments will fix it. dome light fuse to relay, relay 87 and 87a to computer and freq valve.

[SkunkWorks]

Originally Posted by john walker's workshop

if nothing changes when you unplug the O2 relay, you need to deal with that issue first as it may be yuor entire running problem. if the frequency valve is not activated, it feels like you're dragging an anchor. no other adjustments will fix it. dome light fuse to relay, relay 87 and 87a to computer and freq valve.

Hey John, Dome light is on, and I get 12V with car running on relay 87 and 87a. What next?

I can probably conclude that the FV is not buzzing just by feel. Everyone says it's pretty obvious if it's on or not so my "maybe slight fuel pump pulsing" feel = definitely not buzzing/vibrating.

Tony, I checked my pressures back in April and both cold control pressure and warm CP were within spec so I don't believe I have the wrong WUR guts.

Is it possible to check the screen at the fuel pick-up without dumping the entire tank out? I have about 70L in there right now...

[boyt911sc]

Originally Posted by SkunkWorks

Hey John, Dome light is on, and I get 12V with car running on relay 87 and 87a. What next?

I can probably conclude that the FV is not buzzing just by feel. Everyone says it's pretty obvious if it's on or not so my "maybe slight fuel pump pulsing" feel = definitely not buzzing/vibrating.

Tony, I checked my pressures back in April and both cold control pressure and warm CP were within spec so I don't believe I have the wrong WUR guts.

Is it possible to check the screen at the fuel pick-up without dumping the entire tank out? I have about 70L in there right now...

So you checked your fuel pressures in April and they were in spec. So what's the number you got? The fact that you are posting in this forum says you needed help to diagnose or fix your current problem/s. We all needed help.

Your data clearly shows that it took more than 12 mins. to get your max. WCP and still out of spec. It would take about 3 mins. for a good working WUR-072 to get to max. WCP (spec.). The value of your heating resistance of 48 Ohms is not the correct one for WUR-072. You could believe what ever you want to believe, but the WUR you have won't work well with that kind of resistance. Talk to Moozak (Jon) and listen.

Tony

[SkunkWorks]

Tony, I'm not denying that my WUR is out of spec. Just simply stating that I don't believe I have the wrong internals inside of it. Whether or not the internals are working correctly, that's the reason for this thread.

Here are my pressures from back in April. Obviously quite different with what I'm getting now.

Cold = 26 psi/1.8 bar
System = 68 psi/4.7 bar

With WUR plugged in:
30 sec = 28 psi
1 min = 32 psi
2 min = 36 psi
3 min = 40 psi
4 min = 42 psi
5 min = 50 psi
6 min+ = 52 psi

Residual:
Starting = 32 psi
4 min = 26 psi
7 min = 24 psi
10 min = 22 psi
15 min = 21 psi
30 min = 18 psi

[boyt911sc]

Originally Posted by SkunkWorks

Tony, I'm not denying that my WUR is out of spec. Just simply stating that I don't believe I have the wrong internals inside of it. Whether or not the internals are working correctly, that's the reason for this thread.

Here are my pressures from back in April. Obviously quite different with what I'm getting now.

Cold = 26 psi/1.8 bar
System = 68 psi/4.7 bar

With WUR plugged in:
30 sec = 28 psi
1 min = 32 psi
2 min = 36 psi
3 min = 40 psi
4 min = 42 psi
5 min = 50 psi
6 min+ = 52 psi

Residual:
Starting = 32 psi
4 min = 26 psi
7 min = 24 psi
10 min = 22 psi
15 min = 21 psi
30 min = 18 psi

SkunkWorks,

Your SP (system pressure) was @ 62 psi...................Spec. 65.25 - 75.4 psi.
Your WCP (warm control pressure) was @ 36 psi........Spec. 49.3 -55.1 psi.
Your WUR-072 has 48 Ohms resistance....................WUR-072 spec. @ 26 - 27 Ohms.

Your FV (frequency valve) is inoperative as you have discovered when the FP was running. A good frequency valve should be pulsating/vibration when the FP is energized. Either the FV is defective or not getting power from the relay or ground from the ECU.

Correct system fuel pressure, warm control fuel pressure, and frequency valve first and foremost. All the specifications for the fuel pressure are in the book. As for the WUR-072, wish you luck.

Tony

[SkunkWorks]

Thanks Tony.

I'll take a look at my FV per John Walker's instructions and then move onto the WUR. I appreciate your earlier offer of a loaner WUR and I'll most likely take you up on that.

For the FV, I'll check for 12V at the FV electrical connection and throw a dwell meter on it too to verify operability.

Any other checks I should do to find out why it's inoperative?

[Walt Fricke]

Tony
If the bimetallic spring over time gets tired and no longer returns to its full down position when it gets cold again, doesn't that mean that the pressure from the big coil spring acting upward through its metal cup and the little rod, is greater. If the diaphragm is pressed upward more strongly, does not the control pressure rise, because less of the system pressure is being bled off past the diaphragm and into the return line?

Isn't tapping the plastic or bakelite or whatever it is perch for the stud which holds the bimetallic spring down from the outside a way of reducing the cold control pressure? This moves the end of the bimetallic spring down. So a washer between the bimetallic spring and its plastic perch inside the WUR would do the same thing? And lower that pressure? If it lowers it too much, easy to take it out and try to make a thinner one.

I'm not suggesting that this is the way for Skunk to go about things - he's got all those other things out of spec.

Skunk - feeling the FV with your hand when it should be running is the simplest check. If it isn't, then start figuring out why it isn't. Having a working FV is important with this system.

However, having a working O2 sensor is not so important, at least if you don't care about fuel consumption. Most race cars using these FV CISs don't have the O2 sensor hooked up. The default program for the FV works just fine for starting and wide open throttle, and there aren't flat spots in between either. But you'll probably run rich overall (which you can adjust), and as a result rich at the part throttle position, which is where almost all street driving happens.

You should check (or did you) that the mini-brain under the passenger seat is getting its 12V. You've checked the circuit which does that mostly (the light), so probably it does. Next to it is a relay. That is the FV relay - the brain tells it to turn on, and the solenoid in the FV moves one way. The brain says turn off, and the solenoid moves the other way. The frequency with which this on and off happens, combined with the way the on time versus the off time works, controls the bleed of fuel back to the tank, etc. Anyway, this relay has to work in order for the FV to work.

My most recent misadventure in this department was a short in a different part of the dome - er, side - light line, which blew the fuse which controls both. Fussed around with other things way too long before checking the fuse and running down the short.

[boyt911sc]

Originally Posted by Walt Fricke

Tony
If the bimetallic spring over time gets tired and no longer returns to its full down position when it gets cold again, doesn't that mean that the pressure from the big coil spring acting upward through its metal cup and the little rod, is greater. If the diaphragm is pressed upward more strongly, does not the control pressure rise, because less of the system pressure is being bled off past the diaphragm and into the return line?

Isn't tapping the plastic or bakelite or whatever it is perch for the stud which holds the bimetallic spring down from the outside a way of reducing the cold control pressure? This moves the end of the bimetallic spring down. So a washer between the bimetallic spring and its plastic perch inside the WUR would do the same thing? And lower that pressure? If it lowers it too much, easy to take it out and try to make a thinner one.

I'm not suggesting that this is the way for Skunk to go about things - he's got all those other things out of spec.

Skunk - feeling the FV with your hand when it should be running is the simplest check. If it isn't, then start figuring out why it isn't. Having a working FV is important with this system.

However, having a working O2 sensor is not so important, at least if you don't care about fuel consumption. Most race cars using these FV CISs don't have the O2 sensor hooked up. The default program for the FV works just fine for starting and wide open throttle, and there aren't flat spots in between either. But you'll probably run rich overall (which you can adjust), and as a result rich at the part throttle position, which is where almost all street driving happens.

You should check (or did you) that the mini-brain under the passenger seat is getting its 12V. You've checked the circuit which does that mostly (the light), so probably it does. Next to it is a relay. That is the FV relay - the brain tells it to turn on, and the solenoid in the FV moves one way. The brain says turn off, and the solenoid moves the other way. The frequency with which this on and off happens, combined with the way the on time versus the off time works, controls the bleed of fuel back to the tank, etc. Anyway, this relay has to work in order for the FV to work.

My most recent misadventure in this department was a short in a different part of the dome - er, side - light line, which blew the fuse which controls both. Fussed around with other things way too long before checking the fuse and running down the short.

Walt,

The answers are YES to all your questions and statements. Using spacers or washers would work but getting to the finer adjustments using washers is very tedious too. One more thing I like to comment to the above post, FV relay. I refer to it too as FV relay and not as OXS relay. But most people including in this forum refer it as OXS relay and in some notable maintenance manuals.

Tony

[SCadaddle]

I'll just add what my experience with the FV on my '82 SC was like:

1) All FV or OXS relays are not created equal. Tony sent me to schooling on that and it's documented here on the forum under my username. Here ya go:

911SC Frequency Valve problem


2) In my instance, the car was tuned to the right CO level with the Lambda box and the FV working. The wiring from the Lambda box to the FV was suspect as in crunchy and charred in spots, as if the previous owner(s) had hooked something up wrong. For that matter, when I acquired the car, it was tuned to run without the Lambda/FV working. Nonetheless, the box and the FV were put in play and everything was working fine.

Then one day on a drive something happened. The tune changed radically to lean. Nearly undrivable. I limped home about a mile and shut her down. Next day cranked it up and the cold start was smoother than ever, but only for a few minutes. Then the car started misfiring and sitting in my garage the rear end of the car was dancing with the engine running like a paint shaker. Sourced another Lambda box and rewired the section to the FV and it's back running like a champ. Swapped the old Lambda box into another "known well running SC" and the symptoms went along with the box. Maybe there is a bit of info in all that.