CIS: idle rising over time
 

I have dealt with almost every CIS idle problem before, but this one seems really weird.

The idle is steadily rising, by about 200 rpm each complete trip (i.e. to operating temperature, cruising, and returning).

I can set it to 1000 rpm at operating temperature, but then it keeps going up a few hundred RPM per trip, until I have nothing left in the idle adjustment.

It is almost as if the AAR is working in reverse, but I know it is working correctly.

I have checked and set the timing, mixture, and tried to clean the distributor advance mechanism, but it is still happening, and I am stumped.

Anyone have some leads or similar experisnces?

Thanks in advance,

Olivier

Up Up and Away...
 

Olivier,

Sounds like you might have a vacuum leak that is prgressively getting worse. Think about it .. when the engine is cold, the AAR is open and lets in additional air for a fast idle. As the engine warms up, the AAR closes and the idle goes down to 950 rpm or thereabouts. Apparently you have been compensating for the vacuum leak by adjusting down the idle bleed screw until it is completely closed. Look for a bad intake tube boot or maybe even a leaky brake vacuum booster.

Good Luck!

Fred Cook
'80 911SC coupe

Thanks, I will try that. It happens every trip, though, over the past 2 days, could a leak be getting that much worse so quickly.

Olivier

Make sure getting your foot under the gas pedal and lifting up has no effect on the idle.

Doug
75 911S

I will check that, too, thanks. I did check at the throttle linkage, and there does not seem to be any binding and it does easily return to the stop.

Olivier

OK, I found a loose vacuum hose at the AAR and thought that would fix it, but it is still doing it!!!

I checked everywhere for other vacuum leaks with propane, and found nothing.

Just now, I started the car after it sat for 3-4 hours, the oil temp was at 120 and the idle was at 900.

Half a mile later, the oil temp was 150 and the idle was up to 1200.

A mile after than, operating temperature of 180 degrees and the idle was up to 1800!

The AAR is closed all the way even at the starting temperature, so I do not think that is the cause.

Opening the oil cap makes the car run badly by admitting false air, which is as it should be as well.

This problem started suddenly yesterday.

I do not think there are any vacuum leaks and I cannot think of what would be temperature related causing the idle to rise with the temperature.

Anyone have any more ideas? I am stumped and ready to go EFI or trash the whole car!

Olivier

I wonder if disconnecting the deceleration valve would make a difference.. you don't really need it anyway/smog crap..........Ron

Is that something that could suddenly go bad? Should I just disconnect the vacuum line and cap it?

Olivier

who knows how fast it can go bad?
mine was a small vac line disconnect and plug.. your's is probably a different unit, but the same routine.

the point is to eliminate it as a possibility of causing your problem. and mine can be adjusted. which may also help.......Ron

Disconnecting and plugging the AAV did not work, and I am getting really frustrated. I had it at a normal idle at operating temperature idling standing still, but something happening during actual drives in the car is making the idle go back up to 1800 rpm.

I cannot find any vacuum leaks, and the distributor advance mechanism is working correctly.

Does anyone know the most likely culprits for richening the mixture as the car wamrs up during driving?

I can think of the WUR, the Cold Start Valve (leaking?), or something wrong with the mixture control unit.

Has anyone else run into a similar situation?

I am running open loop with the O2 sensor disconnected.

Olivier

Did you check the vacuum advance mechanism in your distributor. This may be sticking as the engine gets hotter.

I found a broken heater wire in the AAR, which I fixed. The fluctuations are now less severe, but still there. The system and control fuel pressures are fine, so I think it is definitely an air leak somewhere. The control pressure did not change with the engine temperature, but it was already over 80 degrees out. The vacuum advance seems to work (the idle rises when I disconnect the vacuum line), but I will check it at higher temperatures.

Thanks,

Olivier

so you tried disconnecting the decelerarion valve as I mentioned above?

Yes, that did not seem to make a difference. My deceleration valve looks just like the AAV, just with one additional (small) vacuum line. It does not seem to match the pictures in the Bentley manual.

I just verified the AAR is about 1/4 open when the engine is cold (first thing in the morning), and closing in about 2 minutes just using the electrical heating element (fuel pump running). The broken wire on the heating element was making it less reactive, and it was only closing after I shut down the wamr engine and the temperature in the engine compartment rose while the car was sitting still. At operating temperature with air flowing, it was cool enough to open a little bit on long drives, I expect. I think that would explain the erratic symptoms (there for some trips, not for others), but I am still having problems.

I also suspect the AAV, but have not pulled it yet. I also noticed that my Control Pressure Regulator has a vaccum connection, which seems to be a feature only on the earlier cars according to the Bentley troubleshooting charts. I am not sure if any backdating was done on my car, but it is an 82 with the closed loop O2 system.

I have also been wondering about the various vacuum connections to the throttle body, as they seem to produce different levels of vacuum. Does anyone have a diagram of exactly which connection goes to which fitting? (I have 3 connections, one for the CPR, one for the decel valve, and one for the distributor advance).

I also verified the operation of the various temperature switches that control the cold enrichment cycle. The duty cycle of the frequency valve is 65% until the car reaches a warmer temperature (about 2 minutes), when it goes to 50.4% in open loop mode (O2 sensor disconnected). I did install a "superswitch" a few months ago that reduces the temperature at which this enrichment occurs for better "moderate temperature" starting. It has worked fine since then, though, and this idle problem came on suddenly.

The way I have the mixture and idle now, it starts and idles low (500 rpm) and rises slowly to 900-1000 rpm on the way to operating temperature. What is unusual about this is that I know the AAR is open when I get the low RPM level, since I have checked it with a mirror, just before starting the engine. Not as severe a swing as before, so it is in a drivable range, but still not right.

I am not sure how I could not have a high idle with the AAR 1/4 open when the warm idle level was 1000. I will try to check the "antler" hose under the air box if I can partially drop the engine. I replaced it a few years ago after finding a small tear in it. I would also like to remove the entire airbox for inspection, but I cannot find a good set of instructions.

This is getting really frustrating, as the behavior seems to be the opposite of what I would expect with certain component failures.

Olivier

PS. There is also an ignition miss that occurs under partial throttle under load, more when the engine is cold.

I checked the resistance of the Magnecor wires, and they all seem to check out for their respective lengths. The cap and rotor are also new.

This problem was there before the idle issue, but I am starting to wonder if they might be related.

Olivier

MINE IDLES HIGH AFTER IT'S ALL WARMED UP...
DO YOU GUY'S HAVE ANY IDEA OF THIS..ISSUE.
ERIC

This really sounds very similar to a problem I had where the distributor vacuum advance mechanism was sticking. If you haven't tried this already, disconnect the red hose. Connect another hose to the distributor and with the cap and rotor removed, suck on the other end and observe the mechanism movement. This should move and return properly. Repeat for the retard blue hose. Run the procedure on a cold and also a hot engine and compare.
The WUR you have is definitely from a pre '78 porsche if it has a vacuum line. Based on my own experience, I have found that the WUR must be properly matched to the year or you will have all types of erratic problems. Check the part number on Pelican.

I cannot understand why someone would backdate a WUR, and why the throttle body would have the fitting for it as well.

I will definitely check the advance again...I tried to lube it just before this started due to the missing problem, and that may be related. I have some Bosch distributor grease, but I have never been able to find a lot of accessible places to put it in there, other than just around the shaft area.

I do know that disconnecting my red hose at idle has no effect on the idle, although disconnecting the blue hose makes the idle increase dramatically. There does not seem to be much vacuum coming from the red hose (never has, really).

Olivier

Current symptoms after the AAR wire fix: the car will idle at 1000 when totally warm, but now has cold starting problems (hunts for a few seconds and then settles into a low idle around 500 rpms, slowly rising to 1000 rpms as it warms up.

I checked the distributor advance and retard mechanisms when cold, and they operate fine. I will check them again when warm when I return home Sunday, as I found the opo-off valve to be a little loose, and I am re-glueing it in now. I had checked it before, and it was a tight fit, but the glue is now cracked and dry.

I verified all of the following are working: all 3 throttle microswitches, 15 degree, 35 degree temp switches, thermotime switch, frequency valve windings.

I cannot find any vacuum leaks at all using propane or throttle cleaner.

I am leaning towards sticking distributor advance mechanisms when hot, as that is the only component I messed with (tried to lube) right before this happened. The broken wire in the AAR was a conincidence that exaggerated the problem, as it is operating fine now.

My CPR definitely has a vacuum connection, which is described in the Bentley manual as providing extra enrichment during hard acceleration. Is this something someone would backdate to? It could not have been an issue of ordering the wrong part, as they had to connect a vacuum line from it to the throttle body.

I cannot find anything on checking the operation of this vaccum connection, and my control pressure did not seem to change at all as the engine warmed up. I took apart the CPR and everything checked out inside, in fact it looked like a new part inside. The heater had good resistance, and I flipped the small diaphragm even though it also looked perfect. The vacuum line just goes to the inside area of the CPR, and I cannot figure out how it is supposed to affect the control pressure. THe rest of the heater connections inside the CPR are also more complex than something like the AAR, with multiple springs and electrincal contacts.

Is rebuilding a distributor straightforward? I am afraid to have springs fly all over the place from my $900 NLA part if I take it apart, but I am sure it is related somehow.

Thanks again,

Olivier

on my system it's the WUR that causes that cold start situation. although mine would also spit ..

but the good news is that I just installed my new shield ......Ron

Although the car is driveable, this is still perplexing me and driving me nuts.

With the idle and misture set perfectly at operating temperature, the car will idle low when restarted cold, slowly rising back to the regular idle just as it reaches operating temperature again. After a cold start (the car fires right up), the idle will rise to about 1800 and then steadily lower back to about 900, which is what I want it to do. That is with it sitting and idling, without touching the accelerator at all. However, manipulating the trottle or driving away, will make the idle go down to about 500 rpms, and then it will slowly rise back to the warm idle level of 900. Even with the low idle, the car drives fine almost as if the idle is at the normal level (there is no stumbling that I have felt in the past with low idles or hesitation)

- The AAR is opening a significant amount for the first start of the day, even in 90 degree weather, and it is fully closed at operating temperature.
- The WUR is providing cold control pressure of 1.2 bar, warm control pressure of 3.5 bar, and the system pressure is 5 bar, all of which is right on spec. The system only takes about a minute to go from cold to warm control pressure.
- The thermotime switch is working and closing at 115 degrees.
- The car pulls a steady 20" of vacuum at warm idle.
- I cannot find any vacuum leaks using propane or throttle body cleaner. I had some small rises in the idle from just under the airbox, but I thought it might be getting sucked into the intake.
- All of the temperature switches and throttle position switches (82 SC) check out.
- The vacuum advance and retard both work under cold and warm conditions.
- I have always driven with the O2 sensor disconnected, and the frequency valve goes to 65% duty cycle during warm up, and then to 50%.
- It definitely seems temperature related, as the idle will rise above the warm setting if I let the car get really hot, like up to 250 degrees. The control pressure remains in spec even at these high temperatures, however.
- I am contemplating removing the airbox to check its bottom and the "antler" hose underneath (replaced two years ago), but that seems extreme for a driveable situation and I am hesitant to pull so much of the FI system apart.
- It seems like there is either a vacuum leak that is only there at cold temperatures that is leaning out the mixture under cold conditions, or something is enrichening the mixture under hot conditions. Wouldn't a vacuum leak when cold increase the idle speed, however?

This problem started suddenly during a single drive, when my wife drove the car to the pool. I heard her pull up witha high idle and thought it would be something simple like one of the vacuum lines to the distributor, but no such luck. She does not remember any emotional events during that drive, like backfires, etc.

I am stumped, nothing seems to make sense with this problem and I feel like I am chasing my tail.

I would love any other suggestions or similar experiences with resolutions!

Thanks again,

Olivier

When you checked the throttle linkage, as reported in a previous post, did you check the bellcrank on the side of the transmission to see if IT was sticking? That bellcrank connects the linkage from inside the car to a linkage that goes up to the top of the engine behind and below the throttle body.

If that sticks, you CAN pull the gas pedal UP with your foot to reduce the idle.

I will check that. I did check at the pedal to make sure nothing was sticking, but I did not check at the transmission. Pulling up on the gas pedal had no effect.

Thanks,

Olivier

If the bellcrank checks OK and you want to pull the CIS, give me a call and I can walk you thru it. I have pulled the CIS in my 80SC twice now. And since we are both in Richmond, we might get together to talk and demo the steps. If you want, give me a call -- I'm in the Richmond book.

That would be great. Were you able to pull it with the engine in the car (lowered only, maybe?).

It is not as intimidating now, as I am becoming more familiar with all of the components as I reach across a (sometimes hot) engine to disconnect and test them.

Where are you in Richmond? I am just west of Short Pump.

Olivier

I'm in Chester, off of Ironbridge Road (Route 10) south of the courthouse. I work at the museum on the Boulevard in Richmond.

Removing the CIS is really simple, but it can appear daunting. Disconnect the fuel supply, remove the WUR and AAR, the flapper position switch, throttle linkage, thermotime valve, brake booster vacuum line, cruise control if you have it, plastic heater ducts, heater motor, and maybe another thing or two. Lower the engine a little, disconnect the manifolds from the heads, and pull the "spider" upwards. With a little wigglin', it comes off as a neat package. Removing the air duct across the top of the unit between the flapper valve and the throttle gives a little more helpful top clearance.

I found that a swivel socket was necessary to get the manifold nuts off. A regular universal was too long to fit the space above the nut.

While the CIS is off, you get to play "discovery" with the things on the back side of the CIS assembly and on the engine in tha "black hole" that you can neither see nor reach into. A valve adjustment would be a good thing to do, plus general cleaning up. You may discover that your your oil pressure light switch is seeping oil.

Brian

I may pull it later in the season, after some project on my other vehicles, since it is driveable. I noticed during the last start-up test that the vacuum level is low (around 16") when the motor is cold, and then up to 20" when warm. I cannot find any vacuum leaks at all with propane or thottle body cleaner, and I checked everything.

Here's a weird behavior: covering the intake with a plastic bag does nothing to even lower the idle, even pushing down on the airbox cover. It idles along happily as if nothing is interrupting the airflow. I cannot see how it could have such a massive air leak and not respond to the propane, etc. I also checked to see that the throttle adjustment screw is in the middle of the range to make sure I am not at one of the extremes compensating for such a large intake or vacuum leak.

Olivier

PS. Brian, which museum? We were just down at the science museum with the kids this afternoon and our neighbor works at the Historical Society.

What happens if you cover both the air intake and the AAR at the same time? If nothing changes....cracked air box?

I know the AAR is closing fully, and I could not get the idle to rise in that area using propane or carb cleaner. I do suspect a cracked airbox now, but it is not responding to any of the vacuum leak finding methods, either. I have doused the underside of the box and top of the fan housing with carb claener with no effect. I am amazed that the car could drive so well with a cracked airbox, too. I may pull it just out of curiosity, it is driving me nuts not knowing what the problem is.

I will try sealing the AAR entirely tomorrow to see if that makes a difference.

Olivier

If th idle does not change when you cover the intake, then outside air is leaking into the airbox plenum then going thru the flapper valve and being measured. That big of a leak ought to be readily visible -- obviously you would have seen it before now.

You disassembled the WUR recently? Sealed up when you reassembled? Hoses hooked up right?

Brian

PS: The Museum of Fine Arts, Boulevard and Grove. Building and construction management, quite involved in our upcoming expansion program.

The WUR is keeping all of the pressures in spec, and it is enrichening the mixture when I apply vacuum to the vacuum port, which I assume is normal and for altitude compensation (lower outside air pressure would enrichen the mixture slightly). It must be sealed well since it will hold this vacuum.

I have always wondered about the AAR sealing, since I have modified (and sealed) the stock plug to use an adjustable nut and bolt. It does not respond at all to the vacuum leak tests, but I may take it out completely to eliminate it once and for all. If it is allowing air in, then I think it would go through the flapper valve and also explain why the idle never rises when the AAR is fully open (i.e. the air is already bypassing the AAR through the leak)

Olivier

Yes, rebuilding the distributor is a straightforward operation. You will want to collect new springs and the plastic that sits on the spring posts prior to your rebuild. Also a pin to attach the pinion back to the dist. shaft.

Use the Search button on distibutor -- plenty to view and see! This operation is something that ought to be done regardless.

John

Could it be that the plunger in the fuel distributor is sticking? Has anyone had that happen before?

Also, I was double-checking the AAR and found that the engine will immediately die if I disconnect the hose from the AAR on the downstream side (towards the oil tank). Is that normal?

Olivier

A sticking plunger would screw up the mixture, and would do it quickly I would imagine. I would think that a plunger that is a little off would result in a significant change in the mixture ...

The AAR diverts "measured air" from the path between the flapper and the throttle to downstream of the throttle plate, creating a throttle bypass but not affecting the mixture. If you disconnect the hose downstream of the AAR, you are opening the intake manifold vacuum to an outside, unmeasured air source and your mixture will not support combustion.

Your increasing engine speed has just about GOTTA be progressive increase in intake air, perhaps unmeasured, from an opening that gets progressively larger with -- primarily -- temperature. But it sounds like you have addressed the possibilities.

Maybe a binding throttle shaft? As the engine heats up, the shaft begins to stick, progressively in a more open position?

(With MY luck, the leak spot would be on the underneath side of the air box -- and it is looking like you have the same luck that I have!)

I would not be surprised if there is a leak under the airbox, but I am surprised it does not react to the propane test either warm or cold, and that the car runs so well in all other respects.

I will pull the CIS after I am sure the other cars are OK for a while and maybe in the winter. Even though the AAR is opening wide (I can see it with a mirror, the idle does not seem to increase significantly, which may be because it is being masked by the air leak or mixture problem).

Do you think something like the FD piston sticking could also be temperature related? I was going to see if it is affected by having a full tank of fuel or not, since I would imagine less fuel would increase the average fuel temperature as it recirculates with less of a heat sink reserve.

Thanks again,

Olivier

I have an electronic air and exhaust hand held leak detector. I think it costs about $125. It works great..........Ron

How does it work and where did you get it, if you don't mind me asking?

Olivier

I'm in the process of chasing a similar demon in my MB 380 SL with CIS...It was suggested that I check for a Vacuum leak and then check for leaky fuel injectors.

I cannot fins any vacuum leaks, but fuel injectors leaking is a possibility. I cleaned them thoroughly and checked the spray pattern a few hundred miles ago, and this problem did not start right after I cleaned them, so hopefully I did not make them worse.

As I checked the patterns, I did not notice any leaking, but it is something new I will consider.

Olivier

http://forums.pelicanparts.com/showthread.php?s=&threadid=118359&highlight=leak+d etector