CIS: idle rising over time

[ohecht]

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

[fred cook]

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

[ohecht]

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

[dougcl]

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

Doug
75 911S

[ohecht]

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

[ohecht]

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

[RoninLB]

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

[ohecht]

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

Olivier

[RoninLB]

Originally posted by ohecht
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

[ohecht]

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

[911SCRS]

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

[ohecht]

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

[RoninLB]

Originally posted by ohecht
The fluctuations are now less severe, but still there.

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

[ohecht]

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

[ohecht]

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

[Eric Mckenna]

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

[911SCRS]

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.

[ohecht]

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

[ohecht]

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

[RoninLB]

Originally posted by ohecht

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.

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