CIS: idle rising over time

[ohecht]

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

[cmonref]

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.

[ohecht]

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

[cmonref]

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.

[ohecht]

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

[cmonref]

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

[ohecht]

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.

[WERK I]

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

[ohecht]

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

[cmonref]

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.

[ohecht]

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

[Jdub]

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

[ohecht]

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

[cmonref]

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!)

[ohecht]

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

[RoninLB]

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

[ohecht]

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

Olivier

[RickM]

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.

[ohecht]

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

[RoninLB]

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

Olivier

More test results, getting scared now!