82 CIS idle dip

[ohecht]

OK, I consider myself almost an expert on CIS now that I have rebuilt it, and I can check and adjust almost all of the components. I am actually enjoying the challenge of keeping it working well, and it is running very well, with one, tiny nagging problem:

Usually, but not all the time, when I come to a light and get off the throttle, the idle will dip down to about 400 rpms for a short second before returning to a normal, steady idle at 950 rpms.

Like I said, not a huge problem, but I would like to know what is going on. The engine is running really strong, but gas mileage is lower due to the richer than stock mixture. Here are my settings and what I have checked:

Total advance set to 35 degrees.
Mixture set to 3.5% with a Gunson, checked several times and consistent.
I drive with the O2 sensor unplugged.
CPR is adjustable, and system and control pressures are right in spec.
Entire CIS was disassembled, cleaned, and all o-rings, boots, etc. were replaced during a engine removal for a new clutch. Injectors were cleaned and two were replaced due to leaks.
Sensor plate height was checked, cleaned, etc.
I have the decel valve disconnected and plugged, and that seems to be the closest item in terms of relating to this symptom.

The only other symptom I have noticed since reinstalling the engine was that the residual fuel pressure does not "last" as long. It is still within spec (it is at about 26 psi after 90 minutes), but it used to stay up there all day. It will go to zero after a few hours, and I have to occasionally turn the motor over for a few seconds to start it when it has been driven that day but not for 3-4 hours. Previously, it would start so fast all the time you almost would never hear the starter at all. I replaced a return fuel line during the repairs, and I can only assume one of my connections there or near the accumulator is not as tight as it was before. I have no leaks, but I will tighten everything again.

Any ideas?

Thanks,

Olivier

[sammyg2]

Sounds like you need to check your decel valve and make sure it is hooked up correctly and functioning.

The factory specs say the car should hold residual pressure for a while, not indefinately.

If it hold pressure for 90 minutes I wouldn't worry about it but if you want to get carried away the possible culprits are
1) a leaky line or fitting
2) pressure accumulator
3) fuel pump check valve
4) leaking injector (s)

[ohecht]

It could be the injectors, I never considered those. The two that were replaced were for leaks (and one was bent!), so maybe one or some of the others are dripping now.

I will keep looking. I have pulled the decel valve before and recently to test that it was working properly, and it was. I could apply vacuum to the small fitting and block the flow of air between the two large fittings, which should be how it works. Is there anything more to disconnecting it than pulling the small vacuum line and plugging the fittings at both ends?

Olivier

[SilverPoly]

I'm with Sammy. My money is on the decel valve. That's what it sounds like to me. If you think it's your injectors, do the carb cleaner test. As for the decel valve, I'd adjust it and go from there.

[ohecht]

How do you adjust a decel valve? I have run it with and without the decel valve connected, and I get the same symptom, so I am thinking it might be something else now.

Could the sensor plate height cause this without any other symptoms?

Olivier

[tsuter]

Increase your warm idle to 1050 and I bet it won't dip anymore.

[911nut]

Oliver, my '82 SC is set up like yours and I see the same problem, but usually after extended city, low rpm driving. After an Italian tune up (burst to near the redline) the plugs clear and idle will drop to 700 momentarily on decel.
Also, my decel valve is not hooked up.

[Paulporsche]

Try these:

Lean out SLIGHTLY. Maybe 3.3 or 3.4% is where your car should be, not necessarilly 3.5

Raise warm idle to 1050. Above may do this automatically

Plug the hose to the top of the Decel valve, not just the valve itself

If reqd, the decel valve can be adjusted by removing it, protecting the nib on the side, and squeezing at its equator 1 millimeter. I have never done this, but I have read that this was a factory recommended fix back then.

Good luck

Paul

[SC-targa]

Why should you raise the warm idle to 1050 rpm?

Jerry Kroeger

[braddb_82SC]

I recently used my Gunsen to adjust my mixture from 0.7 to 3.5 %CO. I immediately started having the same problem you are describing. Idle drops to 400 RPM on deceleration. Normal warm idle is 900-950 RPM. I have to assume the mixture adjustment is causing the problem. I'll try adjusting mixture to 2.5 ppm. I honestly can't tell much of a difference between 0.7 and 3.5 %CO in terms of power anyway.

Deceleration valve has been disconnected for a couple years.

-Brad

[Paulporsche]

I find I get a much steadier, more even idle @ just over 1000 than I do @ just under. I find that when I am running slightly rich I experience surging @ part throttle, a dip in engine speed when I come to a stop, or sometimes a fluctuation in speed when coming to a stop. I also usually experience fluctuations in idle speed when running rich.

Leaning just a little should slightly raise the idle speed. You can then lower it to the speed that produces the best idle for you.

For the past 15 years I have set my CO to just below the threshold of rich idle fluctuation, with the idle @ 1050 and the car runs great.

Each fall and spring I have to make this adjustment as the average temps change.

My car has an SC engine enlarged to 3.2L and has no 02 sensor or cat.

While you may have some other problem (air leaks etc) I find I have always experienced the running conditions I described.

Paul

[stlrj]

I guess you could call it the rich idle dip. Squeezing the decal valve was to cure an annoying high idle hang up which is just the opposite of what you want. If there were some way to unsqueeze it to reduce the spring tension, then you might have a chance of setting it the way you want.

Joe

[Paulporsche]

Perhaps squeezing along the other axis?

By all accounts, the decel valve was only reqd for emissions regs. The car should run fine without it.

Let us know what happened after you leaned it.

Paul

[ohecht]

I tried leaning it slightly the last day or two, and it was the same thing...I turned the screw as slightly as I could counter-clockwise.

This all started just after I set the total advance to 35 degrees and the CO to exactly 3.5% using a Gunson. This was per JW, and the car is the most powerful it has ever been. The only issues have been 15 mpg and this idle dip.

I don't want to lean it out too much with that much advance, though, as I would rather err on the side of rich while pushing the advance that far.

I will try the idle above 1000 next. I have done that before, but usually it "finds its way" back to the 950 setting, probably due to mixture adjustments and ambient temperature changes.

Olivier

[Paulporsche]

Olivier,

I would certainly defer to JW. (no sarcasm intended). Heaven knows, I'm just a DF designer, not a mechanic.

I can just tell you my experiences and experimentation over the past 70,000mi with this engine.

The adjustment I described that I made this spring was about 1/4 turn. If I were to guess, that would be in the area of 1/2 to 3/4%. I usually do this in 1/8 turn increments. Make sure the grub screw is clean and is actually turning.

Since I don't have a DME, 02 sensor, or cat, maybe your situation is different. Perhaps JW can shed some light.

Paul

3.2L SSI Dansk

[braddb_82SC]

I used my Gunsen this morning to adjust my mixture from 3.5 to 2.6 %CO. I also adjusted my idle up from 950 to 1050. I'm not sure which adjustment was effective but my idle dip has gone away.

Thought this might help the thread starter.

-Brad

[ohecht]

Where do you have your total advance set, Brad?

I think it is mixture related, because adjusting the idle is only helping when it is fully warm, and I can tell it is still there, just "masked" by the higher idle. The enrichment from the CPR must be making it happen when cold. I will have to be careful not to lean it out too much with my advance settings, though.

Olivier

[tsuter]

If it was dipping to 400 or so at 950 with the CO that you want, then changing to 1050 and no dip anymore .....well that is not masking.

Unless you say it is now dipping to 500 from 1050 instead of 400 from 950??? And you didn't fool with the CO??

I bet if you lower to 800 or so warm idle it will die on shifts!

Dippity do dah, dippity day!!!

[braddb_82SC]

Originally posted by ohecht
Where do you have your total advance set, Brad?
Olivier

Nothing to radical. Around 30 degrees total advance (with vacuum hoses on).

I've only taken one ride since the adjustment, my 10 mile trip into work. Half of this is freeway time. I can't claim definitively that the dip is gone. It might just be lessened. Some more driving today and tomorrow and I'll know for sure.

-Brad

[Moke81]

I to am having similar problems with my 1981SC coupe, cold start rpm hunts up and down for idel breifly and then goes away then at operating temp it will start hunting for idel again when you push in the clutch. ( it almost dies) I don't think it's a vaccum leak because when I take off oil cap the idel changes noticeably I am going to adjust mixture 1/4 turn counter clockwise to see if that will help, what else do you guys think?