Unusual CIS Behavior

[Paulporsche]

Yesterday my Sig Other and I took a delightful drive thru the area north of Toronto, experiencing the fall colors. The car responded well to the 20C temps.

However I experienced some symptoms that seem @ odds to one another. In fact a search here showed no threads with the following conditions together.

When coming off throttle @ say 60 mph to slow down for a light I heard what I thought was the familiar burbling sound of a just slightly rich condition, followed by a couple of blats from the airbox popoff valve, which would indicate lean. Then, while waiting @ the light, a very slight idle hunt, again indicating very slightly rich. Upon accelerating I heard no pinging or popping and the engine seemed to pull normally. Oil temps were in the 220 to 230 range, which is normal for my engine. Pressure @ 3000 rpm was 40 to 50 psi.

I was able to duplicate the burbling/popping by snapping off the throttle from speed. Gradual lifts didn't produce it.

Obviously I can try control pressure readings and see if they are too high when warm, but how does that explain the slight idle hunt?

Wires, CD, plugs, cap and rotor are only 8000 mi old. Plug gap is .040 w/ MSD. Points are brand new. I experienced some of this w/ the old points.

I will also check for vacuum leaks.

I have recently had the head studs replaced w/ steel.

I am thinking possibly exhaust leaks.

Or maybe there is something to the fuel accumulator and its evening out of fuel pressure characteristics.

I seem to recall something about the air sensor plate going past its desired height and then returning, but I can't recall what that was all about.

Could it be a valve? Adjustment was 8000 mi ago.

Could I have 1 cylinder running too lean? Should I pull the plugs and check them?

Anyone have any ideas on my apparent too rich, yet too lean condition?

[Brian 162]

I have a similar problem as we speak. My car sat for 3 years for a re-paint (long story). I used stabilizer when I left the car.
When I started driving the car this summer I had no problems however I put fuel injector cleaner in the tank.
All was well until 2 weeks ago. My ignition box quit (perma tune). Ignition box replaced. I put more injection cleaner before I went to a DE. Car started to run like crap. Very rich,idle hunting, hard starting, and stalling.
It's been in the shop for a week. My mechanic thinks it's the fuel distributor. He says the mixture seems to change. He is a well respected mechanic (Henry @ Refined Motorsports).

[ljowdy]

Check you fuel filter

[mca]

I don't know if this helps at all but I figured I would throw it out there.

My 82 has always had some sort of blapping / popping when I lift off the throttle quickly. Even when mixture and timing are set correctly, it never changes. Maybe it has to do with having headers? I don't know.

What I DO know is that I hooked up my decel valve for the first time in the nearly 3 years that I have owned it (I pulled a screw out of the vacuum hose and attached to the decel vavle nipple).

Curiously, I get ZERO popping now - I swear on my life, none.

I can't make much since of it b/c I thought the decel was only for emissions control. Oh yeah, I had to replace the entire vacuum line b/c I found a crack in it where it connects to the back of the throttle body.

Let us know how everything works out.

[Paulporsche]

Thanks, guys.

As per my usual routine, I'm going to try the easy, quick and cheap "solutions" first. I'm hoping it's not the FD! I'll have to check when I changed the filter. That's certainly a possibility.

And mca, you reminded me of something I had been considering last night. My decel valve is connected, but it's possible there is a leak. This device seems to cause more trouble than it's worth.

I'm going to check vac leaks, and plug the top line to the DV. I have the older, pear sized one.

I'll let you know if I come up w/ anything.

[Paulporsche]

After further searching I've found that preventing this type of backfiring is part of the function of the decel valve. I'm going to see if adjusting it helps. I'll also try plugging the line.

The burbling and very slight idle hunt should go away when I lean the mixture.

[mca]

Glad to hear it!

Thanks for the update.

[Superman]

Exhaust popping is typically an exhaust leak. Intake popping is typically a lean mixture.

[Gunter]

Originally Posted by Paulporsche

After further searching I've found that preventing this type of backfiring is part of the function of the decel valve. I'm going to see if adjusting it helps. I'll also try plugging the line.

The burbling and very slight idle hunt should go away when I lean the mixture.

I didn't plug the line.
Try setting up the Decel Valve properly.
The adjustable type lets you select the "hang-time" meaning: as you lift off (close the throttle plate) the DV allows more/less air bypassing the throttle plate minimizing the mixture-change.
I believe ccw increases the hang-time, rev and watch your RPM's as they come down.
Set it to your liking.
Ensure that the hoses are good and connected properly.

[the]

Originally Posted by Paulporsche

I was able to duplicate the burbling/popping by snapping off the throttle from speed. Gradual lifts didn't produce it.

My SC used to do that when I ran a cat test pipe (straight pipe in place of the cat). With the cat back on, it wouldn't do it at all.

Not sure what that means, though.

[Paulporsche]

Gunter,

You "read my mind"! I was just out in the garage doing this. I loosened the nut and then turned the port tube 1 full turn CCW. I can't drive the car right now, but @ idle
I don't seem to get any change in "hang time". Maybe I need a little more, which I'll try. I also checked the vac hoses and they seem fine. And I tried undoing the top hose while idling, and then plugging the lines @ idle and I don't get any difference in idle speed. I'm thinking maybe the decel valve is kaput. I also didn't feel any vacuum on the line or the top of the valve. Is it possible there isn't enough @ idle? Or maybe one of the hoses IS bad.

Thanks to everyone so far on this.

the, I think your situation may be due to a change in backpressure when you did this. I haven't had a cat ever on this car.

[cmonref]

As to adjusting the decel valve, I've always assumed that the adjustment affected the preload on the spring that holds the diaphram closed to stop airflow between the two ports at the bottom of the decel valve. Turn the adjustment clockwise, the adjusted piece goes down and increases the spring preload, and vice versa. Increasing the spring preload means it takes more vacuum at the top to open the valve -- or with the reverse thought, it closes with more vacuum. It should therefore "hang" at a higher RPM, then quickly release and goes to idle. On my '80 SC, this results in too large a drop from the release -- and sometimes dropping below idle to the point of stall.

Screw the adjuster OUT and the engine "hangs" at a lower RPM which appears to give a "softer" drop to idle.

If you don't get a change in the "hang", you may be out of the adjustment range. Keep going. Testing mine on the bench with a vacuum pump disclosed that the adjustment range is perhaps half of the travel of the adjusted screw.

As Gunter says, adjust to your liking.

[Paulporsche]

Thanks for that. I'm going to give it a few more turns, checking w/ each one to see what happens. Remember, my attempt here is to prevent that short lived leaning when the throttle snaps shut. That's really, to a certain extent, what I'm going for, rather than any particular type of hang time. I'm just going to play it by ear.

[Gunter]

Make sure the lines are routed correctly:

[Paulporsche]

Just to follow up. I think I have it solved now. I actually did 3 things, all of which seem to have contributed to a solution.

First, I disconnected my vacuum retard from the dizzy. This gave me a nice steady idle @ about 950.

I was then able to richen the mix a little, without getting any idle speed hunting.

I also lengthened the tube port on top of the decel valve, by turning it 4 revolutions. This seems to have increased the hang time for revs @ snap throttle a little.

These changes also seem to raise the revs during warmup phase a little, up to about 1400, which is fine.

I took it for a test drive a couple of days ago in about 17C temps. The car ran great. I'm going out this afternoon, when it will be about 10C. Let's see how it does.