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Jameel's Avatar
 
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CIS Experts: Help evaluate a couple numbers

Spent yesterday testing my brother's CIS for the first time. Some background.

1979 stock SC with 100k. He put 7000 miles on it last year. Ran great. In fact, its faster than my '83 SC which has about 7000 miles on a fresh engine. Ticks me off, but that's another post.

First up, the numbers (form courtesy of Tim's excellent Dummies thread)



The control pressure chart from Bentley:



Further testing not on Tim's form:

-045 WUR holds vacuum well

-AAV is getting 12V with fuel pump running, gate was 85% open at cold, and closed fully within a few minutes of powering it up

-Vacuum leaks are nil, if any. Didn't actually test for any, but idle has always dropped dramatically when checking oil.

-TTS works. Not sure if the cold start injector is spraying, don't really have a way of checking that without invasive surgery. Always started right up in the cold though.

- I have not tested mix ratio yet. It could be off.

Questions:

1. Warm control pressure is 2.7 According to the Bentley chart that's at the bottom end of the range with the vacuum line removed. I did NOT remove the vacuum line for the test, so my number here should be higher, 3.2-3.6. However, I am assuming that with the engine off (no vacuum generated) that the results would be the same as "with vacuum line removed" even though I still had the vac line connected to the WUR. Sound right?

2. I've read through loads of CIS troubleshooting but can't pinpoint a problem this engine has. It happens infrequently, maybe 3-4 times all last year (7000 miles). I'm 90% certain this happens only when the engine is warm. Coming to a stop, clutch in, engine spins down, keeps going down then dies. Restart the car, it won't idle but dies. You have to keep your foot on the gas at a stop to keep it from dying. Adjusting the idle up will prevent it, but then most the time the idle is too high. I'm thinking maybe the decel valve could cause this? If it's letting too much air through on deceleration the mix is too lean and could cause it to die?
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Old 03-19-2018, 05:31 AM
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engine off = no vacuum.

decal valve (DV) has NO effect on mixture, only idle/RPM. it holds it hi when you shift. I removed mine

oh, the 79 is faster

check the dist. check the vacuum pot. make sure the plate works properly. verify timing advance. maybe the weights are sticking you you have a broken spring. (look for the dist to be retarding timing when it should not).

check connection at WUR. could be that the WUR randomly does not get power and the mixture is too rich.

DV idles the engine up. you could remove it and test it. you can NOT blow thru it with no vacuum. vacuum on the port opens it. could be stuck open and randomly closes
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Old 03-19-2018, 06:15 AM
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Thanks for the feedback. I have not tested the DV yet, but I did test the distributor vac pot. It advanced the ignition and held vacuum. I can't verify when it advances because I don't have oil lines on the car yet, so can't run it. WUR connection is good. It warms up as it should. If the DV is stuck open though, or randomly working, then it should just make the spin down of the rpms inconsistent, not change it to the extent that the engine dies, right?
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Old 03-20-2018, 12:41 PM
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Quote:
Originally Posted by Jameel View Post
If the DV is stuck open though, or randomly working, then it should just make the spin down of the rpms inconsistent, not change it to the extent that the engine dies, right?
DV is a designed "false air" leak. If it is stuck open you essentially have a vacuum leak and it will impact operation across the full range.

I doubt that alone would kill the engine but have never experienced a bad DV. Well yes I have and a good wrench squished the pod with a big pair of channel locks and made it seal. That was the 81 style though. I have since just put a golf tee in the hose that pulls vacuum on it.
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Old 03-20-2018, 01:05 PM
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No CIS expert here so my apologies to the true experts for stepping out of bounds ...

Quote:
Originally Posted by Jameel View Post
1. Warm control pressure is 2.7 According to the Bentley chart that's at the bottom end of the range with the vacuum line removed.
I suggest checking the WCP with the vacuum applied, as would be the case in your second question, and see if it's in range.
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Old 03-20-2018, 09:13 PM
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Thanks guys. I will do your checks next, when the car is running in a couple weeks.
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Old 03-21-2018, 04:56 AM
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The DV is not a vacuum leak. All it does it allow air to bypass the throttle plate to raise the idle. The air is still metered air.

It is obviously random. I looked for things that could effect idle based on a change in mixture, timing or air flow.
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88 BMW 325is 200K+ SOLD
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08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
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Old 03-21-2018, 07:49 AM
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So here's an update on the issue of the engine idling super low and dying.

So recap what's happening.

- cold start, engine fires off immediately, idles really low almost to the point of dying (no throttle input) for maybe 3-4 seconds, then rpm pops up to 1100 for maybe 30 seconds, then settles back to about 900. Drive off, and most the time it's fine.

- Drove it dead cold (ambient temps 70-80) the other day and the issue came back. It did its normal start up thing (above), so idling normally when heading off, pulled out of drive, headed to the stop sign at the end of the block, clutch in, foot off throttle, engine winds down and dies. Start engine again (fires right up) and it will do this for the first couple miles, then behave normally.

- Sometimes it doesn't die when you put the clutch in, but just idles really low. But here's the interesting part, sometimes when it doesn't die it will struggle at low rpm for a few seconds, then suddenly it will pop up to 1100 rpm or so and settle back, with the engine warm.

- I'm pretty well read on CIS, but for a non-Lambda CIS (no enrichment, no frequency valve) I can't figure out what aspect of the system could cause an instant increase in idle from 200rpm to 1100rpm. It's almost like a switch is being thrown when it pops up to 1100 on its own. I know that the throttle plate isn't moving of course, so it must be an rapid injection of extra fuel via the cold start injector maybe? And the WUR couldn't cause this rapid change, since it moves slowly to change the fuel pressure.

It's a rather infrequent but annoying problem, especially in traffic. Help appreciated!

Here's a video of what it does. I don't have video of it popping up to 1100 on its own though. But the low idle then die is exactly what it does at stops. I should add that the segment where I give it gas and rev it up to 3k and 4k, I'm completely letting off throttle and it doesn't die. But then at the end I shut it off, restart, and it dies w/o throttle input.

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Last edited by Jameel; 06-12-2018 at 06:34 AM..
Old 06-12-2018, 06:13 AM
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Ok. So I guess this might be the world's most uninteresting thread.....I agree!

So I was looking through an old CIS troubleshooting guide from Jim's site, and I found the problem that describes what's happening, almost exactly. See the last line.



So I went to adjust the plate and discovered the adjusting screw and lock nut were loose. The screw was a little gummed up but turned easily. I could totally see it moving over thousands of miles. So I adjusted it out to move the plate down, and tested the starting. Fired right up, but died immediately. So then I adjusted again and it fired right up and idled immediately at 1100 rpm, then settled back. I repeated the starting procedure about 5 times and it started and stayed running every time. I noticed the spec on the plate says it needs to be right on the venturi transition, or 0.5mm below max. I'm not entirely sure where I ended up, but for now, the problem seems to be gone. I took the car for a 10 mile drive and it didn't kill at stops one time.

If this fixed it, it taught me one thing. The sensor plate position is really sensitive.
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1983 SC Coupe Chiffon White
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Old 06-13-2018, 02:55 PM
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