Calling all CIS experts

[jonbot]

I'm having some issues with my '79 SC, and I'm at a loss at this point and need some help from you guys:

1) I'm experiencing surging during off-idle operation, when it's cold and warm.

2) The idle speed dips down for a second after letting off the accelerator. I have to set the warm idle speed to 1000 rpms, because when it's cold, if it's set any lower, the dipping idle speed after acceleration will cause the engine to die.

So far, here's what I've done:

1) Replaced all vacuum hoses and seals in fuel injection system
2) Rebuilt distributor
3) Installed new plugs, cap, rotor, and plug wires
4) Confirmed vacuum advance is working properly, and set timing to 5 degrees BTDC
5) Checked the CSV for leaking, and there is none.
6) Performed the residual pressure test and it was able to hold the correct pressure for the 10 and 30 minute marks.
7) Set mixture to 3.5 CO%
8) Checked System and Control pressures, here are results:

System Pressure: ~70psi

Control Pressures:
Cold (engine off): 25psi
Cold (idling): 29psi
Warm (engine off): 47psi
Warm (idling, vacuum line connected): 54psi
Warm (idling, vacuum line disconnected): 48psi

How do the numbers look to you guys, and do you have any more tests I can perform?

[MotoSook]

I suspect it's the deceleration valve. That tea pot looking thing next to the throttle body (left of it looking into the engine bay).

[72911]

Yes could be the decel valve, there was a Technical Bulletin (81-07) for the 80-81 model due to unstable idle speed at lower ambient temps, has the decel valve been replaced maybe with one of these later ones? Also I noticed that your CO% was at the high end for `78/79 (1.5-3.5) was this set with the air pump disconnected? You could get better gas milage if set at the lower limit also.

[jonbot]

Originally Posted by Souk

I suspect it's the deceleration valve. That tea pot looking thing next to the throttle body (left of it looking into the engine bay).

This thing?

[jonbot]

Originally Posted by 72911

Yes could be the decel valve, there was a Technical Bulletin (81-07) for the 80-81 model due to unstable idle speed at lower ambient temps, has the decel valve been replaced maybe with one of these later ones? Also I noticed that your CO% was at the high end for `78/79 (1.5-3.5) was this set with the air pump disconnected? You could get better gas milage if set at the lower limit also.

I set it with the air pump disconnected. Also, I set it kinda towards the rich end for two reasons

1) I read somewhere someone said 3.5% was the sweet spot for performance

2) I thought running it towards the richer end would help with the surging.

I am by no means an expert, just tryin my best here.

I just posted a pic of the valve, it looks pretty shiny and new, so maybe it is a later one.

[Paulporsche]

Johnbot:

Yes, that's what Souk was referring to. You might try this:

Loosen the nut on the top of it. Then turn the stem 1 turn CCW, as seen from the top. Retighten the nut, and rev the engine. You may have to repeat this. If working correctly, it should keep the revs up a little longer. If not, you can try disconnecting and plugging the vacuum line connected to the stem on top.

[mca]

FWIW ... I think you are running too rich. I had the exact same issues and everything else was in spec.

In my case, when I would approach a stop my idle would drop low and then bounce back up. Almost to the point of cutting out.

I believe if you lean it just a tad it will solve your problems. Use tiny increments until you get results.

NOT DECEL VALVE ... failing decel valves usually result in high idle. Had experience with this and everything I have read on this board speaks to the same thing - car at operating temps, idle starts to hover near 2000 RPMs.

[Paulporsche]

Actually, upon reflection, I agree w/ mca. I would try what he says first. Even though 3.5 is usually quoted as best for these cars, there is probably going to be some variation, especially since these things are so old.

I would do as he says. Try turning the screw in 1/16 turn increments CCW. Try one, remove the tool and rev the engine. See what happens. You may need another 1/16th, but probably not more. Remember, one full rev is somewhere between 8 and 12% so it doesn't take much.

Let us know what happens.

[72911]

Yes this is my thought also and why I asked about the air pump being disconnected. If it isn't disconnected your mixture ends up being VERY rich and idle will "hunt". I converted a 914 to CIS and had a "hunting" idle solved by reducing mixture slowly. Yes rich can be faster but at $3.20/gallon...

[jonbot]

Originally Posted by 72911

Yes rich can be faster but at $3.20/gallon...

Umm, $3.20 a gallon? I wish, try $3.97 a gallon for premium here

I'll try it out and let you guys know what happens, thanks!

-Jon

[72911]

No worries about the decel valve being the Tech Bulletin style it appears to be the early version....good luck.

[gtihop]

Originally Posted by jonbot

Umm, $3.20 a gallon? I wish, try $3.97 a gallon for premium here

I'll try it out and let you guys know what happens, thanks!

-Jon

Try $5.40 a gallon here is Canada!

[jonbot]

Originally Posted by gtihop

Try $5.40 a gallon here is Canada!

k you win

[nbazalar]

Can anybody give me any insight about the Big Bore Throttle Body that were being offered some years back on the CIS cars??? I've been told numerous things about them not being worth the $$. I've heard they help with the top end performance but really does not make any noticeable difference. I have one of these and my question is should I have it installed or it the difference so negligible that I should sell or trade???
Any advice would be appreciated as I'm NEW to this forum which is GREAT!!!
Thanks for any help!!
FYI my engine has been built to Euro specs:
9:8:1 Pistons & Cylinders
Earlier Larger style intake runners & Head ports Matching
SSI's with Dual inlet and outlet
Elgin SC330 Cam
Euro 0 0438 010 Fuel Distributor
Crank fire ignition
K&N Air Filter

[mca]

Welcome to the forum!

Instead of asking that question within jonbot's thread, you should go here: http://forums.pelicanparts.com/porsche-911-technical-forum/ and the select the "Post New Topic" button.

You will get a lot more exposure that way and won't hijack jonbot's thread.

[wcc]

Maybe a dumb question, but do you have a O2 sensor? I have one on my 930 and it has CIS. It surged at idle and going down the road. Once disconnected I set my own AFR and the surging was gone.

[mca]

Originally Posted by wcc

Maybe a dumb question, but do you have a O2 sensor? I have one on my 930 and it has CIS. It surged at idle and going down the road. Once disconnected I set my own AFR and the surging was gone.

Same with me. My 02 sensor gave a little "trailor hitching" feel. I run without it now. However, it never gave me a high idle ... just crappy surging at constant acceleration.

[jonbot]

Originally Posted by wcc

Maybe a dumb question, but do you have a O2 sensor? I have one on my 930 and it has CIS. It surged at idle and going down the road. Once disconnected I set my own AFR and the surging was gone.

No O2 sensor here, it's a '79.

Update: I discovered one of my fuel injectors is a little leaky, I'm going to order a new one and see if that helps. I'll post my results. Thanks again guys

-Jon

[boyt911sc]

Originally Posted by wcc

Maybe a dumb question, but do you have a O2 sensor? I have one on my 930 and it has CIS. It surged at idle and going down the road. Once disconnected I set my own AFR and the surging was gone.

'78-'79 SC motors are not equipped with O2 sensors. They were installed on later SC's ('80-'83) US models. FYI.

TD

[wcc]

OK, OK, I got it! GEEEZ!

So if you have an O2 sensor, wait a minute, that's right you don't have one do you? LOL!