2.7 CIS Backfire and rough idle

[Jive Bomber]

Hi Pelican friends- I've got a 1976 911S with the stock CIS 2.7 motor (California emissions too). Its began to have a hard time starting, running very roughly when its cold, and popping and backfiring. It does much better above 4,000 RPM, and the low RPM issues do get a little better once its warmed up.

Reading the forums here it sounds like a vacuum leak at the injector seals or intake seals? However I don't have the money to spend on the Smoke Pro to confirm where the leak is coming from: (https://www.pelicanparts.com/techarticles/Porsche_911/07-ENGINE-Smoke_Pro_Vacuum_Leak_Test/07-ENGINE-Smoke_Pro_Vacuum_Leak_Test.htm)

Any advice on where I should start?

Thanks!

[rwest]

Here’s a great thread that has all sorts of links and tips; including inexpensive ways to find vacuum leaks.

CIS Troubleshooting for Dummies

[enzo1]

[QUOTE

However I don't have the money to spend on the Smoke Pro to confirm where the leak is coming from: (https://www.pelicanparts.com/techarticles/Porsche_911/07-ENGINE-Smoke_Pro_Vacuum_Leak_Test/07-ENGINE-Smoke_Pro_Vacuum_Leak_Test.htm)


Thanks![/QUOTE]

https://www.ebay.com/itm/EVAP-Smoke-Machine-Diagnostic-Emissions-Vacuum-Leak-Detection-Tester-NEW/221913989725?hash=item33ab1ac25d:g:wg4AAOSwYlJW52L L

[fanaudical]

The link that Rutager posted is a great place to start.

You don't need a smoke machine to find gross vacuum leaks:

Finding CIS vacuum leaks - the vacuum cleaner pressure test

My "quick" checklist whenever I think that my CIS is misbehaving:

- Verify spark plug and cap/rotor condition.

- Verify ignition timing; if advance seems funny, look for vacuum leaks sooner rather than later

(You'd be surprised how many CIS-suspected problems are really ignition...)

- Verify control and system pressures (a good test gauge is a well-worthwhile investment in maintaining these cars).

- Look for vacuum leaks.

- Check everything else.

Others may disagree with my order, but this is what's worked well for me.

Hope that helps.

[T77911S]

first question is does it have a pop off valve in the air box? if not I would not try to start it again until you put one in.

next is pressure gauges. you need a set if you don't have them.
check pressures.

[Jive Bomber]

Thanks so much for all the advice and links, guys! I appreciate it...

[gazzerr]

If you have a pop off valve it may be leaking or may have unseated itself causing a large vacuum leak.

[Targalid]

77 was the first year with boosted brakes.

[gomezoneill]

Does a 76 have points or hall effect? If it has points then clean and check gap. If the PO put in a pertonix module then pull it out and replace it with points. Set gap and check timing then proceed to check the CIS.

[Targalid]

76 has points. Gomezoneill has a good suggestion since closed points would explain your problem. You assume a fuel problem but really don't know. Exact point gap is not critical but you should have about .012 inch. If you do not see a point gap as the distributor rotates you should correct that first.

[enzo1]

+1^^^

[Jive Bomber]

I figured it was likely a vacuum leak for a few reasons:

1. The car starts, idles and runs better (below 4k RPM) once its warmed up. Often with vacuum leaks the rubber seal or hose swells a bit from motor heat and the performance improves slightly.

2. My fuel injector o-rings are likely very worn as I can wiggle a few of the injectors and my symptoms line up with this troubleshooting guide: 911 CIS Primer - Troubleshooting

On the other hand, I do have Pertronix on all my classic American cars, so I know how nicely it works... Might be worth doing anyway!

[Jive Bomber]

Just an update- I've replaced the injector seals and sleeves, the Warm Up Regulator, checked the timing and adjusted the fuel mixture- There are some minor vacuum leaks from small cracks at the airbox, but not enough to cause this issue-- I'm still getting backfiring, stumbling and dying when I start it up bone cold. After the third crank or so, it will pick up idle and start to warm up. I spoke to a mechanic today that recommended rebuilding the fuel distributor, but before I start chasing that advice, I want to make sure that's the right thing to do. This is so frustrating!

[Jive Bomber]

Also possibilities might be the Auxiliary Air Regulator and the Cold Start valve?

[pmax]

Originally Posted by Jive Bomber

Just an update- I've replaced the injector seals and sleeves, the Warm Up Regulator, checked the timing and adjusted the fuel mixture- There are some minor vacuum leaks from small cracks at the airbox,

No CIS expert here.

I strongly suggest fixing the leaks in the airbox first ...

but not enough to cause this issue-- I'm still getting backfiring, stumbling and dying when I start it up bone cold. After the third crank or so, it will pick up idle and start to warm up. I spoke to a mechanic today that recommended rebuilding the fuel distributor, but before I start chasing that advice, I want to make sure that's the right thing to do. This is so frustrating!

... before replacing any other parts.

[rwest]

Guessing and replacing are very expensive ways to fix CIS. do all the pressure tests that are listed in the CIS for dummies thread and post here for better help.

That thread also has the methods for testing just about every component of the system.

Have you put a sniffer in the tailpipe to see if it’s rich or lean?

[boyt911sc]

Originally Posted by Jive Bomber

Just an update- I've replaced the injector seals and sleeves, the Warm Up Regulator, checked the timing and adjusted the fuel mixture- There are some minor vacuum leaks from small cracks at the airbox, but not enough to cause this issue-- I'm still getting backfiring, stumbling and dying when I start it up bone cold. After the third crank or so, it will pick up idle and start to warm up. I spoke to a mechanic today that recommended rebuilding the fuel distributor, but before I start chasing that advice, I want to make sure that's the right thing to do. This is so frustrating!

Jive Bomber,

Check your cold control fuel pressure and sources of unmetered air. Your air box is suspect. Why was the WUR replaced? Don’t replace the FD unless tested and verified defective. Is your mechanic familiar with CIS? This type of troubleshooting you are doing will be very expensive if you don’t do it systematically. Replacing CIS component after component will eventually fix this problem if you get lucky. Keep us posted.

Tony