Help with troubleshooting my 911sc starting problem

[brian-80SC]

Hi everyone,

I need guidance with troubleshooting a starting problem with my 1980 911 SC. Here's the situation:

Background: Last year I was troubleshooting a loose connection in my factory alarm and as part of the process I tried jumping pins to bypass the alarm -- unexpectedly, when I did this there was an unusual electrical noise from the engine compartment. I documented the sounds here.

I eventually solved the alarm problem at the source (broken wire in the door) and did not need or use the bypass. The problem is that while working on the alarm I started the car a couple times with the bypass enabled -- the first was fine but on the second attempt there was a single loud backfire right on startup. I immediately shut off the ignition but could not find anything visibly wrong in the engine compartment. From this point forward I had trouble starting the car.

The problem: The engine will start, but it takes much more cranking than usual to get it running. At first, it will turn over for about 10 seconds with no sign of starting. After that it will begin to stutter lightly for 5-10 seconds of additional cranking. Then it will begin to run, but roughly. Then after another 5-10 seconds it will start to run normally. The transitions are all gradual. I believe the result is the same whether the engine is cold or warm (need to verify this though since I haven't tried to start since last fall). When running the car seems to drive normally and is not different that I can tell from before the backfire (runs well). I don't suspect there are any other significant problems other than whatever damage was done by the backfire.

Given the backfire I'm of course thinking the airbox may be cracked (no popoff valve was installed -- never had backfire issues before so was not high on my priority list ). I'm currently reading a lot about CIS on the forum and on Jim's CIS primer and there is a ton of information to sort through... So far, the AAR, CSV, WUR and thermotime switch are on my radar as affecting the startup process. Beyond that I know I have a lot to learn about CIS and would welcome any advice on how to go about troubleshooting this... or at least getting started on the right track!

Thank you in advance for any advice!

Brian

[yelcab1]

you have a big vacuum leak. Yoiu have to find it

[Joe Bob]

Backfire on a CIS engine usually results in issues with the air box. IF you have a safety pop off valve in the box. It may be unseated and needs to be reset or resealed. Worst case if no pop off valve it's new air box time. This valve did not come from the factory and is not always installed correctly. Hopefully the PO's mechanic did it right or the PO knew what he was doing when he did it.....

IF you DO have a valve you can fix it w/o replacing the box. A little bit of searching on the inside will give you an answer.

IF the box and valve are good, there are various methods on finding a vacuum leak. Smoke, starting fluid, propane etc. I prefer the ether/starting fluid.

LIGHTLY spray on the suspect areas (in a well ventilated area) when the engine is running at idle. A change in idle will give you a location to start with. Don't assume that is the only leak.

Good luck, report back.

Edit: Oopsie, missed that you have no PO valve....well, use the ether and find the leak.

[ossiblue]

Agree with the two responses above, likely a vacuum leak, so that will be you place to start your investigation.

My suggestion would be to research the methods for locating air leaks on this site. As mentioned, there are several methods that you can use, but I prefer the use of low pressure induced into the system, via a vacuum cleaner hose connected to the intake of the CIS and outlet of the vacuum cleaner. Then, with a soapy water solution, check the air box and all hoses and connections of the intake system.

[brian-80SC]

Thank you yelcab, Joe Bob, and ossiblue!

I looked at the airbox more closely and it is definitely cracked! First i saw there are loose screws along the left side and also inside towards the back. Then I tried pulling up on the box and I can easily separate it along that side as can been seen in the photo.



I assume I will need to replace the box and have started researching prior threads on the process and parts needed. This post has a nice detailed list from Peter Zimmerman. Anything else obvious I should be thinking about replacing, besides breather hoses, or doing to ensure everything is in good order?

Lots of forum reading ahead to prepare for this job!

[Joe Bob]

There is an aftermarket metal box if I recall. Also, the engine needs a partial drop to replace the box. Install a valve.

[Dmitry at Pelican Parts]

Hi Brian - I've attached a DIY guide to troubleshooting the starter system. If you have any additional questions or need more details, please post a comment just below the article and we'll do our best to give you specifics.

Porsche 911 Starter System Troubleshooting | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article


-Dmitry