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Rough idle when starting


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Hi I bought a 911sc 1983 about 9 months ago and it has been great until now.


it is running rough when I start it cold and need to have my foot on the accelerator for it not to stall. However it does start.

how it started:

drove home from work and it was running fine. Stoped at a petrol station on the way home and filled it as it was almost empty.

next morning when I started it, it was very rough

what have I done so far:

changed leads as per the manual I have, they where over 3k ohms.

changed spark plugs and rotor

checked cdi and it has the high pitched sound I also found an invoice from the previous owner for almost $4k to replace the cdi in 2013 so I'm assuming it is fine

checked coil and resistance is within spec

have not checked fuel preassure yet as I went to several places to buy a fuel pressure gauge and no one had one. ( would appreciate if someone could suggest where I can guy a tester in Melbourne)

would appreciate any help/suggestions to get my car back on the road.


cheers Anthony 


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83 were 3.2 right? Check your MAF or o2 sensor. When you put your foot flat it puts it into open loop mode and greatly increases fuel injection. 

Potentially clean injectors when you're checking fuel pressures, youd be surprised at how bad your spray would be after 30 years.

How old is your fuel filter? Maybe when you drove it empty it pumped all the crap that has been sitting at the bottom of the tank? Cheap and easy to replace.

Id check those things, because well you can, all can be done at home.

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It is a 3.0 and it is ROW so it has no O2 sensor. I did change the fuel filter yesterday.

when I put my foot down so it doesn't stall it does run very rich.

i need to find a fuel preassure gauge in Melbourne. Are they universal in the sense of fitting they come with? There are many on eBay however not sure which one will have the correct connectors for CIS

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K Jetronic for the SC's and L Jetronic for the 3.2 - significant difference.

Before you do anything, I suggest you check for vacuum leaks such as a small vacuum hose being dislodged in recent days?  I recall sometimes the hose clamps on the big plastic ducts loosen off themselves.  Any perished rubber vacuum hose ends?  Does the brake booster still hold vacuum?

Has the vacuum advance hose come off the distributor?


Does it idle OK when hot?

Does it start OK when hot?



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Yes , vac leaks are top of the list with rough idling on the 3.0s. Replace all original hoses.

Dirty fuel is always blamed,  but how does it get past a filter?

4k for a cdi? seriously? Surely lots of other work on that invoice? Surely?

The CIS gauge sets should come with a heap of fittings , as long as it's metric.

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I bought the same kit that you purchased tazzieman "universal fuel injector tester" from evilbay

I went though looking for vacuum leaks and did not find anything. However the AAR hose was a bit loose so i tighten it.

The WUR had 32 Ohms which i believe is correct.

The AAR (0280140226) had 21.5 Ohms. I have put it in the freezer for 30 minutes and it is open. I put in on the bench and after about 5 min it was 95% closed. Should it close completely?

By the way the car is still rough and a lot of smoke mainly black smoke.


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It is running rich. I have not touched the mixture screw yet. I'm afraid of stuffing the setting it had. And it was running well with that setting anyway. So something has changed, i don't think it is the mixture screw. However when i look at the plug it is very black.

i tried to upload a photo but the web site states that i can only upload to 330kb, however the photo is smaller than that. I will try to work that one out later.

By the way it runs rough with the engine hot or cold

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So i went and turned the idle mixture screw about 1 full turn ant clock wise and it is idling fine, a bit high. i didn't want to adjust the idle screw yet. Now the question is:

Why did the idle mixture screw need adjusting all of the sudden? Can it wind it self clock wise by itself?

When warm it was idling at about 1000 rpm and the oil level was a bit high on the gauge. So the next question is:

Because it was running so rich for who knows how long could i have a bit of fuel in the oil? 

I will now wait until i get the gauge and test fuel pressure and then try and adjust as best i can the air fuel ratio and take it to someone for the final adjustment and get the valves adjusted as well. i know how to adjust valves but i have never done it on this car i would like to see it done before i embark on that project.



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Normal behaviour is for the car to start on the first crank from cold , and idle a bit high. If it is rich it will "hunt" - up and down continually .

After a minute or so in a normally adjusted car ,  the idle will fall to normal ~950 (use an external tacho) . The automatic choke has done its job.

No idea why this car's mixture appears to have changed. The usual issue is a vacuum leak , which causes poor idling and those not used to CIS cars jump straight to the mixture screw to compensate. This is wrong.

Fix all vacuum leaks , ensure normal cold and hot fuel pressures and then start to think about mixtures.

A valve adjust is within the realms of a newbie but there are some tricks and two methods of doing it. Plenty of info online. Some here bought a valve lash tool from Pelican , which makes it easier.

You probably should change the oil. And the oil level gauge is notoriously inaccurate in many cars; even when it works it must be with a properly warmed engine (20mins driving at least) , on a level surface and idling for at least 30 seconds).

Drain and put 10 litres in. That is pretty close to what you need. NB Don't drop the dipstick in the tank. 






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was it on here that someone went round and recovered a heap of WUR/Cold Start gear from old Mercedes's's's? I've been trying to find the thread

might be worth reading

I reckon it was Shady Speedway.   I plan to do a similar thing. 

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Ok got the gauges on friday and installed them today. Had a few issues, first of all they leaked fuel initially through the quick release and then it sorted itself somehow. The next issue was despite the fuel relay bypassed the fuel pump would not kick in until i moved the airflow plate.

Anyway i tested the System pressure first and it would go to 5 bar (72psi) and then drop to 3 bar (43psi) when i let go of the airflow plate

I then went to test control pressure and it was about 1.2 bar (17psi) went i started the car. It starts straight away however it sat at 900 rpm when cold.

And it took about 5min to get to 4.2 bar (60psi) and stayed there how ever there was hardly no change in rpm i believe it got to 950 rpm max.

The car sat for about 25 min and the preassure dropped to 1,2 bar (17psi). I then started it warm and it started straight away.

So i thought the fuel pressure seems fine, i shall continue to look for vacuum leaks. This time i put a hose through the side hole of the airbox and put it through the airflow plate. I then put the filter in a plastic bag and got a friend to give me a cigarette with which i blew smoke into the air box. and after puffing into it i had smoke come from behind the airbox i believe it was the hoses that connect to the auxiliary air valve, which i have tightened. I then started the car and it is still the same. By this time i had run out of the cigarette. I shall try tomorrow again.

Do you think im on the right path? Just a vaccum leak? Could this be the cause of low rpm with hardly no change when engine hot?


By the way if someone needs fuel gauges im happy to borrow mine

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  • 2 weeks later...

On the weekend i removed the TTS and the CSV and bench tested both. When cold the TTS would activate the CSV for about 3 seconds and then turn it off. When it was over 45 degrees it would not activate the CSV.

I thought maybe i had a leak with the CSV however i put air through it and put the end under water and nothing came out. It was so complicated to remove and install the CSV, i doubt i do it again soon, however i have learnt so much about my car. I'm not a mechanic but i consider myself to be mechanically minded. 

Anyway i went to look for help as this is my daily driver and i need it back on the road. i went to see nIck castone first but he can not see my car until next week. So i then went to Shaun from Jocaro Motors who also specializes in P and he organized a tow truck to pick it up yesterday. So hopefully soon he will be able to tell me what has happened to my car and have it back on the road. 


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They found that it was the fd that is playing up and suggested that we re build the fd and wur and get them calibrated. They also said that we should clean injectors and check valve clearances.

They are going to give me a price to do all of that work. Any idea how much you think it will cost me?

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I have gone ahead with the quote i'm doing a bit more than maybe necessary, however i want to have peace of mind:


$380 WUR

$385 Air flow metre

$715 fuel distributor

$59.28 clean fuel injectors

$69.89 Rocker cover gasket set


$292.50 Carry out checks to find problem

$702 Replace all fuel system

$351 Adjust valve clearances and replace oil and filter

$145 tow truck

$3099.67 Total

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  • 2 weeks later...

So far they have done the valve clearance and checked if i had broken head bolts. Thank God they are fine. i bought the firewall lining and hopefully the full intake will go on once everything has been rebuilt.


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