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Cold running problems

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nick3814

New member
Got the girl back on the road now and she is running great apart from one niggle, when cold the idle is very lumpy without a bit of gas, revving between 500 and 1000rpm almost cutting out for a few minutes then it seems to level at the 1000rpm and if I go for a short drive, turn it off and then restart it it revs right up to 2000rpm for several seconds before coming down to around 900rpm which is quite embarrasing on the forecourt as folk think ya acting like a plonka!

Any ideas what could be causing this??
 
1. ignore what other people think.
2. this sounds like the old cold start SC hunting problem, new cars didn't do this, mine used to do it till I rebuilt the injection and renewed all gaskets and hoses on the top of the engine. I think you will have to live with it but you could try replacing all the vacuum hhoses as these are mostly accessible.
 
Like I say, once its warmed up she goes just champion, might have a look at the hoses as you suggest, cheers Bones.
 
Hi Nick,
My car used to run lumpy, very rich and hunt when stating from cold. (1983 SC) @ around 600rpm and run fine when hot. If I didn't hit the throttle when cold and the engine stoped it would not start unless I floored the throttle until it fired due to flooded engine.
I found that the auxilary air bypass valve had failed. The auxiliary by pass device allows extra air to bypass the normal hot running system when you first start up. It operated via a bi'metal strip which after a short while closes this bypass air off.
Now when I start up the engine runs freely @ about 1200 to 1500 for the first few minutes and gradually reduces after a few minutes to the correct running @ 950rpm.
You might want to check out the auxiliary bypass valve it's on the right hand side of the engine visible under the boot lid. It's not cheep about £80 I think from the Porsche shop. I took a chance and repalced mine and what a difference.
Cheers
John
 
Can you be abit more specific about its location, I have had a look, is it the square grey unit that has a rubber pipe going in one side and out the other with an electronic sensor on it, its on the right side as you look at the engine from the rear, just under the air filter housing??

Thanks
 
From memory, two vertical allen bolts but mine was seized solid. I had intended to service it but left it alone as I feared shearing the bolts. From your original description it doesn't sound like this is the problem but it could be a combination of things. The hoses that conect the airbox to the throttle body are prone to leaks as well and these often need replacing on cars of this age, since renewing all hoses and a number of gaskets on my engine I no longer have the cold start 'hunting problem' , however it's been replaced by what I supsect is low fuel pressure on cold starts and it starts on second twist of the key instead of the impressive first starts at the 'first sniff of petrol' burst into life that used to happen.
 
I have replaced all the small bore vacuum pipes with new silicone piping and its not made any difference so I think I will try this and see where it gets me.

Cheers
 
Hi Nick,
As Bones said you have located the valve and it is held on with two allen bolts.
On my car the two allen screws holding on the valve came undone OK putting , them back needs patients ( 1/4 turn at a time stuff) but the rewards are great. I put the allen bolts in and out several times before putting the new valve on to maker sure the threads were nice and free, if you can remove the valve it's about an hours and a half work all up to get the job done. If you are concerned about the tightness of these allen bolts on your car , 4 or 5mm, I think just try to release them and find out how tight they are, you will need the long end of the allen key to get at them.
I think this valve is worth changing my car now breaths lovely when cold, don't forget the cold start fuel injector makes the cold start mixture very rich you need this extra air to keep the engine running freely when cold.
Have a go you wont regret it .
Chreers
John
 
Well, new valve came this morning (new second hand) spent the last couple of hours fitting it and its made no difference AAAAHHHHHHHHRRRGGHHH!!!! I tried pulling a couple of vacuum hoses off here and there whilst she was running and I noticed the small black plastic disc/valve type thing to the left of the engine was quite hot, it has two vacuum hoses going into it and when I pulled one off the revs rose from about 700 to 800 should this happen? I then took off the vaccuum hose (having placed the other back on) that goes from behind the air filter housing to the left of the engine in front of the black plastic disc/valve into a metal housing that has a couple of sensors/wires coming out the top (I hope this is making sense)and this had no effect on the revs which were still chugging at around 600-700rpm.
It stopped at one time and took ages to start again, having to floor the throttle as John describes. Now I am somewhat baffled, I just hope Porscha-part will take the valve back as mine was obviously ok
 
Does the engine start first hit from cold or chug into life gradually? Once it's running does it hunt for an idle speed? Give me more to go on.
 
Replacing components one by one is an expensive way of fault finding (although they sometimes resort to this at main dealers!!). If I were you I would try and find a local specialist who a. understands the K.Jet system and b. has all the necessary testing gear. Then book the car in for diagnosis. It may well work out cheaper in the long run.

Having said that, if it were my car though I'd also be checking for air leaks. Each inlet manifold connects to the airbox by way of a large tubular rubber seal and these can perish/deteriorate over time. Air can leak in through any cracks thus messing up the mixture and can cause all sorts of funny problems. The six rubbers can just be replaced with the engine in situ but it's a real donkey of a job. Back to my earlier point though, all of us here offering our well intentioned guidance are disadvantaged in not being able to see or hear the car so I think a few quid spent on professional diagnosis is the way forward.
 
Bones she starts first time when cold, fires straight into life and then hunts for a steady idle. After one or two minutes it levels at 600-700rpm and once driven is quite happy to idle at 900rpm. When warm she always fires first time aswell.
I don't recall this problem before the winter when I removed the air pump and all its ancillery components, I wonder if this could have something to do with it?

I really appreciate all the advice you lads are giving me, this forum is a wonderful tool which is increasing my knowledge of this great little motor no end, I'm just getting a little frustrated as she keeps giving me one job after another, I get the feeling I might have to live with this cold start irritation for a while.

Thanks again lads.
 
It sounds like the classic SC warm up pattern, the other component to easily check is the warm up regulator to the left of the throttle body , this can be removed and cleaned. You should have tested the AAR before buying another, much cheaper. At this point do what tripe has suggested get your car along to a CIS specialist for diagnosis, a fuel pressure check should help sort things out.
 
sorry I cannot give any specific advice as I am not familiar with this beast and happened across the thread by accident. (I'm water cooled ,one from over the other side ahem)
You said that your car had been laid up a while and worked fine before so I would concentrate your search on the things that are involved in starting and cold idle. Something that is effected by heat as it clearly starts working properly when it warms up and expands . I would look at any air control flaps and mechanisms on your intake system that effect the air/fuel mixture at startup which may be a bit sticky or need some lubrication. My first thought was the air control valve too but surely it operates a flap in the air intake?
 
Hi Nick,
Just a thought , before you send back the item to Porsche Apart did you check that you had voltage @ the auxiliary bypass valve wiring?
Cheers
John.
PS Dont give up yet.
 
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