I need some help. I have a 86 s4 N/A whenever I start the car and it comes off the high idle cold start the engine shakes violently then shuts off , if I keep my foot on the gas it at anything higher than 1k rpm it will run smoothly. I checked the tps even cut out the plug and hard wired it. I've been told it can be the primary injectors or the thermowax sensor. I'm thinking of changing the injectors. What could it be ? Any help would be much appreciated. Thanks

 

What happens if the engine were hot to begin with? Does it operate normally or much the same as when cold?

 

It starts up and then shuts right back off unless I keep my foot on the gas.

 

Sounds like a vacuum leak

 

Or you might have an issue w/the fuel switch inside the AFM. You could bypass this switch by jumpering the fuel check connector but the vacuum leak also looks very possible as well.

 

Can this also be caused by a leaking secondary injector ?

 

Secondary injectors are not used to start or idle the car as only the primary injectors are used.

 

Ok so a bit of an update. I changed the primary injectors and the leaky secondary one. It appears that the non leaking secondary injector was clogged with debris. The car comes off the cold start and idles perfect now. Now when I hold the rpms higher and let it go the engine starts to shake violently and hunts for idle, then shuts off. While I had the upper intake manifold off I checked the vacuum hoses and they seem fine.

 

still got vacuum leaks.

just because everything looks ok doesn't mean much. a 25 year old vacuum line looks like a brand new line when it is installed, doesn't mean it is ok.

most leaks are at cracks in vacuum line bends unnoticeable til you bend them, or at the old and brittle injector lower seals, or at the old wobbly tired 5/6th port shaft seals, or at some hacked up lower intake gasket install leaving chunks of the old gasket still in place, or the same for lower intake to block... many places leaks can hide.

you could likely offset it by richening the variable resistor, i do it on customer cars where it is obvious the owner's don't want to spend $500 sorting out their intake system. but it only works for relatively small leaks due to age, not someone else's filthy paws being in there and messing with poor gasket replacements for example.

 

Thanks Lyger I'll look into it closer.

 

I had a similar issue after rebuilding. I have a small vac leak somewhere but too lazy to locate it so I richened the idle mixture and now it hunts a bit but wont stall on cold starts.

 

i figured i should update the status on the car. after removing the manifold and pulling the injectors i found that they were blocked up with what i can only assume is rust/dirt(look and felt like coffee ground). after seeing that i pulled the tank, filled with the same dirt/rust/coffee ground looking substance so i sent the tank out to get cleaned, changed the fuel filter, cleaned out the injectors and i also changed the tps ,then started her up. everything seems to be working properly, idles perfect. ill keep looking through my vacuum system. thanks alot for your help guys.

 

Great to hear!