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I have a NA S4 and put a new motor from Atkins rotary inside and can't seem to get it to idle on its own. The motor is stock, the only thing I've done is a emissions delete. I've tried all the basic things like tuning the tps, adjusting idle screw, rechecking timing and everything seems to check out. The one weird thing is it will idle while cold. When I initially start it the rpm will jump then immediately fall to around 800rpm and idle. It does not do any warm up process of holding high rpm them gradually falling as it warms up, I believe that's related to my UIM as my last motor did that too. But once the motor warms up to the point to which the thermostat almost opens it will die. Then upon restart it will start then die. It revs beautifully when given gas and only needs the slightest bit of help from the throttle to stay alive but just can't do it on its own. Could this be because the motor isn't broken in yet or is there more I should adjust?
definitely has nothing to do with break-in. so you probably need to keep looking for the issue.
do you still have the BAC?
Yes still currently have the BAC, it's withing spec and also clicks when given 12volts. I have attempted to change it out with another but it has made no difference. Edit: One thing I failed to mention is ACV valve is deleted in case that affects idle.
That TB doesn't appear to be the issue. Do you hear a hissing noise while the engine is running? it should be vey quiet up front if you are using the stock intake and exhaust. Depending on if youre using an NA or T2 you may have also swapped the primary/secondary injectors.
"I only did an emissions delete" that's one of the biggest causes of idling problems. There are gaskets and holes that could be leaking air, and plus the idle system is integrated with the emissions system.
Do you still have your thermowax & fast idle cam on your throttle body?
When engine is cold, the fast idle cam holds the plates open slightly which is the cause of the higher RPMS when cold. Then when hot coolant is running through the thermowax, it pushes the linkage and the fast idle cam to allow the throttle plates to close and causes the RPM to drop to 'idle rpm setting'. This is in regards to your 'not doing a warmup process'. Check page F4a-56 in the FSM, under 'FAST IDLE OPERATION' for more understanding.
Check these ports for any blockage and the mating plastic gasket, they allow air to bypass the throttle plates that should help idle. Top port should be directly related to the idle adjust screw on the top of the TB of an NA S4
There is also a screw (on mine it was more like a slotted stud with a locknut) that prevents the throttle plates from fully closing. I believe it is just to prevent the plates from damaging the inner walls/getting jammed stuck when closing. I believe from factory these are sealed with like epoxy. I wouldn't touch this just yet though, just check if anything doesn't look right. I cant find anything in the FSM about this, probably because you shouldn't touch it, but you can find info about it searching on the net.
EDIT: Now that I think of it, your plates should be slightly open IF you have your thermowax & fast idle cam set up properly. If they are closed like that, you either have the fast idle cam removed, or it is not set properly. When your engine is hot, your plates will be how they are in your photo. But it should still idle with the airbypass ports I mentioned and the BAC.
Last edited by wilfff; 04-27-23 at 01:25 PM.
Reason: Additional info
04-26-23, 02:51 PM
