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idle drops very low after i slow down and take out of gear

Thu Jun 28, 2012 7:36 pm

Hey guys, so if I'm driving and I need to stop soon I will take it out of gear and coast to the stop sign or whatever I need to stop for but when I do take it out of gear my rpms drop very low. I going to guess 400ish? It drives great and this is my only issu. Anyone have a idea what it is? I hate this because it feels like its going to shut off when this happens witch is almost every time I come to a stop. Mods are gen3 efi,exh cam -6*,ct20b, 3" full exhaust
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Re: idle drops very low after i slow down and take out of ge

Thu Jun 28, 2012 9:26 pm

ive had the same issue then my turbo blew... wish i knew what it was, cant move my car cuz tranny isnt working but i replaced my turbo.. i boosted then the car stalls as soon as i go into neutral. did you check your vacuum lines?

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 4:08 am

Ill have to check them again. As for my turbo, it doesn't smoke and the comp. Wheel looks fine and it boosts great. I'm going to take off my iac (that air valve under the throttle body) and clean it again. It may be fine but I don't k ow what else to do besides the vacuum lines :shrug:

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 4:20 am

One day my powersteering vac lines slid off the valve on the pump. Car ran fine enough while driving in gear, but would start to die while idling.

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 6:30 am

Connect a reliable vacuum gauge, check to see if your vacuum is greater than 17.2 in/hg (inches of mercury)
If you have acceptable vacuum you may need to clean the little ports and holes the throttle body has.
Remove the throttle body but leave the IAC and TPS sensors connected, there is no need to remove and sensors.
Go somewhere away from the car and blast the entire thing with an aerosol cleaner (carby cleaner or degreaser) until you get no more black gunk come out.
You will be surprised how much goo there is inside the passage that lets air through when you are at idle and the throttle is closed.
Before i cleaned mine it was just caked over with pure black goo, and after it was cleaned it idled at 850rpm perfectly.

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 6:48 am

4rsnduction wrote:Connect a reliable vacuum gauge, check to see if your vacuum is greater than 17.2 in/hg (inches of mercury)
If you have acceptable vacuum you may need to clean the little ports and holes the throttle body has.
Remove the throttle body but leave the IAC and TPS sensors connected, there is no need to remove and sensors.
Go somewhere away from the car and blast the entire thing with an aerosol cleaner (carby cleaner or degreaser) until you get no more black gunk come out.
You will be surprised how much goo there is inside the passage that lets air through when you are at idle and the throttle is closed.
Before i cleaned mine it was just caked over with pure black goo, and after it was cleaned it idled at 850rpm perfectly.


I love this idea! I will definitely do this today. As for the vacuum, before I switched over to the gen3 efi and adjusted my exhaust cam to -6* my autometer vacc/boost guage would show -20 vac, aftermarket those mods now it reads -16 at idle (800rpm) then when the rpm raises to 1k it would be around 18-19 vac..... I've asked others why does it go from 7-800 to 1k rpm after a few minutes of driving but nobody knew.

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 7:33 am

My car gets these figures
Dead Cold - 1100rpm - 20in/hg
Still cold but not dead cold - 1000rpm - 21in/hg
Operating Temp - 800rpm or 850rpm - 22in/hg

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 10:35 am

I did a compression check a few weeks ago and 3 cylinders were 190 and the other was 185....ill try cleaning the throttlebody later today and I hope that takes care of it. A while back I was looking into putting some sea foam in my engine but never did. Mabey its time to do it now.

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 11:38 am

4rsnduction wrote:My car gets these figures
Dead Cold - 1100rpm - 20in/hg
Still cold but not dead cold - 1000rpm - 21in/hg
Operating Temp - 800rpm or 850rpm - 22in/hg


Not trying to thread jack at all, but my gauge always reads at least 22in/hg but sometimes it drops to like 26 or 28in/hg?
To be honest, im not sure what this means? Is my timing off?

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 12:50 pm

lalojamesliz wrote:I did a compression check a few weeks ago and 3 cylinders were 190 and the other was 185....ill try cleaning the throttlebody later today and I hope that takes care of it. A while back I was looking into putting some sea foam in my engine but never did. Mabey its time to do it now.


Do you remember which cylinder was low ? hope its not #3
190psi comp pressure is above spec which is good, even 185psi is within spec but when one cylinder is lower presser than the others it makes u wonder....
When u performed the comp test did u crank with WOT and when u reached the highest psi did u let it sit there for a few min, to see if it leaked.

awolo wrote:Not trying to thread jack at all, but my gauge always reads at least 22in/hg but sometimes it drops to like 26 or 28in/hg?
To be honest, im not sure what this means? Is my timing off?


when u say it drops....technically the more inches of mercury you pull the more its going up not down but with the semantics out of the way u can reach 26in/hg or 28in/hg when u are using the engine to decelerate but you shouldn't see such a high figure at idle.
I am not sure what could cause such readings at idle..... possibly your gauge is inaccurate

Re: idle drops very low after i slow down and take out of ge

Fri Jun 29, 2012 11:59 pm

As for my compression test, yes I did the wot and until it reached its peak psi (I think it was 5 or 6 cranks) and yes I let it sit for a few minutes. It was cylinder 3 or 4 but I don't think 5 psi should matter considering it was still 185 psi and the others were even better and even. I bought the Carb/throttle body cleaner earlier and slept a few hours ( I work nights) ill take care of that throttle body soon....

Re: idle drops very low after i slow down and take out of ge

Sat Jun 30, 2012 3:18 pm

just add a manual boost controller and you wont have any problems with those vacuum wires going behind the throttle body and to the sensors. blockoff the other hole with a cap (at the wastegate)

Re: idle drops very low after i slow down and take out of ge

Sun Jul 01, 2012 3:44 pm

Well I took off the throttlebody and sprayed every little tube it had and the butterfly door. It wasn't dirty at all really. The only thing I found dirty was the adjustment screw on top of the TB. I took out the screw with the o-ring and it did have build up. I put everything back together and then it seemed like it idled higher like it had a vacuum leak. I checked everything and it seemed ok. I adjusted the idle screw on the top of the TB and its ok now. Before it would idle at 800 then raise to 1k after being driven for a while, now it just idles at 1k. Smoooothly :) BTW I already have a boost controller

Re: idle drops very low after i slow down and take out of ge

Sat Feb 10, 2018 1:23 pm

I found my old thread and even with a gen3 engine now and a different ecu it still has this problem :twisted:
The car must be cursed with the low idle ha ha ha
I work tonight and then I'm off For a few days and hope to clean the iac then

Re: idle drops very low after i slow down and take out of ge

Wed Feb 14, 2018 6:48 pm

Well I took the iacv completely apart and cleaned it with carb cleaner spray. It wasn't blocked but it did have a layer of crap inside it. Drove it to work and it felt like I had a boost leak and it idled at 1200. Took it apart again, tested the iacv and adjusted it and I also tested my tps using the BGB. Both checked out good.
I did a boost leak test with the key on the ON position so the iacv didnt leak and throttle plate open @20 psi. No leaks :D
I haven't taken it for a drive but it idled smoothly at 1k rpm. I'm confident it's good now :)
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