rpm's not dropping when shifting

[Cirus_93TSI]

iv been looking for cars, since my 91 was t boned and totaled. one that im looking at has a slight problem, and i just want to see what everyone thinks it is before i buy it and start pulling things apart.

when you are up shifting, the rpm do not drop when the clutch/neutral is engaged. rpm hold steady at wherever they were before shifting. when you disengage the clutch it pulls them down. it does drop to an idle when you come to a stop, it seems like it only does it when you are accelerating, and let off the gas and shift. when shifting to neutral after decelerating, the rpm drop to idle. idle is a little jumpy, by about 400 rpm, but this may be because of the cams.
relevent mods are an internal gated 20g, web street grind cams, 660's (afc), 255 pump, o2 eliminator DP, and a 2g mas. c16, boost in high 20s.

to me it seems like it would be a problem on the throttle body somewhere like the ISC, but from my expirances the symptoms the cars with a bad ISC have been different. usually a more erattic idle and an inability to hold idle at a reasonable rpm(ie, idle at 2000 rpm). all accesories have been removed, so theres nothing to turn on to see if the ISC steps up to compensate.

could it be the ecu? or possibly something to do with the turbo/wastegate?(this seems unlikly to me, but it doesnt do it dueing decel with boost reading in vacume) or is it definitly something in the throttle body?

thanks for responses in advance.

 

[flytalon93]

From what you explained, I would have to agree with you and go with the ISC. I would inspect the throttle body area, especially the spring where the cable is hooked to. Flip the butterfly a few times and see if it retracts on its own and freely.

 

[Cirus_93TSI]

the TB butterfly did move the way it should, and im almost positive it wasnt the TB shaft sticking and holding the plate open. actually it moves more freely then any other dsm iv had or driven, it takes a real light toutch to depress the accelerator, other dsm iv owned and driven feel stiff in comparison.
the TB plate used to get stuck open on my 91 cause the jdm TB wasnt very clean, but this doesnt feel like the plate is just stuck open. regardless of what rpm i let off at, it held at that rpm. when my plate used to stick on the 91 i could feel the plate was too far open through the pedal, feel slack in the cable. i didnt feel that in this car, just seemed to idle at whatever rpm i shifted at if i went to neutral.

 

[2.4 turbo Hyped]

You say the spring feels loose? This could be way out there but do you think the airflow demands of the engine are holding the throttle plate open due to the lesser spring pressure??

 

[Cirus_93TSI]

that may be the case, but i doubt it. i would think the engine would fall to the same rpm each time if it were simply the vacume holding it open. it stays at a specific rpm, whatever rpm i was at prior to letting off the throtle.

i guess ill just have to buy like 3 sets of TB gaskets and take it all apart after i get the car home(every time i buy just 1 set i end up needing to take it apart again and have to wait a week on more gaskets from mitsu or extreme). iv got a couple spare throttle bodys with good ISC's on them anyway, i can always try another and see if it fixes the problem.
i just never had a dsm do that before, and wanted to see if anyone had a similar problem with their dsm. thanks for responses.

 

[jott5555]

this would be a bonus in drag raceing..

 

[ArticNemesis]

If the throttle body is that loose, could the turbo be pushing the tb open since you've said it only really happens when accelerating ie( when the turbo is pushing the boost)?

 

[Cirus_93TSI]

this would be a bonus in drag raceing..

i think thats why the current owner never fixed it, he mainly drives the car at the track.

it could be the TB plate being pushed open, but from everyones responses it seems almost certain that the problem is on the TB somewhere, and iv got 2 spare ones.
thanks for the help, ill post up when i pull it apart and fine out exactly what it was.

 

[Seraphix]

check idle control switch, make sure it hasn't backed it's way from the throttle plate. it should be grounded when the throttle is closed.

 

[HondaStalker]

i'm having the same problems, but mine idles like shit, sounds like a race car, and its stock. and before it went to the shop it never did this it just idled smooth but did have an idle surge.