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383 stroker TBI vaccum issues

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383 stroker TBI vaccum issues

playball1956 playball1956
I love my Chevy Chevette! | Posts: 3 | Joined: 09/11
Posted: 09/05/11
10:14 AM

My son picked up a 1989 k2500 4x4 that the previous owner took out the old 350 and installed a 383 stroker. He fitted the 383 with all the parts from the 350. Motor has a hard time staying running at idle, unless you keep the throttle open a little. Check engine light comes on every now and then,scanner pulled a MAP sensor code.We have replaced the Map sensor,TPS,Electronic spark controll module,and EGR sensor.Have re-timed the motor with no luck. Is it a possibility that there is not enough vaccum at the TBI base and therefore MAP sensor is cutting off the fuel due to not enouigh vaccum and so the motor will not Idle? Motor runs fine under power while driving, only problem is at Idle. Any suggestions would be great, Thanks  

redneckjoe69 redneckjoe69
Everything in my driveway’s a Chevy | Posts: 612 | Joined: 02/10
Posted: 09/05/11
01:01 PM

was your tps adjusted properly?    alot of people just bolt them on.  

playball1956 playball1956
I love my Chevy Chevette! | Posts: 3 | Joined: 09/11
Posted: 09/05/11
03:09 PM

Thanks for asking about the tps position, I believe we have it adjusted right but will double check it. Thanks again.  

pepsi1 pepsi1
Big Block power for the win | Posts: 940 | Joined: 03/11
Posted: 09/05/11
10:54 PM

You may have an IAC IDLE AIR CONTROL that is either not working or very dirty, and not alowing the correct air/fuel mix for the engine to stay running. You can remove it and clean the passages for the throttle body and the IAC itself.

When the engine is idling that IAC control is constently moving in and out to adjust that air/fuel mix. Good luck.....  

waynep7122 waynep7122
Everything in my driveway’s a Chevy | Posts: 556 | Joined: 08/09
Posted: 09/06/11
11:34 PM

this is really going to take a scan tool....  yep.. live data is available on these systems...

on the scan tool.
look at the engine coolant temp ..  between 185 and 215F... you are going to need a 192F thermostat with this app..

with the operating temp up...   look at the TPS voltage..   is should be over 0.5 volts and less than 0.98 volts..  work the throttle several times... letting your foot off the throttle each time...  the TPS voltage needs to return to the EXACT same voltage each time you release the throttle ...  if it does NOT..   shut off the motor.. climb up and reach the throttle lever.. open it part way.. then see if the shaft wiggles in the throttle shaft bore..  (O) good tight shaft to bore or loose shaft to bore.. ( O) (O )    the shaft being the O and the bore ( )  

why is this important..  TBI housings have terrible problems with throttle shaft bore wear...  

WARNING WARNING WARNING>. do NOT try to remove the throttle blade screws without removing the exposed threads first.. they are damaged and will prevent the screw from being removed. they just snap off and ruin your day..

when you take your foot off the throttle.. the system reverts to idle programming..  when the Throttle shaft bore is worn.. the TPS drops back to slightly different voltages.. the ecm then thinks you still have your foot on the gas pedal and the system acts stupid...

you will also want to look at the IAC counts...   the what... idle air counts... the idle air control motor is a stepper motor..

with the coolant temp over 185F...  the TPS between 0.50 and 0.98..  you should see an idle air count of between 20 and 50....

if you see less than 20... the throttle is open too far...  and the ECM is trying to slow the engine down to the desired RPM..

if you see over 50...  the throttle is closed too far...  and the ecm is trying to back the idle air control out to increase idle speed..

there is a way to set base idle...  by plugging the passage above the idle air control valve with either a custom shaped rubber plug on a handle.. or by carefully stuffing a dirty shop rag in with a screw driver...   then setting base idle to 500 RPMs...   its much easier to set it by looking at the scan tool IAC counts...

start there... post what you get...  

waynep7122 waynep7122
Everything in my driveway’s a Chevy | Posts: 556 | Joined: 08/09
Posted: 09/07/11
11:25 AM

one more thing.... ON TBI motors.. there is a tan and black wire... with a single inline connector.. either under the dash board up above where the passengers toes would be ... sometimes behind the carpet.. or under the hood someplace... this has to be disconnected to set the base timing...

or you can dead stick time it by bringing the harmonic balancer mark to what ever the base timing should be..   pulling the cap... and rotor...  turning the housing to align the tips of the pick up coil and the tips of the reluctor... this is where the timing should be set at... so you can start the motor on the first try after sticking the distributer in...


playball1956 playball1956
I love my Chevy Chevette! | Posts: 3 | Joined: 09/11
Posted: 09/10/11
06:56 AM

Thanks alot for the info,will be working on it this weekend, hopefully we will find the problem. Will post what we find. Thanks again.  

waynep7122 waynep7122
Everything in my driveway’s a Chevy | Posts: 556 | Joined: 08/09
Posted: 09/10/11
12:37 PM

on another forum.. i posted this info

i would think that swapping to larger injectors would really help...

there are several steps to go through first..

let me see if i can find the chart of TBI injector flow...

or you might contact jim linder services he might be easily able to help you with injector size changes..

i ran into this when swapping up from 4100 caddys to 4500 and 4900..   went to a larger injector in the original throttle body worked great....

you might want to go to the sizes i marked in bold.... its a lot easier than reprogramming the computer.. you will also want to make sure that the system is going into closed loop.. and running off the oxygen sensors...    you are going to need access to some kind of scan tool to verify the system is working properly..

TBI throttle body and injector information

These injectors are rated at the pressure used by the factory, about 13 PSI

Application Throttle bore size Injector color code Flow rate Part #

2.8l v6 1-3/8" Black/White 35 lb/hr GM 5235430 *RPD

4.3L 1-13/16" Yellow/Blue 45 lb/hr GM 5235203 *RPD

5.0L 1-13/16" Green/White 40 lb/hr GM 5235279 *RPD

5.7L pickup 1-13/16" Orange/Black 55 lb/hr GM 5235206 *RPD

5.7L Cop Car 1-13/16" Yellow/Brown 65 lb/hr 17084327-ACR

Late big block pickup 2" Blue/Black 80 lb/hr GM 1708430 *RPD

big block pickup 2" Dark Red/Dark Blue 90 lb/hr GM 5235231 *RPD

during open loop... like when the engine is warming up.. or you are accelerating moderately hard..  the computer reverts to preprogrammed fuel control.. so your system will act lean....

during closed loop...  when the system runs off feed back from the oxygen sensors.. the fuel ratio can be controlled by the ECM.. but its hard for it to go richer.. its much easier for it to be leaner..  


waynep7122 waynep7122
Everything in my driveway’s a Chevy | Posts: 556 | Joined: 08/09
Posted: 09/10/11
12:40 PM

you might want to check out this other post i put up...