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350 tbi help

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350 tbi help

BadBowtie0727 BadBowtie0727
I love my Chevy Chevette! | Posts: 16 | Joined: 01/13
Posted: 03/15/13
10:59 PM

Installed vortec heads...480 lift roller cam...ecu chip...fuel psi reg... Etc... Has stock 305 ecu + tbi...not idling correctly and timing is  good...ideas? Need to change ecu + tbi...or have ecu tuned?  

pepsi1 pepsi1
Big Block power for the win | Posts: 940 | Joined: 03/11
Posted: 03/15/13
11:31 PM

Did you get any codes?

If not I would look for one or more tight valve adjustments.

Check the clearance between the bottom of the valve spring retainer, and the top of the valve guide, and valve seal.

If the duration is more then .230 @ .050 that could cause a rough idle also.


waynep71222 waynep71222
My first time was in a Chevy | Posts: 218 | Joined: 04/12
Posted: 03/16/13
07:23 AM

follow bobs advice above.. you can use a vacuum gauge on an intake manifold vacuum port to see if you have a steady idle manifold vacuum.

do you have a scan tool so you can monitor the data stream from the PCM..

so you can see what the computer is doing with the fuel injection.

here is a generic chart for GM tbi ...

can you make a list from your scan tool... of the data values with the engine idling all the way down the list to open loop closed loop...  the first 13 items.

at idle..


coolant temp

Mat temp


injector pulse width    at idle.. should be 1.2 to 1.4 MS..  at idle..

oxygen sensor  ifs going to be swinging constantly.. from the .1 to .9 V.. how about cross counts.. from another line on the scan tool.. this O2 voltage should not be fixed.. constantly moving..

TPS...  this should be less than 0.95 volts. you did not list the exact model year.. but .075 is a good starting place..  with the engine running.. at idle.. note the tps voltage.. work the throttle a few times.. it needs to come back to the EXACT same fraction of a volt each time.. worn throttle shaft bores are a problem with TBIs... it will cause idle issues..  there are a few ways to fix this..

throttle angle is usually not displayed..  as this is a generic list.. only on caddy TBI systems

iac counts.... this is an import one...  at idle.. engine at operating temp.. 190F to 205F.. TPS at minimum. 0.75 or so..   the idle air counts should be between 20 and 50 counts.

if the idle air counts are below 20... you need to close the throttle stop slightly...

if the idle air counts are above 50... you need to open the throttle stop slightly.

park neutral switch..  verify that when you have it in park.. the PK/N light is on.. when in reverse or drive its off..

integrator   if the engine is running perfect.. should be at 128... this is short term fuel trim

block learn.. if the engine is running perfect.. should be at 128.. this is long term fuel trim
its accepted that these can be 5 to 10 numbers off..  

at idle.. warmed up.. coolant temp over 185F.. should be in closed loop...

please. list the above.. i can usually figure out whats wrong..

if you don't have an OBD1 scan tool.. mention this.. i have a post on a few sites that will help you find a good used one..  

BadBowtie0727 BadBowtie0727
I love my Chevy Chevette! | Posts: 16 | Joined: 01/13
Posted: 03/16/13
11:51 PM

Sry...its a 89 chvy pu..Had 305... Installed 350 and performance parts...was wondering if computer and tbi was different from 305 and 350 that was causing problems...I adjusted valves while running and am pretty sure they are all correct...loosened till clacking then tightened to shut it up then quarter turn further. Friend has engine light reader but the code reader is different probably and will point to my prob. A little frustrating dumping $ and work and still not right and not pleased with the power I added...thinking about carburetor but that's a whole nother headache.  

waynep71222 waynep71222
My first time was in a Chevy | Posts: 218 | Joined: 04/12
Posted: 03/17/13
08:45 AM

this is completely fixable...

only issue might be the lower vacuum because of the cam ..

the 350 uses larger flowing injectors than the 305....

there is also a different prom chip... so thats available also.. as are custom prom chips that can handle the larger cam...

can you get a manifold vacuum reading at idle and post it.. is it around 15 inches of vacuum???
if so.. then its going to work.. probably with an injector change..

there are ways to monitor the system with some digital volt meters...

sometimes you can run across one of these for fairly cheep or borrow one...

more to come...  

waynep71222 waynep71222
My first time was in a Chevy | Posts: 218 | Joined: 04/12
Posted: 03/17/13
09:19 AM

and to answer your question..

finding a proper 5.7 TBI computer with the chip... and a 5.7 throttle body with 5.7 injectors..  would probably  fix this issue.  as long as other parts are not bad..

did you install a new engine coolant temp sensor in the intake??  it has a black and yellow wire on it.. the sensors go bad.. the pigtail connectors go bad.. if you have an autozone around.. pick up an duralast /or wells SU102... ask for it by part number..  under 20 bucks for the pair.. saves 10 or 15 bucks on separate pricing..  polarity does not matter on the wiring on this sensor..

also.. do you have a 192F thermostat installed... so the engine is running at the proper temp for the fuel injection to control the air fuel ratio.. if the coolant is below spec.. it will force the computer to richen up the air fuel ratio...

but in the swap.  things can get left disconnected or damaged in transition..

the engine also will run horrible if the oxygen sensors are not hooked up properly... and/or if they are contaminated...   can you take your digital volt meter... set your meter to either 2 volts DC or 2 volts AC...   with the volt meter hooked to the oxygen sensor.. and the other wire hooked to the engine block or ground on the engine...  do you have the oxygen sensor swinging voltage from 0.1 to 0.9 volts constantly..  this is good...  fixed voltage would be bad...

can you look along the fuel injection harness..  find the 2 or 3 ground wires that connect usually to the intake manifold...  there are some ground connections on the back of the cylinder heads also..

next... engine off.. grab the throttle lever...  open the throttle slightly... then wiggle the lever..

if the shaft moves in the bore more than a tiny fraction of an inch.. it will create idle issues.. as the throttle closes to slightly different angles each time and that freaks out the computer as it never knows when you actually have the throttle closed so it can switch to idle fuel control programming..

if you have attempted to adjust the idle speed by turning the torx headed screw..  you can also cause issues... there are several ways to adjust base idle..   you will notice that there is a passage behind one of the throttle body mounting bolts that is seems to be hissing.. this is normal.  to set  base idle..  you either have to block that passage.. i use a shop rag stuffed in with a small screw driver.. but not deep..  i then adjust the torx idle stop screw to get the SLOWEST idle speed without the throttle plates sticking in the bore as you tip the throttle open..
then unblock the hole.. shut the engine off and restart the engine .. if it does not start.. open the throttle slightly.. take the rpms up to about 2000 .. then slowly.. release the throttle till you get your foot off. this lets the computer learn the proper idle position..


lets look at the injector spray pattern...

take your timing light..  aim it at the injector spray... is it a NICE even pattern ../\  that would be good... if its sputtering..  or off center.. you might have a fuel pressure issue..   the pump is grounded to the top of the frame rail.. and those get dirty..   grounds get left off at the front of the block.. down low.. that go to the frame..  thats an oops.. there may be a ground strap between the body and the frame under the passengers side footwell..

waynep71222 waynep71222
My first time was in a Chevy | Posts: 218 | Joined: 04/12
Posted: 03/17/13
09:28 AM

TBI injector flow rates..  because when the system is operating in OPEN LOOP.. its fuel control is preprogrammed.. it cannot handle the differences in displacement..

engine... tbi colors. flow.    
2.8L~~ | 1-3/8 ~~|Blk/Wht |33 lb/hr |GM5235430*RPD
4.3L~~ | 1-11/16 |Yel/Blu |45 lb/hr |GM5235203*RPD
5.0L~~ | 1-11/16 |Grn/Wht |40 lb/hr |GM5235279*RPD
5.7L~~ | 1-11/16 |Orn/Blk |55 lb/hr |GM5235206*RPD
Cop car
5.7L ~~|1-11/16 ~|Yel/Brn |65 lb/hr |17084327-ACR
Late big block pickup
7.4L ~~| 2" ~~~~~|Blu/Blk |80 lb/hr |GM1708430*RPD
big block pickup
7.4L ~~|2"|Drk Red/Drk Blu|90 lb/hr|GM5235231*RPD  

BadBowtie0727 BadBowtie0727
I love my Chevy Chevette! | Posts: 16 | Joined: 01/13
Posted: 03/17/13
07:51 PM

Jet computer chip installed...cooler thermostat installed...New coolant temp sensors installed...all grounds connected... On my headers there are threads for o2 sensor but blocked off...will need drilled and bottom of cab heat shield needs a little my  o2 sensor isn't hooked up at the moment....I believe o2 sensor, tbi , computer is problem  

waynep71222 waynep71222
My first time was in a Chevy | Posts: 218 | Joined: 04/12
Posted: 03/17/13
09:30 PM


no oxygen sensor... going to run horrible.. has to learn...

depending on the sensor location.. you might also want to install a 3 wire oxygen sensor from a Bosch/Oxygen Sensor
For your 1995 Chevrolet Truck Tahoe 4WD 5.7L TBI 8cyl
Part Number: 15700
Application: Up-stream sensor (located before catalytic converter)
Notes: 3 wire lead
Oxygen Sensor Connector Type: OE Connector
Oxygen Sensor Type: Narrow-Band Heated
just under 50 bucks.

power from the ignition power circuit...
ground to the back of the head..
sensor wire to the existing purple wire..
you will have to dig around for the oxygen sensor pigtail connector... as close as i can figure you will need this from  delco
Part Number:  PT420 might be a autozone/wells 489 pigtail  looks about right but not cheep.
Product Notes:  Inline-To Oxygen Sensor Conn 3-Way female black

i will have to dig deep is you decide to go to the heated oxygen sensor so you keep it at operating temp during lower speed operation...