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Suburban running rough

I love my Chevy Chevette!
Posts: 1
Joined: 07/12
Posted: 07/27/12 05:28 AM

Hello I'm new to the forum and have a question. I have a 1990 chevy Suburban with 350, 4x4 if it matters. It runs rough at idle I have replaced the temp sen., the Egr, Egr sole., new dist., new coil, new wires, new plugs, new idle air control, o2 sen. The truck has 160,000 miles. Runs fine at speed. Also have ckecked for vac leaks. First TBI not sure where to go next. Any help would be great. Thanks  

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I mow my lawn and find Chevys
Posts: 2337
Joined: 03/10
Posted: 07/27/12 05:39 AM

are you getting any SERVICE ENGINE SOON lights?  other things i might check would be the throttle position sensor, fuel filter, and carefully look down the throttle body to see if your getting a good spray pattern out of the injecters.   this might sound silly but have you tried some fresh gas?  verified the correct ignition timing?   just trying to toss a few ideas out there.   welcome to the forum.  

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I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 07/27/12 06:52 AM

aim a timing light at the cone spray of the fuel injectors to see the shape of the cones better..

you may need to move the timing light clip to different wires to see the spray cone properly...

new EGR valve.. cool.. can you hook a vacuum gauge on a long hose.. stick the gauge on the windshield and take it for a drive...  or spin the back wheels above 40MPH.. see if you are only getting vacuum signals when the truck is in motion...

have you changed the intake manifold gaskets...    warning.. if you do so.. pull the thermostat housing.. use a CLEAN shop vacuum to pull the coolant from the engine and intake... there is a big reservoir of coolant under the air passages in the intake.. pours into the motor every time i have tried to lift an intake off..

find a scan tool.... that reads live data on the GM system...

print a copy of this... so you can write your scan tool values down on the proper line to compare...  or post them.. i will look at them and see if i see something out of spec..


you will also want to print this... and follow it carefully.. post your results to the 4 part test...
the computer is ONLY grounded to the engine block.. usually at 3 places... the back of each head and usually on a stud behind the thermostat housing.. please verify that these are clean and tight.. i normally use a stainless steel wire toothbrush to clean the ring terminal faces and the studs/bolts..

VOLTAGE Drop Testing  

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I have an SS396 tatoo
Posts: 448
Joined: 03/12
Posted: 07/27/12 07:11 AM

i have more... the TPS closed voltage is a LITTLE high in the chart above..  i will have to fix it..

it should be UNDER 1.0 volts.. with the throttle closed...

with the scan tool hooked up... looking at TPS voltage...   engine idling..  note the TPS voltage..

open and close the throttle a few times... it needs to drop to the EXACTLY same voltage.. every time.. not a fraction of a volt different...

engine off...  try it also...   again.. it should Drop to the exact same TPS voltage..  below 1.0 volts..

under the hood, engine OFF...    grab the throttle lever.. open it slightly..   see if the shaft wobbles in the housing... if it does.. you will probably need to have the throttle body REBUSHED.. this is NOT a job most people can do at home..  i can as i have tons of tools..

a LOT of carb shops have also shied away from this as they have broken the screws off in the throttle shaft... the extended ends of the screws have to be ground off before attempts at removal...   they just reseal your injectors and hand it back with a blank face...  you can reseal your injectors your self...  its not hard ..

why does throttle shaft play make the engine run bad...

when the TPS does not return to the exact same TPS voltage each time the throttle is closed..

the computer thinks you have your foot on the gas.. so it does NOT revert to the idle fuel trim programming. . where it will drop the fuel injector pulsewidth below 1.4 MS  (milliseconds)

most of these engines will idle at 1.3 or 1.4 MS... warmed up...   if you get more than that.. its flooding the engine with excess fuel because it thinks you have your foot on the gas pedal slightly.

ECT  190 to 215F
TPS below 1.0
IAC between 20 and 50 counts..
In PW  1.3 or 1.4 MS.

if you have a friend with a engine rebuilding shop.. and have some skills... you can pick up a thin wall valve guide liner for the 0.375" sizing..    and a LETTER Y drill .. 0.404"    the shaft is .372" the guide liner is .015 thick for a total of 0.030.. that gives you 0.002 clearance..  i have a 0.375 valve guide reamer that i run thru to get the fit right..  they make these guide liners in many sizes.. if they give you a smaller size.. it will take a LOT more work as you will never get the shaft in..  the screws are also a special metric size.. 2.5MM  not available easily with a large enough head to hold the throttle blade properly..  

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