hello i am new to the forum and i am looking for some help in this idle issue that is driving me insane. i have a 1991 GMC S15 Jimmy 4x4 with the 4.3L V6 TBI(model 220) engine code (Z). i have included every part, test,and troubleshooting procedures i have done in order to give you a better idea of the problem at hand, it is alot of information but i do greatly appreciate any help with this matter, thanks

SYMPTOMS: Vehicle idles normally when first started and until the engine warms up. As soon as the engine is warm idle becomes rough, unstable. The oil pressure will drop very low and become erratic and when any load is put on the engine as in pushing the brake pedal, or dropping it into gear the idle becomes even more unstable. I have replaced about 70% of the vacuum lines during the troubleshooting process. Also engine bay has an excessive amount of heat from the engine. Engine temp gauge never shows overheating, and no service engine soon lights illuminate. I am not able to connect my computer to the truck because it is OBD-1 and I don’t have an adapter cable.

Parts Replaced:
1.) Starter w/power cable: Oct 2012
2.) Belt Tensioner Pulley: Sept 2012
3.) Serpentine Belt: Aug 2012
4.) Spark Plugs: Aug 2012
5.) Plug Wire: Aug 2012
6.) Battery: Sept 2012
7.) Air Filter: Aug 2012
8.) PCV Valve: Sept 2013
9.) Cd Player(aftermarket): Aug 2012
10.) Front Shock Absorbers x2: Apr 2013
11.) EGR Valve: Apr 2013
12.) Idle Air Control Valve: Apr2013
13.) Oil Filter Housing Adapter Seals: Apr 2013
14.) Oil/Filter changed: Apr 2013
15.) Turn Signal Bulbs: Apr 2013
16.) Spark Plugs: Apr 2013
17.) Plug Wires: Apr 2013
VACUUM TESTING
Conditions: Engine cold, air filter housing removed.
IDLE: Needle fluctuated rapidly between 18 and 19 HG, with a consistent duration of movement.
Rapid Movement of Throttle Linkage: Needle dropped down below 10 HG consistent with depression of throttle, needle steadied with no rapid fluctuation.
Slow Movement of Throttle Linkage: Needle initially dropped down in value and then started to rise up to and beyond the 20 HG mark.
Conditions: Engine at normal operating temp, air filter housing installed
IDLE: Needle fluctuated rapidly between 18 and 19 hg
Movement of Throttle Linkage: Needle initially dropped then jumped well over 20 HG (25HG +)
*book indicates possible worn valve guides according to readings of vacuum gauge*
COMPRESSION TESTING
Cylinder 1:
Test 1: (7 KPA/100PSI) to (11 ½ KPA/170PSI)
Test 2: (8 KPA/115PSI) to (12 KPA/175PSI)
Cylinder 2:
Test 1: (7 KPA/100PSI) to (12 KPA/175PSI)
Test 2: (6 KPA/90PSI) to (12 KPS/175PSI)
Cylinder 3:
Test 1: (2 ½ KPA/35PSI) to (12 KPA/175PSI)
Test 2: (6 ½ KPA/95PSI) to (11 ½ KPA/170 PSI)
Test 3: (6 KPA/85PSI) to (11 ½ KPA/170PSI)
Cylinder 4:
Test 1: (8 KPA/115PSI) to (12 KPA/175PSI)
Test 2: (8 KPA/115PSI) to (12 KPA/175 PSI)
Cylinder 5:
Test 1: (7 KPA/100PSI) to (11 KPA/175PSI)
Test 2: (7 ½ KPA/110PSI) to (11 KPA/160PSI)
Cylinder 6:
Test 1: (1KPA/15PSI) to (11KPA/160PSI)
Test 2: (7KPA/100PSI) to (12KPA/175PSI)
Test 3: (7 ½ KPA/110PSI) to (12KPA/175PSI)
*Cylinders 3 and 6 had a very low initial reading on first stroke but rose to normal value by 4th stroke, so I conducted 3 tests for those cylinders to verify results*
-Hit the truck with a timing light as it seems to be dead on-
Fuel Pressure Test (manufacture specs 9-13 psi)
Conditions: Engine cold air cleaner housing removed
1.) Both injectors spray patterns even and consistent.
2.) With fuel pressure tester hooked up, ran a fused jumper wire from positive contact on battery to fuel pressure testing jack in wiring harness, fuel pump kicked on and pressure built up to 10 PSI, pressure held consistently well over 1 minute, tested multiple times with same results.
--Tested the fuel pump relay according to the haynes manual and it passed and indicated the oil pressure switch and fuel pump circut was operating properly--
Thermostatic Air Cleaner Testing
1.) Applied 7 hg of vacuum to diaphragm motor, dampener door closed but did not completely seal off ( about 1/8 in gap at bottom) and held vacuum for well over 1 minute.
IDLE COLD: Door closed but not completely sealed off
IDLE HOT: Door Slowly began to open but not all the way, heat stove door open a little also instead of being closed off, temp was 190 Deg.
2.) Holding thermostatic dampener door open all the way during idle, the idle condition did not change.

 

vacuun needle shouldnt fluctuate rapidly. Indicates a valve isnt shutting all the way, normally results in less than 18-19. Maybe a rapidly closing and opening vacuum line due to it being weak. Check the vacuum line going to the Map sensor. Its related to a sensor ignored in open loop but not in closed loop.

 

that does make sense now that I think about it, I plan to check the vacuum lines to the map tonight and I will start looking into which sensors are ignored in open loop but not in closed.

 

A few things that caused those issues during my Blazer experiences... A ground from battery to motor was loose. Same symptoms. A broken harmonic balance pulley (rubber broke) caused the time mark to jump rapidly in any direction. ECU went crazy trying to keep it all in check. No check engine lights either way. I didn't see in your list that proper timing was confirmed, unless I missed it.