89 c1500 idle issue

[jbrewer]

Alright, I have a fairly high idle on warm up (1500ish on cold days and 1100ish on warm days) after warm up almost always drops low to about 500 and seems to misfire/bog down and truck seems to lose torque/power. The main issue is the low idle and loss of power after warm up. The truck has a 4.3L TBI V6 (non-vortec), and the 5spd muncie-getrag transmission. Approx 266,xxx on the drivetrain and 45,xxx on motor(fyi motor rebuild was done by an inlaw and not professionally so no telling what was done to it ) I've got a list of things I have already done, bear with me it is kinda lengthy..

New air filter, spark plugs(stock AC Delcos; gapped appropriate), cap/rotor, pickup coil, ignition control module, ignition coil, and plug wires, seals and pressure regulater in TBI, fuel filter, fuel pump, lower intake gasket, coolant temp sensor and pigtail connector for it, deleted the clogged catalytic converter, new O2 sensor. I may have left something out?

Fuel pressure is at 11-12psi best I recall and engine vacuum was ok last I checked. Truck burns little to no oil and no coolant. Oil press is around 60psi cold and 40-45 warm. Motor also has proper spark timing. Injector spray is normal. Fuel economy is great, just got over 20mpg hwy

This truck is OBD 1 and has no trouble codes. Also has a few sensors/solenoids: MAP,TPS, EGR, IAC and AIR injection.


Any help is greatly appreciated!

 

[thor124]

Is there any play in the distributor? I've heard of the gear on the distributor shaft getting worn on the 4.3s causing a stumble at some rpm ranges.

 

[Texan]

If I remember correctly, the TBI has a IAC (Idle Air Control) solenoid that is controlled
by the computer. It can get carbon build up on the inside and cause idle problems.

 

[jbrewer]

The IAC has been checked/cleaned when I re-did all the seals in my TBI, also when I had my distributor out of the truck the gear looked fine and had no play with the shaft. I'm leaning toward a sensor going out maybe MAP or IAC? Is there any way to test those? The cost of both is $75 and I would like to save some money if I could

 

[Texan]

Have you checked out the EGR system?

 

[jbrewer]

I haven't tested the solenoid but I do know that the vacuum valve works. Is doing the "suck test" on the vacuum line and acceptable test? That's how I tested it, valve opened and closed along with holding the vacuum to keep it open.

 

[Texan]

The IAC has been checked/cleaned when I re-did all the seals in my TBI, also when I had my distributor out of the truck the gear looked fine and had no play with the shaft. I'm leaning toward a sensor going out maybe MAP or IAC? Is there any way to test those? The cost of both is $75 and I would like to save some money if I could

I am going from memory, which has faded over the years. The MAP has voltage readings based on
manifold pressure (vacuum) that may be measured with a DVM. The IAC is a pulse modulated solenoid
that had some set-up (adjustment) procedure that I can not remember. There are books that address
this issue. Might google it and find info.

 

[Texan]

I haven't tested the solenoid but I do know that the vacuum valve works. Is doing the "suck test" on the vacuum line and acceptable test? That's how I tested it, valve opened and closed along with holding the vacuum to keep it open.

Yes the EGR valve should open with vacuum, and close without vacuum. You said that the manifold
vacuum is good (18-20 in hg). Looses power with warm up. I wonder if somehow the timing module
in the distributor is being effected by heat.

 

[TollKeeper]

I am leaning to the EGR, EGR Controller, IAC and MAP. The only cheap way to get parts for them is to find a 4.3 S series(S10/S15 and Astro), or K series, at a local junk yard, and find the parts used. I loved the 4.3, but when they start acting funny, boy do they act funny.

12 Lbs according to my research is correct for the 4.3. The only other thing to check there would be the fuel pressure regulator.

 

[jbrewer]

Ok I retested my EGR and within a 4 month span it went bad (doesn't hold vacuum). The part is so special for my truck no parts stores in the area had it in stock so I will be picking one up on Tuesday. I'll keep y'all posted on the results!

 

[Envoy_04]

Ok I retested my EGR and within a 4 month span it went bad (doesn't hold vacuum). The part is so special for my truck no parts stores in the area had it in stock so I will be picking one up on Tuesday. I'll keep y'all posted on the results!

Hope that fixes your idle issues! I've got the same non-Vortec 4.3 in my S10, and TollKeeper is right, they're great motors but when they act funny they sure do it up right!

 

[jbrewer]

Ok so the new EGR appears to have fixed my problem When I took the old one off it appeared to be the original with GM stamps all over it! I believe it also wasn't closing all the way and continually dumping exhaust gas in to the intake. However, my truck still seems to idle low(around 500) this possibly is due to the fuel trim being messed up? Also it appears that my MAP is very old since it has a GM stamp on it (original?) so that may be to blame too! Thanks y'all for the input !

 

[Texan]

If I remember correctly, the 89 has the OBD system where you jump two terminals
with a wire and count the flashes of the check engine light for problem codes.
Have you tried this method of trouble shooting? KOEO key on engine off.

 

[jbrewer]

Yep, my truck has OBDI. You jump the one and two terminals on the connector (usually with a paperclip). It flashes the SES light in morse code fashion. A "12" code should always flash to let you know the computer is sending a signal. So yes I have used this method before, however it is not a very sensitive system because the EGR valve on my truck obviously wasn't working but it never through a code, however it would throw a code it you disconnected the EGR solenoid. I usually use the OBDI as a last ditch effort for this reason. Again thanks for the input!