My 1991 S10 4x4 TBI seems to have the plague from you know where! I though I fixed this before, but something, somewhere I am missing. Using a laptop running windows aldl w/cable I am monitoring any available sensor during open/closed loop. Also using infrared laser thermometer, digital timing gun, and vacuum gauge to inspect & verify. The engine has been rebuilt with .030 pistons and a standard lift cam (Melling). Heads were redone as well as tbi cleaned and serviced.

To start off, first cranked when engine is dead cold the rpms raise as usual. There are no hints to any misfire. As it starts to come down (600-800 rpm) while still in open loop is where the problem exists. It will start to hunt with some engine shake (misfire?) and depending on how cold it is outside maybe even stall. I have noticed at he higher end of stated rpm range there is less of the problem. As the engine temp climbs the hunts starts to disappear but a slight misfire will be evident. It also tends to stay in open loop quite a while (as long as it wants it seems) unless I throttle it up a bit for until it reaches closed loop then it idles well and misfire is very unnoticeable if it's there at all. Listening to the exhaust I can hear a tad randomly but in front of the muffle is a leak so I'm not sure if that's affecting it.

Take it out for a drive all seems normal except for a slight midrange bog or flat spot (if that's what you want to call it). Unless I drive quite a distance (chilly weather) when idling for a prolonged period (5 minutes or more than the average stop light) the problem comes back. During this time on the laptop I am noticing it has slipped back into open loop with a rich flag, clear flood/not closed loop flag, and sometimes will also flag closed loop at the same time. The latter which stumps me the most. How can it be both?! It does not shut off, but runs rough again.

My baby has 189,000 on her and I want to keep her a while (not original owner). Here are some things that has been serviced over the course that was in need after verifying was bad:


All new ignition:
distributor
AC Delco plugs @.035
Belden wires
AC Delco cap and rotor
Coil
ICM- was tested the other day until heated. Passed
Timing @ 0 degrees w/wire disconnected


Tbi:
Original assembly and ports thoroughly cleaned.
FPR- replaced including new spring. 12psi steady
Gasket set
IAC- new w/bore cleaned... was sticking
TPS- replaced.... had a bad spot when tested
Map- checked for proper voltage and accuracy
Injectors w/ O-rings.. still have old ones in case
EGR- replaced of course was gunked up. Cleaned out ports
EGR solenoid... original was not operating
ECTS- shouldn't have to explain this one!

All related vacuum lines have been replaced. Vacuum reads @ 21" with slight flicker of less than 1-1.5". Not too happy with that but it doesn't seem to indicate a leak. Blipping the throttle drops down to 0, rises up to 23-25, back down to 18-19, then right back to 21" consistently. Here's where the issue comes in, the laptop at idle during closed or open loop the IAC counts will slowly go down to 0 however there is no change in idle rpm?! During open loop @ this same time shows the rich flag/not closed loop, etc....
Wish I had a smoke machine!!

I temped the exhaust manifold runners and can find no temperature out of whack from the others. Sprayed vigorously around the intake & tbi..... nothing. Temped intake manifold around ects, reads along with laptop. Coolant averages around 177 @ idle to 185 driving with a 195 thermostat. Seems normal

I am running a crappy bosch O2 sensor while awaiting the AC Delco AFS-22 to arrive. Have to be ordered in these parts so I grabbed what I could to keep going. AC Delco should be here today. It reads okay when in closed loop. I can watch it go from .1 to .8 and the truck runs decent. It's when it drops into open loop (avg .45 ) the crappy idle reappears with lower rpm around 550..... prolonged idle cooling engine I presume.

Makes no sense why is the IAC dropping down to O when the rpm is not rising like a vacuum leak should ? And why is the rpm not kicking back up to keep even with closed loop? I have tried adjusting the minimum air idle according to the proper procedure, but am wondering if the "open loop at idle" is inhibiting the adjustment. I am heavily considering converting to a 3-wire O2 because of this. If all else fails I guess I will end up with a shop bill!!!

Another question. In my Haynes manual ( I know it's not the best) it states the IAC comes in two different pintle sizes (10mm & 12mm). Is this true and can this be the problem?

Sorry for the long post as I tried to give as much info as possible. Any ideas people?

 

I would look hard at the O2 sensor which is what you already are doing. I would recommend going to a heated O2 sensor to get it into closed loop operation faster and to keep it there.

 

i would check all the parts that you installed because it could be possible that one of them has failed.

 

Exactly what I am thinking Swartlkk, I'm just a little scared that I am over analyzing or overlooking something. The funny thing is it's not throwing any codes what so ever, but I will try the heated O2 today! Any hints where I can get a harness from if I go to the salvage yard or is that a bad idea?

 

A salvage yard would be a good place to scour for a plug. It doesn't really matter what it comes off of so long as it matches up with the sensor you purchase to put in.

I'd go for a 4 wire O2 sensor which will bring with it a good ground for the heater that isn't the exhaust itself. Just a little added insurance that the heater will do its job properly.

 

Problem still exists.... even after installing the heated O2. I'm really stumped on this one and am starting to think perhaps the intake gasket has gone bad. I do notice a slight loss of coolant though it goes down to a certain level and stops. I've got a few weeks off and may try to tackle it if weather permits, but it's been quite cold even for Florida. If not t then it's off to a hopefully reputable shop to handle. I'm really stumped on this one! Anybody know of usual intake leak symptoms? I'm wondering if the leak is in a runner(s) or between a couple. Hope it's not too serious. What's the usual charge for that type of work?