rnx

Not a thirdgen problem, but it seems that it's the most adecvate forum to ask.

My daily driven truck (1992 S-10 Blazer) has weird (or not actually, reading complaints from internet) problem - it runs way too rich on idle. The O2 stays about 0.95-0.96V and BLM stays mostly at 108. When I drive and log data, it seems to be mostly good - INT around 128 and BLM 122-128. O2 fluctuates nicely and all other indications seem to be normal.

As it idles way high too - about 1400-1500 RPM, I think that it should be something temperature related (yes, I have checked vacuum leaks and there's none), I changed the coolant temp sensor, nothing. My Firebird has IAT sensor, but Blazer don't have it. I have read different information, some say that IAT isn't too common for truck TBIs, others say that it should have the IAT sensor. Rockauto.com list IAT sensor for 1992 S-10 Blazer and it seems to go to the air intake tubing. When I bought the truck, the tube was already missing. I haven't found any information about 16144288 ECM, does it use IAT or not? I checked the PROM module - its AZSM, but I haven't found any information about it from internet either. What else could be the reason for way too rich idle, but normal mixture at higher RPMs?

RFmaster

rnx

Based on GM - EFI cross reference 16144288 is the later version of the 1227747, 1228062 family of ECM. Since it is a truck ECM it should not have IAT wired. IAT usually uses a two wire connector if it ever was present in your truck EFI harness. At ECM connector check Pin C12, usually Tan color wire was used for IAT circuit. It should be sandwiched between Light Green (MAP) and Dark Blue (TPS) wires.

Rich idle could be due to malfunctioning MAP or a vacuum leak in the the line feeding MAP sensor - at idle MAP (stock engine with 16 to 18 in-Hg) should have MAP voltage between 1.2 and 1.8 volts. TPS should read 0.4 to 0.9 volts at idle. You can back probe ECM Pin C11 and C13 respectively to verify that your sensors read what they should.

//RF

rnx

I have replaced all the vacuum hoses, and because of blown head gasket, the intake gaskets are replaced recently too. So I'm pretty sure that there are no leaks.

When I scan with WinALDL, I actually see normal MAP reading at idle (somewhere around 30-40 KPa). It seems that the ECM just don't try to adjust the idle mixture for some reason. It idles way too rich, but there's no indication that ECM would try to lean it out. I compared it to Firebird and it idles fine - after going to closed loop ECM leans the mixture out and O2 reading would change from lean to rich to lean nicely. Could it be faulty ECM?

RFmaster

Take a look at your idle IAC counts. Stock engines will typically run about 20 to 30 counts at idle.

Also, perform a IAC reset (there is a sticky) - If IAC is buzzing continuously during reset then ECM is commanding IAC to close by-pass air passage. I assume that you have cleaned IAC passages and there is a correct gasket between TB and intake manifold.

Take a look at your injector spray pattern - should be nice misty spray hitting walls of TB. If you see any large size droplets then you have an injector that may be gummed up. Another easy injector check - is to disconnect one of the two injectors and fire engine up. If ECM starting adding fuel (BLM's go high) then that injector is probably is OK. If BLM's remain slightly rich - then that injector is delivering way too much fuel. You can figure out what do from there onward.

So, there is no IAT circuit??

//RF

rnx

IAC counts are pretty normal for my eye (below 30). I post little datalog I got idling at my driveway, maybe anybody will find something abnormal I haven't found myself.

RBob

iTrader: (1)

If an IAT is supposed to be there and isn't, the ECM would throw an IAT low code and turn on the SES/CEL.

As for the rich idle, I wonder if it has the correct injectors. An injector pod from a 305 or 350 could easily been swapped on.

RBob.