1992 GMC Sonoma please help Vile...

[zieg85]

I just picked up a 1992 GMC Sonoma 4x4.  I knew it had problems but I thought it might be just a fuel pump.  Some of the symptoms are:  When starting cold, it has a lower than normal idle, sometimes it will die but will restart easily if it does.  It warms up and seems to run great.  After a while you can gun it and the RPM's don't return to idle right away.  When I test drove it, it died on me and could not get it to re-start thinking it was a fuel pump.  Next day it started fine.  Fuel filter is new.

Fast forward to today.  Not going to drive it so I started it and let it run til it reached operating temp.  While looking into the blower motor the truck started to idle 1500 rpm's all on its own.  I went around and gunned it and it just came back to the same fast idle.  On its own it climbed to around 2000 then died.  Went to see if I could hear the fuel pump after I turned the key back on and it built up pressure and shut off like it is supposed to.  It wouldn't start.  Squirted a little gas down the throttle body and it started right up and and ran like it did before with the low idle and slow return to idle.

Two more observations are the water temp gauge isn't registering and the check engine soon light is on.

Any thoughts???

I know nothing on the TBI/OBD1 systems and welcome any help/suggestions to try.  Thanks in advance.  Pretty nice truck for the price, just trying to get it running like it should before I turn my 16 year old loose with it.

[snibook7537]

maybe check the Idol air control valve. That might explain the weird idle. There is also a little port on the throttle body if you plug it off and hold it plugged for about 30-60 seconds it resets something, I believe mass air flow. It is on top is a rectangle, that is how the TBI is on an 84 fiero with a duke and I had similar problems with the idle and it cleared it up. Not saying it will fix or is even the cause of your problem but who knows it might help.

[VileZambonie]

First thing to do is check for DTC's. Do you know how to jump the &b terminal in the data link connector and read the flash codes?

[zieg85]

No I've not had to do so in anything yet.  It also has a digital dash, don't know if it matters at this point.  I thought there might be a "how to" in the technical pages but I couldn't find the procedure.  I know I've read it somewhere but it has been a while.

[VileZambonie]

READING DIAGNOSTIC TROUBLE CODES
The means of communicating with the control module is the Data Link Connector (DLC) located under the instrument panel and is sometimes covered by a plastic cover labeled "DIAGNOSTIC CONNECTOR". The DLC is used in the assembly plant to receive engine information to determine proper operation before it leaves the plant. The Diagnostic Trouble Codes (DTC's) stored in the control module memory can be displayed by counting the number of flashes of the Malfunction Indicator Lamp (MIL) "Service Engine Soon" when the diagnostic terminal of the DLC is grounded. The DLC terminal "B" (diagnostic terminal) is the second terminal from the right of the DLC top row. The terminal is most easily grounded by connecting it to terminal "A" (internal control module ground), which is located to the right of terminal "B" on the top row of the DLC.

When terminals "A" and "B" have been connected, the ignition switch must be turned to the "ON" position with the engine NOT operating. The MIL (Service Engine Soon) should flash DTC 12 three times consecutively. This would be the following flash sequence: One flash, pause, two flashes, long pause, one flash, pause, two flashes, long pause, one flash, pause, two flashes. DTC 12 indicates that the control module diagnostic system is operating properly. If DTC 12 is NOT indicated, a fault is present within the diagnostic system itself, and should be addressed by consulting an appropriate diagnostic chart.

Following the output of DTC 12, the MIL will indicate any DTC three times consecutively. However if no other codes are present will continue to repeat DTC 12. If more than one diagnostic trouble code has been stored in the control module memory, the DTC's will be displayed from the lowest to the highest with each DTC being displayed three times.

DIAGNOSTIC MODE
When the diagnostic terminal is grounded with the ignition "ON" and the engine "OFF", the system will enter the diagnostic mode. In this mode, the control module will:



Display a Diagnostic Trouble Code (DTC) 12 by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon" (indicating that the system is operating properly).
Display any stored DTC's by flashing the Malfunction Indicator Lamp (MIL) "Service Engine Soon". Each DTC will be flashed three times, then DTC will be flashed again.
Energize all control module controlled relays and solenoids except the fuel pump relay. This allows checking circuits which may be difficult to energize without driving the vehicle and being under particular operating conditions.
Move the Idle Air Control (IAC) valve to its fully extended position on most models, blocking the idle air passage. This is useful in checking the minimum idle speed.
FIELD SERVICE MODE
If the diagnostic terminal is grounded with the engine running, the system will enter the "Field Service" mode.
In this mode, the MIL "SERVICE ENGINE SOON" light will indicate whether the system is in OPEN or CLOSED LOOP operation.

If the system is in OPEN LOOP operation, the light will flash approximately two and one-half times per second.

CLOSED LOOP operation is indicated by the light flashing approximately once per second. Also, in "CLOSED LOOP" the light will stay "OFF" most of the time if the system is running lean. It will stay "ON" most of the time if the system is running rich.

While the system is in the "Field Service" mode, new codes cannot be stored in the control module and the "CLOSED LOOP" timer is bypassed.


 
 

[zieg85]

Thanks Vile, I'll check it as soon as I get home from work tomorrow and report.  Have a great evening!!

[zieg85]

One code was found which was "15"...

[VileZambonie]

DTC 15

[zieg85]

Thanks for the info, we'll start testing at first light.  Will report findings.  Is the temp sensor in the head or up top by the thermostat housing?  This has the 4.3L.

[VileZambonie]

It's near the thermostat. the one in the cylinder head is the sending unit for the gauge. This is TBI not CPI right?

[zieg85]

Vin Code Z, 4.3L TBI.  So with the temperature gauge not registering, this would be another problem I take it?

[454ss]

Possible that both temp sensors are out. Or it doesn't have a thermostat in it. Mine didn't register when I got it as well - 1991 Blazer 4x4 4.3 TBI.

[zieg85]

Possible that both temp sensors are out. Or it doesn't have a thermostat in it. Mine didn't register when I got it as well - 1991 Blazer 4x4 4.3 TBI.

Good point, a new thermostat wouldn't hurt anyway...  The blower motor doesn't work so I don't know how hot it actually got coming out of the vents. 

[zieg85]

Happy to report.... The temp gauge issue seems to be the sensor, as I unplugged it and jumped it to ground and the digital gauge, well it started to work, slow climb to hot.  While looking for something else in my abyss... I found a temp. coolant sensor that I bought for another vehicle but never installed.  Put it in and the MIL went off, idled like it should when you first start it up not wanting to die.  Took it for a ride and so far so good.  Runs excellent now...  Need to change the oil/filter, water temp switch in the head, and the drivers side power window switch.  Thanks everyone!!! Another reason why this is the best forum on the net!!!  Will post a picture of the truck with my son once we get it all waxed up and purty....

[VileZambonie]

Glad to hear