Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Your problem can be several different things.I always start with a scan tool to check for failure coses & then view the data with engine off/key on to look at the TPS (should be around .50 v & not moving with throttle closed),engine coolant temperature,& MAP voltage.Thre voltage you see on the scan tool should be the same as you see when disconnecting the MAP & testing with a digital volt meter.
Check the EGR for being partially open (this may require removal to look at pintle)
With engine running look at fuel integrator & block learn (fuel trim) they should be close to 128.If they are higher the computer is adding fuel.When the engine is warmed up the 0/2 should be bouncing back & forth over .500 mv.
My BLM is 135, sorry I forgot to mention i replaced the 02 sensor. Bob
I will do the checks and let you know, thanks for your help.
OK, John, I replaced the EGR valve with a know good spare, also checked for leaks around the TBI. Still same issue. NOTE-when I unplug the MAP Sensor the problem goes away, but MAP Sensor has been replaced. I put a scan tool on the obd1 and checked the voltage on the MAP sensor. when the truck is running it sits around 0.90 volts, when the truck off of, key on, the voltage is 4.70. Reading the tech manual it says engine off, key on, voltage should read .4 volts, could I have a bad ECM??
I will look at this later this afternoon, will let you know the outcome of the readings, etc.
John, you online?
Timing checks good at 0 degrees. I tested the voltage at the sensor connector KEY ON, ENGINE OFF. Circuit 416, GRY Wire (had meter set at 20 DCVolts) meter showed 7.00 volts. NOTE-SCAN TOOL shows 4.70 volts. CKT432 LT GRN Wire 00.6 volts, CKT455 Puple wire showed 00.7 volts.
What is timing after the bypass circuit is reconnected ?
The only voltage reading that is important is the 4.7 V.The other circuits are MAP sensor signal to ECM ,& sensor ground.The sensor ground should test .002 OHMS or lower.
When reconnecting the bypass connector, the timing was not even close to the timing the timing mark.
Ok, so where does this leave me?
Have you tested the MAP sensor wiring from engine to ECM with an ohm meter ?
OK, 170 oms on Pupple wire and 302 ohms on the green wire.
I might of done this wrong,
i only put the ohm at the connector of the sensor and checked there with key on/eng off.
also with the key off it reads, GRY - 006 ohms / Green - 052 ohms / Pupple- 0 ohms
Ok, i will head to the auto parts store and get long enough wire leads for the ohm meter,
Thanks John, i will send you a reply when I finish that.
I suspose we are seeing if there is resistance is in the wiring or it's a ECM issue, RIGHT?
I took my Truck to GM to put on their diag tool. They couldn't figure out what the issue was either. They did tell me that my distributor was cracked, but didn't think that was the issue anyway, long story short, I replaced the distributor with a new one which included a new pickup coil, ignition module, NO HELP, I also replaced the computer with no help. Get this - I replaced the spark plug wires and the problem cleared. I did find a short in one of the wires. (never did find the crack in the distributor)