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We pulled the old blazer out of the field this year after sitting for a couple years. We stopped driving it because the starter went out among other things. I replaced the battery, starter, ball joints, brakes, "fixed" 4x4 (vise grips on cable), cranked torsion bars, slapped on some 31s. It wasn't running great at this point but we took it to the sand dunes anyway. Broke both cv joints and cracked the radiator. Replaced those and kept running it. This whole time it was having problems idling but idling isn't necessary when your throttle is wide open. My buddy scanned it with a fancy scanner and got P0107, P0441, P1107, P1122, P1406, P1509 i think most of these are from us disconnecting sensors while it was running to see it anything changed. He cleared the codes. I tried replacing the IAC with a new one and it didn't help. Then I went to the junk yard and pulled a IAC, MAF, TPS, alternator, coil, ICM, EGR, vaccum purge switch, and computer. I switched everything one by one and nothing has helped. I also had a new cap and rotor on the shelf that I replaced. I have a bluetooth scanner that gives me a P0107 code. got rid of t case vacuum lines and front axel vacuum lines. I have vacuum for brake booster and a small line that goes to the cab I think. The evap canister came off a long time ago. I tested the fuel pressure. I'm at 60 psi key on and it holds for at least 20 minutes. It was still above 55 about an hour later. Running pressure is about 57 58 psi. I pulled all the spark plugs and they all look fine, I believe I replaced them before it was parked. I pulled intake manifold to see what I could see and that's where I'm at now. No broken lines, injector poppets look fine, no sign of a leak/washing out. It will not start just by cranking the key. I need to give it some throttle and stay on the pedal to keep it running. When it's running its rough until about 1500-2000 rpm it seems to smooth out a little.
Sorry this post is all over the place. It not exactly in sequential order either. I've been messing with this for a while now.
You should check your MAP sensor. First check the high and low 5V reference circuit which are the gray and orange wires respectively. If your 5V reference circuit is ok inspect the wires and connectors. At that point you can either replace it or do more detailed testing to confirm:
If that doesn't do it I would check for 1" of strong blue spark at the plugs at idle.
If that does not do it then I would check the other sensor values in live data and compare to ambient/expected values. That includes the ECT, MAF, MAP, TPS.
If that doesn't do it I would look at fuel trims
If that doesn't do it I would look at the O2 sensors in live data.
Key on engine off MAP sensor unplugged pin A of the connector to positive terminal of battery I'm getting 12V = Pin A is the ground (black)
Key on engine off MAP sensor unplugged pin C of the connector to negative terminal of battery I'm getting 5V = Pin C is the power (tan)
Key on engine off MAP sensor unplugged or plugged in pin B to negative I'm getting nothing but to positive I'm getting 12v. Check resistance to ground and it's a dead short 20 ohms.
engine running MAP sensor plugged in pin B to positive I'm getting 14v (running voltage)
If I understand correctly, pin B should be sending between 0v-5v depending on engine speed. I believe I have a shorted wire
Key on engine off you should see 4-5v on the signal wire. engine running you should see 05-2.0V on the signal wire. The signal wire looks shorted to ground with or without the MAP sensor plugged in?
Signal wire is shorted to ground plugged in and not plugged in. I found where the signal wire is going into the computer and ran a jumper wire from there to the sensor and it's still shorted. It must be shorted in the connector.
20 ohms may not be a short, I don't know what the sensor circuit is in the ECM. If you get 20 ohms with the ECM connector disconnected then that's another story but I don't like pulling those big connectors off. I would connect the 5v high and lo ref connections at the sensor (A and C) , IOW power the sensor and read the voltage at the unconnected output (B) at the sensor. My bet is the sensor is dead.
I'm back to messing with this thing. I had it running for a bit by staying on the throttle a bit so it doesn't stall out. I scanned the codes again and now I'm getting map sensor, tps sensor, and shift solenoid sensor. When I searched the shift solenoid code I found the issue could be because of the ignition switch. There was even a thread saying it would cause other low voltage problems. Any input?
Those three or any of the two do not share fused power. 5v ref or lo ref wires so they are not failing because of a common single wire problem. Doesn't mean there isn't a larger mechanical harness problem.
The fix is the same for all three:
Look at the function in live data and see if the values make sense. In the case of the TPS look at the output value through the full range of motion with key on engine off. With MAP look at the value with key on engine off and cold. If the values don't make sense look at 5v ref and lo ref for the sensors and fused power and lo side switching for the solenoid. If that good then check the continuity of the signal wire or low side switching wire on the solenoid. The solenoid gets fused power from Trans fuse 23 10a but so do the other solenoids and many other functions get their power from the ignition switch. You can look at the quality of that fused power and wiggle the ignition switch but I don't think that's it, there would be a dozen things failing. Of course multiple sensor/solenoid failures is a possibility but not the most likely reason.
07-27-2021, 10:37 PM
won't idle
