1990 s10 cold idle problems

[noble527]

i have a 1990 s10 4.3 tbi, 4x4 5 sp. On cold start up it idles rough, until warm, even spits and sputtes driving down the road, once it is warmed up she runs great. Did the egr cleaning, egr solenoid replaced some old vac lines, fuel filter, new gas... still idles rough when cold...Went to put in a new temp sensor for ecm, and it through a code, (code 15) So i put the old one back in and no code no engine light. Tried another new sensor and it did the same thing...could i have a bad ecm? any ideas would be great thank you!

[old_master]

Temp sensor for the computer is located near the thermostat housing. It has two wires: Yellow and a black. Is this the sensor you replaced?

[noble527]

Yes it is

[old_master]

Seems odd that the new sensor set a code, and the old one doesn't. Temp sensors have a thermistor in them. A thermistor is a resistor that changes resistance when temperature changes. If you have a digital volt ohm meter you can check resistance of the sensor to see if it's accurate. This chart is for ECT and ACT sensors. When checking resistance, measure it when cold, warm, and hot to make sure it's accurate at all temps.

[noble527]

I will check, do you think a ECM could be a issue, or do u think that could be why it idleing rough?

[old_master]

Probably not, these ECM's are just about bulletproof and rarely need replacement.

[noble527]

So then if i have good readings what do you suggest? I'm gonna test it with old one in there..

[old_master]

Could be a poor connection at the ECT sensor, or even a ground. There is a ground connection near the thermostat housing. It's possible it was disturbed when you replaced the ECT sensor. The data has to make it to the ECM for it to be able to process it correctly. Certain scan tools will read what the ECM sees and that's a more accurate way of testing. You can read it using a DVOM at the ECM connector. At the ECM: The yellow wire goes to connector "C" pin #10. The black wire goes to connector "A" pin 11.

[noble527]

What do you think about the rough running when cold issue?

[old_master]

When the engine is warm, the ECM primarily uses the oxygen sensors for determining fuel mixture. When the engine is cold however, the ECM ignores the oxygen sensors and relies heavily on the ECT sensor, (it also uses TPS, MAP and MAF sensors). The ECM processes the data from them and uses a predetermined fuel mixture. If the ECT circuit has a poor connection somewhere, the ECM interprets the high resistance as a colder than actual temperature and it will richen the fuel mixture. That might be what's causing the rough running.

[noble527]

ok, something else to riddle you on, this morning went out to start and it was idleing up down just about to die, like it had been, i decided to unplug map sensor, she idled up and smoothly...or atleast better! Whats up with that, its a new map sensor with a new vac line ran to it?? Any ideas?

[old_master]

The MAP sensor sends manifold vacuum data to the ECM. The ECM primarily uses that data for determining engine load. If the engine is surging, manifold vacuum is also surging, the ECM tries to correct it and ends up making it worse. When you disconnect the MAP sensor, granted the ECM isn't receiving data from it, but at least it's not trying to constantly correct for the eroneous data. If there's a problem with the MAP sensor, it will set a DTC in memory.

[noble527]

Dtc in memory? what's that mean? Lol so again what are your thoughts? Generally I have read where people have unplugged sensors while running and if the symptom changes an gets better then that sensor is the issue... Right now I'm pretty much clueless. Thanks for your input i really appriciate it!

[old_master]

Just like code 15 in your first post, DTC = Diagnostic Trouble Code. Disconnecting your MAP sensor covered up the surging idle, but the MAP sensor might not be the root cause of the problem. Disconnecting sensors, without proper diagnosis along with it, will not tell you if a sensor is faulty, that job belongs to the ECM. When troubleshooting DTC's, always start with the first one. After diagnosis and repair, make sure the check engine light stays off confirming that the repair is complete. If the check engine light comes on again, retrieve the code(s), diagnose and repair the first DTC and confirm the repair. Continue like that until the light no longer comes on. The reason DTC's are diagnosed in the order they are retrieved is because the first DTC can cause others to show up. Much like a burnt out bulb in a string of Christmas tree lights: if two of them burn out, it doesn't mean all of them are bad, one bad one can cause the rest to go out. You need to fix them one at a time, in order, until all are repaired.

It's possible that the ECT signal is not being received acurately at the ECM. The ECM has no choice but to "believe" all sensors are reporting acurately. It does "watch for" readings that are out of range. If it detects an out of range condition, it sets a DTC. For example: The actual temperature is 23 degrees, and the ECT resistance is 12,300 ohms, (perfect). If a poor connection in the ECT circuit has 40,000 ohms resistance through it, the ECM will interpret the ECT resistance to be 52,300 ohms which calculates to -20 degrees. The ECM will richen the fuel mixture. After the engine is warmed up, the air fuel mixture will still be rich and a rich DTC will be set, but nothing regarding the ECT because it's still within range. Now lets say that poor connection has 100,000 ohms resistance through it, plus the 12,300 ohms, now the ECT is out of range and a DTC is set. This is the importance of testing at the ECM.

Also, have you checked ignition timing with the EST, (Electronic Spark Timing) wire disconnected? There is a connector behind the carpet just above the passengers feet. Single tan wire with a black stripe, must be disconnected when checkng/adjusting timing. Ignition timing is controlled by the ECM but base timing needs to be adjusted when the ECM is not controlling it.

[noble527]

ok i will do some testing, however i checked for engine codes, had none (ofcoarse until new sensor was installed) Installed old cleared codes none re appeared. I will check resistance of temp sensor. Spark timing? Where is that adjusted distributer? Also what needs to be done to adjust this? Thank you again, will report back with readings.