Rough Idle

So I have 98 Tacoma with the 2.7L and it is a manual 4WD. During a cold start the idle goes crazy jumping from about 600 rpm to 1200 rpm. It takes a couple of seconds for the idle to even out, but sometimes I have to rev the throttle to get it to stop. It is throwing two coded, p0173 and p0125. I have replaced the MAP, TPS, IAC and a few others but the problem still persists. Anyone on here have any ideas?

 

P0125 is insufficient temp to enter closed loop. See if there is coolant in the engine, yes a stupid check but you would be surprised... If coolant is present, what does you scan tool tell you that the ECT (engine coolant temp sensor) is showing? If this shows too low I would suspect a stuck open thermostat. If the scan gauge shows coolant temp above 185 then I would suspect the engine coolant temp sensor, the 2 wire one, not the single wire one, that one is for the temp gauge in the dash. By the time you get around to checking the sensor you could have replaced it.

P0173 is a fuel trim issue. The PCM (powertrain control module) is maxed out in its attempt to trim the fuel, positive or negative, for the motor to run correctly. I find this odd because if there is not enough heat being reported to the PCM, via the P0125, the motor should never go into closed loop mode. Closed loop mode is where the sensors on the motor adjust the fuel, spark, timing etc. Open loop is where a fixed set of parameters are used to get her started and running.

replacing the ECT just might fix the P0173 over a short period of time.

 

I'd be curious if all the vacuum hoses are good and connected. Mine idles horrifically on a cold start if I accidentally unplug one.

 

There's some confusion about P0125. The Repair Manual for my 1996 indicates it means the heater circuit in the oxygen sensor is not working. It actually doesn't have anything to do with the coolant temperature other than it will only throw the code after the coolant is at normal operating temperature. It could be the O2 sensor itself or the wiring.

The repair manual says P0125 is "Insufficient coolant temp for closed loop fuel control" but if you dig deeper into the explanation it talks about the O2 heater circuit.

 

@DrZ I do have an exhaust leak do you think that could be throwing this code?

 

I'd guess yes, if the leak is bad enough. Not enough exhaust going past the O2 sensor will prevent it from heating up. If it is sucking in fresh air that could throw off O2 reading also and O2 sensor would report this as lean and computer would keep trying to add more fuel.

Where is the exhaust leak?

 

The exhaust leak is where the exhaust manifold connects to the exhaust pipe. I have had an issue with it there for about 2 years. It comes lose over and over again no matter what nuts or gaskets I use. @DrZ

 

Same. About to fix mine again today actually. Mine starts to leak every 6 months or so.

 

How do you go about fixing it? I think my exhaust manifold is broken now the stud hole snapped off luckily I have it under warranty. @Russianman92

 

Is there a brace farther down the exhaust pipe holding the pipe to the transmission? If you don't have that I could see it having too much movement where the manifold joins the exhaust pipe.

 

I do think I need to fab another support for it.

 

Good point. That's probably what it is as I do not have it. Toyota discontinued it according to the dealer. I see it available online. Will probably order it. I do have the metal clamp for the down pipe somewhere though.

 

Do you have a link to the transmission brace? @Russianman92

 

hmm... I will have to see if I can test this. On the other model of vehicles I work on the O2 sensor heater circuit trips the CEL almost immedialty on start up, maybe Toyota does it different. If they do, live and learn.

 

https://parts.jimellistoyota.com/p/...igin=pla&partnerDevice=m&userLocation=1015254