Engine idle is rough when it is warm outside

Hi Steven,

You may have a couple things going on here but we need to start with fixing the exhaust leak. That could be throwing off your O2 and effecting the idling issue at a stop light.

As for the rough running when starting when it is warm out, most likely that is your intake air temp sensor. The PCM will only look at a couple of sensors when in open loop when it is first started. These are generally, IAT, ECT, MAF, and maybe MAP. Once the PCM sees the ECT has reached operating temp, it will start monitoring the o2 sensor data and this is why it smooths out because it sees that it is way to rich when it switches to closed loop.

Here is a resource that will help explain this further. Step 4 and 5 are most important in your case.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

In my opinion, you need to get a scan tool and hook up to the vehicle to look at what these sensors are doing. Plus, I imagine you have some DTCs. However, there are ways to test some of the sensors without a scan tool. Here is a video for the MAF:

https://youtu.be/p0ocaDmcAIM

Here is one for the IAT and ECT:

https://youtu.be/nksLy9MS0IQ

I suspect what is happening is, the intake air temp sensor is reading wrong which is why it only does it when it gets warmer outside. Or the mass air flow sensor is reading more air then what is there so the PCM is adding more fuel.

Let me know what you find and we can go from there.

That Steven. This is great information. If this issue started after you put this crate engine in, you may be missing this intake temp sensor. The PCM is looking for it and is recording it as -40. This is indicative of an open in the wiring or sensor. If it is not there, then this would explain the open. Either way, the PCM is looking for it and not seeing it. This will effect the way the engine performs in open loop when it is warm outside. When it is cold, the engine will run a little better because the -40 is not as far off so it may not be as noticeable. I see that the GM TBI crate engines are direct replacements for this vehicle but don't see where it says the IAT is not needed.

The next thing is the MAP seems off. I assume the MAP in your recording is measuring in vacuum not pressure because at key on, the MAP matches your BARO. Then quickly your MAP shows load on the engine or possibly throttle input, even though your throttle remains at idle. The PCM is command an RPM of 890 but clearly the RPM is ranging and this indicates a rogue sensor that the PCM is looking at.

If it were me, I would investigate the intake/manifold air temp sensor and then replace the MAP and retest. This is assuming that when the crate engine was installed, the PCM was re-calibrated for the engine change. What engine was in this prior? Was it a Vortec engine?

So if the PCM is looking for and not getting a good IAT sensor (in this case it is calling it a Manifold Temperature Sensor) then that can definitely effect open loop operation. The engine has to have some sensor that gives it what the ambient or intake temperature is. The reason is, the PCM uses ambient/intake temp, engine temperature and MAP to determine (guess) what the best amount of fuel is needed to operate properly until it gets up to temp so it can start looking at o2 data and others. At that point it will adjust and the engine smooths out.

Based on what you are saying, your issue is at open loop. Everything involved in that is, PCM, MAP, any intake temp/ambient temp sensor, and Baro. If Baro is reading properly, the MAP has been changed, you don't have an intake/ambient sensor (still think we are missing something here) then I would assume the PCM is just not interpreting this data correctly.
This could cause it to over fuel, so it commands your injectors to open too long, which causes your RPM to range, which explains why your MAP is all over the place.

If you are going to replace the PCM, I would research what PCM has the best software for the replacement engine you have. Not just replace it with the one for the way the vehicle was built. Maybe call the engine supplier and ask if they offer a PCM to go with that engine. Again, I understand that this was a replacement engine for this vehicle but the fact that the PCM is looking for something that doesn't appear to be there, something has to be different.

Taking a step back, you said this didn't start until you replaced the engine. If it happened pretty quickly after replacing the engine then we need to find out what is different between the last engine and this one, because clearly it is not acting properly.

Great to hear the idle is smoothed out. I wonder if this PCM shows that manifold temp sensor as well? Just if you have time send another live data with this PCM. I am just curious.

You are correct. Base is 0.

It does not seem low on idle with no load. Put the parking brake on and block the tires, put it in gear so it loads the engine and see what the advance goes to. However, the key thing is how it is running. You can bump it up a little for base timing if you like. You can play with a little bit but I would not go to far because then you will start affecting acceleration and fuel mileage. However, if it runs smoother at 2-3 base timing which puts it up to around 10-12 with advance then go with that. Basically, the engine will tell you where it is happy so you are looking for when it sounds really smooth or what they call a rolling idle.

That clears it up. Thanks.