Engine stutters after warm up

Hi DASCON. Dandy descriptions and thank you for listing the engine size and mileage. Unlike other sites where anyone can chime in, here this becomes a private conversation between you and one or two experts. When you posted the follow-up, it moved your question to our list of questions that have already received replies. That makes it look like you are in a continuing conversation already, and most of us will not butt in. Most of the other experts will not see it or have a chance to reply. That will not get you the help you need. I only read it thinking I might learn something.

I can add a few comments of value, but consider reposting your question so it shows up as a new one waiting for an answer. Engine performance is not one of my specialty areas.

Contrary to common belief, oxygen sensors typically will not cause such noticeable running problems, but their readings do indicate the results of those running problems. Look instead at the other sensor readings to see what the engine computer is responding to. One in particular is what the computer is requesting for idle speed. If you see "idle steps" or "throttle percentage" go up when idle speed goes down, the computer is trying to raise idle speed but without success. Years ago, when fuel additives were not so effective, we saw a lot of air passages around the throttle blades plugged with carbon. That prevented proper operation of the idle air control valve.

If you see the requested idle speed go up when actual engine speed goes up, the computer is asking for that in response to something. Check if the intake air temperature sensor and coolant temperature sensor are reading correctly. Temperature sensors have a very low failure rate because there's just one component inside them. More problems are caused by resistance from corrosion between mating terminals in a connector and adjacent terminals in a connector.

Chrysler is the only manufacturer that never needed a mass air flow sensor to make their engines run right. For all other brands, that sensor is the main fuel metering calculation. There cannot be any leaks in the fresh air tube between the mass air flow sensor and throttle body. Vacuum leaks cause the same problems. If any air sneaks in that does not go through the mass air flow sensor, no fuel will be calculated to go with it.

GM makes a lot of use of knock sensors. At the mileage you listed, it is possible the timing chain is sloppy and is slapping against the cover. The knock sensor will detect that as spark knock and the engine computer will retard ignition timing to try to stop that. Look at the requested timing on the scanner to see if it is bouncing around.

There seems to be a lot of performance problems solved on GM vehicles by attending to ground wire connections. Computers usually have multiple ground wires and they are for different circuits. Do not waste your time trying to measure resistance in those wires and connections. It will be far smaller than the normal resistance of your meter leads, even when it is excessive. Instead, measure the voltage on those wires while the systems are in operation. A resistance that is too small to be measured will cause a voltage drop that can be measured.

Watch on your scanner to see if it shows whether the system goes into closed loop or stays in open loop. The coolant has to reach a specific temperature, then the computer switches to closed loop. The only difference is in open loop, the oxygen sensor readings are ignored. Those sensors have to reach 600 degrees before they start to work and the assumption is they reach that by the time the coolant is warmed up. The computer calculates fuel needs based on all the other sensor readings, with the mass air flow sensor being the major contributor. In closed loop, the front O2 sensor reading is just used to fine-tune the average mixture. The rear sensor is only used to monitor how well the catalytic converter is doing its job. That one has nothing to do with engine performance.

You can also look at fuel trim numbers to see how much the computer needs to vary the fuel metering calculations from those that are pre-programmed in at the factory. High positive numbers means it feels the need to add more fuel than expected. That could be, for example, in response to a vacuum leak. High negative numbers means too much fuel is going into the engine and the computer is trying to reduce that amount. Causes can include a badly-leaking injector or fuel pressure that is too high. A spark-related misfire will send too much unburned fuel into the exhaust system, but that should be detected as a "specific cylinder misfire" or a "random cylinder misfire".

As Paul Harvey used to say, "I told you more than I know". I can do a real good job of explaining theory of operation, and I can share stories I have heard and read about, but the real specialists can find clues in minute sensor reading changes the rest of us would overlook. Some of the people here are real good at analyzing all kinds of sensor data and they have tricks to force artificial lean and rich conditions to see how the computer responds. That is why it was in your best interest to re-post your question. I will keep an eye on your new question to see who replies. I have two or three people in mind already.

Hi DASCON,

I have reopened this question in the cue for another expert to answer.

Renee

Well it seems I'm late to the party, story of my life. Low coolant temps cause the PCM to keep sending extra fuel because the engine is running "cold". However I've never seen it cause the type of symptom you posted earlier, like an on/off switch causing the O2 sensors to drop out.
I have seen that before though. It was due to the wiring harness at the rear of the engine rubbing through at the corner of the engine and the O2 signal wires getting shorted to the block. It only acted up when the engine would warm up because the wire insulation would get softer. The harness T's off at the back to come over the top of the engine and carries a couple sensor wires and wiring to the alternator. If you replaced the thermostat you may have moved that harness enough to fix the issue. I would check the harness to be sure that wasn't the reason for the O2 related items. It's nothing some liquid tape and wrapping the area with some good Etape won't fix if the wiring id damaged.

Either way, you did a good diagnosis on the T-Stat.

Not that I know of, I've posted half a book before and it all showed up. Take a close look at the wiring around the rear of the engine and down to the sensors, if they are shorting it can cause a lot of problems with the exact symptoms you are having. The O2 sensors shorting causes the ECM to go nuts, especially if they toggle in/out with different values every time.