Rod Bender

If it were only the temperature guage not working but the engine also has a misfire after it is warmed up. I believe the misfire when warm situation is supposed to be caused by a problem with the temperature sensor. However, it is my understanding that the temperature guage and the temperature sensor are separate. I have checked connections and they seem tight. I have replaced both oxygen sensors after getting an oxygen sensor code. Is it merely a coincidence that both these situations have occurred or am I missing something. The car is a 96 Z28.

Injuneer

What was the specific numerical code for the O2 sensor? There are numerous codes, some of which are caused by problems other than the O2 sensor itself.

Many causes of misfires when warm.... not limited to the CTS sensor. There are two coolant temp sensors - one in the driver side head for the dash gauge, one in the water pump housing for the PCM. Which connections did you check? And of course, a complete check involves checking the resistance of the CLT sensor, and verifying the PCM is supplying the correct voltage at the sensor connector. See Shoebox's guide:

4th Gen LT1 F-body Tech Articles

Of course there are other possible causes of a misfire. So.... no misfires on cold start? Only when the engine is fully warmed up? To what temperature? Under what condition - at idle? .... crusing at part throttle? .... at wide open throttle? ... at what RPM? ... when lugging the engine or pulling up a hill in the wrong (too low) gear? .... all of the above?

Rod Bender

Thanks Fred. I checked that the wiring was connected to both sensors. i will read what Shoebox has to say about reading voltage requirements. As far as how the car is driven, just like I always have. I do not remember what code was given. At Auto Zone and one other parts store they both said oxygen sensor, so I replaced the oxygen sensors.

GaryDoug

Do yourself a favor and buy a cheap code scanner like the Autel MS310: https://www.amazon.com/Autel-MaxiSca.../dp/B013WB7RVS

For some reason this link goes to a page that says Amazon doesn't have any in stock. That is incorrect; just do a search for Amazon Autel MS310 and you will get the correct link for the one at $23

The MS310 reads the trouble codes and most of the OBD2 engine data as well. Most cheap ones only read the codes.

Rod Bender

Thanks Gary. But do I really need it. Don't the code readers that Auto Zone and O'Rileys use do the same thing?

GaryDoug

Evidently not if you can't remember what they said or maybe they didn't even let you see it. Also, theirs does not show the engine data. I'm sure you spent more than that on the O2 sensors which you may not have needed.

Injuneer

...

+1

Rod Bender

Thanks Fred. The code reader at Auto Zone Read "PO131 HO25-11 (Bank 1 Sensor 1)Circuit Low Input " and under "Suggested Parts"," fuel pump". The clerk said it did not really mean I needed a fuel pump though.

Rod Bender

Well I finally was able to run the checks on the coolant temperature sensor and wiring to it. The sensor's reading and the wiring were right on Shoebox's numbers, so something else must be causing the part throttle miss I get when the engine is warmed up. You mention the CLT sensor. What is that?

Rod Bender

I read on some engines the temperature gauge sends information to the temperature sensor. However, with just one lead to the temp sending unit is that possible on the Lt1?

Injuneer

The gauge is in a circuit that provides 12 volts reference to the sensor. The sensor resistance varies inversely with temperature (a thermistor), causing the voltage on the circuit to vary, and that drives the gauge. No information is sent to the sensor.

A DC circuit needs a positive voltage source and a ground. The single wire supplies 12 volts to the sensor. The engine block supplies the ground. The reference voltage can vary to some extent with battery voltage, and the ground can vary depending on the contact of the sensor with the block, corrosion, the quality of the block to chassis ground wire, etc. A bit sloppy, but accurate enough for the dash gauge.

Not peculiar to an LT1, typical for most dash temp gauges.

The temp sensor for the PCM needs to be more accurate than the gauge circuit, so the PCM supplies an accurately regulated 5 volt reference, with 5 volts on one wire and a ground wire, hence a 2 wire sensor.

Rod Bender

Thanks for the explanation. I am getting current to the sending unit. Is it possible to tell if the problem is in the sending unit? However, this still leaves me with my main issue, which is a seemingly rich running situation after the engine warms up. I performed the tests Shoebox suggests and the temp sensor and pcm seem to be operating properly.

Injuneer

If by "sending unit" you mean the temperature sensor, Shoebox's how-to covers testing the sensor. And your are not at any point testing for current. You have to verify 12 volts to the gauge sensor connector (connector off the sensor), 5 volts to the water pump/PCM sensor. Then test the resistance of the sensor per the Shoebox chart (connector off the sensor).

The key question is whether the PCM is getting the correct coolant temp input. You can only get that info by connecting a real time scanner (not just a code scanner) to the PCM and looking at what the PCM says the coolant temp is.

What did you do about the O2 sensor code?

Rod Bender

Thanks for the reply. Yes, I followed Shoebox's specs and suggestions for testing the sensors and current to them. The Auto Zone code reader showed " PO131 H025-11 (Bank 1 Sensor) Circuit Low Input. I then replaced that sensor. Later, when it gave that code again I replaced the other. It sounds like I need to take the car to a shop that has a system analyzer as you suggest.

Injuneer

There are OBD-2 live data scanners available for $75. There are apps for smart phones. A dealer will charge you more to scan the PCM with a Tech-2 than the cost of the do-it-yourself live data scanners.