Newbie to LT1 and trying to diagnose a cutout/bucking problem with my Triumph TR7 that the late PO did not finish. From the screenshot you can see the excel file where the changes occur #1074 to #1075. There is a big time jump, Coolant temp goes from 169 to -40, Fan1 Relay and Fan2 Relay both turns on(They don't exist on the car, a manual fan switch is in place).
My BIN file opened with Tunerpro showed standard Fan Enable Coolant Temp of Fan1 Lo 107.8 and Fan2 Lo 113.

What is happening? Is my coolant temp sensor defective or the non existing Fans turning on cause the -40?

Scan9495

 

Just a guess - possibly the PCM stopped reporting on the data wire for 2 minutes, or your computer stopped recording. Never saw this happen in the 100s of data logs I've reviewed. Did you try and run another log after this one? Is the PCM still functioning normally?

 

Thank you Fred for replying so quickly. I didn't run the car after recording this anomaly. The car always starts and runs. There may be some issues with the PCM, I am still in the process of trying to test each area out but mainly trying to get drivability now so I can test for other things.

The car feels underpowered, bucks and acts like it had a fuel cutoff around 3500rpm when accelerating. With the MAF sensor disconnected, it will pull through to 4500rpm without no problems but still feels underpowered. Brand new NGK plug wires and spark plugs. It always drives well with no bucking when cold so I haven't checked fuel pressure yet.

Today is the first time I hooked it up to Scan9495 to check on the MAF and noticed the strange time jump, the temp sensor reading and Fan1 Fan2 relays. I am new to the LT1, so I am unsure if there is/is anything else out of place with my scan log. I am planning to get a new coolant temp sensor from Autozone(Duralast is all they have in stock). I can take another log after to rule out the temp sensor.

If you see anything out of place, please let me know. I don't even know where to look for as to why the Fan1 Fan2 relays are activating going from 169 and then into -40

 

Forgot to add, I had a failed flash(dropped comm) with the PCM and used the Unbrick Kit from LT Flash to restore it recently. But the bucking/cutoff was there before the failed flash incident.

 

The lapse between 1074 and 1075 was ~124 seconds. Where did that data go?

Check temp sensor and 12 volt supply:

http://shbox.com/1/4th_gen_tech2.html#ect

 

No idea. That's why I am posting it here to find out. The temperature change from 169 to -40 (between 1074 and 1075) caused a stumble and erratic idling. I was logging to see if the MAF was working properly and was completely surprised by the coolant temperature sensor. I'm also surprised to see the Fan1 and Fan2 relays turning on when they don't exist. I am going to assume I have a bad coolant temp sensor or an electrical malfunction from the Fan1 Fan2 relays shorting out the coolant temp sensor.

I will pull another log tomorrow before I swap out the sensor with a Duralast sensor from Autozone.

 

DTC 77 tells the PCM the relays aren’t there. The 0/1 indication of the fan relays may just be the PCM tripping the internal switches to activate those relays in programmed logic response to DTC 15.

I added the coolant temp test procedure in previous post.

 

Ok, that makes me feel better. It makes sense the PCM would trip the relays in response to the DTC15 from the temp sensor.

Thank you, I will test the CTS per the procedure tomorrow. It will be a huge relief if this is the problem that's causing the bucking

 

The fan and coolant temp sensor circuits do not overlap. The fans are operated through PCM connector “A”.

http://shbox.com/1/1995_pcm6.jpg

The coolant temp sensor feeds through connectors “B” and “C”.

4th Gen LT1 F-body Tech Articles

You've got some issues with the Bank 1 O2 sensor. After 1075 the PCM entered closed loop. It normally can’t enter closed loop until a timer (206 seconds from start) times out. That occurred during the missing data. When the PCM goes into closed loop, the O2 sensors should start to toggle back and forth between lean (~0.1xx volts) and rich (~0.9XX volts) Your Bank 1 sensor seems stuck at ~0.6xx-0.7xx volts. The PCM tries to correct the (apparent) rich condition by lowering the Bank 1 long term fuel trim (LTFT). Your LTFT drops to the minimum value (108) pulling out 15% of the normal fuel, then starts dropping the short term fuel trim (STFT) to pull out even more fuel. Yours drops way down. Bank 1 was probably running way too lean after 1075.

Test the temp sensor and reference voltage before starting to replace parts.

 

Thank you for looking at the log and explaining them. For a newbie that is like getting an advanced bonus. Yes, I noticed when I first started up the car, the right O2 was ready very quickly while the left O2 was not. I was planning on getting the O2 sensors replaced anyway because the O2 sensor wires were touching and sort of wrapping around the shorty exhaust headers. I moved all the wires away but who knows how long they have been touching.
I read all your guides on the PCM and scanning. Hopefully, I can get it all figured out and have the car running the way it should be soon

 

I tested the temp sensor, and the ohm reading was way off with an ambient temp of 71. I replaced it with Autozone SU109, and there is no more funny business going from 169 to -40. The O2 sensors will be replaced tomorrow, and hopefully, there will be a nice test drive.

Since I have your attention, I have been meaning to ask about something I have observed since I took ownership two months ago. When the car is idling, every once in a while, the idling will change like a burp for a split second and then back to normal. On Scan94/95, you can see the Fuel, O2, and Closed Loop values go blank and then come back. Is this normal?