The 88 had been running a little odd, hesitation and mild stumble, so I ran a scan with DataMaster, I noticed no knock counts or anything else, however the Coolant reading from the computer was way wrong as was the target AFR.

The coolant reading starts at 117, according to something the scanner calls Coolant STRT. It rises somewhat normally until I see the engine fall into closed loop, and then the coolant temp reported by the computer is 246. It remains there basically all the time.

It then starts the main fan, so I'm sure it does believe the engine temp really is that high.

The in-car gauge, which is connected to the sensor in the head, is reading as you would expect. It rises until the tstat opens, holds there, and then temperatures are very normal. Car hasn't seen 220F on oil or coolant. The secondary fan is set to 180 and it does come on at the appropriate time, but its in the head as well.

The target AFR reported by the computer is reading 24-25 on DataMaster's screen. Way wrong.

I thought perhaps the CTS had failed, I disconnected it and the reading went to 30 as it should've according to the manual, so I replaced it. I sprayed contact cleaner on the pigtail, and the problem remains. I traced the wiring back within the engine bay but noticed nothing out of the ordinary.

Is there anything else I'm not thinking to check before i further investigate wiring? Could there be something up with the chip program?

 

........... Not sure what your set-up is , but make sure your CTS is in water and not in some air pocket in the cooling system ... closed loop likely coincides with the opening of the thermostat and the CTS may become exposed to air at that point ...........

 

No the Closed loop operation is not connected with tstat opening.

Closed loop is enabled when the ECM registers the right range from the O2, the timers have expired, and a temperature is reached. That temp is much lower than tstat opening.

 

hmmm sounding very familier to my old problem here, do me a favor and put scanner on it again and wait till it has that problem , shut off engine and turn key back on. does the scanner still read way high? if so waste no time, leave key on scanner hooked up and go out and pull the cts plug and test the volts dc on the two wires take note then test each wire to ground like the block and take note then hook cts back up and verify the scanner still reading it as high, then let us know your findings. i think you will find the same odd results as i did but we will see

 

Sounds like the CTS is putting it into limp home mode. Does the spark advance become very low ?

 

Total timing is set at 30* and remains steady, it basically never changes. This is because we found this motor knocks with anything more than 30.

There is no Code 14 or whatever the CTS code is.

 

That sounds strange. Has your harmonic balancer ring shifted? What do you have your base timing set to?

You said earlier the CTS showed 30° with the connector disconnected. I hope you meant -30°. Even that is high. I would expect something around -50° (I never tried it -- I guess I should). Another test you can do is to disconnect the connector and short the two pins in the connector together. You should get a very high temperature, like 350° (I should do that test too).

[EDIT] Instead of doing the test I looked at the CTS tables in the code. According to those tables the lowest temperature the ECM sees is -40°C = -40°F and the highest temperature is 151°C = 304°F. That would be the extremes -- CTS input shorted or open [/EDIT]

 

Base timing is 6*, theres nothing wrong with the balancer. This timing is the same as it was in the 86, timing is not the answer.

 

First question is can the data be trusted? Based on what has been posted, this is questionable.

Try and confirm the values of the troublesome coolant temp, startup coolant temp and target AFR signals with another scan method such as TunerPro, just to rule out an issue with datamaster.

Has the bin file been modified in a way that would change the information in the datasteam?

For both coolant temp and startup coolant temp, open circuit value should be -40 F and shorted value should be 304.25 F

Its good that the startup coolant and coolant temp have the same inital value, so it suggests that they are using the same singal, so may just be a scaling issue with Datamaster.

For the coolant temp signal, the ecm toggles between two pullup resistors (4k and 348) on the coolant temp circuit. It is possible that there is something wrong with the toggle function (either sw or hardware) that is skewing the displayed results.

This does not explain the strange AFR targets, however.

Is Datamaster scaling correct?
Has datastream been modified in bin?
Is pullup resitor toggling working in ecm?

 

I would say the Datamaster is correct, the scan with it and another scantool read the same temperatures. Only datamaster shows the target AFR however.

Is the resistor inside the ECM, how can that be checked

Does the AFR target change to those numbers if the coolant temperature is so high? I would not think so.

 

assuming all is correct, have you reloaded the bin or swapped out the ECM?

 

From what I can determine, the 4k pullup and table is used for cold temperatures up to 50 C. The 348 pullup and table is used for warm temperatures down to 40 C.

From observing my own logs, it is somewhat common to see the Target AFR occasionally glitch to a value of 24-25. This seems most likely to occur when coolant temp is below approx. 50 C during warm up.

I'm still not clear on what causes this intermittent data glitch, but it sounds like you may be somehow stuck in the glitched state. It may also be somehow related to the pullup resitor circuit toggling.

If all else fails, try another ecm if you have access to one.

 

The ECM remembers the temperature is sees at startup and calls that the STRT temperature. It uses that info to decide how fast to degrade the fast idle speed and other stuff related to coolant temperature (spark and fuel). The STRT temperature comes from the CTS sensor as usual.

I was really surprised when I found that toggle function in the code. It actually measures and stores both values each time around the loop and then decides which one to use (which one is more accurate) based on the temperature. It's a clever way to get around the horribly nonlinear response curve of the CTS. All this is done inside the ECM. It's actually two resistors: 348 ohms and 348 + 3650 = 3998 ohms.

 

..... Yet another example of how the mad scientists at GM have over complicated their ECM ... I am sooo glad I switched to a Holley C-950 controller ...... my laptop is dusty ..........

 

Well I traced the problem with the CTS to the ECM, replaced that, and now the Coolant reported to the scanner and to DataMaster is correct.

However, the Target AFR being reported to DataMaster is still 25. Is the target a simple parameter that you could mistype into the Moates software he was using?

 

... What is the TPS showing during all of this ? ... If it thinks its near WOT it will cause the computer to think flood clear and command 25:1 AFR ................

 

TPS is at 0.54V from idle and progresses normally to WOT.

 

Took the car out today and logged with DataMaster, nothing stands out as being wrong on the screen except that the Target AFR is still staying in the 25-26 range.

With the ECM changed some of the hesitation issue are gone though, Im wondering how the computer uses this target?

 

....... Time to spring for a wide band O2S .............

 

It was tuned with a Wide Band O2.

Can someone tell me exactly how the ECM uses this target value in its trims?