So I’ve had some serious trouble with my car running awfully until it gets to operating temperature (~190F) and then running fine. Stuff like hesitation and rough idle. I’m thinking the coolant temperature sensor is a good place to start-I feel like it’s got to be a sensor because it runs fine once it’s up to temp.

where is the temp sensor that takes to the ecm? Would it be possible for that to be bad, but for fans to still come on properly and temp gauge to operate correctly? Can someone send me a photo of where this sensor is located? Thanks!

edit to say I have a 1985 with the L98 engine.

 

When the car is cold and not up to temp, it is considered open loop.
that is to say none of the sensors except throttle position sensor and coolant sensor is reporting to the computer.
in closed loop, all of the sensors report to the computer, and the computer is then making hundreds if not a thousand decisions a second to make your car run good.
in open loop, the temp sensor’s job is to alert the computer that the engine is up to temp, and nothing more till it gets into closed loop.
the prom in the computer is the “reference” that the computer uses to calculate timing and mixture, etc. when cold and warm.
check to see that yours is correct for your car.

 

This car is completely unmodified. Also the issue has appeared and gotten much worse since I’ve owned the car.

 

The Coolant Temperature Sensor is one of the most important sensors in your L98. It is used in both the "Open Loop" and in "Closed Loop" as it is important to many calculations, especially the air fuel ratios.

During "Open Loop" the following sensors are used.
Intake Air Temperature sensor
Coolant Temperature Sensor
Throttle Position Sensor
M*** Air Flow Sensor or MAP if equipped.

When the engine switches over to the "Closed Loop" it then goes from using all the sensors to primarily the Oxygen sensor and the Fuel Injectors going through the ECM.
This information is from the book: Corvette Fuel Injection and Electronic Engine Management, by Charles D. Probst, Published by Bentley Publishing

On your L98 engine the Coolant Temperature Sensor is located up on the front of the plenum. It has three wires going to it and should not be too hard to find. Be sure the connections are very clean and if needed clean them with an eraser or DeOxIt which will wipe away decades of corrosion in one swipe.

Remove the CTS and verify the signal it is sending to the ECM, the resistance value crosses over to a Temperature on the chart in the FSM to tell you what temperature it thinks it is.

The Intake Air Temperature should be verified as well as it too can screw up the calculations. I would have to refer to the FSM for testing the IAT but I would definitely check it to be sure it is working properly. A lot of people like to relocate this sensor as it is mounted in a place where it gets hot and stays hot and this affects the readings. A cooler IAT signal will allow the engine to make more power.

The MAF might be a good thing to test as well. Unplug the MAF and start your Corvette, does it run smoother or better? If it does that would point to your MAF sensor. The MAF sensor has two Relays that control it. One is for the power control of the relay and the second relay controls the Burn-Off Function. Any time you replace the MAF be sure to replace BOTH relays at the same time. I have seen the burn off relay fail without triggering a warning light or Code. It is important as it keeps the tiny wires (1/10th of a mm) clean of any contaminants. When working in the dark the little wires glow yellow as it is burning off the crud built up on the wires. If the burn off has not happened in a while or you have something it can't burn off then the MAF would start to suffer. You want to be sure that the MAF is working properly as it measures the volume of air being ingested by the engine's combustion process.

Best Regards,
Chris

 

^^^^^^^^^^^^^

This is incorrect information ....

I believe early '85 already used the 25036979 (connector style) but check your FSM and Circuit 410 YEL and 452 BLK are correct wire colors. At this time of it's life I'd think it's been replaced at least once and there would have been something resembling a 25036979 will be found. Regardless look for 2 wires YEL and BLK. Don't forget COLD START SWITCH is also on the intake manifold nearly 'side by side'. Very different.

GM 25036708 would likely have been an original.

 

Thanks for all the good information! I’ll have to check the intake temperature sensor as well. Where is that located on the ‘85? In the plenum?

I will replace the MAF and potentially the burn off relay as well. I actually have both of those sitting around (from a donor car) since I know they’re hard to find. I’m ok with throwing a couple $20 parts at it. I really think it’sa temperature sensor since once the engine warms to operating temp it runs fine. Note that this fits NOT mean that it runs well once it goes to closed loop at 140F, but at 190F it runs fine.

 

What intake temp sensor? The wires and sensor mentioned are the ECM temp. The other in the intake is for 'Cold Start' - if you've no issues with first start of day cold don't worry about it. I believe it's a very hard find. YEl & BLK - 410 & 452 is all that needs to concern you. I wouldn't suggest multiple replacement at one time (especially used). You asked ECM TEMP - it's been provided. You've an FSM, use it.

*** Switch next to the sensor you asked about which I mentioned as CSS (TAN & PPL / 832 & 806) Ignore it. 6E3-30 section of the FSM might be interesting reads and research for you. If you can't find reference in the 6E3 Section of FSM - worry not.


Most everything in post #3 isn't relevant or even interesting info! Ignore it!

 

thanks for the info. Sounds like a plan. I might replace coolant at the same time since it’s all gonna drain anyway right?

and yeah no trouble with cold start. I meant is there An air temperature sensor i should be thinking about in addition to coolant temp?

 

I've replaced a CTS twice with very small loss of coolant. Just had to top off each time.

 

Just an update on this. I swapped out both the CTS and the IAT (Intake Air Temperature) sensor. They're the same sensor, got both for ~$10 total from RockAuto, so figured why not? Very little coolant was lost from the CTS, I actually didn't even bother topping off, there's plenty in the reservoir.

Unfortunately, I drove the car before swapping the sensors and it was driving much better today-the ambient air temperature is about 15 degrees cooler than it had been when the car was acting up, and I've noticed in general the car runs better in the mid 70F range. So I didn't notice much difference after swapping the sensors, but at least it didn't get WORSE, so I'm thinking that I at least didn't hurt anything and the old sensors looked original to the car, so it can't hurt having 30 year newer sensors-right? I still had a little bit of a surging idle, not sure if that's related or not.

Next up I'm replacing the O2 sensor. All these sensors are cheap, and I don't mind throwing a few of them that should PROBABLY be replaced anyway at this problem.

I'll update if I start having more trouble with it, here's hoping that it's solved.

On a slightly related note-I always have trouble getting the plenum bolts back in, and I'm pretty sure there was a vacuum leak until a tightened one of them down a little more (I had to remove the plenum to get to the IAT sensor). Does anyone else have that problem? I mean it feels like they're getting cross-threaded or something.

 

Welp-still wants to stall after a cold start. Once it’s up to temp still runs great.

 

Hello Bfenty,
What temperature does it transition to running properly? Is it running poorly in the "Open Loop" or is it in "Closed Loop" as well.

 

it’s definitely worst in open loop, but there’s not a magic fix at a specific temperature. It used to immediately run better when it hit 140F, but now it takes until 170-180 to clear up. It’s also intermittent and seems to be better or worse based on ambient temperature (which makes very little sense to me…the engine operates at high enough temperature that 10-20 degrees ambient shouldn’t make a huge difference).

 

Ok so weirdest thing. I got really into my car this weekend and replaced lots of fluids and the O2 and MAF sensors. That’s pretty much every engine sensor replaced now. None of that made much difference to how the car runs.

On a whim I replaced the ECM with another I had from my donor car. There was an immediate difference in performance. The car idles much better, power is instant, and the car feels like it’s woken up.

the only thing i can think, is that when i first got my car I swapped injectors with my donor as part of fixing it up. The donor injectors were not stock and may actually be 30lb injectors. That’s outside the tolerance of the stock computer to compensate for. The donor ECM most likely was tuned for the injectors (i have receipts saying it was purchased at the same time as injectors, but not why). I don’t know how to tell if that’s the case, but swapping ECM has made one of the biggest differences of anything I’ve done to the car. It added a very noticeable amount of power and I’m thrilled.

 

That is Great news!

Now you can get out there and drive it before it gets cold!

Best regards,
Chris