Coolant Temperature sensor failure - Code 15 (Page 1/2)
br1anstorm FEB 19, 12:26 PM
I have a 1988 Fiero Formula with stock 2.8 V6 engine. It's been garaged over the winter and little used over the past 4-6 months. Normally it starts and runs perfectly normally. Just recently however it seems to have developed a problem.

- the SES warning light is on and when checked via the diagnostic socket it gives only a Code 15;

- it needed more gas-pedal than usual to start, and it backfired, idled roughly and stalled a couple of times;

- with plenty of gas it revved up and ran fairly OK, but once warm the idle remained high (around 1400rpm);

- the idle speed did tend to fluctuate unpredictably, and once the warm engine stalled it wouldn't restart.

All this seems to point to a bad coolant temp sensor (CTS) giving no (or low voltage) signal, which tells the ECM the temp is below -30 degrees. So it feeds a rich fuel mixture into the engine which effectively floods the cylinders with unburnt fuel. Hence starting and idling problems, backfire, and high idle.

Okay, so I have to replace the CTS. First - how to find and access it. Not easy or obvious. Both this post from 2012, and this YouTube video give helpful guidance. But before I start - and I still have to order the replacement CTS - I need to clarify this mystery around the mounting of the CTS inside an additional brass bush or bolt. This additional fitting seems to have no part number, is not supplied with the new replacement CTS and according to the YouTube clip, is a standard plumber's fitting from a hardware store!

I'd like to know - (1) is that additional bushing or bolt always present? (2) Given that access and removal of the CTS is already a real challenge, any tips or tricks for extracting it without any parts breaking off or jamming in the hole? And (3) - this might be an obvious question, but is it possible to replace the CTS without draining down the entire cooling system?
1985 Fiero GT FEB 19, 12:55 PM
I was just putting my top end back together yesterday, I seem to remember that bushing, but it doesn't need to be removed, the coolant sensor just screws into it, unscrew the coolant sensor, replace just it, and that's all you need to do, I think the bushing is to make it more durable, as you expect to replace the sensor, and if it was just threaded into the cast aluminum that might strip. And you don't need to drain the coolant at all, some will drain out, but that can be topped up through the thermostat housing. It should just be a 3/4" (I think) deep socket with some extensions as needed, the sensor is horizontal protruding from below the thermostat housing over the belt toward the passenger strut tower
Vintage-Nut FEB 19, 01:20 PM
GM 25036979 / Sensor, engine coolant temperature for ECM

When you get a 14 or 15 code; don't waste your time - REPLACE IT!
{Many members didn't get a 14/15 code and reported a 'high' idle issue that was a 'mystery'. By replacing this sensor, their idle problems were solved...}


quote
I'd like to know - (1) is that additional bushing or bolt always present? (2) Given that access and removal of the CTS is already a real challenge, any tips or tricks for extracting it without any parts breaking off or jamming in the hole? And (3) - this might be an obvious question, but is it possible to replace the CTS without draining down the entire cooling system?



1) The bushing is called a reducer fitting as the sensor is smaller than the manifold threads / When I replaced my; the reducer didn't move and only removed the sensor.
2) Unbold and Remove the EGR Solenoid and push the main wire harness out of the way.
3) You don't need to drain fully, but yes, you'll lose some coolant to the sensor level.

GM use a sealant on the threads; if not, I use high-temp RTV

[This message has been edited by Vintage-Nut (edited 04-15-2025).]

Patrick FEB 19, 01:34 PM

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Originally posted by br1anstorm:

- it needed more gas-pedal than usual to start



"than usual"? It shouldn't take any gas-pedal at all to start the engine. If you've always had to manipulate the gas-pedal when starting the engine, that indicates to me that there's been a long standing issue.


quote
Originally posted by br1anstorm:

All this seems to point to a bad coolant temp sensor... Okay, so I have to replace the CTS.



"seems"? Have you not scanned the engine and/or run WinALDL to see for sure what reading the CTS is reporting to the ECU?

Or at least use a multimeter to check the sensor.

[This message has been edited by Patrick (edited 02-19-2025).]

olejoedad FEB 19, 02:12 PM
Before spending time and money on a sensor, check the wiring.
Sensors seldom go bad, but when a car sits for long periods, rodents have an opportunity to feast on the wiring.
br1anstorm FEB 19, 03:30 PM
Thanks to those who posted helpful advice, and especially the picture....

To answer @Patrick's points - yes, usually, normal starting requires minimal gas pedal. So the current situation required more than what was usual. But it hasn't previously been an issue.

Scanning the engine? Well, if I were a full-time workshop engineer with all the relevant equipment to hand, maybe. But I'm not. I have WinALDL on a 20-year old Windows XP laptop somewhere in a storage container. I may get round to unearthing that and trying to see whether it still works and what scan-readings it might provide. But not this week.

Using a multimeter to check the CTS readings? Yeah, but first I have to get in/under/around/and at the connections. Not the easiest of locations to access. One reason why I sought guidance from those who had already been there.

Checking that mice haven't eaten the wiring? (@ olejoedad) .... Yes I know that can be a hazard. But my carport seems (touch wood) to be a critter-free zone. I regularly check on the car and over the winter run it round the block occasionally: so it hasn't exactly been hidden away in storage and neglected. As for visual checks of the wiring, see point above about access. I will indeed be taking a look when I can.

Meanwhile, it's a 35-year old car. The CTS is undoubtedly original. It has been there all those years. It's not an expensive part. They are known to fail. Some indeed recommend replacement on a preventative/contingency basis. See also @Vintage-Nut comment above. Delivery of a replacement part will take several weeks. So I am minded to order a new CTS anyway, assuming I can manage to get at the old one and remove it safely.

[This message has been edited by br1anstorm (edited 02-19-2025).]

Vintage-Nut FEB 19, 07:23 PM

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So I am minded to order a new CTS anyway, assuming I can manage to get at the old one and remove it safely.



Good man, and yes - I bet you can...
1985 Fiero GT FEB 19, 08:15 PM

quote
Originally posted by br1anstorm:


To answer @Patrick's points - yes, usually, normal starting requires minimal gas pedal. So the current situation required more than what was usual. But it hasn't previously been an issue.




Starting should never, ever require you to touch the gas pedal, except in the one situation where it has gotten flooded (cranking for a long time without it catching in really cold weather), and pressing the gas pedal down will disengage the fuel injectors, allowing it to clear the flood. These are not carbureted, and do not require any throttle input to start, as idle/start air and fuel are entirely controlled by the ECM, if it does, that would indicate a problem.
Vintage-Nut FEB 19, 09:45 PM

quote
But it hasn't previously been an issue

1985 Fiero GT FEB 19, 11:57 PM

quote
Originally posted by Vintage-Nut:

But it hasn't previously been an issue



Yes, but that could point to a more complex solution because the "problem" has more symptoms, one of which is that it requires throttle to start, which is not how it is supposed to work, hence is an issue, whether it is annoying enough to look into and fix, or not, it is technically an abnormal issue, that adds a symptom to any potential fixes. All of the symptoms (difficulty starting, rough idle, backfire, stalling, etc.) could also describe things like a failed TPS sensor, and it is possible for sensors to fail partially without triggering a code, which also might make it need throttle to start initially, whereas I don't see the coolant temp sensor doing anything that would require you to add throttle on startup.

[This message has been edited by 1985 Fiero GT (edited 02-20-2025).]