Hi,
It`s a 92 LT1 6 speed.

I think I need to replace the ECM but thought I would just check on here first in case there is any other possible cause.

The digital coolant gauge now only reads about 90 F when fully warm (T-stat open and analogue gauge half way) - but when stone cold in the morning in 35 F temps it still displays 45-50 F - when it would normally have read "LO"

The car won`t idle when warm (it either sputters out completely or keeps revving at 2000 RPM) and you can`t give it any more than light gas or it will bog down.

Every 10 mins or so the digital gauge will read properly (ie. 190-210 F) when warm and the car will run fine again for a few mins.

I read some posts on here and replaced the front coolant sensor (near the water pump) - but it is still acting exactly the same.

So I disconnected the front coolant sensor connector again to see if the coolant gauge would read "LO" when not connected at all - but it still read around 45 F.

I`ve checked the continuity of the 2 wires from the connector end up to the ECM end and it all checks out ok - I wiggled the connectors and harness about a bit as well to make sure.

So, if it`s not the sensor, the connectors or the wiring anywhere up to the ECM, is the ECM the only explanation?

Could bad wiring to the digital gauge cause interference or feedback to the ECM and cause this?

I just wanted to make sure that there was nothing else to realistically check before replacing or repairing the ECM.

Thanks for reading.

Cheers

Dom.

 

I'd have to double check but I don't think you get a "lo" on coolant temp., only on oil temp.
I've never seen Lo on my 92, always reads in the 40's.

Disconnecting the sensor (or even jumping it to ground) won't show you anything.

92's have been said to have ground problems on the web. I've never verified it but have cleaned up every ground on mine.

Hopefully you have an FSM.

And you absolutely have to pull the codes.
Your idle and gauge problem may be related but maybe not. And without pulling codes, everything is just a guess.

 

The Digital readout is from the Coolant Temp Sensor which the ECM uses to control fuel delivery and fans (non a/c). If the reference - Yellow/5 volts - is shorted to ground, the ECM sees 300 degrees and sets a Code 14. If it's open, the ECM sees -38 degrees and sets a 15. Any wackiness in between isn't monitored and there are no other Codes for this Sensor, so what you're Digital is showing is being interpreted by the ECM as normal.

You can verify the 5 volt reference with a DVM and you can generate the Codes by disconnecting the harness (with it running) which should set the 15 and then grounding it which should set the 14. You might then spend 12 Bucks on a new one and simply plug it into the harness to see what it reads. It should be ambient or close to it. If that works, R & R the old one. If that doesn't fix it, it probably needs a new ECM, but you might scan it or at least ohm out the signal wire between the Connector and ECM pin and of course make sure the Connector is in decent shape. Any resistance will skew the signal and the fueling will be incorrect meaning it's not going to run - or run very well.

 

Replace the sensor that is screwed into the waterpump.
Its not working correctly.

 

Hi thanks for all the replies - sorry this is a bit long but I have to explain it all - also sorry it`s taken a while to post back but I wanted to make some progress first - and then there`s the time difference.

Anyway....

First off I checked for codes and there`s non - except for the "C12" which I read it is meant to display when there are no errors.

I also checked with another sensor and within about 10 degrees all 3 read similar.

I was reading through SunCr`s post and thought I had better checke everything gain with the engine running - b4 I did it all with the ignition on but not the engine running - not least because it won`t really run properly but sometimes will idle - although at 2000 RPM.

Well, I double checked and eveything is still the same

I unplugged the sensor connector and it still wouldn`t read "LO" - just 45F again. I grounded the yellow wire - and it still wouldnt read 300F or "Hi" - just 95 F again.

Then whilst I was working with the wires, I saw the hood starting to come down! I dived out of the way and got out in time before it crashed down. Unfortunately the ECM did not! Balanced on top of the hood latch it got a pretty hefty whack! - and wuld u believe, the revs returned to normal and the temp display started reading 195F as normal

I disconnected the sensor wire again - and this time it did display "LO". However 195F is the max I could get it to even with the analogue gauge starting to get well towards the 3/4 mark - so something was still wrong - and obviously the cooling fan didint come in when it normally would have.

Just to check I turned off and checked for codes again - and got the code 15 - low coolant temp - whihc u would expect disconnecting the sensor connector.

Anyway I went back to restart - and ofc the coolant temp was back to reading just 90F again (only about 5 mins later) - and the idle was back up to 2000 RPM.

I wiggled the whole ECM about to c if the hood crashing down temporarily disturbed a wire but couldnt get it back working again.

So, as a last resort I gave the ECM another "not so gentle" whack with a rubber mallet and everything started working again - normal idle and normal temps. I was holding the ECM in my hand whilst doing this so the vibratiuon can`t have disturbed anything else in the engine compartment - so I think it now has to be the ECM

Whenever I turn the car off and then back on again - the temp gauge is all wrong and the idle at 2000 rpm again - but I can repeat the process and give the ECM a whack and it all works again until I turn the engine off.

Maybe seems like a dry joint or something ?

I think repairing the ECM myself is a bit too much for me - so its a choice of sending it off or getting a used one. I ran the part number through google and it comes up with the same number for 93 LT1 F body`s - so will they work as well?

Also do I need one for a manual car or can I just swap my chip over from my ECM ?

One last thing - there are a few numbers on the ECM - at the top it says Serv. No. 16159278. Underneath it says 863861. Next to that is M212601423 and at the bottom is 16173861. Which is the critical number for reference ? I used the top one to get the match from F-body`s and stuff....

Thanks again for your help - I will post back for sure and let u know what happens.

Dom.

 

Sounds like a weak solder joint or cracked board. Sometimes you can confirm by aiming a hair dryer at it - though that usually only works if there's a "hot" problem (yours seems to be hot and cold).

 

Sorry to tell you that your car will be down as long as it takes to get it repaired.
The 278 is manufacturer discontinued and scarcer than hens teeth.

You can try to find a used one from one of the recylers to get you up and running.

Good Luck!

 

Hi,

It was the ECM.

I bought a used one and it is working again now

Sure was a wierd problem though!

Thnaks for everyone`s info

Dom.

 

Oddly enough, just started having a very similar problem on my 89 C4. The car warms up and runs fine, but the gauge readout on the dash will read erratically between 130 and 170 degrees. I'm going to start with replacing the sensor and then go from there. I hope it is not the ECU.

 

'89 is different and the Temp Display is from a sender in the right head between #6 and #8. Disconnected, the Display will show LO and grounding the Pigtail will get you 300. On the OP's car this sender controls the Analog gage which your '89 doesn't have, and the '89 does not display the Coolant Temp Signal which on this Year is screwed into the front of the Intake. Anyway, sounds like your Vette has a bad or intermittent connection or the Display needs to be replaced, though it might still be the Sender itself. Disconnect/ground and see what it shows. Make sure the Connector is in decent shape.