Check Engine Light query

[mr_s]

I have a 1997 K2500 Suburban, and when I go to start it all of the lights in the cluster light up momentarily and then all go out. The 'Check Engine' light doesn't come on at this point (the bulb is good, I can trigger it by unplugging various sensors, so I know the light itself is good). Just wondering if this is normal, or could it point to an ECM fault (I have another issue with it not going into closed loop, so it won't pass emissions here in the UK) and am beginning to suspect the ECM is at fault.

Anyone out there with a 96/97 vehicle, ideally running a Vortec 5.7 that could check their truck for me please?

 

[b454rat]

My 96 lights come on when turn the key. Goes out once started, it does come on when there is a fault...

 

[df2x4]

I can check my Suburban for you when I leave here in a few minutes. It's a '97 GMC K1500 with the 5.7L.

 

[mr_s]

Thanks @df2x4 and @b454rat, but it turns out it's just me being a dumbass... The Check Engine Light comes on and goes out as it should. However, if you turn the engine off and turn the key back to 'on' then the light doesn't reappear. If you take the key fully out, leave it a few seconds then turn it back on, it lights up again as it should.

Back to the drawing board as to what my open loop issue is...

 

[df2x4]

Glad you got it figured out. Just to clarify I did check and my suburban did illuminate the CEL at key-on. Didn't check after shutting it off like you described but I'd assume what you saw was probably normal.

 

[Schurkey]

I have a 1997 K2500 Suburban...
...not going into closed loop, so it won't pass emissions here in the UK) and am beginning to suspect the ECM is at fault.

Back to the drawing board as to what my open loop issue is...

What does the scan tool say?

 

[mr_s]

@Schurkey The scan tool reports no fault codes, all it says is 'Open Loop due to insufficient temperature'. The scan tool reports coolant temp as being read correctly (replaced the thermostat and ECT sensor just in case, as I have no idea how old they are), o2 sensors are brand new and reporting voltage (voltage changes when throttle is applied, so they are working - just not being read by the ECM due to being in open loop).

Things I've tried so far:

Changed ECT and thermostat - no change

Heating up the ECT sensor more quickly by plugging my old one in and heating with a hot air gun (to see if there was an issue with how long it was taking to warm up), heated up to over 200F within about 1 minute, but the problem persists.

My next thought is to unpin the ECT wire from the ECM (Pin 23 red connector) - just to see if the scan tool is getting the ECT from elsewhere. I suspect this will do the same as disconnecting the sensor, but I'm running out of ideas...

My only other thought is that I've got a bad ECM and it's time for a 0411 swap!

 

[AK49BWL]

The ECM doesn't get its ECT reading from anywhere except the sensor on the thermostat housing.. There are two ECT sensors on these.. One in the head that feeds the gauge, the other on the thermostat housing on the intake that feeds the ECM... You're checking the right one? Just a thought. It should switch to closed loop at 131*F or so, unless it's colder outside then it'll hold off to around 145.

Have you by chance tried disconnecting the battery for a while then trying it again? I can't think of anything else that would keep the system from entering closed loop.

 

[mr_s]

@AK49BWL Yep, I figured that would be the case, but I'm running out of ideas...! Not had much chance to work on her over Christmas, but I may get a few hours this weekend.

I'm definitely checking the right sensor - I removed the connector to the other one (in the block) and the dash gauge stopped reading. I disconnected the one by the thermostat and my reader showed -40. I grounded it and it read 150 (celcius as that's what I had my reader set to at the time). Both when disconnected and when grounded, it threw up a check engine light and associated code.

I had disconnected the battery for a while (istr it was a couple of days, but I could be wrong), maybe I'll disconnect it now and leave it overnight again, just to check.

Thanks for the reply, it's all helpful - even if it's so I cam double and triple check I'm not going insane!!

 

[mr_s]

Just a quick update on this (having finally found a few minutes to work on it!), I removed pin 23 from the red connector on the ECU, and as predicted it had the same effect as unplugging the ECT sensor. I've now disconnected the battery and will leave it a couple of days, see if it helps. If not, I guess I'm looking at a replacement ECU, or a 0411 swap...