Engine Cutting Out

**srgould41** · 10-17-2021

Hi all,

Trying to troubleshoot an issue before shot gunning parts. The question isn't the TBI, per say, but related. I'm happy to repost in a different forum if needed.

I have a 1990 Police 5.7 in an FJ40 trail vehicle. This year it has only been out of the garage twice. A year ago I posted about the engine stalling. I had thought it was an injector issue, but after lots of hunting I determined it was a wiring issue. The 4 wire plug that plugs into the distributor ICM was missing insulation and the bare wires were corroded. The high resistance led to lots of voltage drop and the return sense wire was not making a connection at all triggering a fault code. I replaced the wire harness and it drove great afterward with no more fault code for the sense signal. I have only driven it maybe four or fine days since.

Yesterday I went on a four wheeling trip. The whole trip the check engine light kept coming on and going off without performance degradation. I connected Tuner Pro today and the fault code is for a low coolant temp. I don't believe this is related to my current problem, but does need to be solved. While monitoring with Tuner Pro today the temp seemed fine so I am guessing it's intermittent, either a failing sensor or bad wire.

Now to the larger problem, while driving home the engine lost power several times for a few seconds and puffs of black smoke came out the exhaust. The worst one was for several seconds, but the engine came back on as I was drifting onto the shoulder. It happened a total of 6 times in the two hour drive before I got home.

First question just to make sure, a low coolant temp error would not cause the engine to cut out, right? Seems ignition related to me.

My thought on the engine cutting out issue is related to my previous distributor wiring issue. I suspect the low voltage/high resistance may have degraded the ICM.

What are your thoughts and suggestions?

Thanks

**tayto** · 10-18-2021

Was it a code 15? do you have a known good coolant temp gauge to confirm the engine was up to operating temp? sounds like the CTS is bad or there is some wiring fault on that circuit. datalogging should tell you if that sensor is doing its job or not. i have seen sensors fail and cause the vehicle to run very rich, because it thinks the coolant temp is cold.

**srgould41** · 10-18-2021

Yes, code 15. I do have a temp gauge and it was reading normal. I am going to test the wires, end to end, before replacing the CTS. I ordered a replacement ICM, just to be safe. My spare is pretty old and I would not trust it for more than getting off a trail.

**NomakeWan** · 10-18-2021

The temp gauge is driven by a different sensor than the one the ECM/PCM uses, so the dash gauge working doesn't mean the ECM is getting the proper temp. In fact, the Code 15 blatantly states that it is not receiving any signal whatsoever from the CTS, and has gone into a fallback mode. I'm not certain about the L98, but if it's anything like the later LT1, the fallback mode for complete CTS failure is...sadly, nothing other than throwing the code and, on an automatic, applying the TCC early. You should find the FSM for your '90 5.7 (L98?) and check the diagnostic page for DTC 15 to be sure. I only have the 1994-1995 5.7 (LT1) FSM, so that's all I can go by. But if in 1994-1995 the only thing that happens is throwing a code and applying TCC early, I would assume 1990 would be exactly the same. Doesn't make sense for GM to do something more advanced on a far more primitive computer system.

However, you are correct that a failed-low-temp CTS should not cause the engine to not run at all. It will cause it to run pig rich, and will cause it to never enter closed loop operation, but it should still run. If it does not run, something else is going on.

I agree that a total cutout sounds like ignition. Check all connections between distributor, coil, ICM, and ECM thoroughly. Again, the FSM can actually help you with the kinds of readings you can get with your multimeter to check each of these components. Or you could just throw parts at it, like replacing the ICM. But 100% start with the wiring since you already know that was a problem before. And if that CTS is any indication, it sounds like your wiring in general is suspect. Perhaps a rodent decided to have a snack. Best of luck.

**srgould41** · 10-18-2021

Thanks for the reply. True that I have two temp sensors, one for the ECM and one that runs the gauge and electric cooling fan. Part of the problems I have been solving come from using a donor harness. It was in fairly good shape overall, but did have some issues such as the bare wires on the ICM plug. I plan to spend some time this week checking wires and voltages again and doing some data logging to see if I can see the intermittent temp failure. Glad to know the CTS would not cause the stalls. Didn't think so, but it's nice for validation.

**JimCT_9C1** · 10-18-2021

Sharing my experience with bad coolant temp sensor (CTS) on my 95 LT1 -
My CTS failed low (constant, not intermittent), but not failed to the point that a code was set.
Flooded out the engine - wouldn't run when warm. Could start it with flood clearing mode (throttle to floor, no fuel) but would flood out after running a few seconds. Pulled the CTS connector to cause a hard failure. Started and ran well enough with code set and no CTS to get me home.

Testing without the sensor in play (and thus wiring as well) may help narrow things down.
Note my experience was with LT1, so others can chime in on any differences to your application.

Hope this helps -
Jim