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I have a 1993 Chevrolet C1500 with a TBI 5.7/4L60. There is a variety of strange things going on with this truck, so I thought I would give this forum a try. A couple years ago, I rebuilt the engine and kept everything bone stock. After installing the rebuilt engine, I started immediately having issues with the truck flooding out and not wanting to run well. I eventually came to the conclusion that the coolant temp sensor was bad. I replaced the sensor, and the truck ran absolutely perfect from that point forward. However, a few months ago, it started having new issues. The first problem was that the engine would turn over, but not start. I determined that it was injecting fuel and getting air, so the issue was no spark. It is also worth mentioning that when I would crank the engine, the tach on the dash would go up to 5k rpms and then twitch around (the truck was not running). I replaced the whole distributor, the plug wires, and coil. The truck then would start, but would not run well at all. The tach would still twitch at times, and the truck was now flooding out. Even while driving down the road at 60-70mph, it would sometimes act like it was about to stall out. I ran the codes on the ecm and got a coolant temp sensor code as well as a knock sensor code (the knock sensor code has always been on, but the engine used to run perfectly regardless. If anyone has an idea as to why this code would be there, I would appreciate any help). I replaced the coolant temp sensor, and the problem continued (the code never went away). I have now replaced the coolant temp sensor 3-4 times. It has not helped. Additional parts that have been replaced are the injectors, coil, air idle control, fuel pressure regulator, and throttle position sensor. As of right now, the truck will start fine when it is stone cold, but then nearly stall out when you give it throttle. Once the engine is warm, it cannot start without flooding. However, the engine seems to run alright after startup when warm. I am completely baffled by this truck, and I know this is a very long and confusing issue. I would greatly appreciate any help. Thanks
 

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I'm wondering if you don't have a bad wire or ground at your ECM which might be causing the weird tach issues as well. I would also look at the O2 sensor being an issue. On my 95' I had some weird issues with it running really rich and running bad when warm. Turned out to be my O2 sensor, once the ECM started running in closed loop it would start stuttering and running really really rich. (Enough to make your eyes and nose watery). I was able to use an older GM OBD1 scanner and watch my sensors with it. I was able to see that as soon as it started using the O2 readings was when the problem got worse. You might try unplugging the O2 sensor and seeing if it makes any difference since it should keep the ECM in open loop mode and ignor the O2 readings and some other sensors.
 

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Discussion Starter #3
I'm wondering if you don't have a bad wire or ground at your ECM which might be causing the weird tach issues as well. I would also look at the O2 sensor being an issue. On my 95' I had some weird issues with it running really rich and running bad when warm. Turned out to be my O2 sensor, once the ECM started running in closed loop it would start stuttering and running really really rich. (Enough to make your eyes and nose watery). I was able to use an older GM OBD1 scanner and watch my sensors with it. I was able to see that as soon as it started using the O2 readings was when the problem got worse. You might try unplugging the O2 sensor and seeing if it makes any difference since it should keep the ECM in open loop mode and ignor the O2 readings and some other sensors.
I will definitely try that. Out of curiosity, where did you get a GM obd 1 scanner? I can’t find anything that will scan this truck, and I’ve been sticking to the paper clip method. Thanks
 

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Out of curiosity, where did you get a GM obd 1 scanner?
You can go on FleeBay and search for a Snap On MT2500 (red brick) or comparable. Another avenue is using a laptop, TunerPro RT, and an ALDL/USB cable for a total cost of ~$60.

Yes, check your grounds, for the ECM, at the T-Stat. When you rebuilt the engine did you remember to install the ground strap from the right side head to the firewall? Which CTS are you replacing, the 2-wire one by the T-Stat or the 1-wire one in the left side head between cylinders 1 & 3? Is your Knock Sensor still hooked up? That wire is prone to heat and corrosion being down by the starter.

It is also worth mentioning that when I would crank the engine, the tach on the dash would go up to 5k rpms and then twitch around (the truck was not running).
That is normally an indication your ICM is wigging out. What brand distributor did you use? I've had a Spectra Premium waste its ICM in less than 2000 miles, and a Pertronix Flame Thrower coil go out about the same time, in my 88 5.7L. I sprayed a little water on the coil body while idling and it almost died. I'm not sure if the coil took out the ICM but after replacing them with quality AC Delco parts, I've had no related issues since.
 

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Discussion Starter #5
You can go on FleeBay and search for a Snap On MT2500 (red brick) or comparable. Another avenue is using a laptop, TunerPro RT, and an ALDL/USB cable for a total cost of ~$60.

Yes, check your grounds, for the ECM, at the T-Stat. When you rebuilt the engine did you remember to install the ground strap from the right side head to the firewall? Which CTS are you replacing, the 2-wire one by the T-Stat or the 1-wire one in the left side head between cylinders 1 & 3? Is your Knock Sensor still hooked up? That wire is prone to heat and corrosion being down by the starter.


That is normally an indication your ICM is wigging out. What brand distributor did you use? I've had a Spectra Premium waste its ICM in less than 2000 miles, and a Pertronix Flame Thrower coil go out about the same time, in my 88 5.7L. I sprayed a little water on the coil body while idling and it almost died. I'm not sure if the coil took out the ICM but after replacing them with quality AC Delco parts, I've had no related issues since.
I had no idea that there were two coolant temp sensors. The one that I’ve been messing with is next to the thermostat. I’ll check on that first because the code that the truck is throwing indicates that the CTS is bad. Thanks for the info
 

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The one that I’ve been messing with is next to the thermostat.
Sorry for not specifying, the one by the T-Stat is the one that feeds the ECM, the one on the head strictly feeds the gauge.
 

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I borrowed an OBD1 scanner from a friend, im not sure where he got it though, it said Auto X-Ray on it.
 

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2nd on crispyt suggestion it’s related to going into closed loop.
you can jump pins on ALDL and cel flash will tell you when ECM goes into closed loop if trying to confirm. I think pins a&b. Google GM ALDL closed loop and you‘ll find it.
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Also, when was the last time you checked your thermostat?
 

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The truck is running perfectly again. I felt a bit stupid once I figured out what it was. All I did to fix it was put electrical grease on the Collant Temp Sensor connector. Even though the connector looked fine, I guess it wasn’t making a good connection. Sometimes the biggest problems require the smallest solutions
 

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That's probably not a long term fix, clean, or replace the connector and you'll be happy longer.
 
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