1988 K1500 5.7l won't idle

[cbrown04]

The truck runs for 30 seconds to a minute beautifully then begins idling rough and shuts off. It will restart however sometimes takes a bit. NOT a problem in cold weather, only above 80 degrees outside! It's driven weekly. The parts cannon has been lit and fired by me and 2 different mechanics. Below is what has been done/replaced over the last 3 years:
- new distributor
- new plugs/wires
- new fuel filter
- fuel pressure is good
- IAC valve
- coolant temp sensor
- throttle body rebuild kit
- new injectors
- vacuum lines
- PCV
- MAP sensor
- O2 sensor
- ECM swap (then original put back)

Sensor readings appear normal when measuring resistance but O2 sensor seems kind of all over the place. We're at a loss! Thank you for your help!

 

[1990Z71Swede]

There are two coolant temp sensors, one for the gauge and one for engine management. I assume you don't have an OBD-1 Scanner, so before starting to look for one, you could try Measuring that sensor with a Multimeter, resistance should change gradually as the engine heats up.

Sounds like it is loading up with fuel, that is maybe the reason the 02 gets crazy. If the sensor measures fine, maybe there is a wiring issue for that signal to the ECM. If so your Scan tool hunting season just started

I'm not a TBI expert, the above is Generic troubleshooting that applies to most computer controlled engines, even the two temp sensors part is actually quite common.

 

[cbrown04]

I had no idea there were 2 temp sensors. Also, where would you find an OBD1 scan tool? Seems like the one GM dealers used doesn't exist anymore. Thank you!

 

[1990Z71Swede]

I had no idea there were 2 temp sensors. Also, where would you find an OBD1 scan tool? Seems like the one GM dealers used doesn't exist anymore. Thank you!

On a TBI truck you have one on the driver side cylinder head, that is for the Gauge. The ECM sensor is on the inlet manifold. They are different, so you cant swap them.

I will let @Schurkey answer the Scan tool question as I´m a little curious myself. At least curious of what to get in the year of 2023 . I think he uses an Snap On Scanner, A pro style type of deal. There is also a GMT400 Community tool floating around that you maybe can borrow.

In the meantime there is this:

Obd 1 scanner options

I’ve got a 95 gmc k1500 with a random reoccurring code 32 (a common goose chase on these guys according to our identifix program at work and this site) Point is I’d like to get a scanner for data retrieval. I have a moates aldl cable but I don’t like to use my laptop. I’d rather use a...

www.gmt400.com

 

[mojokoly]

I am also facing this situation. I did the same and it didn't work.

trap the cat​

 

[cbrown04]

So Monday, it was 60-65 degrees outside. I had driven the truck to work and back no problem (as is the usual for cooler weather). I pulled next to my other car to jump it off (you can tell I have a yard full of gems) and after attaching the jumper cables and wiggling to ensure a good connection...it's almost as if I had "triggered" the idling problem. It did it's usual rough idling up and down, up and down, then stall. Perhaps there is an electrical problem and I upset it?

It has the original harness from 1988 I'm assuming. Does anyone know what the ground strap is for...from the firewall to the frame on the passenger side?

 

[Schurkey]

I will let @Schurkey answer the Scan tool question as I´m a little curious myself. At least curious of what to get in the year of 2023 . I think he uses an Snap On Scanner, A pro style type of deal.

I used a Snap-on MTG2500 for about two decades. Strongly related to the MT2500, which is the "community" scan tool. When that died, I upgraded two generations newer, to a Solus Pro. My MTG2500 had 6.2 software, good to 2005. The Solus Pro happens to have 8.2 software, good to 2007. My newest vehicle is a 2003, so that's good enough for me EXCEPT for auto-bleeding the ABS on my '97 K2500. Neither tool will auto-bleed the '97 ABS. They'll report on ABS sensors, ABS codes, etc, but not auto-bleed the '97. Both would auto-bleed my '03 Trailblazer, though. I'm told that auto-bleed of earlier ABS functionality was added in a major software upgrade in 2010; I know the 14.2 software will auto-bleed my '97.

after attaching the jumper cables and wiggling to ensure a good connection...it's almost as if I had "triggered" the idling problem. It did it's usual rough idling up and down, up and down, then stall. Perhaps there is an electrical problem and I upset it?

Clean and tighten both battery cables. Assure that the cables--including any small wires attached to them--have good connections, no corrosion and not loose.

It has the original harness from 1988 I'm assuming. Does anyone know what the ground strap is for...from the firewall to the frame on the passenger side?

Grounds the firewall to the frame, which is then grounded to the engine, which is then grounded to the battery.

 

[cbrown04]

Update on the 1988 silverado not idling correctly....after two different mechanics and $500 in parts, I cleaned the battery terminals and battery posts with a wire brush and she's been running like a champ ever since. I thought I had performed a voltage drop across the terminal and post with my meter but must have thought the reading wasn't significant. How embarrassing!