Hot start issue with 2000 Silverado 1500

[DarkNinjaSG]

Hello! I’m back again!

I am having a problem with my 2000 Silverado 1500 with the 4.8l. Whenever I hot start the engine, it will drop down to ~300rpm for a few seconds then jump back up to its normal idle of ~550. Almost stalls but the engine doesn't vibrate horribly like you'd expect it to for being at such a low rpm. Won't ever get close to stalling again afterwords when I drive it.

I was having intermittent high idle issues that ended up being caused by the IAC and I replaced it. Did it before and still does it after replacing it. The MAF sensor reads consistently and both upstream O2 sensors bounce between 0.1v and 0.9v as they should. Cleaned throttle body already and that didn't help. Spark plugs and plug wires have also been changed (maintenance items). The long term fuel trims for both banks stay below 2%. There also seems to be a weird idle vibration that feels like it might be a misfire every second or so but I don't have any codes showing up (for either issue).

Any help would be appreciated!

 

[ORVietVet]

Let the engine get cold and before starting, unplug the coolant temp sensor. Not the sender, the sensor. What does it do?

 

[DarkNinjaSG]

Let the engine get cold and before starting, unplug the coolant temp sensor. Not the sender, the sensor. What does it do?

I’ll have to check that when I get back home. On a trip right now and am just trying to get ideas of what to check when I get back. Thanks for the suggestion!

 

[DarkNinjaSG]

Let the engine get cold and before starting, unplug the coolant temp sensor. Not the sender, the sensor. What does it do?

Unplugged the sensor after it’s been sitting for a few days and it starts up good. Goes to 800 rpm then drops down to 600.

 

[ORVietVet]

If you can, let it get cold again and then plug back in and restart and see what it does.

 

[DarkNinjaSG]

If you can, let it get cold again and then plug back in and restart and see what it does.

will do

 

[ORVietVet]

Hopefully you paid attention when you unplugged it and looked for any looseness, filth and/or corrosion at the sensor. You want to be sure there is a tight clean fit.

 

[DarkNinjaSG]

Yeah I didn’t see anything around it and the plug was tight. Just started it again and it’s doing something strange now. When I let it warm up to temp the long term fuel trims on both banks went negative. Only after it warmed up though and the rpm dropped to 600. Before that the fuel trims were at 0. Started up fine with it plugged in too.

 

[ORVietVet]

This means the ECM inputs are sensing too rich on fuel and is trying to slow down fuel delivery.

Is this happening with the sensor plugged back in? You said the connection was good but did not say if you plugged back in. If this fuel trim problem happens with sensor plugged in and if does not happen with sensor unplugged, then I say the sensor is suspect. Sounds like O2's are doing their job.

The coolant temp sensor is the choke for fuel injection system.

Talk tomorrow.

 

[DarkNinjaSG]

Sounds good. Forgot to put that I did plug it back in. I’ll unplug it again tomorrow and see if the fuel trim is off with it unplugged. But from what I recall it looked like they were fine.

 

[ORVietVet]

More Information for GM GENUINE 2134333

Use Together w/ Mating Connector PT1798; Mating Connector Use 88987993; CONNECTOR PLASTIC ON PART MAY BE WHITE OR BLACK IN COLOR; 2-WAY MALE

More Information for GM GENUINE PT1798

If the connector design looks like this, use the Genuine GM sensor and the recommended connector.

Do not use generic parts for this.

 

[ORVietVet]

Make sure, when you do the connector, to do a good clean tight connection, solder if you can and heat shrink, or use the connectors and heat the connector to seal out moisture.

 

[ORVietVet]

Keep in mind that with the coolant sensor plugged in, and on a cold engine start, the system is in open loop and it will push more fuel/rich readings and then when the engine warms up enough and the oxygen sensors get warm enough, the system gets in to closed loop and the fuel trims lean down. The coolant sensor is part of the equation for the ECM to get engine temp readings and make adjustments accordingly.

 

[DarkNinjaSG]

Keep in mind that with the coolant sensor plugged in, and on a cold engine start, the system is in open loop and it will push more fuel/rich readings and then when the engine warms up enough and the oxygen sensors get warm enough, the system gets in to closed loop and the fuel trims lean down. The coolant sensor is part of the equation for the ECM to get engine temp readings and make adjustments accordingly.

Sounds good. The connector still feels like it’s sealing to the sensor good. Think it’s still good to reuse the old connector in that case?

Heres what I did this morning:

I started it up this morning with the sensor unplugged and let it sit for like 10 minutes and the ltft never went negative. I then plugged the sensor back in and let it idle for a while again and the ltft on both banks went negative again. With the engine fully warmed up and the sensor unplugged the engine doesn't dip to 300 rpm, but if I plug the sensor back in it does. I disconnected the fuel pressure regulator vacuum line and didn't see any fuel coming out of it (with sensor plugged in). This caused the ltft to get closer to zero cause of the vacuum leak , but when I blocked off the vacuum line it still didn't go as negative as normal. The idle vibration was worse when idling with the sensor unplugged. I'll post some videos of my ultra gauge readings when it's idling with and without the sensor plugged in.

 

[DarkNinjaSG]

Sounds good. The connector still feels like it’s sealing to the sensor good. Think it’s still good to reuse the old connector in that case?

Heres what I did this morning:

I started it up this morning with the sensor unplugged and let it sit for like 10 minutes and the ltft never went negative. I then plugged the sensor back in and let it idle for a while again and the ltft on both banks went negative again. With the engine fully warmed up and the sensor unplugged the engine doesn't dip to 300 rpm, but if I plug the sensor back in it does. I disconnected the fuel pressure regulator vacuum line and didn't see any fuel coming out of it (with sensor plugged in). This caused the ltft to get closer to zero cause of the vacuum leak , but when I blocked off the vacuum line it still didn't go as negative as normal. The idle vibration was worse when idling with the sensor unplugged. I'll post some videos of my ultra gauge readings when it's idling with and without the sensor plugged in.

And I do know it’s probably not the best to start a warm engine with tha coolant sensor unplugged but I didn’t do it for too long

 

[ORVietVet]

Yes, replace the connector with the sensor. GM says to do it, then do it.

 

[DarkNinjaSG]

Alright, will do. Just ordered the parts. I'll post an update when I install them.

 

[ORVietVet]

Hope it takes care of the problems. Make sure to post the fuel trims, cold and hot engine, when the new sensor is installed and plugged in.

 

[DarkNinjaSG]

Will do! Thanks for the help!

 

[DarkNinjaSG]

Installed the new sensor and harness but it still does it. Doesn’t drop quite as low in RPM as it used (only drops to 400 now). Doesn’t affect drivability so I might just leave it for now. The fuel trims seemed to have fixed themselves though. They’re back at +1% for bank 1 and -.78% for bank 2.