Weird Idle issue

[92K5]

Hello all and happy thanksgiving to everyone

Last night while driving home from the holiday festivities my service engine soon light came on. The truck was running great still so I didn't think too much of it. This morning, I started to truck up to move it around the yard and it would not dip below 1100 Rpms while idling. I sat there for 3 minutes waiting for it to kick down and I never did... I took it for a short drive to get it to operating temp and the SES light came back on again and this time it started to idle a little rough but nothing major. It ran fine while accelerating but would idle lower when stopped.

I have read some stuff online that points to the IAC or the EGR valve. I have not tried to check the codes yet because I haven't been outside with the truck since this morning.

I read that there is a way to check the IAC valve with it being out of the throttle body, so I think I may try that.

By the way, the truck in question is a 1992 K5 blazer that is 100% stock with 189k on it. It has the 5.7l/700r4 in it.

I did clean the TB and replace some hoses that were in rough shape, but I didn't clean the IAC.

There is also a distinct whine sound that you can hear when the truck is running, it sounds like a power steering issue but the pump is fine and has full fluid and doesn't fluctuate at all when steering or driving occurs its just a constant whine. I don't know if the idle issue and this are related, but I just want to give the most info possible.


Thanks,

John

 

[someotherguy]

Before jacking with the IAC...check the codes.

My bet will be you have a CTS going bad.

Richard

 

[92K5]

In reference to a coolant temp sensor? I had to replace both of them in the least two years... Does the coolant temp mess with the idle?

 

[someotherguy]

The one on the intake manifold absolutely does. It's how the computer knows what to set the fuel mixture at. Cold engine, warm engine, hot engine...throughout the entire operating temperature range.

If you replaced that one not too long ago then it is -probably- still OK; double-check the harness grounds near it on the intake. There are some attached at the thermostat housing and next to it on the intake. If these aren't in top notch shape, fix 'em.

Check the codes.

Richard

 

[92K5]

Richard,

I just pulled codes and got a code 15 which indicates a bum temp sensor. I had no idea that it would cause this issue, I really appreciate your help though. I replaced the top one 3 years ago according to my receipt log. I'll replace it either today or tomorrow and report what happens.

 

[someotherguy]

CTS's tend to go bad very gradually over the years, delivering a false cold reading due to increased resistance. The computer accepts this as valid data as it has no other sensor to reference against. They can go bad and cause a too-rich mixture to the point of stalling the engine and still never set a code. When you DO have one that sets a code, it's usually because it has shorted out - closed or open. As long as the mixture is still sufficient for the engine to run, and the data coming from it doesn't change within a given time period, the computer knows it's most likely gone bad and will set the code.

Replace that sucker and all should be well. Buy a good one - Delphi or AC Delco.

Richard

 

[92K5]

Just an update,

I replaced the coolant sensor, cleaned the ground, unhooked the battery to reset the computer...

Started it up, and the idle stayed around 1000rpm (i assume because it's cold today in Michigan), then gradually fell off. I drove it around for a solid 30 minutes and there was no issue with an SES light.

I pulled codes and didn't get any aside from code 12. So for now I think my issue is resolved.

However, the whine under acceleration is still an issue which is pretty annoying. I don't even know where to begin looking for that.

Thanks again for your help Richard.

John