Question regarding high(er) idle

[kir_kenix]

I know this is a long post and I apologize in advance. I wanted to give enough info so I could rule out as many issues as possible. If anybody has any idea why my idle has increased 100ish rpms or point me in the direction I should investigate I would appreciate it.

I have been driving and logging the data on my 1997 Chevy S10 for over 70,000+ miles now, and thought I was pretty familiar with it. I noticed several tanks ago that my idle had increased from 925-950 rpms to around 1,000-1,025 rpms on a fully warmed up engine (whether coasting or in neutral, clutch out at a stop). This could have been going on for a long time, but I rarely idle for any length of time in closed loop, and if I coast with the engine on I don't usually pay attention.

I still didn't really think it was a big deal because the weather has been cold, and I've been running high blends of ethanol. I have read before that running higher blends of ethanol can, in some vehicles, cause a slightly increased idle because the CPU compensates in different/strange ways. I don't know if this is actually true, but I just assumed it was by what my Scangauge reports.

However, I happened to find some baseline information from when I first set up my Scangauge April, 2011 with nearly identical temperature to today. With identical (within 2*F, within 50ft elevation) outside temperature, running e-10, I noted that my idle was averaging 925 rpms with the engine at 198*F and I was burning .4 gallons per hour. Today with the truck (e-10 in the tank again) at 198-199*F I was running an idle of around 1025 rpms and burning .4-.5 gph.

On my baseline 3 years ago I had recently replaced the plugs, wires, 02 sensors, coils, fuel filter, air filter, TPS sensor, and ran some Seafoam through the engine via vacuum line. I just recently swapped out plugs and performed another Seafoam treatment at the end of March. Tests are about as identical as I can make them 3 years apart aside from the wear and tear on the engine and components I installed. I do now have a WAI, but intake air temperatures were within 10*F on both tests.

My first thought is that one of my 02 sensors is probably going wonky. No CEL yet, but one or both could be getting there. Before I go and put in new O2 sensors is there something else I should be looking at? I did not log the TPS reading in 2011 unfortunately, but it is reading 1-2 right now. I have an OBDII diagnostic tool that allows me to capture a "snapshot", but I really don't know what I should be looking for specifically, because I don't have anything to compare it to.

Anybody experience anything like this in the past? Any likely culprits that I may be overlooking? 100ish rpms probably isn't the biggest deal in the world, but it is causing the truck to burn that much more fuel. If its something as simple as tossing on a new TPS or 02 sensors I would be fine with that, but hate to swap them out if there is something else I should be looking for.

[Frank Lee]

One time I had higher than usual idle speeds caused by a failed engine-to-chassis ground strap. Another, a gooped-up IACV.

[Hersbird]

Vacuum leak somewhere will cause high idle and sometimes no other problems. The air just is bypassing the throttle but the fuel injection just adds the gas necessary to keep the mixture right.

[bestclimb]

Well the first two responses cover what I would start with (vac leak and idle air control)

[kir_kenix]

I will trace all of the vacuum lines tonight. I did do a "once over" under the hood yesterday...checked the brake booster because I had it disconnected a few weeks ago.

Hadn't thought of the ground problem Frank! Good excuse to to the big 3 upgrade, even if that is not the problem.

[kir_kenix]

I traced all the vacuum lines last night...sprayed water from a squirt bottle to see if it was getting sucked in anywhere. Monday I think I'll upgrade all of my grounds to see if maybe I have a bad connection somewhere. If that doesn't do it, I guess I'll look at the IACV.

[arcosine]

Maybe your pvc valve?

[baldlobo]

i'd ohm the coolent sensor. to me it sounds like your not exiting cold start.

idle for most cars/trucks is normally 650-750

[kir_kenix]

Problem went away. Thinking I have a bad ground somewhere or the IACV is on the fritz. Idling around 900 again when I coast with the engine on. I'll look into it again if/when the high idle returns.

In positive news I just returned my first ever 40mpg tank! I think they will be few and far between because of the effort it took, but at least I know it is doable.