1987 Chevrolet 350 tbi high idle

[iamtherealJayy]

I figure this has been discussed before but I haven’t found a definite answer, I’ve got a 1987 Chevrolet V20 with a 350 tbi paired to a sm465, and it idles around 2000 on a cold start and comes down to around 1200-1400, I’ve never got it to get below 1000. If I’m rolling to a stop and put it in neutral it shoots back up to around 2000 and eventually comes back down. If I put it in low and start rolling and hit the brakes I can get it down to around 800 before the truck starts shaking and push the clutch in and it violently shoots back up the rpm. I’ve replaced the map sensor, engine coolant temp sensor, and throttle position sensor so far with no significant changes. I’ve sprayed sea foam throttle body cleaner around the throttle body and vacuum lines with no increase in rpm, so no huge vacuum leaks. I’ve been daily driving the truck for over a year now and I’m just trying to get a little better fuel mileage from it and I’m thinking if the idle is a little lower it’ll help a slight bit. Currently gets around 8-9 mpg

 

[gmbellew]

I'd be looking into the IAC, it controls idle. you could check the base idle first with the iac plugged. it could be that somebody prior messed with it and the IAC is actually good.

 

[iamtherealJayy]

What do you mean check it plugged? I’ve read some stuff about unplugging things and seeing if something changes but I haven’t gotten to the iac

 

[gmbellew]

first try resetting the IAC. turn ign to on for 5 seconds, turn ign to off for 10 seconds, then start the engine.

I'd also check for vacuum leaks and fix any you find.

if none of that works....

if you can get access to a scanner or ALDLdroid, you can check the IAC counts and see if they are appropriate at idle.

 

[iamtherealJayy]

I just did the ignition on 5 seconds ignition off 10 start and it did nothing sat at 2000 and slowly came down to around 1200-1400. There are no vacuum leaks as I can find. I sprayed majority of a can of sea foam throttle body cleaner around the throttle body with no increases in rpm

 

[gmbellew]

check all the vac lines...don't forget about the evap cannister.

the initial high idle that drops down is normal operation, it is just the RPMs are way too high.

if you have access to a scanner, check IAC counts. if not, you could just replace the IAC (I'd recommend a Delphi) or you need to check base idle (sometimes called minimum idle).

 

[iamtherealJayy]

How would I go about checking the base idle? And the vacuum canister is on the radiator core support correct?

 

[gmbellew]

the evap cannister is driverside on core support.

TBI Idle Speed Adjustment

TBI Idle Speed Adjustment

forum.73-87chevytrucks.com

 

[iamtherealJayy]

Is there any way of checking any of it without a scanner? I don’t want to spend a bunch of money on a scanner tool that im not going to use a lot. I could spend $15-$20 on one no problem but I got on here to ask questions because I was tired of dumping money into sensors that can’t be returned

 

[Rusty Nail]

You could use a paperclip but doubt you'll be able to return it after use.
You could run the codes but you'll want to watch LIVE DATA.
Maybe.

If you like old GM vehicles you'll have to stop farting around and buy ya a BRICK!

They are effing cheap these days. I bought mine on teh ebays.

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[iamtherealJayy]

I’ve done the paper clip in obd1 to read codes before and truck has no stored codes.

 

[Rusty Nail]

Yeah...live data is the only way to watch the IAC.
Check out a Snap-On MT2500.
Think I gave $40 for one.
I thought 'IAC" also when reading your post..there is the reset procedure but sometimes it pays to remove the part and clean the PINTLE.
Reckon it needs adjusting. There are many you tube videos about adjusting IAC PINTLES.
Might could turn it out a little bit - the pintle needs unscrewed yes?
Look into that, man.
Hope this helps!

 

[iamtherealJayy]

I think I could dump $40 into it but the throttle position sensor was $60 and non returnable. I’m just tired of replacing stuff and it not helping, although the map sensor and engine coolant temp sensor helped the running of the truck, it originally would run rough on 87 pump gas and not idle so it ran on 93 majority of the time but now runs fine on 87 just idles too high for my liking. It idles at what I think an average cold start idle should be

 

[Rusty Nail]

Ya know - on second thought- it is imperative the engine be at running temp and out of CLOSED LOOP when manipulating the IAC.
If not, all results downstream will be skewed and I do mean ALL.

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Make certain the thermostat is open when adjusting idle speeds, that cant be overstated. Adjusting the idle correctly takes a long time- it sure as hell aint no 5 minute deal, its gotta run a long time.
This may be the root of your issue, incorrect adjustment procedure.
8)

 

[iamtherealJayy]

I’ve never messed with the idle adjustment other than throttle position sensor, it’s got a 160 thermostat in it. It doesn’t take long to open with it constantly sitting at 1200+ rpm