350 TBI high idle

[gmbellew]

Just went for a drive and it warmed up and switched to closed loop. With the IAC unplugged it idles like it should. Once it was warmed up it would idle at 1000 rpm in drive completely stopped. When I shifted it into park, it started idling at 1450 rpm and slowly went back down

idle will always jump when you return to park bc the load comes off the engine. it could jump to maybe as high as 950-1000rpm. but should settle down fairly quickly and likely be in the 750rpm range fully warmed on a stock computer chip. in gear idle will be in the 525rpm range, also stock chip. IAC should be 5-30 in gear stock chip. and in park should be close to that IAC range also. all those are for fully warmed up and ac off.

does TPS return to "idle" voltage quickly or does it hang? could be a linkage issue if it returns to idle rpm slowly after revving?

your IAC counts didn't go to 0, did they, with 1000rpm in gear warmed up? bc it really sounds like some sort of vacuum leak...

edit - those coolant temps are too low. you should have a 195deg thermostat in there. cold coolant temps will mean more fuel and a higher idle....I'd be looking at the temp sensor itself or the thermostat to make sure it isn't stuck open.

 

[JACK34]

Turn the idle screw out a little.

 

[Fleetwood]

idle will always jump when you return to park bc the load comes off the engine. it could jump to maybe as hogjbas 950-1000rpm. but should settle down fairly quickly and likely be in the 750rpm range fully warmed on a stock computer chip. in gear idle will be in the 525rpm range, also stock chip. IAC should be 5-30 in gear stock chip. and in park should be close to that IAC range also. all those are for fully warmed up and ac off.

does TPS return to "idle" voltage quickly or does it hang? could be a linkage issue if it returns to idle rpm slowly after revving?

your IAC counts didn't go to 0, did they, with 1000rpm in gear warmed up? bc it really sounds like some sort of vacuum leak...

No IAC has always been really high. You can see on the pictures I posted how high it is. Its at least above 60 all the time.
TPS is good.

 

[Fleetwood]

Turn the idle screw out a little.

I tried that. It worked for a second until the IAC unwound itself and sped things up again

 

[Fleetwood]

It's not necessarily hurting things, but it isn't helping the OP either....

@Fleetwood I assume if you plug off the IAC passage the idle will come down?

If that is the case then it's on the computer side. If the idle is still high with the IAC port plugged, then it's a mechanical issue such as a vacuum leak or miss adjusted throttle stop, stuck EGR ect.

If it's on the computer side pull the IAC valve out clean the seat out in the throttle body and install a new IAC valve.

Make sure the timing is set correctly.

Also don't forget that these engines learn their idle. You need to warm the engine up and cruse at about 40 - 45mph steady for a bit. I think you need something like 5 seconds.

Looking at your read out I'd be looking at that open loop and low coolant temp. If it won't fully warm up, it'll never properly learn the idle. So then a thermostat maybe in order or possibly a temp sensor.

Hopefully some of this is helpful.

I havent had it up to 40 yet so would going faster for a bit help?

 

[AceHanlon]

Sell it. TBI trucks are junk. I sold my 94 it drove me crazy. Get a older truck with a carb on it.

Do you ever get tired of disparaging TBI's that are, in fact, not that bad of a system. You seem to forget how old these TBI's are. They are not that hard to work on.

 

[JACK34]

Do you ever get tired of disparaging TBI's that are, in fact, not that bad of a system. You seem to forget how old these TBI's are. They are not that hard to work on.

No not at all after all the trouble I had with mine that's a hard no!

 

[Fleetwood]

The last high idling drive was the first time it had started since having the battery unplugged for a few hours. I started it a couple times and took it for a cruise at speed. I guess the computer has learned the idle now because it runs better than it ever has.

 

[gmbellew]

The last high idling drive was the first time it had started since having the battery unplugged for a few hours. I started it a couple times and took it for a cruise at speed. I guess the computer has learned the idle now because it runs better than it ever has.

yes, it will eventually learn the iac position. the quick way to do it is to force it closed by jumping A and B on the ALDL with key on. then it knows what fully closed is and where 0 is. if you messed with the idle stop screw, just make sure your IAC counts are in the 5-30ish range warm and at idle (lower end of the range preferred) and that the TPS voltage is still under 0.75 or so at rest (closer to 0.5 the better, just not over 0.9v).

 

[84-C20]

All of what I have seen from @gmbellew I fully agree with and it looks like you are well on track to having it running correctly.

 

[Mango]

@gmbellew I Have some of the same issues , High Idle will kick dows some when i give a good rev in park, In the moarning with block heater on all night starts up with the correct idle drive to work 10 miles park high idle?? I recently replaced the intake manifold gasket, Removed the IAC and blasted it with carb cleaner!!!
Re set the IAC and worked idle speed down test drive back to the old high idle?

 

[Mango]

Figured it out the Cruse control linkage was the culprit!
Checked idle screw and was not touching throttle plate
A quick adjustment and all good now

 

[Frankenchevy]

I messed something up on my 88 burb.

I hooked up my Moates cable for the first time yesterday. I was wanting to check the CTS value of the computer vs what my gauge read. Being impatient and wanting it to get up to temperature quicker, I held it at about 1800rpm in park for a couple minutes. Now it idles high in park/neutral.

When you first start it, it’ll high idle at 975-1000 which seems normal. Take it for a drive and put in back in park and it wants to idle at 1700 rpm. You can hear the IAC open up and suck air in park big time.

I jumped AB to reset IAC until it was fully closed, then unplugged it, fired up the burb and adjusted idle to 450. After that I shut it down, removed the AB jumper, plugged in the IAC and fired it up. Hot idle in neutral was still close to 950 rpms, idle in gear started out at 750, then you could hear the IAC open up and it brought the in gear idle up to 950. Put it back in park, IAC starts to open and it goes up to 1850 again.

TPS is at .71v or 4% at idle. IAC value is in the 140s when it climbs to 1800 rpm in park.

Any ideas? Most of the vacuum hoses are new. Truck didn’t start with these symptoms until I held it at 1750 rpm for a few minutes about 30 seconds after starting it with the Moates cable hooked up.

For now, I’ve reset the IAC again and disconnected both negative battery cables.

 

[gmbellew]

does the TPS show you are back at idle? There is an “idle/off idle” indicator the ECM reports out. Does it show you are at “idle” when it does this? If the IAC isn’t closing to control idle speed, it is probably not an issue with a vacuum hose or a leak.

 

[Frankenchevy]

does the TPS show you are back at idle? There is an “idle/off idle” indicator the ECM reports out. Does it show you are at “idle” when it does this? If the IAC isn’t closing to control idle speed, it is probably not an issue with a vacuum hose or a leak.

Where do you think I’d find that in WinALDL?

Here’s a shot while it’s idling high in park:

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There are obviously many more screens and I did datalog the whole thing.