TBI slow to return to idle

[chris989]

Hello everyone,

I have searched the forum and could not find the answer i need.

I have a 89 suburban, 5.7 TBI automatic. I have replaced every sensor (ACI, throttle position, egr, ETC.) and my truck still idles high and is slow to return to idle after removing it from gear or tapping the gas. It will return after 10 seconds but this makes forward to reverse a real pain. I have search for a vacuum leak with carb cleaner and starting fluid. It does not appear to have any leaks.

The truck does occasionally jump between 2nd and 3rd gear a few times if the throttle is just right when i accelerate very slowly. Makes me think I have a leak but can not find it.

The motor runs smooth as silk and starts instantly every time. When i was looking for burbs I noticed a few others running this way also but i thought they where in need of a sensor. Now that i bought one i realized maybe it is an issue with early model TBI???

Any help would be great,

Thanks
Jon

[thirdstreettito]

I have the same problem with going back to idle. But only when it's cold and high idle is on. Other than that, it goes right back down.

[Mr Pops]

I'm no mechanic, but I think there's two temp sensors on this motor. One for the gauge and one for the computer. If the computer thinks it's cold, it may never come down from the high idle. Could be a bad temp sensor or maybe an air lock in the coolant giving it a bad reading..?

[chris989]

Hello Pops,
i changed the the coolant sensor too.

yesterday i changed the electronic spark control and knock sensor and I will test drive after the brakes are fixed.


I have my suspicions about the timing. advancing itself via the computer causing the idle to rise, and as the timing corrects itself the idle returns.

All of the motors i have built in the past have been carbureted. even with my msd units i have not seen the timing swing like this. Sure they jump depending on the set curve but the engine is controlling the timing and this feels like the it is happening in reverse.
After setting the timing, and connecting the wire the timing will advance itself to 20 plus degrees and as i watch the idle come down so does the timing return to 8-10 .
I know all of this will get better gas millage when it is correct but right now i am dreaming of a new MDS.

[Mr Pops]

Yeah, I'm pretty sure you have to unplug some little brown wire on the drivers side firewall right above the brake booster prior to setting to timing to 0 degrees. Then it advances by itself once plugged back in..

[Big 'ol burb]

I have an 87 TBI truck and mine idles high also. So far(thanks to searching this forum), I have replaced the coolant temp sensor(the one in the top of the intake manifold), and the throttle position sensor. Also I tried jumping the A/B terminals on the plug under the dash. I checked for intake leaks. No luck. Any suggestions would help me too. Thanks.

[chris989]

Big o burb,

I just returned from a 700 mile loop of death valley to yosemite and my burb still idles high. There is nothing more i can replace. I have check and replaced everything and the motor holds 20 inches of vacuum steady.

Just for thoughts;
At 8000 feet i could barely take off from a stop and the acceleration was horrible.

The truck did average 14 miles a gallons with trails, stop and go plus highway.

The motor is smooth as glass, never a miss or sputter.

Mine will finally return to an idle after 10 seconds of taking it from gear or starting.

I noticed the from a stop the converter holds the engine rpm very low. Maybe my computer compensates for this and tries to raise the engine speed. This struggle continues and then, when I put it in park, the engine jumps up and then takes the 10 seconds to return. This is only a theory, but if someone rebuilt this trans years ago and put a "heavier" converter in it is could cause the low rpm stain.

I will report back

[chris989]

converter in it is could cause the low rpm stain.

I meant strain.

Anyway, I was told the trans is in great shape and to look at the catalytic converter. I had it tested and was told is was 50% clogged. Not sure how scientific the test was but i think i will cut and bypass it before spending another 300.

Anyone experience the "clogged cat"

[chris989]

New Cat, and no better,...

I started a new thread as i think this is a timing issue.


http://67-72chevytrucks.com/vboard/s...40#post5106340

[chris989]

big ol burb,

I have been on the hunt for the answer,,, and I have some resolution.

For now i have the IAC unplugged. I jumped terminals A&B, turn the key on for 10 seconds (drives the IAC pin closed), unplug the IAC, then turn off the key, unjump A&B.

My truck now has normal throttle response. The IAC slows RPM return to improve emissions when there is a vacuum spike (letting off the gas fast).

I am currently looking into a manual way to energize the IAC (fast idle) via a switch, Just like some of my older Diesel work trucks.

I also am looking into editing my bin files for the computer. I downloaded tunerproRT and can now log my data real time.
There are a host of parameters that control the idle (in gear, vacuum, temp, etc) that are available to edit.

I am not a pro at it yet but i will figure it out, For now with the IAC disconnected it idles at 800 park, 525 in gear.

Jon

[walkerscott]

do you have a check engine light and trouble codes..

Do you have an operational VSS

[walkerscott]

I noticed the from a stop the converter holds the engine rpm very low. Maybe my computer compensates for this and tries to raise the engine speed. This struggle continues and then, when I put it in park, the engine jumps up and then takes the 10 seconds to return. This is only a theory, but if someone rebuilt this trans years ago and put a "heavier" converter in it is could cause the low rpm stain.



assuming you have 700r4 if your driving steady speed steady throttle pressure down the high tap the brake does the rpm go up ~300 rpm

if so the tcc is working properly

The IAc is working because when the selector is shifted from park to any gear the rpm will adjust

check out the tuning tables for idle adder in park

[Dually Sublazer]

Hey everyone! Im a new guy here and am curious if the O2 sensor could have anything to do with it. I have a 92 TBI454 running on the old 350 harness with a custom chip. When i put the exhaust back together (fabbed from the Winnebago doner of the 454, 16,700 miles!) the O2 bung ended up a couple of feet farther back. It works but i get occasional O2 codes which go away after punching the accelerator. Said all that because mine idles VERY high when cold but acts better hot. Mileage should be better but im gonna try a heated O2 and report back. If i understand correctly, the O2 sensor regulates fuel mixture and will default to VERY rich if not working optimally. Just a stab in the dark......
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[chris989]

hello dually,

Funny you should ask,, I was just going to post about my outcome. I have another post here about idle issues so i will surmise both.

1) 02 sensor,,,,, My older truck has a single wire 02. It needed a three wire as it was to far back and would not heat enough to enable the computer to go into closed loop. My sensor was just aft of the y pipe and that is not close enough. I used a 02 from a 1995 S10 4.3L truck. Not sure if that is your problem but it is a easy fix and worth it. Just run a +/- that is energized with the key and you are done.
I want to thank Brian at Harris performance for the correct info.

http://www.ebay.com/itm/Walker-3-wir...sories&vxp=mtr

2) stall converter---- My burb 5.7/400 turbo came with a GM-8 converter. (remove the converter cover, rotate until you find the stamp con the converter.) GM-8 is a 1100 rpm stall from the factory. Maybe mine was beginning to fail but it would not power brake past 875 rpm and when in gear it would choke the motor to 400 or less rpm, forcing the computer to compensate.. I installed a Hughes XTM coverter (approx 1800 stall) and MAGIC,,, my trucks driveability increased 100 percent.

http://www.hughesperformance.com/ind...d=52&Itemid=53

3) I also installed from Moates, the chip adapter and use tuner pro to log and view data. MAJOR help for any tbi application. without it I am not sure it is possible to work on one of these trucks.
With tunerpro i was able to see the park/neutral switch was not working (adds a few rpm when put in gear).

http://www.moates.net/

Anyway,, I hope this helps a few others,,, I chased my headaches, asked all of my local shops and no one knew a thing, All guesses or the obvious (vacuum leak stuff). Checking or changing the other sensors is all good advice. There are relatively cheap and most likely original, But for me it was the 02 and a new converter. The converter made the greatest improvement 200$, 02 necessary improvement 75$, Moates not necessary but awesome to have 300$ +/- depending on extras.

You can message me , send you phone # if you want help,,,
Good luck,
Chris

[Dually Sublazer]

Awesome! I havnt changed my O2 sensor yet cause i have a dual exhaust to install and will move the bung then. Brian is who i got my chip from. He's tons of help and told me about the heated sensor. Im curious about my converter now...... Last couple times i drove it, I noticed if in gear and stopped idle is smooth. If i let it roll (inches) with brake applied to hold it back, the engine surges. Like a couple hundred RPM's, up and down. Stops when yhe truck does or if i take off...... New converter may be in order.
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