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Idle problems

de3en16

de3en16

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utah
First off this problem is on a 1990 Blazer TBI 350 and 700R4. I set the base idle to 450 rpm with the IAC and the truck in gear i got an idle of 750 once the IAC was hooked back up. drove the truck and idle was 750 in park and 550 in grear. Now the problem, shut the tuck off and start it back up i get and idle of 1200 to 1500 rpms. I can put the truck in drive, 1, or 2 and get the rpms down to 550 in gear but if I try in park or nutral i get code 24. The back up lights come on when I put it in R and i replaced the nutral safey switch. After i get the rpms down i can drive around all day and every thing is fine shut the truck off and its back to 1200 to 1500 rpms.

Could this problem be in the computer? Whats the right computer for my blazer?

What else could this problem be caused by?


code 24 is if i remeber right is the computer sees the truck moving but the vss isn't sending a moving signal if the truck is not moving its the nutrarl safety switch.


Thanks for any help
 
Did you unhook the IAC when you set the idle? Was it fully warmed up, with good gas and clean airfilter/fuel filter and no vac leaks? TB all clean and injectors spraying correctly?

I set the idle once and it wasnt tuned up correctly and I went through hell and high water to figure it out again. Also the IAC has to be fully closed and unhooked to set the idle. Does the speedo work?
 
454Sub said:
Did you unhook the IAC when you set the idle? Was it fully warmed up, with good gas and clean airfilter/fuel filter and no vac leaks? TB all clean and injectors spraying correctly?

I set the idle once and it wasnt tuned up correctly and I went through hell and high water to figure it out again. Also the IAC has to be fully closed and unhooked to set the idle. Does the speedo work?
Click to expand...


Being a 90, it runs a DRAC assembly with a speed sensor on the tail shaft of the t-case vs the optical sensor in the dash. If his speedometer is working, more than likely his DRAC is going to be sending the right signal to his ECM as well. I kind of suspect that he may actually have a code 42 which is an ignition error code. It can be set by a ton of different things, but the most common is disconnecting the EST bypass to set the timing. Even after the active alarm clears and the CEL turns off, the truck has to be started 50 times before it will forget that code on it's own without a reset and will display it when you check the codes manually.

The number one suspect when it comes to an elevated idle on these trucks is a vaccum leak. Check the base plate gasket and the throttle shaft bushings very closely. It doesn't take a very large leak to make your RPMs come up significantly. Also don't count out your vaccum booster for your brakes and the various emissions hoses.

How are you forcing your IAC closed and keeping it there? There is a certain procedure to follow when setting the minimum air setting on these trucks.
 
I did unhook the IAC and made sure it was up to temp the air filter was off. My speedo works but it dosen't read right do to a gear change. Ive replaced the IAC twice to make sure i didnt get a bad one.
 
de3en16 said:
I did unhook the IAC and made sure it was up to temp the air filter was off. My speedo works but it dosen't read right do to a gear change. Ive replaced the IAC twice to make sure i didnt get a bad one.
Click to expand...

When setting minimum air, you need to jumper the ALDL port like you are checking for codes and wait 30 seconds to let the IAC close fully (you should be able to feel it vibrating a bit if you touch it while the ALDL port is shorted). Unplug the IAC and fire the truck back up. Set the idle RPMs to the minimum the engine will run smoothly at operating temperature. This should be between 450 - 550 RPM.

Plug the IAC back in and remove the ALDL jumper. Un-plug the positive battery lead for 5 minutes to reset the ECM and clear any codes. Fire the truck back up and it should settle the RPMS between 750 - 800 RPM in park. Dropping it into gear should drop the rpms momentarily then raise them back up to 750 - 800 RPM again.

If the truck still revs up when you start it or put it into neutral, you need to start looking for your vaccum leak. Hook a vaccum gauge up to the engine and see what kind of vaccum you are pulling at an 700 rpm idle. Should be around 22" HG. If it is lower than that, you need to look for vaccum leaks.

Was the truck running properly before you touched the idle setting? Have you done any other mechanical work lately, tune ups etc?
 
I jumpped the pins on the conector under the dash turned the key waited 30 seconds then unhooked the IAC turned the key to off took out the jumpper and started the truck and put it in gear with the park brake set and the wheels blooked and set the ide to 450 shut the truck off hooked up the IAC and started the truck and it had an idle of 750 drove the truck over 20 miles with no change till I shut the truck off.

Ive tried to check for vaccum leaks with crab cleaner, I chaged out the gasket for the TBI unit to intake when I installed a spacer from CFM Tec with no change to my problem. I'am planing tomorrow to plug off all the vaccum ports then checking again for a leak.
 
When you jump the ADAL port, you have to have the key on for the IAC to move, just so you know.
 
I always thought that with a vaccum leak I shoudnt get the RPMs to drop when its in gear I thought the RPMs would stay high.
 
vac leaks always raise RPM.

Maybe PVC valve is plugged up, or maybe a vac controlled item is leaking or not operating correctly
 
also check to see that it is actually seated. for some reason mine will take a a couple of tries to get it to seat sometimes.
 
There are several things it could be, but I am slightly suspicious of the computer's engine temp sensor.

It will idle it up when the engine is cold, and I suspect, when you first start it. If it was not getting a good reading from the temp sensor, then it might keep thinking its cold.

Also, if the IAC was dragging slightly, it might not return to idle position after it idled it up from start.

Can you "blip" the throttle and get it to idle down?

Carb cleaner, WD-40, or an unlit propane torch will find vacuum leaks.
 
Yeah Thats why Iam going to start all over tomorrow with the basic stuff 20+ year old truck it could be about any thing.
 
If i dont hold the brake and let the idle come down in drive it will stay high evan after driving it 20 miles. First start up of the day it will idle about 750 then go up to about a 1000 then on up to 1500 as it gets up to tep. I have to put it in gear to get the idle down I cant get it to ome down in park or nutral I get get code 24 after a little bit but that code clears after driving a few miles.
 
Update

Checked for vaccum leaks, unhooked all the vaccum lines and checked one line at a time no leaks that I could find. Checked nutral safety switch for crossed wire or out of adjustment, no change. Fordum taked about the coolent tep sensor thats the one in the intake right in front right? I did replace that one not to long ago but i have another to change it out with. What should I look for when I swap it out again?
 
It sounds like your IAC is getting confused or stuck for whatever reason.

Maybe is your TPS sending a bad signal to the ECM causing it to dump more fuel, if its not a vac leak, the only way for it to raise the idle is its either dumping more fuel for whatever reason, or the IAC is not operating correctly and letting extra air in when it shouldnt.

Yes, its the 2 wire sensor on the intake by the t-stat. Should have a ground right next to it for the ECM and the harness.
 
Diagnostics for all the TBI codes is here:
http://coloradok5.com/forums/showthread.php?t=240425

Wiring diagrams here:
http://coloradok5.com/forums/showthread.php?t=302678

Scan tool and Data Logging with laptop here:
http://coloradok5.com/forums/showthread.php?t=302680

Your blazer should have a 1227747 ECM.

What it is doing is normal. IAC logic works completly in gear and only has a high low setting when not in gear so it revs higher. P/N wire grounded in P/N and open in gear. Changes IAC logic in ECM so engine revs higher for warmup of engine, O2 sensor and Cat.

Although with a good Minimum Air Adjustment on my 1990 Suburban I get about 1000 RPM P/N and closer to 600 RPM in gear.
 
Would it mater if I don't have a cat and a stock O2 sensor? Or could a heated O2 sensor help?
 
If O2 was problematic would likely have a problem across the whole spectrum of engine operation. As mentioned, only time you might see a problem specifically at idle related to the O2 is if the motor is dropping out of closed loop at idle (IE getting cold, poor sensor, etc.) and even then, on a properly tuned motor (factory) even if it's going open loop at idle, it shouldn't run worse.
 
de3en16 said:
Would it mater if I don't have a cat and a stock O2 sensor? Or could a heated O2 sensor help?
Click to expand...
Stock O2 sensor and no cat will not interfere.

Jessie James said:
You should only need a heated O2 sensor if you have headers.
Click to expand...
I've proven that wrong 2 differant ways...

One if tune is dialed in correctly you will go open loop when O2 sensor cools off and not notice it because fueling is correct.

One with coated headers the stock/un-heated O2 sensor stays hot in long tube headers at idle.

But in long tube headers the O2 sensor is moved further downstream and INT Delay Vs Airflow needs to be increased and INT Delay bias at idle.
 
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