Sorry...tried this "Q" over on the "TBI" forum......tried a few of the suggesrtions, but it didn't help.

Maybe there will be someone over here who can give me an idea or lead.

'89 Firebird Formula, 305 TBI, auto. VATS has been properly disposed of, underhood is still all factory original.

It starts, idles and runs perfect until a short bit after the fan kicks in....then it won't idle, or sporadically idles correctly. Still runs down the road just fine...engine only quits quits when I stop and the car drops to idle. If I shut the engine off and wait a few minutes, it idles fine....for a few minutes. Then it starts all over again.

Anyone got an idea? TPS seems to be operating correctly.... I put in a new O2 sensor.

The car has sat idle for about 4 years with only an occassional start-up. Could corrossion on some actuator or sensor be a contributing factor?

I've replaced the fuel filter...but its' been a while. Trash in the tank, perhaps?

I've been told on the TBI forum to check the "IAC".....I have no idea what that is or where to look for it!

I have a weekend to figure this out....18 year old stepdaughter starts driving it Monday!

Thanks

 

Just a few more bits of information.

You mentioned that the engine starts and runs fine initially. Does that mean that the idle RPM starts high, then slowly drops as the engine warms up?

Also, since there are no changes to the factory control system other than VATS, is the ECM storing any error codes? What are they, if any?

 

Idle starts high(about 1200 RPM) and slowly drops to around 450 at full warm....as I understand, this is as it should be.

No codes.....I only wish there were!

 

i have an lt1 camaro and it runs on maf and speed density. i know that when it starts up that it runs speed density till it warms up then after warming up it switches to the maf system well me not being knowledable with tbi. and not knowing if it actually has anythis or not. but i do kow that in my car if you unplug the maf it forces it to stay in speed density and if your car even has that you could try.

disregard this post im a moron :lala: :lala: :lala: :lala:

 

If the engine RPM is higher at startup and slowly decreases, at least the IAC is working. It may be sluggish, but is at least working. That's good, since you should be able to clean and lightly lubricate it so that it reacts at normal speed. That alone may solve some of your problems. If not, you'll have a reliable IAC and can discount that as a possibility.

No DTCs isn't a bad thing either. It means that at least all the sensors are reporting some signal within an acceptable range, and all the necessary load devices are connected. It may make it a little harder to diagnose, but at least the hardware is working to some extent. You should at least get the MIL lamp to turn on when the ignition is on and engine is off, however.

The fact that your startup RPM is only 1200 and hot idle is only 450 is a problem. Target idle RPM should be closer to 600-650 in DRIVE on an automatic. If you're relying on the dash mounted tachometer for this information, that is understandable, since they are notoriously inaccurate. If you are using a handheld tach, you have a problem somewhere.



Your IAC is located on the right rear corner of the TB. You'll have to unplug the connector, unscrew the IAC, then start cleaning the TB. A small, stiff brush and carburetor cleaner will be your tools. Once the TB is cleaned (both the IAC air passages and throttle bores) you can move on to disassemble the IAC and clean/lube it.

Rock the IAC pintle back and forth gently while pulling on it lightly (in the direction of the spring). After about two minutes of this, you should have the IAC pintle and spring loose from the IAC body. Using the appropriate spray cleaner (one that is safe for electric motors), you should be able to flush the carbon, dust, and junk out of the IAC motor hole where the pintle was residing. Allow this to dry thoroughly while you clean/brush the varnish off the pintle gear itself - yes, it's a gear, not a screw thread.

Once everything is clean and dry, apply one drop of clean engine oil to the pintle gear/stem, then reassemble the unit by pushing in slightly while you rock the pintle back and forth. Retract the pintle fully against the spring before you install it, or you may damage the pintle and IAC unit when screwing the unit back into the throttle body. Make sure the gasket is serviceable and in position, then reassemble. Reconnect the electrical connector.

Start the engine and allow the cleaner to burn out of the cylinders. While you're there, you might as well check the TPS voltage to make sure it is within the correct range. You may have to reset the throttle minimum air position is the TB was excessively dirty, then set the TPS voltage again.

 

Good info.....I'm an idiot when it comes to "puter cars"!

Have a friend with a "code reader". Had him check it, came up "MAP sensor. Replaced the MAP sensor...it helped, but didn't cure it.


At least, now, I have a code.

34.......Vacuum sensor. Where is it and what does it look like? My Haynes Manual reads like stereo instructions, so I have no idea what I'm looking for!


By the way....car is now properly tagged and titled(only 2 1/2 years later!), and 18 yr. old stepdaughter starts driving it Monday.

I KNOW this ain't a major problem(it WILL re-start and run), but I'd like to have it right for personal reasons!

 

FS,

Your '89 TBI shoudl have only one sensor for that purpose. The MAP (Manifold Absolute Pressure) sensor IS the sensor that monitors engine vacuum. A '34' means that it is reading a lower than expected absolute pressure ( higher vacuum) at a given throttle position and engine RPM. That's not a usualy failure, and is generally caused by a TPS voltage being too high, a clogged or incorrect PCV valve, or poor connection at the MAP sensor itself or in its wiring harness.

If you have a digital voltmeter and eithe a vacuum gauge or vacuum pump, you can test the sensor output at a known vacuum (or MAP) and compare it to the table:

 

I don't have the testing instruments you refer to...

Let me just spell out the symptoms with a bit of history.

This car has not been tagged in over 4 years(read....not driven). I have discovered that once you're on the road it runs fine...... fresh start in the driveway will give you fits....especially after its' warm.(it don't like Drive...at all!)

Once its' been driven, everything works fine.

I am of the opinion that corrosion is probably the leading cause of my problems....perhaps just driving it will help!

NM