I swapped a 350 TBI in my 1988 Firebird Formula and have had this erratic idle problem ever since. It originally was a 305 TBI and it never had this problem. It now has a 1991 Firebird 305 TBI computer and PROM burnt by Tunedperformance, injectors were also swapped from the 350 along with rebuilding the throttle body and all new gaskets on the engine. It usually fluctuates the idle until its warm and then its good from there. I've driven the car for a year like this and it runs fine when warm, yet still chasing the problem. We unplugged the MAP sensor which put it in limp mode and it idled perfect. So we replaced the MAP sensor and it still idled erratic. Anyone have an idea or suggestion?

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well, if it runs good when it heats up?

probably 'cause it starts trimming, or 'cause a hot engine needs less timing to idle properly..

call your tuner, it's probably incorrect idle timing or fueling.

 

Ok thanks, I'll have to check base timing

 

Well kind of an update, I checked the timing and its on the money. So I'm going to change the coolant temp sensor and see what happens.

 

couple of questions

what is your Fuel Pressure?
was the IAC and TPS relearn procedure done after the throttle body was rebuilt?

i know these are simple questions but it will give a little more insight to the issue

 

I agree with above, must make sure the mechanical stuff is correct before going any further. The best chip cannot correct a mechanical issue.

Fuel pressure should be 13-14 PSI
Timing should start at 0* or TDC, although many engines with stock spark tables in the chip make best power at 4-8* BTDC.
You will need the minimum air rate screw uncapped for this.
Perform an IAC reset, jumper A to B in the ALDL connector, turn the key to run, but do not start, wait 30 seconds and unplug the IAC with the key still on, with the IAC disconnected start the car. It will likely idle very, very low, maybe 350-400 rpm. Adjust the torx screw on the TBI to allow the engine to idle at about 500-525 rpm in Park/Neutral with the a/c and all heavy electrical draws turned off. Shut the engine off and plug in the IAC. The IAC will now have a proper point of reference for the larger 350 engine.
I have been able to get some "un-tuneable" cammed 305 and 350 combinations to idle and run well enough on the stock EPROM to drive and fine tune by performing the correct mechanical adjustments to allow the ECM to do its job correctly.

Surging is usually a lean air/fuel mixture (relying on too much airflow through the IAC from lack of proper throttle plate opening can cause this) or too much timing.

 

Fuel pressure will NOT be higher than 13psi seriously seriously doubt this. If between 11-13psi your fine. Ive seen tbi motors 'running' on as low as 6psi.

 

Anything under 13 is not enough pressure for wide open throttle. Don't care what the factory pressure is, just stating what it NEEDS to be to run properly.

 

I've never seen mine above 13. Mine would run on around 6 when the pump was going out. Would surge like a dog though.
I've never seen mine hit 13 at idle. Even with the new tank, TPI pump, filter, and strainer I just installed for the L30 motor, my FP was sitting at about 12 with a gauge hooked up right after install. How do you get it up to 14 in stock form fast?

 

Guys, stock could be anywhere from 9-15 psi depending on where the Pressure was set from the factory, no two TBI vehicles were set exactly the same(the stock regulator was not meant to be adjustable by a consumer)

With that said, this guy has a 350, he's gonna need more fuel pressure to feed larger injectors and a bigger motor, I would assume he would NEED over 11, more towards 13-15 PSI for a 350

My L03 would idle with 3 psi pressure(split hose in tank), unplug map sensor and it would idle fine, but as soon as you touched the accelerator it would bog and die

With a working pump my factory setting is 10 psi, with an adjustable regulator I can crank it to 15 or more

if you want to bump pressure, do the fp regulator mod or buy an adjustable TBI regulator, you can change spring rates as well to get a little more pressure out if needed

 

The replacement GM regulator from a 1992 G20 van was set right at 12.5 PSI. The GM replacement regulators had a tamper resistant torx screw that allowed them to be adjustable. I have also seen multiple stock spring spring rates and lengths depending on year and application. There are even some springs that give 28-32 PSI. Then of course there is the broken spring special that only gives you about 6-8 psi and horrible running.

 

This is a replacement adjustable regulator. It is also very easy to turn this regulator into a vacuum referenced unit.

 

I know i was just letting the op know that if he sees some where between 11-13psi its running normal. Maybe not optimal but i just wanted to let this guy know he doesnt NEED to see 13-14 psi as ive never seen a tbi running stock higher than 13psi. I will admit though 13psi is very low even for a stock tbi setup. Like fast is saying you can bump FP up to about 15psi on the stock motor and gain power just from that. Im running 61PPH right now on my 350 TBI and stock FP is at 11.5psi, im running very lean (verified by EBL wud) at 3k rpm. My DC rating is hitting 105% This motor can rev to 5500. i NEED lots of more fuel

 

First thing that looks unusual besides the hunting idle, is the low cold idle, the stock TBI ECM usually have a 1200-1400 rpm cold idle, this one seems to be in the 800 rpm.

 

Considering the early TBI ECMs do not actually control the idle RPM until the engine is warm, this is not suprising. Probably needs the throttle plates open more.

 

I do not know much about the stock TBI EMCs as I only did data logging with them. Though I came across several 87-92 TBI cars and their cold idle is noticeably higher then the warm idle. Always thought it was the ECM commanding it, as it disappears with EBL.

 

Sorry I kind of forgot about this post but I solved the problem. It just needed the throttle plates opened a little more like Fast355 said. Wish I would have known a longgg time ago cause Im building a 5.3 for it now lol!

 

What happens to that L05 when the 5.3 goes in?

 

Im probably not going to put it in anything. It runs good, all new gaskets except for the headgaskets. Typical it needs valve stem seals but just puffs a tiny bit when you rev it up. The bearings were all good I was told the engine had high miles but its been fantastic for me

 

I guess I could have commented on this as well, but missed it. When the engine is cold the older TBI engines merely command IAC steps vs coolant temp, its not until a specified coolant temperature that the IAC actually starts controlling the idle speed using ECM logic. With the cold engine the ECM command say 115 counts. Lets say for example on a stock 305 that gives 1,100 rpm cold. With the larger displacement 350 moving more air per revolution the idle may only reach 950 rpm. The ECM will not attempt to adjust it, because it is merely using IAC counts vs temperature.