Hey guys,
I’m having a heck of a time trying to diagnose what is going on with my 1991 5.0 TBI Firebird Base Model.

When accelerating under any load the car seems to not fire properly or maybe it’s bogging (I’m not sure) but it accelerates fine in Park and Neutral. It is almost the same feeling as if you were driving a stick shift, and you were in too high of gear for the lower speed you were going - it just seems to not get any acceleration / power.

If you press and hold the accelerator for too long, it will keep bogging or whatever it is doing, and it will eventually stall out.

The only code I am getting is GM 12.

I have replaced the following to try and remedy this problem but none have seemed to work.

Rebuilt and cleaned entire TBI
New AC Delco fuel injectors
MAP Sensor
Throttle Position Sensor
IAC valve
EGR Valve
EGR Solenoid
Inner Air Temp sensor (air charge)
Engine Temp Sensor
Fan Switch
02 sensor
thermostat
Distributor
Spark plugs and wires
All New fuel lines, fuel filter and upgraded pump.

I have replaced all vacuum lines hooked to TBI and EGR
Installed new fuel line with an external regulator that regulates pressure at 13-14 psi (even under load) I’m getting plenty of fuel. I Platted off the internal fuel regulator (it was doing this problem before I messed with the fuel regulator)

The only solenoid I haven’t replaced is the charcoal purge canister.

What in the world am I missing here?

Someone mentioned the torque converter… why would this affect acceleration under load?

Any help would be much appreciated,

 

Quick try:
pinch the return fuel line and see if the car will run better if it does your fuel pressure is too low. Put a fuel pressure gauge on it. Should be 10-15 psi

 

I'd look into an exhaust blockage too, maybe the cat?

 

I’ll definitely try that. Thanks for the advice. So I’ve already installed a gauge and it stays right at 12-14 under all circumstances. But I haven’t tried to pinch it to see if it does anything.

the only issue is - it idles fine and I can rev the motor fine in park and neutral.

 

So I failed to mentioned I installed a new high flow cat with a 3” cat back. No blockage. I have taken off the manifold when I changed the 02 sensor and all seemed fine.

Might be a good time for me to convert to headers lol.

 

Check the ignition timing and make sure it advances when you rev the engine.

 

So I have set timing to 6 degrees per service manual and it does seem to bounce up to about 12-22 degrees when revving the motor in park. But that is only revving to about 3500 rpms.

I’ve had others recommend adjusting timing. I’ve tried advanced and retarded. Honestly, when it was right at 6 degrees it ran a tad longer until it stalled out compared to advancing to 12 degrees or so.

Thanks for the input.

 

tbi base timing is tdc

 

That’s correct, then the manual recommends 6 degrees advanced after 0 degree base timing (with the tan and black wire disconnected)

 

🤔 what ? Base is set with est disconnected. The ecm will advance based off coolant temp, throttle position and load

 

maf ? Tbi uses a map sensor . This is throttle body injection?

 

Nice - this is good advice. So I set timing at 0 with est disconnected at TDC. Turn the car on then I rotate the distributor with the engine running until the timing line is on 6 degrees advanced.

At that point, I reconnect the est wire.

Should I be doing something else?

appreciate it.

 

whoops - yes MAP sensor. Throttle body.

 

engine warmed up set base to tdc with est disconnected. After it’s set shut the engine off and plug the est back in. Clear code by disconnecting the battery if needed for 5-10 minutes. Idk what book you are using but throw it away.

 

This is pretty much what I have done, minus disconnecting battery. I’ll give it another shot And set everything at TDC 0.

The only step that I have added (after your instructions) is I set timing to 6 degrees advanced by rotating the distributor, which is recommended for 3rd gen’s I think? I do this while the engine is idling (wire still disconnected).

It sounds like I set at 0 and then let the computer do the rest?

Maybe I’ve misinterpreted forums on here and the Haynes Book.

Noted on tossing the Haynes book, lol. It has come in handy for drum breaks and suspension parts.

 

Your right set base let the ecm advance from
there. 6 degrees is for tuned port injection.

 

Have you tried a new stock ignition coil? Have you looked at your ignition control module and the paste under it?
I once upgraded my coil and had all kinds of weird problems until I went back to a stock coil.

Bad paste on the icm will cause some issues but the car usually just shuts off when things get warmed up and the icm is bad or paste is dried out.

are your plug wires in the correct order?

 

Appreciate the response - Yes, when I changed the stock distributor with a stock Delco distributor - it came with an ignition coil and icm - Stock replacement.

Apologize if this is a novice question - what do you mean by paste? I’m assuming the grease that came with it? Lol

Also, spark plugs are good to go. Triple checked those.

Thanks

 

Yes, the special grease that comes with the icm, it goes on the bottom metal side of the module and acts as a layer between the metal and the distributor itself.
so the plug wires are in the correct order too? I once had an old Monte Carlo that acted weird at highway speeds due to two crossed plug wires. I was hoping it was something simple for your benefit, but I feel stumped too now. Last ditch quest, have you examined the plug wires for burn spots and in the dark with the engine on and revved up, to check for any arcing or voltage jumping from the wires to a metal surface?

 

Nice! I’ll check the grease, I definitely neglected to grease all of the areas you mentioned.

As for the wires / all are new 10mm dragon fires. So I believe they are just fine and my order is correct. I’ve triple checked and confirmed with a few of my neighbors that are savvy. Rotor is pointed at #1 cylinder at TDC and number 1 wire is located properly, along with the following fire order with the corresponding wires.


But I’ll definitely try the “in the dark” trick to check for arcing / etc.

Yesterday, I had a neighbor mention I may have an unmonitored air leak in the throttle body or intake - even though I rebuilt that. I guess I can throw a leak test on it and see. Just I highly doubt that’s the case since it only has issues when accelerating.


My last effort at this is testing all spark plugs and their gaps, if it ends up being something as simple as that I’m going to be pretty pissed lol.

Appreciate everyone’s input, my last hurrah will be checking the spark plugs, and just checking compression psi on each cylinder.

 

Did you check the air intake to make sure there are no blockages?

 

Sure did, perfectly clear. It brings in air like a boss. Someone also mentioned a fault inner air charger / thermostat (whatever that annoying little sensor is inside of the air box). I’ve replaced it, but someone mentioned those are really unstable with temperature change. Any truth to that?

Thanks,

 

I am leaning towards the ESC
Electric Spark Control box

 

I haven’t tested this yet! I haven’t tested the knock sensor yet either and it looks pretty weathered. This will be tomorow’s to do list. I’ll keep you posted
thanks,

 

is the est retarding timing or something? Sorry if I missed that

 

Update everyone! Since I’ve started this thread…

I set base timing at zero and then reset base idle.

Changed all spark plugs and wires to performance brands. I gapped the spark plugs to 035.

Installed headers with knew 02 sensor

I swapped the engine knock sensor just to be safe.
I have a new ESC box coming in the mail soon.

Good News - is the stall outs have stopped!

BAD NEWS - is that the car (when in gear only) will not get past 1200 rpm’s and it starts to bog under load and I can’t get any rpm’s built up unless I throw it in neutral, rev it to about 3000 rpm, and then slam it back in gear (I know this is awful for the car, but it’s the only way I can get a little more speed in the car). I am getting absolutely no engine codes.


The car idles a little rough when its cold, but as soon as it heats up it purrs and I can rev to whatever I want in park or neutral.

I can’t tell if it’s a timing issue because I am only having the issue when I’m driving in gear.

Everybody, thoughts please, I’m exhausted trying to figure this out!

https://youtu.be/g8QIxGrAhJw?feature=shared

Thanks,

 

It kinda sounds like you have a dragging accessory whine. Maybe pull the belt and check if the accessories turn freely.

 

The timing isn't going to make it bog.
Timing isn't going to make it stall
ESC won't either,
Or the grease under the ICM.
Nor the whining alternator or PS pump or whatever that annoying sound is.

We got a WAG fest going on here.

That car is starving for fuel. IDK if I trust that fuel pressure gauge, or if it's connected into the wrong point on the fuel system (or maybe the Port going to the TBI unit is the wrong one and you have ~14 lbs, but not at the TBI? You can hear the sound/symptom of it starving, the RPM starts to fall off, then you can hear the injectors blast for a second, and then the engine recovers. It's starving for fuel. You need to figure out why. What happened to the car when the symptom started? What mods or changes did you make? I would re-run that same scenario that you did in the vid, but with a fuel pressure gauge installed in the line going to the TB....of even AT the TB. It looks like you may have a single fuel hose running to the TB...do you? If so, what have you done w/the return and FPR on the TBI unit?

That car is starving for fuel.

 

I'll admit that fpr setup...well I can't asscertain how it's setup from the video...

TBH when we had the dragging alternator, the engine had a laboring crank and wouldn't start at all. Changed battery and starter at some point in the venture. Had us stumped for a few days troubleshooting in the parking lot.

Since I'm throwing darts, on one of my daily drivers, I dealt with an intermittent stall that only happened when slowing to a stop for 9 years! Ahhh it happened couple of times pulling out of the driveway, too. Never fails well not never (read-on) but everytime it happened, slapped it into neutral, key off, key on and crank. Always re-started so I dismissed a lot of things that could possibly be wrong with it. I did the same exact parts cannon but the problem persisted. Well on Dec 30, 2025...stalled on the H1 and went through my not so SOP to restart but this time it didn't restart. Troubleshot it down to a bad coil. Ever since the new coil went in..."knock on wood" the intermittent stall has not reared it's ugly head. I'm by no means saying to go buy a new coil plus failing coils normally don't behave like mines did (usually all or nothing) but there's some ohms checks that you can perform before making that decision.

 

Appreciate the response / so I have the return line hooked up to the return port of the fuel regulator that sends the unused fuel back to the tank. Single main fuel line going back to the throttle body. I have a internal fuel regulator “block off plate” installed. (See link)

https://www.hawksmotorsports.com/tbi...s-motorsports/

Brand new fuel pump and filter. It was doing this long before I upgraded the fuel system. I have even gone as far as installing another pressure gauge (farther down the supply line right before the Throttle body inlet) and it’s right at 13-14 even when accelerating in park. I can’t tell when driving under load and in gear.

The whining noise is the intake sucking in air. It’s wild, but after it warms up it’s fine. But once I drive it still has the popping exhaust symptoms while in gear.

When I start the car - it’s almost like is surge idles then the ECM tries to correct with two much air? Hence the whining / hissing noise.

Also, the car was doing this before the mods. (I have installed the mods to try and remedy this problem). Lol.

Still the main problem remains — when I accelerate under load it’s almost as if I were driving a stick shift and trying to accelerate in the wrong gear and it just makes a popping noise out of the exhaust. ( I have confirmed this isn’t engine knock). If I press the accelerator harder or hold it
down without letting off it will eventually will bog so low it will stall.

Take look at this video at idle. Without the whine (only happens when I accelerate under load). Totally fine and it will allow me rev however I want in park or neutral.

I just hope it’s not a TCC issue?

I will get an in car video this afternoon.

 

I appreciate the input, anything helps at this point. I even at a neighbor who works for a mechanic come check it over.

He thinks it’s something with the ECM trying to compensate air to fuel ratio when the engine is in its open / warm up loop. He said even the IAC might be crap or the TPS. The wining noise is coming from the intake through the throttle body when accelerating on. All belts and pulleys / bearings are in great condition.

take a look at this idle video, let me know what you think.


Appreciate your help!

 

Can you get a video of the popping out of exhaust sound? First thought is the cylinders are firing when they're not supposed to be.

 

Man, that sounds like my son's '91 Firebird. Same issues, though, we haven't done as much on it. He's only had it a few months, but it does stall out mainly at highway speeds. We have it at a mechanic now. I'll post on here if he discovers anything.

 

It seems like all of your opinions on your fuel pressure are based on what you can see with no load on the engine (idle, revving the engine in Park or Neutral). Your issue doesn't occur with no load, but your "test" is without a load. So it passes the test. Your issue is UNDER load, so your test should be too. Duct tape a camera pointed at that gauge and take it for a drive. Aside from a second remote gauge where you can see it when driving, that's the only way to know for sure.

New pumps have been known to be bad or weak right out of the box. Did you replace the rubber lines inside the tank along with the new pump? They have been known to split (a failure which may not be obvious without close examination), and should be replaced with the pump.

Not saying the pump or pressure IS your issue, but asking if we can eliminate those variables from our thinking.

I see you have an aftermarket air cleaner. Did the factory air cleaner housing have a sensor in it? Where is that sensor now?

You mentioned your 02 is new along with a header install. Is the 02 in about the same spot as in the factory system?

An OBD1 capable scan tool would sure help in this case...it's the only way for YOU to see what the ECM is seeing, and look for erratic 02 values, etc.

 

I mean, it's idling perfectly. How are "the cylinders firing when they're not supposed to"? AKA, wires hooked up wrong.

No, the thing is starving for fuel. It needs more fuel. I'd bet $20 bux that it's backfiring through the intake when he steps on the gas in gear.

I think that I would start by putting the stock FPR back on.

 

Will definitely do this afternoon.

 

Can’t thank you enough for your input. I’ll get a camera in there this evening and try to get a good video of it under load.

The fuel pump and rubber hose connections, all 6an connections are perfect in my opinion. No leaks, proper size fuel hoses, clamps, you name it. It’s an AC Delco pump that offers a little bit higher psi. Without it connected to the regulator / the pump sends out about 28-35 psi. I installed this for the starving fuel theory when it was my stock tbi,gas lines, and pump set up.

Yes, I have properly drilled out the size for the Inner air charger temper sensor (people call it so many different things) and I have ohmed it out properly to make sure the resistance is correct at cold and hot operating temperature.

the o2 sensor is right where the headers collect together about 6/8”s from the start of Y pipe connection. Again, it was having this problem well before I installed the headers, new fuel lines, new air filter. That’s why I’m stumped. I was trying to cancel out everything you are saying to check by process of elimination​​​​​. Nothing has availed lol.

So I have the GM plug-in code scanner. But all it shows are the codes on the dash. (Which is none) except for 12.

i thought a faulty O2 sensor and a rich / lean condition could through a code.

would a fault TCC throw a code?

Thanks a bunch,

 

Definitely keep me updated!

 

 

Take a look at this @DynoDave43 @Tom 400 CFI the first clip is idling.
Then I set the car on jack stands and put into gear and revved. This is that whining hissing noise you hear.

 

Okay, so I just heard back from the mechanic. He's suggesting two possibilities: first, the ignition coil or possibly the fuel pump especially since when we bought it the tank had been replaced. He said it doesn't even look like the right tank so it might possibly not even be the correct fuel pump. Another issue we're having is gas pouring out from the filler neck and that's not even when it's full. That just kind of happens at random. I'm not sure if anyone else has had that issue. But as far as it stalling at higher speeds, I'm gonna start with replacing the coil and see what happens from there. Best of luck to you.

 

@Miketee
Glad you were able to find out some of the issues there.

I went through that same rabbit hunt with spark, timing, coil, distributor, rotor, the whole nine yards. Unfortunately, wasn’t my issue (Atleast what I’ve been told)

As for the fuel tank - mine is stock and has the stock in tank set up. Has the sock filter and rubber grommet and small hose assembly.
You can run higher rated pumps, you just have to regulate fuel and return pressure with an external regulator. The gas sloshing around through the filler neck is alarming! Best of luck - post some videos when he gets it up and running. 👍🏻

 

Do you have a scan tool that you can put on it?

 

@Tom 400 CFI Just an Obd1 tool that triggers the flashing engine light on the dash. No codes except 12. Which I believe is normal flash test sequence code.

So I’m assuming this is not necessarily a true scan tool but simply a code tool. Do they make an actual realtime diagnostic scanner for Obd1? I didn’t think those existed.

Also, I got a video of the fuel gauge while driving under load. No needle movement at all from 13-14 psi. I’ll upload it tomorrow.

That leads me to believe one or two things that are not working properly. But I am certain now it is starving for fuel under load: not to mention the smell of the exhaust is more carbon than raw fuel.
1. The TBI is just old and shot (not convinced because I installed New Delco injectors and rebuilt the entire thing with a Delco rebuild kit)
2. The IAC /TPS is faulty.
3. ECM sending wonky signals?

I am so close to scrapping this damn thing and installing a performer manifold compatible with a nice carburetor or even save for a few months and just convert to a Holly Terminator X FI system.

 

They exist. There are at least two, Snap-On makes one, I have an OTC Monitor 4000. Both have a "data Stream" feature and can record and play back data. This would be a huge help in being able to "see" what is going awry. If you have good FP...and it SEEMS like you do, then I can't help but wonder about the MAP sensor, and/or the TPS. You can test those individually, but a scan tool makes it way easier.


3.5: ECM receiving wonky signals.

 

I don't have my factory service manual (fsm) here with me, so I'm not sure if TCC can set a code or not in our model years. OBD1 lasted until 1995, and continued to grow. So one always has to be careful when "consulting the internet", but this GM chart of OBD1 codes does show a couple of circuit fault codes.



I'm no expert, but my experience with TCC has been that it won't engage, or it stays engaged when you come to a stop. I've never seen one cycle on and off, that might create a sort of surge. Not saying it can't happen, just hasn't been my experience.

Were you able to get a video of fuel pressure gauge under load? Just trying to eliminate possibilities.

Code readers are OK, but the scan tool will allow you to see live data, like this....



There is no input for fuel pressure, but you will see coolant temp data (as supplied by the sensor), throttle position sensor, etc.

 

@DynoDave43 @Tom 400 CFI
Awesome info - I just looked up one of these scanners and it definitely might be worth the couple hundred dollars.

I plan to keep this car for quite a while. (My son loves it and my father used to have one). So it’s became the fun weekend project for all three of us.

I did get a video of it under the hood - I thought I uploaded to YouTube but I just checked and I did not. I only uploaded the one in gear on Jack stands.

It shows the fuel pressure hovering around 13-14 psi and when I easily accelerate the psi 12. When I really mashed it, it maybe dropped to about 11 or so. Would that much fluctuation cause that sound it’s making and feel like the car is stumbling?

 

A cheaper alternative to a mt2500 is moates xtreme aldl cable 1 and tuner pro rt with a $61 adx file. That’s if you have a Windows laptop.Fuel pressure on tbi iirc is 10-12.

 

boy that sounds like weak fuel pressure, try spraying carb cleaner down the tbi as you rev it. Any chance the map sensor is on a wrong port or clogged port? Tee in a vacuum gauge there.

 

What are you considering low ?