[ChaseK]

Ok I bought a 92 Firebird 3.1 around a year ago and it wasn’t running when I got it. The P.O. Had replaced the entire distributor, plugs, IAC, and fuel filter trying to fix a misfire and one day it just stopped being able to crank and he sold it to me. I fixed the no crank and got it running again and am in the middle of trying to figure out the misfire.

The problem I’m having has some really weird symptoms I’m going to list everything I have noticed but it might not all be important
1. A really bad misfire at idle that gradually gets worse as the car warms up
2. If I fluctuate the throttle around just above idle the misfire gets significantly worse sometimes causing the car to die
3. While it’s misfiring if I open the throttle just the slightest bit the car immediately revs up to 3000rpm
4. If I unplug the tps the car immediately revs up to 3000rpm
5. It has a much higher fuel consumption while misfiring
6. If I let the car get up to operating temp the tach starts jumping around (like up to 5-6k rpm) even though the rpm is staying relatively consistent
7. Check engine light with code 33

I fixed a few vacuum leaks throughout the car. That didn’t change anything. Then, after some research I replaced the ICM and I finally heard the car run “normal” I drove it around a bit and nothing seemed bad I parked it and disconnected the battery to clear the check engine light. Later that day I started it back up and it was back to its old habits. I went back into the distributor to look and see if anything was different and it wasn’t so I put it back together and started it back up. It ran great like a normal car for about 2 months during that period I changed the plugs again and wires with them. randomly a couple days ago it started again and ever since it’s been off and on running good and bad I haven’t been able to figure out what causes it to switch between running good and running bad
Any advice would be greatly appreciated and thank you to anyone who read that whole thing

 

[ZumpTA]

You didn't mention any stored codes when you got the car, but mention clearing the codes. I'm assuming the Check Engine light is on again. If so, there are stored diagnostic codes waiting for you. All you need to get at them is a paperclip or piece of wire as prior to 1994 it was possible to "flash" the codes on the instrument panel.

Here is the procedure as documented by shbox.com; let us know what you find:

​

 

[ChaseK]

You didn't mention any stored codes when you got the car, but mention clearing the codes. I'm assuming the Check Engine light is on again. If so, there are stored diagnostic codes waiting for you. All you need to get at them is a paperclip or piece of wire as prior to 1994 it was possible to "flash" the codes on the instrument panel.

Here is the procedure as documented by shbox.com; let us know what you find:

View attachment 233165
​

When I bought it it had codes 22 33 34 35 but nothing has come back except code 33

 

[ChaseK]

When I bought it it had codes 22 33 34 35 but nothing has come back except code 33

I also have plugged in a snap-on obd-1 scanner and the only thing I’ve learned from that is that the TPS is within spec

 

[ZumpTA]

Seems to mean the same thing as on my '93, which it should being OBD1.

Meaning: Manifold absolute pressure (MAP) sensor – circuit malfunction
Causes: Wiring open circuit/short circuit to positive, hose blocked/leaking, MAP sensor, ECM

You need to use a Factory Service Manual to run through the diagnostic flow-chart for this code and determine which cause fits the 4 potential causes fit. I know '93 is a different animal, but the same diagnostic steps should apply. I'm sure the "voltages" are the same. If not, there's "generic" instructions all over the internet for diagnosing MAP issues; but the flow-chart in the '93 manual should be much closer if not an exact match to the procedure you need to follow. I'd start there unless you have a '92 Factory Service Manual handy. Chilton & Haynes have nothing on the Factory Manuals.

Failed MAP system can most certainly cause your symptoms and negatively impact fuel economy.

Here's the link to the 4th gen manuals, best I can offer; grab the '93 manual and go to page 6E3-A-64

MediaFire

MediaFire is a simple to use free service that lets you put all your photos, documents, music, and video in a single place so you can access them anywhere and share them everywhere.

www.mediafire.com

Looking around, I did see mention of MAF being a potential cause as well. Something about it reporting more air entering the system than possible. Just for grins, you might want to put a multi-meter on the MAF and see if it's within spec too. They didn't put a MAF sensor on the '93, so I don't know the first thing about it. The '94 manual might have the information and measurements you need to test it though.


Good Luck!

 

[ChaseK]

Seems to mean the same thing as on my '93, which it should being OBD1.

Meaning: Manifold absolute pressure (MAP) sensor – circuit malfunction
Causes: Wiring open circuit/short circuit to positive, hose blocked/leaking, MAP sensor, ECM

You need to use a Factory Service Manual to run through the diagnostic flow-chart for this code and determine which cause fits the 4 potential causes fit. I know '93 is a different animal, but the same diagnostic steps should apply. I'm sure the "voltages" are the same. If not, there's "generic" instructions all over the internet for diagnosing MAP issues; but the flow-chart in the '93 manual should be much closer if not an exact match to the procedure you need to follow. I'd start there unless you have a '92 Factory Service Manual handy. Chilton & Haynes have nothing on the Factory Manuals.

Failed MAP system can most certainly cause your symptoms and negatively impact fuel economy.

Here's the link to the 4th gen manuals, best I can offer; grab the '93 manual and go to page 6E3-A-64

MediaFire

MediaFire is a simple to use free service that lets you put all your photos, documents, music, and video in a single place so you can access them anywhere and share them everywhere.

www.mediafire.com

Looking around, I did see mention of MAF being a potential cause as well. Something about it reporting more air entering the system than possible. Just for grins, you might want to put a multi-meter on the MAF and see if it's within spec too. They didn't put a MAF sensor on the '93, so I don't know the first thing about it. The '94 manual might have the information and measurements you need to test it though.


Good Luck!

Thanks for the advice I do have a 92 factory service manual along with both chilton and Haynes manuals I will look into that tomorrow I think the map was also replaced by the PO as well though

 

[ZumpTA]

I think the map was also replaced by the PO as well though

Frankly, doesn't matter if he did. So many electronic replacement parts these days have a short service life, erratic operation, or are DOA. I say these days, it's been a good 15/20 years, but the last 10 have been hardest on Quality / Reliable electrics. Unless you personally purchased a reputable brand from a reputable source, always question it. Even if you bought the best brand from the best retailer, there's still no guarantee you've got a good component with a long service life on your hands, just much better odds.

Pretty much all production of everything has either moved to China, or if it was previously in China, offloaded to a "cheaper" production facility; most likely sub-contracted to Africa. Africa is quickly establishing itself as "China's China" for manufacturing after all.

Most of the "Genuine Delco" stuff on Amazon and eBay is counterfeit now too. The real "New Old Stock" items are very few and far between at this point.

The Duralast electronics from AutoZone are still of reliable quality, as are their Duralast Gold mechanicals.

 

[Injuneer]

There's additional wording in the DTC 33 code description - “….signal voltage high, low vacuum”. Look at the code description in the manual to see what the threshold values are for high circuit voltage and low vacuum. If you have a vacuum gauge, hook it up to a full vacuum port. Check the harness wires for 5 volt reference voltage. An intermittent “open” in the reference voltage wiring can cause high signal voltage.

If you can read the MAP and barometer from the Snap-On scanner, the vacuum can be calculated by subtracting BAR from MAP. Just make sure the units are consistent- “Hg, or kPa. MAP at idle should be 30-35 kPa. BAR depends on the elevation above sea level and local weather conditions. At sea level BAR should be in the range of 95-100+ kPa. Issues with the lifters or rockers can cause low vacuum.

 

[ChaseK]

I looked at the flow chart in the factory service manual and it says to correct the rough idle before doing the chart. I am thinking that the reason the code 33 is thrown is because of how bad the car idles when it decides to misfire. Non of the other codes it had when I got it have come back unless I unplugged the iac or tps I’m going to keep looking through this manual and try a couple other things it recommends

 

[1993TransAm]

OP: Does the vehicle "hunt" for an idle, or just idle rough?

 

[ChaseK]

Ok I am more confused than ever now as I tried doing what the factory service manual said to do in the flow chart and it says to correct the rough idle first and to go to the symptoms section and the first thing in the symptoms section says to read the block learn on a scan tool which I did and it was 128 with the car off and 128 with the car on according to the book that means that it is running rich but the scanner says that the air file ratio is at 14.5/1 at idle. Also the book says to unplug the map sensor to put the Ecm into “backup” mode. When I did that the car reved up to around 2,800 and started climbing consistently to where I had to shut it off around 3.5-4k after I plugged the map back in it idled back down but was still missing really bad and after driving it around the block it started backfiring every couple seconds but stayed running. This car has me running in circles every time I feel like I’m getting somewhere it changes things up I don’t even know if I’m dealing with a fuel, spark, or, vacuum issue anymore. Everything seems normal the only thing I can think might cause all of this is maybe the ECM ?

 

[ChaseK]

OP: Does the vehicle "hunt" for an idle, or just idle rough?

It doesn’t hunt just misfires really bad it sounds like it’s got a giant cam lol

 

[1993TransAm]

Might you have a stuck valve or two?

 

[ZumpTA]

Might you have a stuck valve or two?

Good call. I'm now wondering if the "new distributor" has a faulty ICM. Without knowing where it was sourced from, "new" doesn't carry much weight. Did they buy the $300+ Pertronix or equivalent distributor? An MSD? Or did they cheap out with the $78 Cardone unit.

With these things sitting around $300 from reputable manufactures and a brand new Cardone unit sitting alongside them at 25% the cost, and no-name units selling for even less; I have my suspicions of what's installed, and I certainly wouldn't trust it until I verified it. Even if they got the $178 Summit Racing "Special", I'd question it.

Given the intermittent nature, I want to lean towards "electrical". In my experience, mechanical failures are rarely intermittent, but, I'm not an "internals" guy, so; Yeah.

I'd personally be doing diagnostics on the "new" ignition components, specifically the ICM that came with this new distributor.

 

[ChaseK]

Good call. I'm now wondering if the "new distributor" has a faulty ICM. Without knowing where it was sourced from, "new" doesn't carry much weight. Did they buy the $300+ Pertronix or equivalent distributor? An MSD? Or did they cheap out with the $78 Cardone unit.

With these things sitting around $300 from reputable manufactures and a brand new Cardone unit sitting alongside them at 25% the cost, and no-name units selling for even less; I have my suspicions of what's installed, and I certainly wouldn't trust it until I verified it. Even if they got the $178 Summit Racing "Special", I'd question it.

Given the intermittent nature, I want to lean towards "electrical". In my experience, mechanical failures are rarely intermittent, but, I'm not an "internals" guy, so; Yeah.

I'd personally be doing diagnostics on the "new" ignition components, specifically the ICM that came with this new distributor.

I’ve already replaced the ICM with a new AC delco replacement unless I’m thinking that I either have a bad ground somewhere that I don’t know about a bad pick up coil or a bad ECM the car almost seems like some days it’s happy and others it’s not when I replaced the ICM it ran the same way and the day after it ran great and it ran great for atleast a month until randomly it became intermittent again

 

[Injuneer]

Ok I am more confused than ever now as I tried doing what the factory service manual said to do in the flow chart and it says to correct the rough idle first and to go to the symptoms section and the first thing in the symptoms section says to read the block learn on a scan tool which I did and it was 128 with the car off and 128 with the car on according to the book that means that it is running rich

Nothing above is correct. There are multiple BLM (block learn multipliers, or long term fuel corrections) cells. If the ECM has been “reset”, or the power to the ECM is lost, the BLM values reset to 128. 128 is a neutral point - it is indicating neither rich or lean. It results in 0% correction to the fuel flow.

A number above 128 indicates the O2 sensor was reading “lean” (significantly below 450 millivolts), and the ECM corrected the lean condition by adding fuel by increasing the injector pulse widths. A number below 128 indicates the O2 sensor was reading ”rich” (significantly above 450 millivolts), and the ECM corrected the rich condition by subtracting fuel by reducing the injector pulse widths.

Most likely your ECM was reset, and the BLM's defaulted to 128. They aren’t indicating anything. The idle BLM's are only meaningful if the vehicle was allowed to idle, fully warmed up, for 10-15 minutes.

….but the scanner says that the air file ratio is at 14.5/1 at idle.

The scanner value it the target A/F ratio the ECM is trying to supply. The BLM's indicate whether the actual A/F ratio is above or below the target. The target is typically 14.7:1 with the engine fully warmed up.

Also the book says to unplug the map sensor to put the Ecm into “backup” mode. When I did that the car reved up to around 2,800 and started climbing consistently to where I had to shut it off around 3.5-4k after I plugged the map back in it idled back down but was still missing really bad and after driving it around the block it started backfiring every couple seconds but stayed running. This car has me running in circles every time I feel like I’m getting somewhere it changes things up I don’t even know if I’m dealing with a fuel, spark, or, vacuum issue anymore. Everything seems normal the only thing I can think might cause all of this is maybe the ECM ?

Did you consider making the simple tests I recommended in post #8? Why not?

 

[ChaseK]

Might you have a stuck valve or two?

I guess it could be that but the engine surprisingly doesn’t chatter very much for having 180k on the clock

 

[ChaseK]

Nothing above is correct. There are multiple BLM (block learn multipliers, or long term fuel corrections) cells. If the ECM has been “reset”, or the power to the ECM is lost, the BLM values reset to 128. 128 is a neutral point - it is indicating neither rich or lean. It results in 0% correction to the fuel flow.

A number above 128 indicates the O2 sensor was reading “lean” (significantly below 450 millivolts), and the ECM corrected the lean condition by adding fuel by increasing the injector pulse widths. A number below 128 indicates the O2 sensor was reading ”rich” (significantly above 450 millivolts), and the ECM corrected the rich condition by subtracting fuel by reducing the injector pulse widths.

Most likely your ECM was reset, and the BLM's defaulted to 128. They aren’t indicating anything. The idle BLM's are only meaningful if the vehicle was allowed to idle, fully warmed up, for 10-15 minutes.



The scanner value it the target A/F ratio the ECM is trying to supply. The BLM's indicate whether the actual A/F ratio is above or below the target. The target is typically 14.7:1 with the engine fully warmed up.



Did you consider making the simple tests I recommended in post #8? Why not?

I did check the vacuum with a gauge and it was in the “good” section of the gauge even though it was fluttering a bit with the misfiring if I remember correctly on the scanner the Map was in the 60s kPa and the BAR was 104 kPa I did not get to check the map reference voltage because I don’t have a multi meter rn I had a friend borrow it I should have it back tomorrow to check that voltage Sorry for not updating you on that

 

[Injuneer]

60 kPa at idle is not good. A stock cam, correct valve adjustment, no misfires should be at 35 kPa at idle. 104 BAR less 60 kPa MAP indicates 44 kPa vacuum = 13”Hg vacuum. Should be pulling 20” Hg. Check valve adjustment if you've already verified condition of spark and fuel.

Does seem to indicate the MAP sensor is OK, but why won’t DTC 33 clear? Can’t figure that out. I'll have to read the factory service manual again.

 

[ChaseK]

Ok I found a massive vacuum leak today at the brake booster the hose was completely off of the port I must have popped it off by accident while I was trying to get to the back of the engine for the ground on that side that made it back to how it was running before. I also checked the harness plug and I have ground and I have that 5v reference from the ECM. Also I did confirm that while the engine is misfiring the vacuum drops significantly (around 10-15 hg) but while it is running good the vacuum is in the green on my vacuum gauge (around 20 hg) also I looked into the O2 sensor reading and it is hovering right around 450mv. I let the car idle for around 15-20 minutes and get up to temp where I noticed that the block learn had changed to 156 not sure if that means anything because it was only at operating temp for a shorter period. Maybe it is something to do with the valves because The engine from what I know has around 180k on it I need to do valve cover gasket’s so whenever I get to that I will check on them. Side note are these engines supposed to run at around 230 degrees Fahrenheit ? My car has a electrical fan and it doesn’t kick on until around 230-235 but once it kicks on the temp quickly falls to around 180 before the fan shuts back down.