FlameFeather

I have a 91 Trans Am, 5.0 305 Tuned Port Injection. (Automatic Transmission with the OEM more MPG ratio rear gears, if it matters.) I have just replaced the plugs and wires, cap and rotor, as well as O2 sensor but none of that helped. In fact, it's gotten worse.

It starts up runs fine for the first time of the day until it warms up. Then it misses real bad and will stall out at stop signs, traffic lights, and any time the RPMs dip below 1000 or 800 or so (like going slow down a hill). Most of the time it will start back up after stalling out, but sometimes it takes a bit of turn over/cranking and some times you will have to hold down the accelorator to get it to start. There was one or 2 times where it did not want to start back up. Now I'm worried I may have broke something trying to get out of holding up traffic. (I reved it up to get started, held it in neutral and dropped it gear. I don't think my drivetrain liked that too much. Neither did I).

Just tonight it has gotten as worse as it's ever been because going down the road I lost all power and went from 60 to 25 mph at the drop of a hat, and when I'd push the accelorator it wouldn't do nothing (got more out of it by easing into a sweet spot on the accelorator than pushing for more gas). It would also pop and backfire a little bit (also complete with plug pinging) and you could feel the pop through the gas pedal. The more you pushed the more it would pop and act like it was dying. But if you let off a bit, it seemed to catch power better. This could be that I'm practically driving on 4 cylinders, but it hasn't done this before even with the misfiring and stalling out. These problems are continue to progress to get worse.

There was also a subtle fuel smell that you could smell. Especially after parking it.

We've checked fuel pressure and that seems fine. We've pulled the plugs one at a time and it seems that the entire passenger side is missing. We've ran the codes and I've got a code 12 and a code 45. What are those codes? I know it's running too rich.

The next thing we need to check is compression.

I've only replaced the O2 sensor on the mainfold. Is there another one behind the cat?

The fuel pump had been replaced about 5 years ago, so I doubt it's that.
This car has had a history of getting a rich check engine light. I had a fuel injector replaced 7 years ago that never seemed fix the light issue. However I think the mechanic may have mistaken which injector, or some other error.

So I'm wondering, what are the odds that this is a Fuel Injector problem? Or is it possibly the EGR Valve? Or maybe something else.... what does it sound like to you guys?

I love this car and I'd hate to watch her die like this. This is my first post, and I plan on sticking around and getting to know everyone here. So all you guys who help with this will become my newest best friends! I will be greatful for your help.

Rolling Thunder

Well I kinda want to say its fuel injector problem. Reason being ignoring everything else for a moment you mentioned one side of the motor wasnt firing. TPI is a batch fired system firing 1/2 the injectors at a time. First ide check for +12v and ground at the fuel injectors and ohm them out and see what your get.

the code 1-2 is basically an everythings ok code you will get that one even when everythigns ok and the code 45 supprise supprise indicates the cars running rich.

there is only 1 oc sensor none after the cat

camaronewbie

I'd check continuity on all the plug wires first. Everytime I've seen backfiring, it was electrical and not fuel related. It's amazing how a new set of wires can be bad right out of the box. Double check firing order, and make sure the wires are seated on the plugs fully. Might also check ignition module in the distributor.

F-Body Demon

iTrader: (1)

I was having a similar problem with my TPI, It would stumble backfire and die. Needed a fuel filter....im not kidding lol.

FlameFeather

Thanks for the replies guys. I mentioned that TPI only fired half it's injectors at once to my Dad and he wanted to know why and how that is. So I figured I'd ask here. How does TPI work in that way exactly?

I was thinking of re-checking the plugs wires as well, because despite swearing up and down he's sure, my dad (who did the tune up and is a long time drag racer/mechanic) has been through a traumatizing head injury from an accident 6 years ago that makes me wonder if he may have simply made a mistake. (His memory isn't what it was before the accident.) He says he's sure, but still... I'm going to double check. Know where I can get a chart of the firing order?

Do you use a Multimeter to check the OHMs? I'll need to get one.

Also, if the Fuel pressure reading was fine, could it still be a fuel filter, fuel pump, or fuel injector?

Thanks again everyone for your help...

FlameFeather

I just tested the OHMs in each fuel injector but I'm not sure what the results mean. I've heard they should all be within .5 of each other and none of them were. I tested first cold, then warmed it up to operating temperature and tested it warm with the engine off. Here is what I got...

Chart explanation: OHMs reading - (Passenger side)Cylinder__Cylinder (Drivers side) - OHMs reading

16.6 - 8________7 - 11.6

3.9 - 6________5 - 16.1

16.1 - 4________3 - 14.7

10.7 - 2________1 - 15.3

Note: Cylinder 6 fluctated from 3.9 to 4.0, and Cylinder 3 fluctuated from 14.8 to 14.8.

Hot...
11.0 - 8________7 - 11.5

1.8 - 6________5 - 16.7

11.5 - 4________3 - 12.0

8.3 - 2________1 - 13.3

The readings were alot lower when it warmed up, and they were all over the place. None of them were within .5 like I heard someone say in a youtube tutorial.

What does all this mean? Do I have some bad fuel injectors?

Rolling Thunder

Well resistance changes with temperature so im not so concerned about that but what i am concerned about is the one that reads 3.9-4.0 that one is way too low and needs to be replaced. Ide replace them all as a set personally as like you said they should be relatively close to each other. I dont recall exactly what the spec is off hand but again to me i can say with certanty you have at least one that is blatantly bad and the others although they may be OK i would replace them as well as they don't look great either. Also if one failed you can be assured the others probably are not far behind.

FlameFeather

What types go with the 305 engine? I've looked around a little and all I saw was 24 lb but doesn't that go with the 350/5.7L?

Rolling Thunder

Yea factory 305s are 19 pound injectors your right the 24s are for 350s.

FlameFeather

Thanks Guys. I relayed this info to my dad who's a hardcore drag racer and has been working on these types of cars for well over 25 years and has way more experience then myself, and he still has questions. He suspects something wrong with the ECM.

Would the ECM give these same symptons if it were bad?
Would the bad injectors cause the ECM to go bad?
And finally, how do you test the ECM to see if it's still good or if it's malfunctioning?

Rolling Thunder

Well although if the ECM was bad it could cause any number of problems even this one theoretically i suppose. Although i highly doubt the ECM is bad as there pretty reliable units and the results from ohming your injectors explains your symptoms as it was what i was thinking it was to begin with. Whats happening is the ECM has limitations on the amount of current it can put out to drive things like injectors. When you have an injector like that 4 ohm one it essentially shorts out that entire bank of cylinders which explains why that side isn't firing. No you said it was also running rich which at a glance wouldnt make sence seing as how 1/2 of your injectors arnt firing. However keep in mind the O2 sensor tries to maintain the proper a/f ratio. the only way its going to be able to do that if half the motor is just pumping air is to over enrich the side that is working which explains why its running so rich. So really although the ECM can cause any number of things i firmly believe its the injectors and even if the ECM was bad as well you still need to change the injectors why not do the injectors and see how it goes from there? The only way really to test the ECM is with a test fixture but for cars these old its not going to be something you'll be able to run down to your local parts store and have done most likely although no harm in asking I guess. Beond that its just replace it and see but the ECM is the very last thing to suspect in all cases. Especially considering you have some other known problem that could explain it all. Now could the injectors kill the ECM? In all likely hood no. Im not exactly sure what drives the injector outputs of the ECM but i believe its current limiting output meaning even in the presents of a short it wont kill itself. Now i could be wrong about that and if its not current limiting its plausible that it could short out the driver for the injectors but again if you replace the injectors and it still dosnt work right then other possibilities could be explored.

Toehead

You are correct. The ECM has thermal limits on its injector drivers. This means that they will shut down if they are shorted. The 3.9 ohm injector is likely shorting that entire bank and after the driver heats up, that whole bank stops working. You need at least one injector, but it may be time for a set.

Also, when it dies, have you noticed any tach irregularities? Does the tach suddenly drop to zero or cut out? The reason I ask is that the ignition control module also causes hot cutout problems.

Edit- I also saw that one injector dropped to 8 ohms when hot. That is also too low. Looks like you should get a set of new injectors... People seem to have good luck with ford injectors.