1991 Firebird, TPI engine,

Problem symptoms
- Runs pretty much ok until you put your foot into it, then it just falls flat. Top rpm limited to about 2500 or so. As long as you are not pushing it, seems normal.
- Sometimes erratic idle, 500 up to 900 or so, normally about 700~800? Last month or two.
- Sometimes runs rough at slow speeds. Last month or so.
- Smell gasoline occasionally at stop or slow speed, no visible leaks at engine or on ground. Can’t smell from engine area right after stopping. Seems to come from interior air vents ! Last couple of weeks.
- Check engine light was on while max speed dropped to about 35 uphill, bad cutting out. Day was very hot in desert, had set by road for about 1/2 hour to get tire changed, this happened soon after restarting.
- After very carefully easing throttle, shifting as needed, light cleared after about 15 minutes, seemed to run much better, but I couldn’t check it as I had on a “space saver” spare, limited to about 50 mph. Ran seemingly ok like this for about 250 miles at about 50 ~ 55. Still had somewhat rough and erratic idle and low speed. Code was "lean condition".
- All gauge readings normal at all times. no further codes after the one time.

Fuel filter was plugged bad, now new filter installed. Fuel pressure ok, throttle position sensor ok, catalytic converter / muffler ok (runs same with it all removed). Plugs new.

Maybes :
- Fuel tank vent / vapor canister / canister lines plugged - Try with fuel cap removed ?
- Low voltage to ignition circuit?
- Bad / loose connection to control boxes, etc.
- Remove two red wires (CB and radar detector) plugged into aux radio connector - should be no problem, but just to make sure - CB has some backfeed into the circuit, regular radio plays for about 3 seconds after switch turned off.

HELP?!?

 

First of all, I can see why this problem is perplexing your mechanic. My first thought would have been plugged cat. Youve already been there. The lean code tells us that your fuel supply system is delivering insufficient amount, that your fuel management system is not functioning correctly, or that you have an ignition system failure. Its important to note that O2 sensors do not detect HC. Fuel/air mixture thats gets throught the engine unburned will look to the O2 sensor like air. The O2 voltage will go low as if the engine is running lean, even though its loaded with unburned fuel.

First of all I would like you to try running with the fuel cap either off, or loose to elimminate the tank vent/pressure control valves as possible suspects. I have not seen much trouble with these but lets be sure. Then, I would like you to connect a fuel pressure gauge, a spark tester, and a scan tool so that you can read them all while driving.

I have a fuel pressure test set with an extension hose that allows me to tape the gauge to the windsheild for easy viewing during this test. You may need to assemble something for this. Be sure to use high pressure hose. A long spark plug wire will allow you to connect the spark tester so that it can be taped to the windsheild as well. Use the "flash bulb" type in line tester for easy visibility. I want you to recreate the failure while monitoring fuel pressure and spark. At the same time, I want you to connect a suitable scan tool and monitor Integrator, BLM, O2 voltage, and MAP voltage. This can be tricky so take care. Do not watch data at the expense of driving safely. Somewhere between these factors, you will find your problem.

The likely causes are: Spark drop out, O2 sensor reading false lean(shorted) causing system to go over rich, killing power and causing fuel odor. Fuel system failure causing pressure to drop off at higher engine loads/speeds. MAP sensor reading inaccurate, causing lean or rich condition.

Spark drop out may be caused by a faulty coil but more likely by a primary ignition failure such as in the ICM. If you find there to be an ignition failure, I suggest replacing the ICM and coil together as one can cause the other to fail.

 

The shop read the code, but hasn't yet given me the actual number, just said "lean", supported by the horribly clogged fuel filter. That code hasn't returned since the first time in the middle of the desert, and no others have showed up.

I'll report the actual code later, if they have it, as well as any results, or my incarceration due to induced insanity and wallet dehydration...

THANKS AGAIN!!!

 

Solution apparently found - bad #4 injector. It passed all tests, but when unplugged, car ran good (for a 7 cylinder. ,much better than about the two that it felt like) for a short test drive, much longer than all the previous test drives when it did not run good.

Apparently the malfunctioning injector confuses the ECU and then it screws up the overall running.

I'll know for sure tomorrow after the new injector is installed, and will confirm here.

 

Confirmed - runs well now. Never would have thought one bad injector could make it run so bad.

 

If tested the injector would likely reveal a shorted coil which was pulling down power to the other 3 on that bank. I hope that your mechanic replaced the injectors as a set. If not youll be back soon. Sorry to be so rash about it but its proven true too many times. A blueprinted set from Southbay or FIC (both TGO sponsors) is about $260. Of the items I listed, I appologize for forgetting injectors. Maybe the most common cause of lean condition. Man Im getting old.

 

Replaced just the one, seems to work fine. I told him to check the others carefully, visually and electrically and any other way he had. He doesn't have a way to check flow. This one did check ok by resistance, found it by disconnecting it.

Where are Southbay and FIC located, I want to do the full set if it happens again. Big problem is that I have ONE car, and waiting a week to get a shipment just doesn't work.

Thanks!

 

Southbay is on the forum by that username. FIC I dont know for sure. You might try a search.

 

Here is FIC's # 770 888 1662

 

Final Update -

Just a few days later, another injector (#7) went bad. I followed up on my plan, got a full set of 8 rebuilt injectors from Southbay.

After installation, VERY much better running, even from when all 8 were working before. Both smoother low rpm, and high rpm power. Now a couple of weeks later, all is still well.

Southbay was excellent to work with, and good prices.