I recently posted about my 91 305 TPI Trans AM failing emissions and my car running rich.

I pulled my plugs and 3 had black tips. The front 2 and passenger side rear. They also smelled of gas. I verified that each is getting a spark....and when a plug is pulled when the engine running, there is a noticable difference.

So they are firing...just not firing right. And if the sparks good..then is it bad injectors?

If this sounds right, then how do I check those injectors?

 

Slow 91,

You are running rich. Check the oxygen sensor. If it hasn't been replaced in the last 30,000 miles, now would be a good time. If you have a data logging scanner, you can read the O² sensor while the engine is running. I'll bet you're getting below 300mV pretty consistently. The common failure mode of this type sensor is that the signal degrades over time, slowly reporting more and more rich mixture from the lower voltage output. This would "fool" the ECM into thinking the engine is running lean, and the ECM will add fuel to compensate. Sound familiar?

If the oxygen sensor is correct, the output voltage should be well over 600mV, reporting a rich mixture. If this is the case, the ECM would be trying to lean the mixture. If the O² readings are correct and you're still too rich, you may have an air leak into the exhaust, through either failed gaskets, holes, or the air injection system.

You should also check the EVAP canister and purge system, since a saturated canister can force lots of fuel into the intake and cause your symptoms.

If all that is correct, you should monitor the fuel pressure just after the engine is shut off. If the fuel pressure drops more than a couple of pounds in the first five minutes, there is a good chance that you have a leaking injector(s). If the fuel pressure holds for 15 minutes, forget about the injectors for now and concentrate on the control system.

Another easy way to check the ECM and oxygen sensor control loop is to allow the engine to warm up, then force the ECM into Field Service Mode by grounding the diagnostic terminal of the ALDL. The MIL lamp should flash on your instrument panel. If the system is in closed-loop mode, the MIL will flash about once every second. If the sytem is in open-loop mode, the MIL will flash about every 2½ seconds. You'll have to warn the engine more to achieve closed-loop. Once in closed-loop, the MIL flashing faster than once per second indicates the O² sensor is detecting a rich mixture and the ECM is trying to lean it out. Flashing slower than once per second indicates the O² is sensing a lean mixture and the ECM is trying to enrich it.

My guess is that your ECM thinks the mixture is lean.

Waiting for results...

 

thanks for the info vader...

I just replaced the oxygen sensor. With my scan tool, the signal bounces above and below 450mv during closed loop. From what I understand this is the correct signal. So the ECM is compensating.

I havent gotten a guage to test the fuel pressure yet.

From the emissions graph, the HC's seem to peak when the gas pedal is released. I've also replaced the cat and preformed a full tuneup withen the past month.

here is some more info from my scantool...
TPS was 0.58 Volts at closed throttle
BLM was 113
Coolent Temp changed from 180 to 230 degrees F
IAC was 031 at idle
The injector pulse width was 1.7ms at idle
Loop status was closed
MAP volts were 1.70V at idle
MAP was 12 IHG at idle
MAT was 140 degrees F
The Spark Adv bounced between 11 - 19 degrees at idle but jumped to 30 degrees when I rev'd the engine

 

So it's back to the injectors or other source of excess fuel.

Check the EVAP canister purge, FPR diaphragm (pull the vacuum hose while the pump is running), check for air leaks in the exhaust and vacuum leaks, and power balance test the injectors by unplugging them one at a time. Don't forget to get a good fuel pressure reading.

Aside from that, the MAP signal seems a little high (low vacuum), so that may be a hint of a leak somwhere.

 

Slow91, If your car has the original injectors that it came from the factory with, they are Multecs (gray plastic bodies with a silver band). These are notorious for going bad and this could be the cause of your failing the emissions test.

Take a DVOM and ohm test each of the injectors. They should all test around 16 ohms. My bet is that you will find that the three cylinders that had black plugs will have lower ohm readings. If this is the case, you will need to replace the injectors and I would suggest replacing all eight of them and not just the three that are reading low.

Shadow

 

if you want to save some money on injector replacement, a few people have reported sucess running ford injectors in their chevy TPI setups.

http://cq-search.ebay.com/search/sea...t&BasicSearch=

 

thanks for the help guys...

I got a wavier for emissions today, so I'm good for 2 years. And I am gonna take your recomendations and test/fix those injectors when it gets a little warmer outside....its gonna snow tonight in chicago.

Here is a tip for you all...If you print professional looking receipts from your computer. The emissions place usually wont check.

At least mine doesn't.