1994 350 TBI Running Super Rich!

[pitzer86]

Hey guys, I'm kind of new to the forum so here is what I got...
1994 K2500 Cheyenne 5.7 TBI
Engine is running really really rich. So rich that you smell like gas after you drive it! It doesn't miss and Idles really well for how much fuel its dumping. The power is really bad though. Doesn't really want to pull itself in overdrive. I'm a decent mechanic and have worked on these TBI engines quite a bit in the past but this one has me stumped. I have a snap-on Solus scanner so I checked the OBD1 live data while truck is running and warm and everything seems to be in spec. the coolant temperature sensor is operating as it should, O2 sensor is switching between .1 and .9 volts as it should, the MAP sensor is reading about 8-9 inches and voltage hovers around .9-1.9 at idle. IAC stepper motor is operating and Injectors are misting a fine conical shape of fuel and fuel pressure is running about 11psi. The fuel trim is showing 126 and I'm not sure if that is correct, I haven't been able to source what the correct spec for the fuel trim should be. I've been reading up on forums all over the internet and a tbi running rich seems to be a common occurrence but most seem to have a bad coolant sensor or MAP sensor but mine seems to functioning well as far as I can tell from the live data. Any help or information would be greatly appreciated as I don't to simply start throwing parts at the truck. Thanks in advance

 

[AuroraGirl]

The oxygen sensor "SHOULDNT" be switching if your truck is smelling like a gas spill

that means your sensor is reading a lot leaner than it is
that can be from a leaky exhaust to bad sensor

You must be registered for see images attach

OEM heated one on the left, high mileage, slow to react but it worked
new denso on right

 

[pitzer86]

so if i have a cracked manifold it could cause it to run rich?

 

[PlayingWithTBI]

The fuel trim is showing 126 and I'm not sure if that is correct, I haven't been able to source what the correct spec for the fuel trim should be

128 is Stoich, above 128 is lean, below 128 is rich (126 is acceptable). When you see 126 and O2 sensor voltage going high and low, are you in Closed Loop instead of Open Loop? Do you see INT (Integrator, AKA Short Term) and BLM (Block Learn Multiplier, AKA Long Term) on your scanner? Do you see Cross Counts too? That's how many times the O2 sensor "crosses" over/under .45V in a given time. Over 6 Cross Counts is usually good enough. FYI - when in OL it runs off the VE tables, when in CL the PCM controls AFR based on O2 feedback and VE tables.

Are you running W/O a CAT? That would give you the gas smell too.

 

[pitzer86]

Im not seeing any of those other parameters on my scanner and truck has converter

 

[Schurkey]

"Rich" smell is usually misfire.

If it was misfiring on the passenger side (even-numbered) bank, it might not affect the single O2 sensor in the driver's side (odd-numbered) exhaust manifold.

That would account for poor power, too.

But I'd expect the engine to run rough.

A failed catalyst will cause offensive tailpipe odors. Verify that the outlet of the catalyst is hotter than the inlet. If not, it's done and you need a replacement AFTER you figure out what killed it. (Which could be excessive mileage, given that the truck is ancient.)

 

[Drunkcanuk]

Is the smell coming from the tailpipe? Or from around the cab area? The rubber fuel hose that connect to the TBI could be the culprit. Ethanol wrecks them, vapors start leaking through. Just a thought.

 

[pitzer86]

I have a small crack in drivers exhaust manifold, ordered one today, will follow up after install and see if it fixes the rich condition.

 

[Frank Enstein]

If the O2 has more than 50k miles or you aren't sure replace it when you do the manifold and clear the block learn by disconnecting the battery. Then try it.

 

[Supercharged111]

Lean stinks too.