bakerstreet

I have a 92 rs 305 tbi automatic with 149,000 miles on it (everything stock), & it keeps failing hc, co & nox during emissions testing. I've owned the car for over 15 years & the car runs great & has always run great. The exhaust has always smelled bad, but it doesn't smoke nor does it burn any oil. Inside the tail pipe is black & sooty, but it has never smoked at all. I now live in an emissions county & now i'm having to deal with emissions.

It's all tuned properly, rebuilt tbi w/cleaned injectors (oem 305 injectors), new distributor, fuel pump, cts, map, plugs, wires, air filter, o2 sensor & timing is @ zero degrees as per specs. Fuel pressure is at a steady 12 psi, the tbi sprays perfectly shaped cones w/o any dripping. The egr funtions as intended, it doesn't leak any fluids & the cooling system is in great shape as well.

I replaced the spark plugs & o2 sensor & they looked normal, without any soot or any indication that the car was running rich or lean.

I replaced all of the above parts (even though they were all within specs).

No exhaust leaks, cat is about 3 years old, there are no codes, & except for the exhaust smell, this car runs perfectly.

I've been datalogging, & my closed loop results are all in spec except for the blm's. The o2 sensor voltage swings high & low as it should, yet the blm's goes straight down to 108 after it goes into closed loop.

I even tried another (salvage yard) ecm & the blm's are the same using that ecm as well.

Here is a screenshot of my datalog:

IAC-29

Coolant temp-205 degrees f

Speed-0 (just idling)

Map-1.1v (31.8 KPA)

RPM-750

TPS-0.49v (-1.2%)

Integrator-127

O2-constantly switching between .1 & .9mv

Batt-12.9v

Knock counter-2

BLM-108

IAT-110 degrees f

Everything is within specs except for the BLM's, & according to the datalogs it doesn't matter if the engine temperature is 180 degrees or 235 degrees, the blm always goes to 108 after the engine has warmed. The integrator mostly stays @ 126-128 but ocassionally.

According to the blm's, the exhaust is rich, but it does not smoke at all.

This car has always ran perfectly.

Can u guys think of any other issues that may be causing the blm's to stay @ 108 in closed loop?

jaychevyman

Start by checking the egr valve....I failed smog a couple years back and I just pulled the egr out, cleaned all the carbon and replaced it with a new one since the diaphram was stuck....car will run alot healthier as well

ex-x-fire

First make sure its running 190-200* going down the road.
Make sure the map senor hose is good, hook up a gauge to it, it should be 19-21" at idle.
Make sure the timing advances when you rev it.
Check the ecm grounds, these can be at the back of the heads or near the t-stat area.

bakerstreet

The egr valve was replaced a few months ago when i replaced the intake manifold (because it had a slight crack near the thermostat) with another factory one. There are no vacuum leaks, the timing advances when i rev the engine, the engine temperature is always steady @ 195-205 degrees or so when in closed loop.

I've already checked the ecm grounds at the ecm connectors & they were all good. If there were problems with the ecm grounds at the back of the heads, wouldn't it show no ground at the ecm connectors?

I will get a vacuum gauge & check the engine vacuum & post the results.

bakerstreet

Has anyone else had this problem of 108 blm's on a great running, non smoking engine?

bakerstreet

I checked the engine vacuum & it's within specs @ 20" at idle, so does anyone else have any ideas? I have a new bosch o2 sensor (bosch), i've replaced the o2 sensor several times since i had the car & each time i used the $20.00 bosch sensor.

Although winaldl says my blm's are 108, the car doesn't smoke at all, the spark plugs & the o2 sensor never has any black soot on them, yet the tail pipe has that black soot.

Could the bosch o2 sensor be causing the rich condition all these years?

ex-x-fire

Are you very sure those 305 injectors? If so, then your fuel pressure might be alittle high. It can be as low as 9 psi & be ok. Maybe some one put a stiffer spring in the regulator or made it adjustable.

RFmaster

Question - does your BAT voltage increases from 12.9 to 13.8 Vdc as you rev engine to 1500 RPM????

13 PSI is good - reducing fuel pressure is ill advisable.

//RF

ex-x-fire

The spec is 9-12psi. Why is it ill advised to lower it as long as its still in spec?

RFmaster

Very simple ex-x-fire - at lower fuel pressure fuel injector spray contains large droplets of fuel which results in non uniform air - fuel mixture entering cylinder combustion chambers. As fuel pressure is increased the size of fuel droplets in the injector spray decreases resulting in a fine fuel mist. This results in a better air - fuel mix overall. Further, larger fuel droplets will drop out from inlet air stream - resulting in fuel pooling in the intake manifold. 13 PSI is well withing fuel pressure gauge reading tolerance (+/-10% on good day) on top of observers parallax.

Back to the tread.

BLM 108 indicates extra fuel is getting in. We'll have to assume that he has stock 305 sized injectors. He also indicated that his O2 is switching in closed loop between 0.1 and 0.9 volts at idle. This would indicate that ECM is operating in CL. The last time I had situation similar to this when I had a fuel pod assembly with a hair line crack. It would run OK for a while then all of sadden BLM drop like a rock and engine run pig rich! After breaking down TB and looking at all parts under magnifier it turned out to be the front plug in fuel passage would leak once TB warmed up. Replacing fuel pod assembly solved that problem. This goes into extreme category. A common cause for rich condition are leaky injectors or 350 injectors installed in place of 305s.

BTW I run my TBI's at 16 to 18 PSI, stock tune is off by 12-15 points at idle. However, I do not run stock tune.

//RF

bakerstreet

I check the fuel pressure often & it stays @ 11 psi, even while driving. The battery voltage is over 13 volts while driving. I checked the injector part numbers (5235279) & they are the ones for the 305, & the injectors were sent out & cleaned while i rebuilt the tbi.

Could the bosch o2 sensor be causing this rich issue? I have always used those $20.00 bosch sensors & have always had this rich condition. But again, the car runs & drives great, but just won't pass emissions.

One thing is, that the exhaust seems to smell the same wheter it's in open or closed loop. When i first start the car in the mornings & after it goes into closed loop, the exhaust seems to smell the same.

RFmaster

Check ignition system - make sure that all cylinders are firing and spark plug wires are routed to correct cylinders. A misfiring cylinder will dump unburnt fuel and O2 into exhaust stream. That may explain high HC and CO levels and stinky exhaust even though your idle vacuum level is excellent!

Sorry, but can not comment on single wire BOSCH O2 sensors.

//RF