After my mass-air conversion I thought I'd pass inspection with flying colors. Instead I learn that I have unburnt fuel in my exhaust. What could make the truck run rich? The O2 sensor is not throwing any codes. The only codes that may pertain to the problem are

32 (O,M) - EVP signal is/was low (EGR valve and sensor are new, but haven't measured voltage/resistance in the EVP snsor, yet).

66 (M) - Mass Air Flow (MAF) signal low (I backprobed the MAF and the voltage is around 1.1V at idle and increases with depressing the throttle. Looks fine to me.)

What else could cause a rich condition, that's not detected by the O2 sensor? I'll double check the MAF voltage at the computer and will ty to measure the voltage at the O2 sensor as well as the EGR stuff. Somehow I have the feeling it's something else, though. Only what?

 

a clogged or partialy clogged cat would cause it to run richer. I recently had to take my smog test with a partialy clogged cat and squeezed by.

 

Actually running rich would be the cause of the clogged cat, not the other way around. That code for the mass air flow sensor could cause a problem, if the maf isn't measuring the air correctly it can make it run rich.

Was the check engine light on during the test? If it was on due to those codes then it will run a little rich because it's in open loop.

Do you have the rest of the test results? The co, co2, and o2 readings can also help figure out whats wrong.

-Jon

 

I bought the cat 10k miles ago, so it should be fine.
I didn't get the other codes of the test, because the guy was so nice to tell me after the 15mph test that I was failing and if I didn't want to come back another time (he didn't charge me a dime, either).
The funny thing with the MAF codes is that I backprobed the signal lead and it reads over 1V at idle and increasing with increasing throttle. I'll have to double-check my wiring for the MAF.

The other engine codes I'm getting are
67 (O,M) - Park/Neutral circuit fault (it's a manual and it was in neutral. Where do I double check that?)

85 (O,M) - cannister purge solenoid. (1988 trucks didn't have that solenoid. Shouldn't make much difference, should it?)

95 (O,M) - Fuel pump: open, bad ground or always on (I checked the ground from the computer to the chassis, it's perfect. I even checked it from several points in the harness. What other grounds could there be that I have missed?)

My Check Engine Light doesn't come on at all anymore (I wish I knew why, I've tried everything). Today, I'll measure at the test connector if it would be coming on if it worked.
I'll also check fuel pressure and triple check all the EGR parameters, vacuum and O2 sensor voltages.

Thanks for any pointers! Keep 'em coming!

 

UPDATE: I checked vacuum at idle: just over 18 inches of Hg. I checked the EGR valve position sensor: 4800 Ohms (should be just under 5000) closed, 400 Ohms open (should be about 100). If the EGR is open at idle, engine stalls (it should). Voltage at EVP sensor harness is 5V (should be between 4 and 6V). The EGR valve loses 1inch Hg per about 10-15s if I apply about 5-6 inches Hg (should be about 30s). In short, even though the values are not exactly within specs, they shouldn't cause a 32 error message (hard fault at KOEO).
Triple checked all grounds and connections to various sensors. All checked out fine (between 1.4 and 0.4 Ohms).
MAF error message did not come back.
Still can't check engine running codes (there is no voltage across self-test connectors), so there's got to be a problem somewhere in the wiring. What other grounds are there? The one that gives me the 95 error (PCM to ground) is less then 0.5 Ohms.
Couldn't check fuel pressure (will give the %$^# gauge back to Autozone as soon as I'm done typing).

Does that give anybody an idea what else I could check?
Any help is dearly appreciated!

 

Unbelievable but true! After a long day of troubleshooting I passed!!!
Party!!
While checking the EGR system out, I sealed the EVP sensor onto the EGR valve with high temperature silicone. That must have made all the difference for the sniffer test, even though it didn't reflect in the engine error codes <shrug>.

However, I didn't pass as well as with the speed density system a year ago (2000 rpm for SD, 2500 rpm for MAF):

Pollutant | Standard | SD reading | MAF reading
HC (ppm) | 170 | 14 | 90
CO(%) | 2.11 | 0.03 | 0.2
CO2(%) | | 14.2 | 13.0
O2(%) | | 0.3 | 13.8
Nox(ppm) | 1677 | NA | 951
Dilution | (>6) | 14.3 | 13.2

Can someone please decipher those values for me??

I also noticed a ping/spark knock at 1400-1700rpm with slight acceleration and at WOT (but not inbetween). Should I change to premium fuel? (Timing is just under 10 degrees BTDC.)

Oh yes and the fuel pressure is just under 30psi for KOEO and KOER. It increases with thrittle opening and it also increases if I unplug the vacuum hode from the fuel pressure regulator.

Thanks so much for any help, guys!

 

For the exhaust gas readings - HC is the amount of unburnt fuel in the exhaust, most of this is taken care of by the cat. CO is sort of a measure of the air/fuel ratio, looks like you could be running slightly lean. CO2 is a measure of engine efficiency, the higher the better, about 14.5 - 15% is what you would see on a brand new vehicle. NOX (oxides of nitrogen) is something that is only formed when combustion temps are above 2500 degrees, your nox is a little high which makes me think that either the egr isn't working perfectly, or your timing is off, or you are running a little lean. The timing advance or lean mixture could also cause your pinging.

I haven't done any exhaust analysis recently but thats what i could pull out of memory.

-Jon

 

Great explanation! Thanks so much! So what I apparently have is a fairly efficient engine that's running slightly lean. I figure it's running lean since the EGR should be fixed and the timing is even below the stock 10° BTDC.
That reduces my problems to finding a fix for the lean condition. Could that be the slightly low fuel pressure? It is at just under 30psi at KOEO and KOER and increses with engine rpms/throttle, so the pressure regulator should be fine, no? I'll check my manuals as for fuel pump diagnostic procedures. Anyone with another idea?