TMZIrocZ350

I just failed emissions for the second time because of high NOx. Just changed the O2 sensor before the second time around and it dropped the NOx considerably, but not enough. 614 is passing and got a 704. I checked vacuum lines to egr and I can't get an vacuum from the TB. I pulled off the TB and cleaned all the passages real well and made sure the vacuum lines are all set. I also checked the egr by bypassing the silenoid and nothing happened. I would expect the engine to stumble a little at idle. I am out of ideas, any help out there.

DENN_SHAH

iTrader: (1)

some more info on the car would help.
do you have a factory EGR valve or is it one from the aftermaket?
where are you located at? if your in Cali, it makes a difference on what you can do with the timing.
posting the other numbers can help even though they will be after the cat, CO, CO2, HC & O2 all mean something & can help point out a problem.

the vacuum port won't show a vacuum reading at idle, its a ported source so you need to open the throttle a little. you will probably need a vacuum gage to to check it, some are pretty low, like around 6~7 inches or so.
at 704 i think your EGR system is working.
normally when the EGR isn't working you see much higher NOx numbers, like 1500 & up.
carbon deposits can raise NOx.

check your timing, a little too high can have a big effect on NOx.
if you can, pull some timing.

TMZIrocZ350

Hey thanks for the info.

The car is an 89 IROC with a 350 TPI.
I am in CT and to the best of my knowledge the timing hasn't been changed, but I will check. It is also the original EGR valve.

Here are the numbers for the 2 tests:

Test #1
HC
Limit 88 PPM
Emissions 24 PPM

CO
Limit .28
Emissions .02

NOx
Limit 614
Emissions 1041

Changed the O2 sensor after first test and went back
Test #2
Same limits

HC 24 PPM
CO .02
NOx 704

The NOx dropped by quite a bit with the O2 sensor change. The other thing that I am worried about is that the car puked a fuel pressure regulator a little while back and was running a ton of fuel through the car. I am not sure if this could have burnt up the cat. The cat is aftermarket, but I don't know what brand or if it high flow or not(bought the car that way).

Damon

That's odd. The line on the TB where the EGR vacuum line hooks up should have vacuum at all times (full manifold vacuum). It's only when it gets to the EGR vacuum solenoid (near back of engine, pass. side) that vacuum gets "regulated" under control of the ECM before being sent to the EGR valve itself on the intake. The valve itself won't be sent any vacuum at idle- only at part throttle, under load, and on a warm engine. Otherwise, no vacuum to the valve (valve closed).

I'd be checking why you don't have any vacuum at the TB EGR vacuum port before replacing any parts downstream. Are you hooking it to the correct vacuum port?

TMZIrocZ350

Damon

I have the vacuum lines hooked up correctly. Also I did not mention this before, but the car does not throw any codes for the EGR. The only code I have ever got was for the speed sensor when the speedo was broken. I think the next step is to get a full diagnostic from my go to guy. Better off spending the money on that and not replacing a bunch of parts I don't need.

DENN_SHAH

iTrader: (1)

Damon, some motors use a full time intake source, many use a ported source.
the reason for that is when your on the gas intake vacuum goes away, by about 1/4 throttle you have no intake vacuum but you can still use EGR to help lower NOx & reduce spark knock.
with a ported source you still have some vacuum to open the EGR valve.


TMZIrocZ350, with the CO & HC numbers you have with 704 NOx i will almost guarantee your EGR valve is working right.
normally engine temp doesn't have a major affect on NOx, but your pretty close, so it is something to check.
the cat may have gotten hurt from the raw fuel. also some aftermarket cats have problems with NOx.

some other things that can cause higher then normal NOx are, the EGR valve & ports maybe a little dirty causing a small drop in the amount of EGR. combustion chamber deposits can raise compression enough to bump NOx up.
you may be running just a little bit lean. the fuel mixture can be off by a small amount & give good CO & HC readings but cause the cat to not do much for NOx. also, running a little lean raises combustion chamber temps. it may be time for an injector flush.

check the timing, pulling 2 degrees may possibly get you a pass on NOx.