Desperate for CA Smog help - 98 Frontier with reman engine will NOT pass smog due to high NOx

[svizh]

Hey everybody,

This is a repost from the clubfrontier forum, and I am pretty desperate for advice. My beloved 1998 Frontier SE has been at the mechanic for going on 3 months now. We are both at our wits end with it. I am trying to get funding through the CAP program to fix the smog issue/help pay for the new engine, so it needs to pass with this mechanic. It is mainly a smog test shop, and he's not a Nissan specialist, but he has been very communicative and understanding with me calling him every couple days.

The mechanic put a new/reman engine in (it had a blown head gasket) but it will still NOT pass CA smog due to high NOx, reading around 1000ppm. He says it's running fine, but is hesitating intermittently, which is when the high NOx readings happen.

Parts replaced/checked: exhaust manifold (replaced with Magnaflow), cat (replaced with Magnaflow), upstream O2 sensor (checked/new within the last 5 years), downstream O2 sensor (replaced with Bosch), EGR (checked), timing (quadruple checked), fuel filter (replaced), PCV (checked), coolant sensor (replaced), BPT valve (replaced - lowered NOx readings but not enough), MAF (cleaned), intake (cleaned), hoses (checked), knock sensor (replaced)...

No codes.

I have asked him to check the fuel pump and injectors next. A buddy of mine, who is a great mechanic but lives out of state, suggested there may be some gunk in the tank. It was sitting for a good 6 months last year.

What the F is going on! Any advice?

[fontana dan]

Has the mechanic checked live data on the fuel trims or o2 reading during this "hesitation" to get a better idea of what is going on?
Fuel pressure? Injector balance test?

[VStar650CL]

High NOx with normal HC and CO is virtually always from an EGR problem. It may be an issue with the plumbing or the BPT and not the EGR Valve itself. None of that stuff is expensive, I'd replace all of it.

[svizh]

Thanks for your replies.

He hasn't mentioned the fuel trim data but he has checked the O2 readings. I imagine he will check the trims today, but I will ask him to next time we speak.

He doesn't think it's the EGR and has cleaned it and checked the solenoid -- however, he was going to try to replace the EGR anyway but couldn't get the hard line off. I'm not totally convinced at this point that it's not the EGR, but not sure where that leaves us as I am not sure what to tell him about the stuck line.

[VStar650CL]

The EGR's entire purpose in life is to reduce NOx, it has no other reason for existence. High NOx along with HC or CO can mean other things, but when HC/CO are normal, you'll virtually always find an EGR fault at the bottom of NOx.

Alternatively, you could ditch communism and move to a free state.

[svizh]

The EGR's entire purpose in life is to reduce NOx, it has no other reason for existence. High NOx along with HC or CO can mean other things, but when HC/CO are normal, you'll virtually always find an EGR fault at the bottom of NOx.

Alternatively, you could ditch communism and move to a free state.

Makes sense to me. I just went down there to talk with him some more. He'll be checking the fuel trim/injectors and fuel pump next.

I asked about the EGR and he couldn't get the nut off the tube connected to the valve itself, not the one (from my understanding) that's usually stuck on the intake. He didn't want to risk damaging the tube. I told him if the fuel system is working ok then he should go ahead and replace the EGR. Guess I'll see if he can find a replacement tube as well, and if not, I will have to find one.

As far as the communism... well, I actually got this truck for cheap from my comrade if you can believe it. It helped build our community space, which is part of the reason I'm so attached to it. As an anarchist myself, I agree with you that the state (especially CA) has done jacksh*t for me, my beloved truck, the environment, and for all the people with way bigger issues. And my favorite taco spot just got dinged by the health department and can no longer cook their meats on a grill and it just doesn't taste the same, but I digress...

Thanks for the help.

[VStar650CL]

According to NissanPartsDeal most of the EGR parts are still available, including the main tubes. The valves are pricey but RockAuto has them cheaper, as well as the gaskets. The only things that aren't available are the coupler passage (KA) or guide (VG), and those can likely be cleaned up.

And my favorite taco spot just got dinged by the health department and can no longer cook their meats on a grill and it just doesn't taste the same, but I digress...

The new(some) normal. (sigh)

[svizh]

Unfortunately the EGR tubes available on NissanPartsDeal are for the VG33E, not for my KA24DE.

I went to the junkyard today and no luck on a tube. On the bright side, I did find a pretty good condition headliner (sans cloth) to replace my crappy one.

The mechanic sent over these readings off the O2 sensors at ~800 and ~1813rpm, and the fuel trim readings: https://postimg.cc/gallery/tT0ChWN

Both banks are reading constantly lean. Does this point to one or two faulty O2 sensors? Injectors? The upstream O2 sensor was last replaced pre-catastrophic head gasket failure, and the downstream is a new BOSCH.

Tomorrow I'll ask him if he did a smoke test and if he's SURE there's no vacuum leaks. I would still like him to replace EGR but not sure what to do about the seized tube.

Thanks again.

[VStar650CL]

Oh, that bites, they don't have the CARB tube, just Federal. Next best thing is to check the plumbing by manually applying vacuum to the valve at idle, if the plumbing is decent it should stumble severely. Little or no response indicates a blockage.

If your guy says the CO/HC is normal, I seriously doubt you have a vacuum leak. Leaks pretty much always cause HC that's high and unstable, and usually low CO. Small leaks will also cause elevated STFT at idle but little or no elevation at higher RPM, and if I'm reading your pics correctly, you're running similar plusses at both throttle settings. That contraindicates a leak.

[Niti QX4]

If you have a tubing bender, bend an EGR line from NICOPP to get through the test. It will hold up more than long enough and you can move on with your life.

[svizh]

Thank you for the tip on the NicCopp line, will definitely use some if need be.

After testing the O2 sensor and wiring, the mechanic thinks the issue is with the ECU. The downstream O2 sensor is reading at 300mv, even when it's disconnected. He also checked the ground. He's going to do a couple more tests.

I'm not convinced that the ECU was malfunctioning when I brought it to him, pre-engine swap, but I don't have a definite way to know.

I'm also not convinced the malfunctioning ECU is the culprit of the high NOx readings. From my understanding, the downstream O2 sensor simply indicates whether the cat is functioning, and does not affect engine performance. I would still lean more towards the EGR and tube needing replacement. I'm wondering if I should tell the mechanic to replace the EGR and tube before the ECU.

[VStar650CL]

It's not normal for the downstream O2 to be "stuck" like that, but the ECM's do have an internal "fixed bias" and an open wire could also cause that behavior. However, the cat must be warm and lit for the rear O2 readings to mean anything. The rear O2 will sit there flatlined until warm, and it should be throwing a code after some driving if it's truly misbehaving. Since it isn't, I suspect that's barking up the wrong tree.

[Niti QX4]

I agree with you. Tell him to replace the line and leave the ECU alone for now. One thing at a time.

Then measure the results.

[svizh]

Quick update, mechanic still can't get the damn tube off. Think I'll head over there tomorrow to try and get it off myself to save him the trouble..

In the meantime, I've posted a couple places seeking the EGR tube (part no. 14725-F4503) and am keeping an eye on the junkyards.

Anyone know if a tube out of a manual trans would fit? The part numbers are different.

[Niti QX4]

How did it go?

[svizh]

I went there on Monday and got the 30mm nut off the tube on the valve side after some PB plaster and double-wrenching. But the mechanic didn't want me to take it off the manifold side since it's aftermarket and he said the threads aren't exactly right.. Wish he would've mentioned that before I went down there. I looked in the tube, was coated in carbon but didn't seem clogged. Sprayed some brake cleaner in it but didn't do a whole lot else to it. Wish I could've taken it off completely, but since the manifold is new, and he swears he cleaned it out, I'm hoping it's fine.

It was closing time before I got the new EGR put on. He finished it yesterday I believe, but I am still waiting on smog test results and pulling my hair out!

[Niti QX4]

Thanks for the update. Best of luck on the results.

[svizh]

Still not passing with a new EGR, NOx numbers are the same at about 1000ppm.

Going to swap in a new PCM next and then take a look at fuel injectors if that doesn't solve it.

Wondering if a PCM from a '99 will swap into a '98 with no issues?

Feeling pretty defeated...

[svizh]

Thinking it over more, and I would really rather not replace the PCM. Seems like a big headache, having to either get a replacement programmed or ship out the existing one to be repaired, especially considering I'm not convinced it is the issue.

Just to reiterate, the mechanic believes the PCM should be replaced because the voltage on the downstream O2 sensor is reading at .29V, even when it is unplugged. My understanding is that the downstream O2 sensor only exists to see if the cat is functioning and does not affect engine performance. Am I incorrect?

I think the main issue is that it's intermittently hesitating, as well as the High NOX/Low HC/Low CO readings, which would point to running lean. I have not seen the hesitation myself, the mechanic described it as such. The mechanic doesn't think it's lean, not really sure why.

Reposting the fuel and air readings in case anything sticks out:
RPM - 788
ECT - 179F
O2 MON - LEAN
O2 MON S2 - LEAN
B/FUEL SCHDL(mS) - 1.14
O2 HEATER - On
TPS(V) - .44
O2(mV) - 710 (.71 V)
O2 S2 - 290 (.29 V)
MAF(V) - 1.50
INJ(mS) - 3.38
O2 HEATER S2 - Off

RPM - 1813
ECT - 177F
O2 MON - LEAN
O2 MON S2 - LEAN
B/FUEL SCHDL(ms) - 1.15
O2 HEATER - On
TPS - .52
O2(mV) - 790 (.79V)
O2 S2 - 290 (.29V)
MAF - 1.89
INJ(mS) - 3.13
O2 HEATER S2 - Off

Short term fuel bank 1 - 11.7%
long term fuel bank 1 - 9.4%
Intake manifold absolute pressure - 9.3mHg
@812 rpm

Latest emissions test (15mph)
HC: 3
CO: 0
CO2: 15.5
O2: 0
NOx: 1008

[VStar650CL]

Check the wiring to your "stuck" HO2S for an open circuit, and ohm the sensor itself. As I mentioned earlier, all ECM's have a fixed internal bias for the O2's. Nissan ECM bias runs around 300mV when open. It's very unlikely the ECM is responsible, and on most Nissans the ECM uses the rear O2's as a cross-check for the front sensors. That means a flatlined (but not shorted) rear O2 can confuse the ECM.

Note that low HC does not indicate a lean condition. Even a tiny amount of lean misfire will drive HC higher, not lower. What you might have is a bad rear cat. Most Nissan rear cats are "high reduction" Rhodium types specifically aimed at NOx. It's not something we "free state" technicians need to account for very often, but it occurs to me that it might be your issue.

[svizh]

Thank you for clarifying about the rear O2, especially the ECM bias and confusion. I will let the mechanic know about that and will ask him about the wiring. He said he checked it, and the ground. Maybe the sensor I got from O'Reilly was bunk.

And great info re: the lean condition, good to know about the low HC. However, he did replace the rear cat with a Magnaflow CA compliant cat.. which was pretty painful to my wallet.

[Niti QX4]

1. You mentioned hesitation and the higher readings happening then: Is it possible fuel flow interruption? Maybe put a gauge on the fuel rail and watch for pressure fluctuations?

2. So, your O2 sensors are indicating a "lean burn" condition. Lean burn produces NOx due to higher combustion temperatures (and is the whole reason for the EGR system).

Try this for grins. Go one heat range colder on your plugs. And then if that doesn't work, go to a lower temp thermostat. All of this is to lower engine temp in order to help lower combustion temperatures to reduce NOx. This is only for the test though, as I would go back to regular plugs if the cooler plugs "fixed" the NOx.

Another option is to use water injection to lower combustion temperatures, but with the standard plugs.

If it were me, I'd try the water injection first.

https://www.roadandtrack.com/new-cars/c ... orsepower/

https://www.motortrend.com/how-to/impp- ... injection/

And you don't have to buy an expensive kit either. You can make your own with a washer bottle, a small pump, and a spray nozzle. People have been doing this since the 1960's due to higher compression and poor gas quality. Since you are just trying to pass the test, it doesn't need to be active after the test either. You could use your existing windshield washer bottle, and just get an auxiliary pump for short term use.

Oh, and as a last thought, if they still have your old cats, have them install them after the new CA cats to maybe help with the clean-up.

[svizh]

Brief update -- Looking for a decent remanufactured ECM.. Waiting to get the correct part number from the mechanic.

I think fuel delivery issues are the more likely culprit, but after insisting on replacing the EGR and it not making a difference, I'm just going to roll with ECM swap for now.

Thanks again for your help and I will keep everyone updated.

[Niti QX4]

Any updates on your progress?

[svizh]

The saga continues:

Finally received the PCM this weekend and mechanic got it installed today. No change.

Mechanic is going to call the state and thinks they will give me a smog waiver. I'm going to try to get the truck back from him and work on it myself, but without the extra money from the state, I'm not sure that I will be able to afford to. Not loving the idea of shelling out thousands of dollars for a truck that still has issues.

I was leaning towards fuel system issues -- although the mechanic ran it on propane and it still hesitated. Could it still be fuel injectors? Maybe a wiring issue? I'm just at a loss here. Since the readings did not change between the first engine and the second, it must be a part that was reused in putting in back together.

[svizh]

Another quick update, the mechanic called the state and says he wants one more day (probably more like a week) to try and fix it. I guess the state doesn't wanna give up their money to a truck that doesn't pass smog and needs a waiver.

I told him my last ditch ideas are:

An issue with the distributor/wires/plugs
Radiator or water pump clogged from head gasket sealant causing high temps
Fuel system delivery issues (injectors and o-rings, fuel injection pressure regulator, fuel pump)
Bad MAP sensor
Bad IACV
Issues with EGR vacuum lines he might have missed, EGR gasket, or solenoid issues (he had mentioned something about the solenoid a while back)

Appreciate any input.

[Niti QX4]

When was the last time the testing machine was calibrated?

Is he running the tests right?

When was the mechanic last certified to test emissions?

It seems crazy to me that all the things done have yet to lower that one number any further. Especially since he used propane also.

[svizh]

It is crazy!! I don’t know specifically when he was last certified but he was one of the only mechanics on the list for this state program. He has great reviews and is always very friendly to me, and has tons of people there getting smogged with no issues as far as I can tell.

Hoping he can find an issue with the injectors so we can be done with this.

[Niti QX4]

...Hoping he can find an issue with the injectors so we can be done with this...

No doubt! Good luck with this - my fingers are crossed for you

[svizh]

Thank you!

So the mechanic called and he is going to replace the TPS. He says it controls the EGR solenoid. I did some search on here and looks like he may be right, although it was mentioned only one time. I'm curious about this and am going to do more search.

From what I understand, the logic here is that the TPS sends a signal to the PCM, which then activates the EGR solenoid given the proper conditions. With a malfunctioning TPS, the PCM will not send the signal at the correct time, and the EGR will not activate. The TPS being the issue makes sense given that a) the EGR system has been replaced, cleaned out, and checked and is fully functional and b) the mechanic thought something might be wrong with the PCM.

Also looking back to the fuel/air voltage readings he had sent a while back -- the TPS is reading at .5V at 1800RPM. The FSM spec says it should be reading at *4V* at WOT. Seems like something he should have noticed!

Fingers crossed.