Emission Test Failure

[Weatherman]

So I finally took my '91 explorer to get emissions tested (VA) after working on it. All the emissions components were in place. I did add JBA headers, a Car Sound (MagnaFlow) direct fit cat, and a Flowmaster Cat-back. It failed for NO readings, but everything else was low.

Results were: (first # = reading, second # = limit)

Speed...........15 MPH...........25 MPH
RPM's..........1392 RPM........1484 RPM
HC(ppm).......18/134............21/130
CO%...........0.01/1.08........0.01/1.34
NO (ppm).....1985/888........1677/809
Dilution............14.1..............14.1

Shop diagoned problem as failed converter(s), saying there should be minimum 100 degree difference across each one. Actual readings were 23 degrees and negative 100 degrees.

There's only 100 miles on the exhaust, and now I'm going around and around with MagnaFlow relating to the converter.

Anyone else got insight on catalytic converters, or experience with aftermarket converters? I searched through the board, and someone said one converter is NO only and the other everthing else.


- Ben

 

[hunterdan]

Check your EGR valve, its main job is to cut NO levels.

Dan

 

[MrShorty]

Except, a '91 doesn't have an EGR system.

The factory setup uses two cats. As I understand it, one is for reducing NOx and the other for reducing hydrocarbon emissions. Most aftermarket replacements are designed to replace both cats with a single unit that performs both functions. Is the cat you purchased designed as a direct replacement for one of the units, or designed to replace both units?

The standard test for a converter is to measure the temperature of the exhaust stream upstream and downstream of the converter. Downstream temperature should be significantly hotter. The shop who tested yours, for obvious reasons, believes the converters aren't working. What is Magnaflow's response to the technician's diagnosis? Many muffler shops will test Cats for nothing, you might get a second opinion. I'm not sure what would cause a CAT to fail the above test but still be good; perhaps the folks at Magnaflow have some ideas on testing the CAT?

 

[Weatherman]

The replacement looks just like the original (2 converter bodies). Right now Magnaflow will not take it back unless they have a local dealer check it to verify that it doesn't work. I'm still looking for a dealer that can test it... So I asked Magnaflow what they are looking for, testing wise. Their response is the temperature difference (which I already have). Right now they are trying to figure out whether the temperature should increase or decrease across the catalytic converters (no joke). Still waiting for my call back.

I'm also considering other setups just so I can get the thing back on the road (though I don't really want to eat the cost). Dynomax has one (direct fit) but their fine print says something about cutting the flange off of the Y-pipe (which sounds like hacking my new headers- um, no). Catco makes one, which Summit has for $180 that I may consider. Then there are other ones out there that are really the same thing with a different name.

- Ben

 

[Billy177]

new converters will NOT solve a nox problem there is a problem with your EGR system

 

[Weatherman]

I took the shop's word for it and swapped in an OEM cat. It did about the same. And as stated before, 1991 Explorers do not have EGR.

My temperature guage is low - I replaced thermostat - nothing changed.

There is no smoke, fuel pump is brand new, O2 sensor (1991 = only one sensor) is brand new, all of the emissions components are in place, it has no codes, etc - even the air box.

Any thoughts anyone?

 

[MrShorty]

Replacing the cat didn't do the job? NOx is a result of excessive combustion temperatures. Running a little lean can cause combustion to be hotter, and on these engines can also cause them to ping. I think I would check some of the same things that cause the engine to ping (dirty MAF, vacuum leaks, loose lower intake manifold bolts, etc., carbon buildup in the cylinders).

 

[Weatherman]

Yeah, I'm going through the thing now error checking it...

For what it's worth, the Walker replacement cat (OEM) blew about 200-300 ppm lower than the Magnaflow unit did. Still doesn't cut it, since I'm at 1700 instead of 2000 where I was at before, but need to be under 888. My other 1991 explorer with 50k extra miles on it blows about 450 ppm with the OEM unit.

Thought I would throw that out there in case anyone is curius - it looks like the factory cat blows a little cleaner, but not much. Obviously other demons at work...

Thanks guys,

Ben

 

[Weatherman]

In case anyone needs the information in the future, here's what I learned about catalytic converters:

The first cat removes (converts) CO and HCs. The more you have, the greater the temperature difference. Therefore a dirty exhaust causes a +100 difference. A cleaner exhaust (like mine, at least the HC/CO part) only heated things up 23 degrees.

The second cat removes NOx. Again, the more you have, the greater the the temperature difference (but in reverse direction). Since my engine puts out way too much NOx, that cat was working as much as it could, resulting in the 100 degree drop.

Jury is still out on things... Vacuum is fine, fuel pressure fine, etc. When I get it solved I'll post so that no one else has to deal with this crap.

- Ben

 

[410Fortune]

how are your spark plugs? Wires?

Manifold vacuum test was fine? I am curious how you tested this, your issue could be related to a leaky intake gasket, the lower intake gasket is known to cause problems on these 91-94 engines as they get older......start by checking the torque on your lower intake manifold bolts..

Do you have any exhaust leaks with the JBA headers?

Are you still running the stock intake tube and air box?

 

[Weatherman]

Found out from the shop that it's running lean and the timing is advanced. Makes sense with the high NOx.

I've been through every joint in the exhaust and cannot find a leak anywhere (I'm assuming I could feel a leak with my bare hand right at the joints). Putting my hand over the tailpipe gives a lot of pressure, and no discernable leaks.

Intake is stock except for a BBK throttle body. At one time it had a large MAF with open element. I switched it back to stock MAF body, box, everything, reset computer and drove it 100 miles since then (open element would fail emissions visual check). Even the little bimetal sensor/preheater/diverter is working right with vacuum.

I'm wondering if I killed the TPS somehow, or the BBK unit itself is causing the problem.

Fuel pump was dead when I went to start it a few weeks ago, so I put a new one on it. Fuel pressure reads 40 psi at key on, holds pressure for a long time, then drops to 34~35 psi when running or just applying vacuum to regulator.

Vacuum reads fairly steady at 20~21 in. Quickly revving throttle drops it down to nothing but it comes right back to 20~21. Also I cannot hear a vacuum leak anywhere. I will check the lower manifold bolts though.

Spark plugs are new, regular Autolite, gapped properly.

I looked through diagnosis info on sensors (IAT/ECT/CKP/IAC) and I'm thinking about checking into those (voltage/resistance readings).

I read somewhere that dielectric grease in an O2 sensor wire connector could cause O2 sensor reading problems because the sensor is vented through the plastic connector (I do use the grease on connectors). Sounds like far fetched crap to me, but who knows?

This is driving me nuts...

 

[Billy177]

I would pull the spout connector or whatever it is called, that shouldretard teh timing and bring your nox levels down. also clean your maf at the same time

 

[450rwhp]

t

 

[Weatherman]

Pulled the SPOUT: base timing was about 9 degrees (within the 8-12 range). Timing light shows spark on all cylinders.

I replaced the O2 sensor and plug wires.

My father has a '91 explorer that recently passed emissions, so I swapped the throttle body, ICM, MAF, and IAC from it. Also reset computer.

I tested the ECT and IAT sensors with digital meter, and they referenced to reasonable temperatures. I also tightened the lower manfold bolts (none were loose, but all got torqued to the max of the range (18 ftlbs or so).

Now, it throws a CEL, with code 41 (O2 sensor always lean). I can turn off light but it always comes back within 10 miles or so.

Still can't hear a vacuum leak anywhere. I've hosed down every joint in the engine with carb cleaner looking for a response (to indicate vacuum leak) and still show nothing.

I'm out of ideas at the moment. Only the JBA headers and flowmaster exhaust are non-stock. Everything else is stock.

 

[MrShorty]

Is that 41 both CM and KOER? What's fuel pressure? Is your O2 sensor functioning?

 

[Weatherman]

Yeah, KOEO and KOER. Brand new O2 sensor (shop had diagnosed lean condition before I changed O2 sensor though). Fuel pressure is still 40 psi static, 35 psi running (or with vacuum on pressure regulator). Book says pressure should be 30-40 psi, so I don't think it's that. My old fuel pump was dead a month ago, and I put a new one in (changed my filter too - old one was clean though).

Also tried pulling octane block and it still threw a code.

- Ben

 

[Weatherman]

Time to give up?!?!?!?

New fuel pump, pressure regulator, filter, and injectors. Pulled fuel rail, upper, and lower manifold, cleaned thoroughly, and replaced all gaskets. Vacuum tested and smoke tested intake - absolutely no leaks at all (before and after new gaskets). I can run the hell out of it and fuel pressure will never go below 30psi, not even for a second at heavy load. Compression test, coil test, every sensor tested or verified, different [TB,MAF,ICM], new wires, colder plugs, non-platinum plugs. Decided that it was sucking the preheater open too long one day so I plugged that vacuum hose. That fattened it up for about 30 miles (cut NOX from 1800 to 1000 ppm) but I guess computer readjusted and it went back to its regular lean condition before too long. Now it reads 1600 ppm no matter what I do. Talking to the shop, we got an idea and they drove it on a 45 minute round trip, brought it back, tested (failed), then used infrared thermometer on each primary tube - all hot and all within 20 degrees, so it's running uniformly lean. I've also driven it about 400 miles (illegally) since all this began a month ago. Computer always talks to me. I unplug sensors, it tells me they're unplugged, I hook them up, it says things are all OK. The "always lean" CEL never came back either.

Do I just give up on it?

 

[410Fortune]

Wow
has a compression test been performed?
how many miles on this truck?
Injectors may be old not spraying enough fuel?

this is a tough one as it seems you have tried jsut about everything I would have.

Any coolant loss?

 

[Weatherman]

Yeah, all cyls within 150 to 170 psi.
Already replaced injectors (if nothing else, I've at least got new style injectors now, which won't clog in the future).
Truck has 140k miles.

Can't really tell coolant loss. I keep draining the system to work on it. And of course, state wont let me tag it to drive until emissions test passes. At this point it looks like I'll buy a waiver (good for 2 years).

I just wired one of those autometer A/F gauges into the O2 lead. It doesn't really show a damn thing, but at least I might be able to guess what the computer is seeing from the O2.

Could a sensor wire fray somewhere in this thing, shorting, and cause problems (without throwing a CEL)? Anyway to look into this sort of thing without a breakout box?

Ben

 

[410Fortune]

on a 91 only wiring problems with emissions sensors will throw a CEL for the most part

At this point a break out box might not be a bad idea, the $80 or so shop charge might be worth every penny

You could always remove the entire wiring harness and inspect it yourself