I/M240 Repairs: 1985 Jeep CJ5


The NOx Failing Jeep

This vehicle was a Nightmare to repair.

 

 

But before we get into the horrific details, some background information is in order:

Heavy Tampering

The Jeep was missing just about every rubber hose under the hood except the distributor vacuum advance, the carb bowl vent, and canister vapor return lines. Some GOB (Good Ol' Boy) had gutted the CJ's plumbing at some time. The double and triple vacuum solenoids (load input to ECM and AIR control) were also missing as was the decel valve, 5-port vacuum switch (EGR control), the delay valve, the vacuum reservoir, and scads of vacuum tees. We were surprised to find the EGR valve and converter still in place. Needles to say, with all this stuff missing, the feedback fuel control, AIR, EVAP, and EGR systems were malfunctioning or not working at all. We had our doubts about the CAT too--lots of miles on it.

How Did It Ever Pass?

This made for an interesting situation because the Jeep came from a "basic" I/M area--one where a tampering check was part of the I/M program. It had a valid "basic" inspection sticker on the windshield at the time. Now regardless of what the tailpipe emissions were, the Jeep should have failed its inspection because of the extensive tampering. A little snooping on our part revealed the Jeep had been recently purchased by a woman from a used car dealer. The dealer was a licensed I/M inspection station and the I/M inspector worked for the dealer. Obviously the inspector ignored the tampering and passed the car to make the sale. Tampering inspection information, in Colorado basic I/M areas, is manually entered into the I/M test computer by the inspector--so it's easy to fudge this part of the test. Herein lies one of the faults of a decentralized I/M test scheme--it's relatively easy beat the test. The tailpipe emissions happened to be within the standard--the official vehicle inspection report (VIR) was in the glove box--and its emission information is reproduced in the following table.

Idle

2500 rpm

Standard

HC ppm

33

203

400

CO %

.01

0

1.5

CO2 %

17.1

16.3

NA

O2 %

1.14

1.58

NA

An IM240 test at our lab revealed the true emissions:

 

IM 240 grams/mile

NCVECS Standard (Tier1)

HC

0.64

0.8

CO

10.8

20

NOx

8.2

2.5

CO2

376

NA

This illustrates an inherent deficiency with tailpipe tests--they don't measure NOx. So in the Jeep's case, a severely tampered, high NOx emitting vehicle passed the tailpipe test no problemo.

High NOx

Actually, the high NOx wasn't surprising given the obviously lean air/fuel ratio, excess oxygen in the exhaust, advanced initial timing, and inoperative EGR system. The question was, "Where do we start?" We began by resetting the timing to the spec of 9° BTDC and running another IM240.

 

IM240 grams/mile

HC

0.408

CO

8.736

NOx

5.166

CO2

386.92

That took a few grams per mile out, but not enough. It looked like we were going to have to do something about that gutted vacuum system. So much stuff was missing though--and affecting virtually every emission control system, that our only recourse as we saw it, was to restore the Jeep's vacuum system back to original. We replaced the triple and double solenoids, the decel valve, the 5-port vacuum switch, the delay valve, the vacuum reservoir, and the hoses and tees in about two hours time and at a list price of about $233 in parts. (We had to wait several days to get the parts from the dealer, however).We made sure all he new stuff would work, then ran a third IM240.

 

IM240 grams/mile

HC

0.45

CO

8.254

NOx

5.752

CO2

376.4

Things go to Hell in a Hurry

You don't have to be a gifted student to see all that time and money didn't help the NOx one bit! The nightmare was beginning to rear its ugly head. Yeeaahhh! I HATE it when you perform needed repairs and the problem remains. But what can you do but dig in and try it again?

We had hoped that repairing the load inputs to the ECM would restore closed loop fuel control and shift the lean A/F ratio richer to produce a decrease in NOx. No way, Jose´. So now we focused on fuel control, specifically, the carburetor. The Jeep's fuel mixer was a Carter 1-1/4 BBD feedback 2 barrel carb. This unit incorporated a feedback controlled, stepper-motor actuated, air bleed to modulate the A/F ratio. It was called an actuator in the parts books. The actuator's pintle could be observed through the carb's air horn when the air cleaner housing was removed. The pintle didn't move--it was parked in the lean position.

We checked a wiring diagram and a couple of shop manuals attempting to understand how the actuator/ECM combination worked. The books didn't tell us much--big surprise. We used an oscilloscope to check each wire to the actuator for the pulsed square waves we thought should be there.The waves were present on two of the four wires. We hoped this was correct and replaced the actuator. (It listed for $110). Guess what? The new actuator behaved the same way. More nightmares! That's what happens when you're working on an unfamiliar system with inadequate reference materials.Why do the manual suppliers assume technicians are a bunch of dorks that can't understand theory? Have they ever tried using their information to fix anything? We took the old actuator apart, figured out how it was supposed to work, (should have had square waves on all 4 wires) and discovered its mechanism was busted anyway! Sheesh!

A Process of Elimination

Now, by what had been an expensive process of elimination, we knew we had an ECM problem. We removed the ECM from behind the Jeep's dash--that was fun--and took a look at its circuit board. One integrated circuit (IC) was smoked. We supposed that IC to be a driver for one of the stepper motor's windings. After waiting a month for an ECM, the new $275 unit arrived. We installed it, the actuator worked, and we ran an IM240.

 

IM240 grams/mile

NCVECS Standard (Tier 1)

HC

0.98

0.8

CO

16.557

20

NOx

2.876

2.5

CO2

261.85

NA

STILL didn't make the NOx standard and now the damn Jeep was an HC failure too! We quit--we had to--we were out of money allocated for the repair of this vehicle. The Jeep probably needed a converter at this point.(What else was left?!) We told the owners that if they would buy a CAT, we would install it for them. That never happened, however.

Besides sleepless nights, this vehicle cost us $618 in parts (list price) and 5.5 hours of time. Caramba!

FYI: We have repeatedly found that on a carbureted vehicle, all emission controls have to function, the A/F ratio must generally be in the 14s (which at our altitude often means some sort of carb recalibration, like rejetting, metering rod adjustment, air bleed modification, etc.), the timing has to be at spec (both initial and total), and the CAT has to be good (which usually means new) in order for carbureted cars to pass stringent Tier 1 IM240 cutpoints.

Moral

Be careful what you agree to work on. A heavily tampered vehicle such as the Jeep may not be worth your time!!!!


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Text Copyright © 1996, Colorado State University; Vampire Head from Brad Middleton at www.vampyre.wis.net/vampyre; Pinky and the Brain © Warner Bros.


Rev. June 3, 1996

HMC