Fixing Cars to Tier 1 Levels--
Does the Car Need a CAT?

Arizona has been IM240 testing cars to phase-in standards since January, 1995. The Arizona standards are similar to the Colorado phase-in standards illustrated in Table 1.

On January 1, 1997 Arizona will go to "Tier 1" IM240 standards. These standards may be similar to the Colorado "Final" IM240 standards depicted in Table 2.

While it is rare that a converter needs replacement to make phase-in IM240 standards, this will change when Tier 1 arrives. Many cars will need a CAT to make the 15 gpm CO, 0.7 gpm HC, and 1.5 gpm NOx Tier 1 standard--after all other problems that can cause excessive emissions are repaired.

NCVECS has two rules of thumb for meeting Tier 1 standards:

1. If the vehicle is carbureted, and has failed Tier 1 IM240 standards, it will almost always need a CAT to make Tier 1, in addition to other necessary repairs.
2. If the car is fuel injected, it probably won't need a CAT. Fuel control on fuel injected cars is so much better than what a carburetor can deliver, that CATS can function efficiently into the 150K mile range.

NCVECS uses four steps to fix a car to Tier 1 levels. They are:

1. Is everything working?
   • Compression, ignition, timing, fuel pressure and volume, other emissions control systems besides the CAT
2. Is the mixture in control?
   • What's the A/F ratio with the AIR system deactivated?
     • Measure at idle/2500; or on the dyno/open road at 50 mph.
   • Figure the Engine-out CO.
     • For F.I. cars: 5 gpm CO per 1000 pounds of inertia weight ( IM240 test weight). Find inertia weight (IW) on the Arizona Trace Print-Out. Inertia weight IS NOT the same as GVWR! Don't use GVWR!
     • For carbureted cars: 7.5 gpm CO per 1000 pounds of inertia weight.
   • Example 1: To find engine-out CO for a fuel injected full size pickup truck with inertia weight of 5000 pounds, divide 5000 by 1000 which equals 5, then multiply the 5 times 5 gpm CO. This gives an engine-out CO for the fuel injected pickup of 25 gpm CO.
   • Example 2: For a similar inertia weight, carbureted pickup, the engine-out CO is (5000/1000) x 7.5 = 37.5 gpm CO.

Engine-out CO is used as a goal to work toward in fixing a Tier 1 IM240 failure, and as a benchmark for determining bad CATS as described in step 4.

1. Compare engine-out CO to measured (IM240) CO.
   • Are they close?
     • Yes--don't fix it.
     • No--the CO is too high--fix it and retest (mass)
     • No--the CO is too low--leave it, but be aware of possible problems such as: high NOx, increased HC due to lean misfire, and lean driveability problems.
2. Finally, look at the measured (IM240) HC.
   • If the HC is greater than 1.0 gpm, and the measured CO is close to the engine-out CO, the CAT is not working to Tier 1 levels and needs replacement.
   • Conversely, if the HC is less than 1.0 gpm, AND the measured CO is less than the engine-out CO, the CAT is working.

In other words, if the engine-out HC and CO appear to be coming through the CAT untouched-the converter has to be bad. A reference chart is included at the end of this handout that lists typical pre- and post-CAT HC, CO, and NOx values in grams/mile for carbureted and fuel injected 4, 6,and 8 cylinder engines.

Let's look at two examples of the method in action: first, a Colorado car in which the above method was used to track an IM240 repair and identify a bad CAT; and second, a hypothetical repair of an Arizona car using actual Arizona IM240 data.

The Colorado car was a 1984 Pontiac Firebird, 5.0L V-8, manual 5 speed transmission, no A/C, and 132,086 miles on the odometer. The inertia weight for this vehicle was 3625 pounds. The initial IM240 results and Tier 1 standards are given in Table 3.

The car had multiple problems: no CEL or data stream, an open M/C solenoid, a bad TPS, disconnected MAP sensor hose, and disconnected AIR system for starters. The expected engine-out CO for this vehicle was 27.2 gpm. Replacing the missing CEL socket and bulb fixed the CEL and data stream problems-the ECU was good. The M/C solenoid and TPS were replaced. The M/C solenoid was adjusted to 33 degrees dwell at 3000 rpm. This gave an A/F in the 14s. At this point a second IM240 was performed. The results are given in table 4.

Obviously the engine-out CO was passing through the converter untouched. This was verified by the HC and NOx values which also indicated a "no change" of engine-out HC and NOx through the converter. The next round of repairs included repair of the AIR and EGR systems-and a new converter. Everything has to work to make a Tier 1 standard. The third and final IM240 results are given in Table 5.

For Arizona technicians, this method of identifying bad CATS will probably be most useful when fixing marginal Tier 1 IM240 failures. The second example illustrates this concept. A 1985 3.8L V-6 carbureted Pontiac LDV comes into the shop. It has failed Tier 1 IM240 HC, CO, and NOx standards. The actual Arizona IM240 data is shown in Table 6. (The A/F ratio and IM240 retest data are estimates of their true values--actual data was not available).

The inertia weight for this vehicle is 3375 pounds. The calculated engine-out CO for 3375 pounds of inertia weight is:

We compare engine-out CO to measured (IM240) CO. They are about equal (21.3 ~ 25.3). Let's say the A/F ratio at 2500 rpm is 14.5-that's good. From the A/F ratio and measured/engine-out CO comparison, we know we do not have a mixture control problem. We also know our engine out CO is apparently passing through the converter untouched.

Let's assume a quick scope check shows the engine and ignition to be OK. A scanner reveals no trouble codes. Additional checks show other emission control systems to be present and functional--although we better check the EGR system closely--because the NOx is about 1 ½ grams higher than our expected engine out NOx feedgas.

Lastly, we look at the measured (IM240) HC. It is 2.16 gpm. That value is greater than our benchmark of 1.0 gpm. In fact, comparing the IM240 HC to the engine out HC in our feedgas chart indicates the HC is passing through the converter untouched too. The CAT has to be bad. Unfortunately, we don't have the Arizona retest data for this vehicle. If the CAT was replaced , and the EGR system repaired, we could expect retest numbers like 0.2 gpm HC, 5.5 gpm CO, and 0.9 gpm NOx.

This method saves the technician time in that no Delta T, or intrusive CAT testing is needed. The method is also useful for charting a technician's progress through an IM240 failure repair, if repeat IM240 tests are done between repair steps.Repeat tests may negatively affect a technicians "report card", however.

Following are some examples of cars with good converters and deteriorated converters. Notice the HC gpm and compare the measured CO gpm with the calculated engine-out CO gpm. It also appears that NOx often tracks HC but not always-see the 1992 Ford F-150 below.

Clean Cars-Good CATS:

Cars With Deteriorated CATS:

Copyright ©1996 Colorado State University

Rev. August 16, 1996