What could be the problem

[Bowed]

1991 silverado K2500 w/5.7 and 4l80e. Miles 75xxx


passed emissions today by a hair . By the time I got home (10 miles) the catalytic converter was red hot . Can somebody read something in the the test figures . Any info is a help . Thanks


loaded reading
(HC) PPM 119

standard to pass
(HC) PPM 300


Loaded reading
(CO) % 2.99

standard to pass
(CO) % 3.00


Idle reading
(HC) PPM 111

standard to pass
(HC) PPM 300


Idle reading
(CO) % 0.74

standard to pass
(CO) % 4.00



The 2.99 is probably heating the cat . Why ?

[imjeff]

Was it under load on the way home? Cats get hot and they get hotter when they are plugged.

[Bowed]

Originally Posted by imjeff

Was it under load on the way home? Cats get hot and they get hotter when they are plugged.

For the test ... load is being driven at normal road speed while the rear wheels are on rollers in the test bay . So driving it home was the same ( load ) as the test load , thanks .

[Bowed]

I forgot to mention that in the last few hundred miles , new parts are

fuel pump & filter

coolant temp sensor

distributor module

195 degree thermostat

Delco plugs


The engine is not overheating , just the cat and the truck runs fine except for the problem stated above .

[Bowed]

Well I was able to figure out , the readings mean the truck is running rich . That would also be why the cat is hot . Any hints on what to check first on a rich TBI ?

[JohnO356]

I'd start with the IAC, but first check that the injectors are sprying a nice even cone shaped mist.

[speedygonzales]

An IAC can not contribute to running lean or rich, particularly if it is idling without stalling. Other than idle, the IAC is out of the picture anyway.

A test on rollers is NOT like driving the truck on the road. There is no weight that is being moved and no aerodynamics involved. So don't fool yourself (or try fooling us).

Don't go assuming your cat is any hotter than it should be unless you test it's temperature with something like a IR thermometer.

If you get a chance, redo the test with the air filter removed.

[ChevyTech]

The most common problem on TBI trucks when they go rich is the coolant temp sensor.

The computer uses the coolant temp to determine how much fuel to inject and how fast the idle speed should be.

It is common for these sensors to fail.

To test the coolant sensor, unplug the wire connector and test the resistance across the two terminals of the SENSOR with an ohm meter.

If you see any coolant leaking out the terminals replace it.

If the terminals in the connector are corroded replace the “pigtail”.

On the TBI trucks the sensor screws into the intake manifold, on the front passenger side, by the thermostat housing.

Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or 45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.