DIY Test for Clogged CAT Convertor & Backpressure

[BNaylor]

Here's a simple do it yourself (DIY) test that can be performed to see if your CAT convertor is clogged or if you have excessive exhaust system backpressure or restriction:

Tools needed: vacuum gauge, tee connector

Connect a standard engine vacuum gage into a vacuum port off the intake system. You can tee off the fuel pressure regulator or purge valve lines located on the engine.

Start engine and maintain rpms at 2000 - 2500 rpm.

Watch vacuum gauge and reading should stabilize and stay steady.

If vacuum reading starts dropping or decreasing, it indicates either a clogged CAT convertor or restriction/backpressure in the exhaust system.

This simple procedure could save you a lot of money and headaches trying to figure out why the performance of your Grand Prix has degraded. Many times a clogged CAT convertor will not set off a odb-ii DTC error code or SES/MIL light. Its also a good way to determine whether a stock exhaust system is sufficient or whether you should mod.

Vacuum gauges are available at most nationwide auto parts stores. Cost is approximately $15 - $30.



Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

[kustomkid54]

Originally Posted by bnaylor3400

Here's a simple do it yourself (DIY) test that can be performed to see if your CAT convertor is clogged or if you have excessive exhaust system backpressure or restriction:

Tools needed: vacuum gauge, tee connector

Connect a standard engine vacuum gage into a vacuum port off the intake system. You can tee off the fuel pressure regulator or purge valve lines located on the engine.

Start engine and maintain rpms at 2000 - 2500 rpm.

Watch vacuum gauge and reading should stabilize and stay steady.

If vacuum reading starts dropping or decreasing, it indicates either a clogged CAT convertor or restriction/backpressure in the exhaust system.

This simple procedure could save you a lot of money and headaches trying to figure out why the performance of your Grand Prix has degraded. Many times a clogged CAT convertor will not set off a odb-ii DTC error code or SES/MIL light. Its also a good way to determine whether a stock exhaust system is sufficient or whether you should mod.

Vacuum gauges are available at most nationwide auto parts stores. Cost is approximately $15 - $30.



Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

[GTP Dad]

Thanks for the information Bob! It will be quite helpful for those folks with performance problems who either refuse to believe the cat is bad or just want to diagnose their own car.

[BNaylor]

Originally Posted by kustomkid54

I know you're just pulling my leg. Ok Jim, if it will make you feel better here's a pic of a typical automotive vacuum gauge. Its marked from 0 - 30 inches HG of vacuum. The green zone (normal) is around 15 - 22. Many gauges are color coded - green, yellow, red. Normally any reading out of green is bad.

When the CAT convertor is clogged the reading on the gauge will slowly decrease to yellow or below 15 - 22 or even into the red zone towards zero. Any decrease is abnormal based on my test procedure and indicates a problem with the exhaust system.

Other than that, posting detailed pics of the complete procedure is a little time consuming. The procedure is intended for the average DIY'er who wants to save money and get a handle on the situation without depending on the dealer or some shop.

I remember in the old days, we used these to tune our engines and set ignition timing, etc. Good item to have in the tool box.







Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

[BNaylor]

Originally Posted by GTP Dad

Thanks for the information Bob! It will be quite helpful for those folks with performance problems who either refuse to believe the cat is bad or just want to diagnose their own car.

No problem GTP Dad. Definitely helpful if you want to convince the dealer you may need a new CAT especially before the 8 year/80K emissions warranty expires or even after 80K miles so corrective action can be taken before wasting money on other parts that may not resolve the problem.

The procedure worked for us. Got a friend with a '00 GTP and he was ready to trade it in and give up. We ran the test and then took it to a mom n pop muffler shop. They cut off the CAT and we found it was fried. It was so clogged to where you could not see any light through it. Put an aftermarket hi-flow CAT on and its like a new car again. The original problem that caused it was a bad MAF sensor causing severe misfire.

I hope the procedure comes in handy for someone out there. Lots of CAT failures on the forum lately.




Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials

[hoot49]

how about a picture of where you attached the tee on the engine?

[BNaylor]

Originally Posted by hoot49

how about a picture of where you attached the tee on the engine?

When I get a chance I'll try posting pics later, but it depends on your engine. The test is for GPs with a Series II/III 3800 but is applicable to other GP models and engines. Its the same principle.

On the 3.1 or 3.4 engines, you should have at least two vacuum lines that you can tap into or tee right on the intake manifold. One is off the fuel pressure regulator and another off the line that feeds the tranny vacuum modulator on 1996 and under models.




Thrasher CAI, DHP v1.0 PCM,
3.25 SC pulley, CAIT GMAF, 2.5 DP/Hi-Flow CAT,
u-bend delete, 160 TS, ZZP Stage 1 TB,
TB spacer, MSD 8.5mm wires, Autolite 104s,
Goodyear Gatorback Serp. & SC Belt,
Nitto NT555R Drag Radials