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Discussion Starter · #1 ·
My ex 91 B4C has a over heating rear Cat that has melted the mat to the floor

The mechanics are puzzled.

They have replace the O2, MAP sensors and the idle air control.

It runs good when cold but surges between 500 and 1000 RPM when warm. Also uses lots of gas causing the cat to over heat. Since this is the same engine as a Z28 I would like to know if anyone else has seen this problem.

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Al 95 Z28
 

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Could be anything.
If there were a vaccum leak, the O2 sensor would sense the oxygen in the exhaust and richen up the fuel mixture.

If there were a plugged injector or ignition problem. The O2 sensor would sense the oxygen in the exhaust and richen the mixture.

A bad fuel pressure regulator?

Have they plugged in a scan tool and checked the computer data?
David

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Discussion Starter · #3 ·
They have tried just about everything with scan tools and found that the chip has a 3 digit number that is not registered. This has to be investigated further. GM Canada was previous owner before me. New owner is now walking.

The car did run for 9 years with no problems so it has to be a fault of something.

Compresion, leak down ok. Fuel pressure ok.

They have followed all normal diagnostic proceedures and have contacted GM.

Distribultor replaced due timing variation because of shaft wear.

The mechanics have a lot of info and do performance work.

Idle injector pulse width changing but the reason for this is not know. Disconneted injectors one at a time and nothing changed.

I talked to the owner of the shop since last post so this is new info. Wife took the first call so info was limited.

I have electronic training in an other industry but high performance cars are my passion and this has me baffled. As the car is 65 miles away everthing is by remote control.

Thanks for the input
 

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it's pluged up.take the o2 sensor out and put a pressure gage in it and see what kind of back pressure you have.???? think it should be under 3 psi.
 

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Easiest way to check a cat for plug-up is to find a shop with a temperature sensor gun, hand held. A goot smog/tune-up tech will have one.

Aim at hot converter, engine running, out of gear. Inlet reading should be about 100 degrees or so COOLER than the outlet. If the inlet is hotter than the outlet, the cat's plugged. Cats raise the outlet temps, so this is a valid test.

If not, there is a problem upstream. Could also be plugged muffler, and hand in the exhaust stream at pipe end test is warranted.
 
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