Plugged Cat
Thread Starter
Pro
Joined: Oct 2006
Posts: 537
Likes: 0
From: Englewood Ohio
Plugged Cat
I have a 1990 Coupe That has a problem with full pedal accelaration. The car shifts into passing gear and revs to 2400 rpm and then hits a wall. If I back off a little bit then car will start to accelarate. It also idles rough. Could this be a bad Cat? Also would it be the Cat or pre Cats? How can it be tested and is this a job for a muffler shop to test.
Burning Brakes
Joined: Feb 2004
Posts: 829
Likes: 39
From: sun prairie wi
ON MY 89 when cold I could get it to 3500rpms.As it warmed the r's went down and no power.The exhaust comes off fairly easy if you can get it on a lift.My right cat was breaking up and clogging the main one.When I pulled the main off it was broken in half.After total replacement my vette has never performed this good.I wonder if it had been gradually clogging?BTW your old cats are worth $100-$150 if you can find someone to buy them.
Le Mans Master
Joined: Oct 1999
Posts: 6,366
Likes: 34
my 90 I lost the pre cats, both of them at the same time. it took out my main cat.
sometimes you can hit it with something like a rubber mallet and listen to pieces. no need to beat it to death.
I couldn't get over 2000 rpms. I was doing 35mph in 6th gear, basically coasting to get home.
sometimes you can hit it with something like a rubber mallet and listen to pieces. no need to beat it to death.
I couldn't get over 2000 rpms. I was doing 35mph in 6th gear, basically coasting to get home.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
I don't thin the clogged cat would much effect idle but could be two issues at work here.
You can test the cat by inserting a pressure gauge in front of the cat; pressure should not be more than a few PSI above atmospheric.
You could be experiencing a fuel problem. You can test for this with a gauge on the schraeder valve and then simply hammer the accelerator (with the vacuum line disconnected from the regulator and plugged). When I had a clogged filter and would see maybe a 5-7psi drop in pressure; after the new filter pressure stayed rock steady.
You can test the cat by inserting a pressure gauge in front of the cat; pressure should not be more than a few PSI above atmospheric.
You could be experiencing a fuel problem. You can test for this with a gauge on the schraeder valve and then simply hammer the accelerator (with the vacuum line disconnected from the regulator and plugged). When I had a clogged filter and would see maybe a 5-7psi drop in pressure; after the new filter pressure stayed rock steady.
Burning Brakes
Joined: Aug 2006
Posts: 1,123
Likes: 1
From: Rockland County NY
Wait till nighttime and take it out for a spin. After a few miles, pop the hood and take a peek at the pipes. If the cats are clogged, either the cats or the pipes before them should be glowing red.
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
Glowing or hot cats are normally a symptom of engine problems that can lead to damaged or plugged converters. Once cats quit working they no longer create heat from catalytic action and in fact a cat that is cooler than normal is a check used by emission shops to diagnose failed ones. The vacuum method is more reliable for determining a clogged exhaust.
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Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
With engine at normal operating temperature disconnect the EGR valve and measure intake manifold vacuum at idle speed, then raise RPM to 2000 and hold while observing the gauge, a vacuum drop of more than 1" indicates an exhaust restriction.
The direct pressure method mentioned by 65Z01 is even better if you can obtain the low pressure gauge necessary to do it, and adapters to take a reading. An old style fuel pressure gauge used on carburated cars should work if you can find one, the adapter part is trickier since any leak will invalidate the result. You can remove the air injection check valve and connect to the exhaust manifold port to take a reading.
Backpressure at idle should be less than 1 1/2 PSI. and should not exceed 3 PSI at 2000 RPM.
The direct pressure method mentioned by 65Z01 is even better if you can obtain the low pressure gauge necessary to do it, and adapters to take a reading. An old style fuel pressure gauge used on carburated cars should work if you can find one, the adapter part is trickier since any leak will invalidate the result. You can remove the air injection check valve and connect to the exhaust manifold port to take a reading.
Backpressure at idle should be less than 1 1/2 PSI. and should not exceed 3 PSI at 2000 RPM.
Thread Starter
Pro
Joined: Oct 2006
Posts: 537
Likes: 0
From: Englewood Ohio
I just went out and check fuel pressure and it is at 40psi. I pulled the oxygen sensor out of the pipe and let it idle and the car idled very well. I feel that the cat is plugged. The local muffler shop said they can remove cat and test it. If it is good they will put it back on, but I think it will be bad.
Instructor
Joined: May 2007
Posts: 109
Likes: 0
From: Ostersund (Sweden)
Hi Guys!
I just noticed this thread and I think that I got the same problem.
I started this thread about my engine is getting weak after hard driving: http://forums.corvetteforum.com/show....php?t=2052596
I just went out to see if anything was glowing - and it was!
Mostly on my left side, it was glowing on the pipe going in to the cat.
It was glowing on both sides where the pipes connect to the engine ("headers"?) but maybe that's normal after hard driving? What do you say?
/ Marcus - 1991, Automatic
I just noticed this thread and I think that I got the same problem.
I started this thread about my engine is getting weak after hard driving: http://forums.corvetteforum.com/show....php?t=2052596
I just went out to see if anything was glowing - and it was!
Mostly on my left side, it was glowing on the pipe going in to the cat.
It was glowing on both sides where the pipes connect to the engine ("headers"?) but maybe that's normal after hard driving? What do you say?
/ Marcus - 1991, Automatic
Instructor
Joined: May 2007
Posts: 109
Likes: 0
From: Ostersund (Sweden)
I just went out and check fuel pressure and it is at 40psi. I pulled the oxygen sensor out of the pipe and let it idle and the car idled very well. I feel that the cat is plugged. The local muffler shop said they can remove cat and test it. If it is good they will put it back on, but I think it will be bad.
Thread Starter
Pro
Joined: Oct 2006
Posts: 537
Likes: 0
From: Englewood Ohio
I found I have bad gas for one. I bought a hand pump and pumped about 2 or 3 gallons out of bottom of tank. I noticed that I could not smell strong gas oder. I got some on my hand and held it right up to my nose and it still did not smell like gas. I had my wife smell it and my brother in law and both said it did not smell like gas. It was suppose to be Sunoco 260. It should have been strong. I have just under 1/2 tank left by the gage. Do you think octane boost will help so I can run it out or should I just try to pump out all I can?
Thread Starter
Pro
Joined: Oct 2006
Posts: 537
Likes: 0
From: Englewood Ohio
I found I have bad gas for one. I bought a hand pump and pumped about 2 or 3 gallons out of bottom of tank. I noticed that I could not smell strong gas oder. I got some on my hand and held it right up to my nose and it still did not smell like gas. I had my wife smell it and my brother in law and both said it did not smell like gas. It was suppose to be Sunoco 260. It should have been strong. I have just under 1/2 tank left by the gage. Do you think octane boost will help so I can run it out or should I just try to pump out all I can? 
