clogged cat converter?

96 Chevy Blazer, 4.3L vortec; 199,600 miles.

Dont know where to start...
For some time now my service engine light has been anywhere from flashing, to off, to staying on all the time. Mostly staying on all the time. I've had it coded numerous times and always got the typical "multiple misfire" answer. It would miss and jerk when shifting through first and second. This it only happened radomly, for the most part, it ran fine at other times. I did notice a jerk at high speeds every now and then.
Now, I've been hit with a $250 tow bill because 40mph is not a safe speed to drive on the interstate. :smile: No I wasnt trying to improve my gas mileage, thats as fast as it would go with my foot to the floor. I only went maybe a mile or so like this because the engine started overheating. Here's what happened; Driving at a cruising speed of around 70+mph, no jerking or missing, no A/C. I noticed the speed droping everytime I hit a slope, temperature gage started to creep up above normal. I pulled off at a gas station, filled up, checked fluids everything fine. Pulled back on interstate and then moaning and straining started. Spitting, sputtering and with my foot on the floor could not get above 40mph. Then the temp gage shot to the red, I immediately pulled over and expected it to die completely. It didnt, it still runs but just not good. Had to have it towed about 80 miles to the house.
Well, all the good old boy shade tree mechanics I talk to thinks its the Cadalytic Converter stopped up. But I thought I'd run it through the forum to see what you guys think before I start throwin $$ at it. Any suggestions?

Behind the manifold on the drivers side is the schrader valve to check the fuel pressure- CHECK IT First- Running and not running. The multiple misfires is associated with fuel NOT Cat Conv. PSI should be running 54-60 psi shut truck off and watch the gauge does it stay at 58 psi or does it drop-?
If drops chnage the the cpi unit under the manifold and the fuel lines feed/supply- let us know the results..

The missfire problem should be fixed.
Missfire is aleading cause of cat problems.

For a test on clogged cat or exhaust.
Check engine vacuum at idle and at 2K rpm.
If vacuum drops below idle vacuum at 2k you have a restricted exhaust system.

And as said check that fuel pressure for sure.
MT

I checked the vacuum, but please verify that I had my gauge hooked up right. I grabbed a tube that what running to this ball shaped mechanism that is mounted to the hood itself (What is that thing anyway?). I took the tube loose from there and thats where I plugged my gage. I was getting 16in-Hg at idle with engine at normal operating temp. At higher rpm (no tach-I guestimated about 2k) I was getting about 18in-Hg. Then it would shoot up past 20 when I let off the accelerator then back down to 16.
So if I hooked up correctly this means I have no exhaust blockage, plus someone told me I should have smelt a sulphur smell if the cat. was clogged.
My next check is the fuel pressure check which I'm having trouble locating a tester. I may have to bite the bullet and buy one. I'll post results when I get them.

The ball or can tank is the reserve tank.
Not a real good place to check.
Best to get rigt on the intake or close to TBI or book right into the bake booster vacuum.

Smell,from cat does not say it is plugged up.
A good vacuum check or back pressure will tell you.

You need to check that fuel pressure for sure.
Borrow rent or buy a gauge.
AZ and some parts places have loaner tool.
The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

Where might I find the "bake booster vacuum"? :p I bet you meant the "brake booster vacuum".

Dont mean to sound ignorant but what is the TBI?

I dont know if I could make it 20-30 min on the highway. One of the issues was it started overheating as soon as it started bogging down. But yesterday, while running the first test it idled for maybe 15 minutes and didnt get above normal so I'm at a loss with what is up with that. Overheating may only occur while under a load. I would have thought bogging down and overheating would be a sure sign of a clogged cat, but thats why I'm here.:confused:

Where might I find the "bake booster vacuum"? :p I bet you meant the "brake booster vacuum".

Dont mean to sound ignorant but what is the TBI?

I dont know if I could make it 20-30 min on the highway. One of the issues was it started overheating as soon as it started bogging down. But yesterday, while running the first test it idled for maybe 15 minutes and didnt get above normal so I'm at a loss with what is up with that. Overheating may only occur while under a load. I would have thought bogging down and overheating would be a sure sign of a clogged cat, but thats why I'm here.:confused:
Yes brake.
The big vacuum line that runs to brake booster.
The TBI is where the gas linkage butterfly is.

Plugged in where the brake booster hose plugs into the intake. 17 in-hg at idle, just as I started to increase rpm it would drop to 15 then jump up to around 18 or 19.

Sounds good.
If vacuum a 2 K steady rpm does not drop below idle vacuum exhaust is OK.
Move on to fuel pressure and other stuff.
MT

Cold start pressure = 60psi
Engine running pressure = 55-56psi
One the road pressure = 59-61 psi
Shut it off and doesnt drop below 60psi

Changed fuel filter, checked all the above again with no change.

What next?

Fuel pressure is 5 or more lbs low.
Is it still acting up on the road?
Any check engine lights on?
Is the coil good hot blue spark and good hot blue spark to all plugs?
How are they and what mileage and or what kind of spark plugs?
MT

Fuel pressure is 5 or more lbs low.
Is it still acting up on the road? YES
Any check engine lights on? YES For some time now my service engine light has been anywhere from flashing, to off, to staying on all the time. Mostly staying on all the time. I've had it coded numerous times and always got the typical "multiple misfire" answer.
Is the coil good hot blue spark and good hot blue spark to all plugs? Whats the easiest way to check that? Take each plug out, attached wire and hold to ground?
How are they and what mileage and or what kind of spark plugs? I'll have to check that one. I think they have about 20-30k on them.

Check fuel pressure on the road when it is acticg up.
Check direct fuel pressure at fuel filter with fuel pump good and hot after 20-30 minutes running..
A good fuel pump will have 85-95 direct fuel pressure.

They make spark testers for checking spark.
Check for spark at coil wire and coil.
Pull plug wire and check at all plugs.
It should have a lod snap and blue spark.
And jump well over 1/4 inch.
Pull plugs and look at them.

If not Ac-Delco repace them with AC-delco.

Also check for codes and post back code no.
MT

My vehicle is doing the exact same thing, same symptoms, same thing with the check engine light going on and off at random.

I can add to this discussion though - I just changed my plugs, plug wires, and air filter, and it's still doing the same thing.

If anyone figures anything out, let us know.

My vehicle is doing the exact same thing, same symptoms, same thing with the check engine light going on and off at random.

I can add to this discussion though - I just changed my plugs, plug wires, and air filter, and it's still doing the same thing.

If anyone figures anything out, let us know.

Best to.
Start a New Thread on yours.
Give all of the info on it .
And someone will help you.
MT