Catalytic Converter

After replacing engine, tranny and all related sensors I'm still having what I consider to be a lack of power up to 2000 RPM. Acceleration on the interstate can be a bit dramatic...ie cruising at 2100 RPM 75 MPH to accelerate it seems to jump to 3000 to get any power with a bit of pedal travel in between.

For those that don't remember....I put a new crate engine and transmission I bought the whole kit from GM with the following.

1998 Chevy Blazer VIN W 4WD 4DR

New Radiator
New Thermostat
New Waterpump
New MAF
New EGR
New IAT
New TPS
New Alternator
New ECT
New Coil
New Plugs (Delco Plats) and Wires
New MAP

The Intake and EVAP, wiring harness,Ignition module and PCM were reused...After a warranty issue with the CAM GM did the injectors and poppets as well.

I also have replaced the Distributor, PCV, Vac hoses, 3 O2 sensors, Belt, tensioner, Fuel filter and battery during installation. Fuel pump is 1 year old.

While I know it seems excessive, I had a new engine and tranny so I thought since everything was original I might as well and start from new.

As I've had 2 major episodes of misfires since I bought it new I'm thinking the Catalytic converter is done but am looking for another opinion.

The Post-catalyst O2 sensor is showing a definite pattern matching the Pre catalyst on bank 1...although not as dramatic...switches just as often.

GM LTFT are B1= -2%, B2 = 0%
CAM RETARD is 0 Degrees (thanks to MT)
No Misfires Recorded
No Current or pending codes.

Tried the socket in the tailpipe trick...didn't seem to make a difference.

I'm thinking of a Walker for $ 130.00 from Advance.

Mike

Downstream o2 matching the upstream? That sounds like a bad converter, not a pluged one to me.
If you think the cats plugged, take out both upstream sensors from the pipe and see how it runs. Runs better = bad cat. Runs same = ok cat. (not nessecarily good, but ok)
Socket in the tail pipe trick? Enlighten me. Im curious.

I read somewhere on this board that if you put a large socket into the tailpipe and the idle doesn't change due to the increase in back pressure then it points to an already restricted exhaust system. i used a 1-1/16 and it made no difference.

The Downstream sensor is not the same amplitude as the upstream...but it's not the gently rolling wave it used to be. Very choppy

Hard to explain but during acceleration it seems like something is holding it back.

One other thing is my temperature went from 199 degrees on the old engine to a consistent 205 on the new one. I can also notice a slightly increased exhaust sound from the wheel well area.

EGR tests fine but the GM Mode 6 Deceleration test comes back as 33.xxx seconds (spec shows that 15-25 seconds is ideal).....backpressure ????

GM Mode 6 catalyst efficiency test is showing a value of -32.768 seconds (warning on my scanner) not sure what the value means though.

The Catalyst I/M test passes. No Codes

If I rev it to 3000 K RPM you can also notice a slight almost deep popping/grumbling sound in the exhaust on the decel....only noticeable at idle and if you open the throttle fast and release.

I also have a 98 blazer, the catalytic convertrrs are famous for goin out on this year model for some reason. It had a major loss of power. Replaced it with a magnaflow ($50 for the converter off e-bay and another $50 to have it installed) and it ran just fine. Thing is, the new converter made the check engine light come on. And i have yet to figure out a way to make it go off. My advice would be if you replace the converter, get the same kind that you take off, not an aftermarket one.

Not sure what a deceleration test is...

If the PCM is passing the catalyst monitor then the cat is probably fine.

If you still think is plugged, try taking out the front 02s and dirving the truck. If it runs better with an opened exhaust, the the may be plugged up.

Another way to test the cat is with a temp gun. With engine warmed up and idling, the exhaust should be 50-100 deg hotter at the rear of the cat.

Not sure what a deceleration test is...

If the PCM is passing the catalyst monitor then the cat is probably fine.

If you still think is plugged, try taking out the front 02s and try driving the truck. If it runs better with an opened exhaust, the the may be plugged up.

Another way to test the cat is with a temp gun. With engine warmed up and idling, the exhaust should be 50-100 deg hotter at the rear of the cat.

I also have a 98 blazer, the catalytic convertrrs are famous for goin out on this year model for some reason. It had a major loss of power. Replaced it with a magnaflow ($50 for the converter off e-bay and another $50 to have it installed) and it ran just fine. Thing is, the new converter made the check engine light come on. And i have yet to figure out a way to make it go off. My advice would be if you replace the converter, get the same kind that you take off, not an aftermarket one.


I considered the Magnaflow based on the comments, but thought the Walker Direct fit is what was OEM. Did you check your post catalyst O2 sensor....that's the only way the catalyst efficiency system knows to throw a code if it's converter related.

I did have the code checked, it is P0420. The light came on about a day after i had the new converter installed. The code is for Catalyst system effiency below threshold (Bank 1). And i've been told that the new converter is what tripped it. But i haven't replaced any 02 sensor's, gas mileage is 19-20 on average so i don't worry about it too much.

I did have the code checked, it is P0420. The light came on about a day after i had the new converter installed. The code is for Catalyst system effiency below threshold (Bank 1). And i've been told that the new converter is what tripped it. But i haven't replaced any 02 sensor's, gas mileage is 19-20 on average so i don't worry about it too much.

Very possible could be the post cat o2 sensor....the PCM temporarily enriches the mixture, measure the delta betwwen the pre and post cataylst sensors then leans it out and measures again. If the threshold is exceeded on the post sensor it determines the catalyst is bad....theoretically could be the pre sensor but you'd have driveability issues.

Post sensor is usually only feedback and doesn't affect driveability...only real place it hurts is during emission testing.

I'm thinkin it may be an o2 sensor. The way i see it as long as gas mileage is good and everything is running good, i'm not going to worry about it to much. After all, this vehicle just rolled over 200k miles so it will be ready for retirement sometime soon. Besides, here in Arkansas, we have no emissions test or inspections of any kind.

mdrush,

at highway speeds you won't have a lot of power at 2100rpms. This engine doesn't really start touching the power band until around 3500 so from what I am hearing you have a normal running engine.

Well...I put on the Walker cat.... and muffler as well while I was at it (had loose baffles). Sounds very quiet and the post cat 02 is almost flatline now. MAP sensor went from 19 at idle to 24 and an intake vacuum test which was 15 (just before cat replacement) at 2000 RPM went to 20 and remains steady. Very easy swap with the direct fit.

Performance seems better at low end....fuel economy to be determined. All those misfires on the bad GM Cam may have done the old one in.

May be unrelated but after resetting the fuel trims the LTFT hasn't moved off zero in 100 miles....

Mike

So in essence you have increased back pressure. That is what your readings are saying.

Also, LTFT will have nothing to do with and fuel measurement when your computer hasn't even went through the full relearn which will take about 250-400miles.

So at this point, indications are good, with the increase in vacuum and decrease in MAP. I'm not sure what you mean by "flatline" with regard to the rear O², but there should be some activity and crosscount there. Does the O² heater test complete shortly after startup? Can you get the rear sensor a little more active by "blipping" the throttle several times for 10 seconds or so? There should be SOME activity at the rear sensor.

MAybe he just meant the reading is constant?

MAybe he just meant the reading is constant?

When I say "flatline" I meant there is some movement between 450 and 600 mv but it's very gradual and sine wave like in shape.

My mistake Blazer LT, I transposed the MAP numbers it should have been from 24 to 19 Kpa.

10-4 on the LTFT, although I'm reading the GM LTFT Cell average not the ODB2 trims...still, to your point sampling over 100 miles is probably not enough...

After my interstate drive today...I would characterize the change as saying that performance is more consistent throughout the 0-2500 RPM range and ironically the shift points seem to have moved...although I can't say it makes sense. Things definitely are quieter.

post mortem on the converter shows a definite rattling when shaken and some unknown metallic particles (flakes) on the windward side of the honeycomb. (possibly from the old destroyed engine ????)

Bank 1 catalyst efficiency went from -32.768 seconds to 0.112 seconds..

Still can't explain why the IM test passed on the old converter.


Mike



Mike

Probably within their "acceptable" standards.