Motorheads Help!

[Sutty]

I have a 98 GMC K-1500

It is showing two DTCs P0420 has been there a while Cat Con issue I believe. (Ouch Spendy)

The second one P0305 which I believe is a missfire on number 5 cylinder.

Doing a little research on the net it sounds like it could either be the catcon or a bad O2 sensor.

Any tests I can do to isolate the problem?

Thanks,

Sutty

[VMS]

Not sure if this will help or not, but if you have a clogged catalytic converter, you tell by punching the gas really hard. The truck should shift down as usual and be able to pick up speed. If it does not, the converter is clogged and holding back your power (at least this is what happened to my 96 silverado when the intake manifold gasket drew in antifreeze over the course of a winter).

I'd pull the #5 plug and see what it looks like. If it is wet, you have a misfire, which could be the plug, the wire, etc. If it is sparkling clean, it could be a clogged injector, or it could be drawing in antifreeze... I'd get an cooling system pressure test. I'd start with the plug and see if it is getting spark. If not, replace with a new one and try again. If sparks, put back in and see how she runs...

Not fun diagnosing the problem further than the code, but hopefully this is a start for you...at least in one possible scenario.

Steve

[Jeremy airjer W]

Which motor?

Can you feal the misfire?

If you can feel the misfire than start buy removing the plug wire to see if there is spark. I would carefully do this while the engine was running myself (yes you can get hit with 30kv + and yes it can tickle) one to verify the dead cylinder and two to kind of guage how much spark I halve.

Pull the plug to see if it is worn out or fouled.

Do you ever have an extended crank after shutting the car of for 10 - 15 after a full warm up?

I have had a couple of the newer chevs with the dual cats where one cat breaks up and will block to flow of exhaust enough to cause one bank or the other to misfire (either 1-3-5-7 or 2-4-6-8-) usually 1-3-5-7 for some reason.

How many miles are on it. You would be surprised at how long manufacturers will honor your emissions warranty which is a lot longer than your "standard warranty".

[Sutty]

It is the 5.7 350

I can feel it miss, it runs pretty good but I would describe it as just a tiny bit off after it warms up.

I did do a tune up last summer plugs,wires,cap & rotor. It has about 180,000 so I doubt they will cover the cat/con I read online they would cover them to 80 so I am 100 behond that

By an extended crank do you mean where it will continue to run or sputter after shutting off? If thats what you mean no it doesn't do that. I can stomp on the gas and it will downshift and pick up speed.

I also was recently loosing coolant and it was recommended to redo the intake so that was just done and I forgot to check when I got to work today but the coolant level had been steady.

[Jeremy airjer W]

Extended crank is an unusually longer crank time before the motor actually starts.

If you just had an intake gasket done I think you may have a poppet going bad or maybe plugged. I think this is a central port injected fuel system (CPI). It is somewhat common for the poppets to stop working after this service is performed. I've litterally done hundreds of intakes with the CPI setup and have had a couple go bad, but am aware of quite a few.

What kind of plugs did you install?

Check the basics. If nothing seems out of the ordinary than get it to a shop. You could easily spend over $400 dollars throwing parts at it and it could be as simple as a leaky fuel injector or a corroded plug wire terminal.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

Bank 1 is the side that the number one cylinder is on. Do you hear a hollow rattle when the engine is at idle? If you wack the cat does it rattle? (these can get incredibly hot!!)

[VMS]

If the manifold gasket was leaking, it might cause some problems with the O2 sensor... If you can stomp on the gas and it kicks in and puts you in your seat, the converter is not gummed...but, if the converter is not doing it's job like it should, that would cause the O2 sensor to be misreading, hence the code.

Regardless, if the catalytic converter needs replacement, the O2 sensor at 180,000 miles is probably rusted in pretty tight...to try and save it, would be extremely difficult IMHO, Too bad they still didn't have emissions testing facilities for diagnostic purposes...that would tell the story...

Steve

[Sutty]

It starts fine, although now that you mention it there have been a couple of mornings periodically where it starts and stops right away. About the third or fourth time I started it would stay going but it only happened a couple of times but it turned over right away It did not crank before starting. Just stopped as quickly as it started.

I put in autolight double platinums, and I should have mentioned that I had the catcon code (420) before I had the intake done. I had brought it in to a muffler shop and he thought it was just the donut gaskets (they were loose) that was maybe 6 weeks or so ago and the intake job was maybe a month ago.

It takes a while for the code to come back when I clear it.

Uh I can't say I have whacked the cat I hear tell that sherrifs are gettin in trouble for that. Shall I wack it when it is hot or cold or doesn't matter? If it is rattling at idle it is not real loud.

[Sutty]

I whacked both of the cats and they do not rattle.

P0305 is coming on regularly now the 420 has not come back on in the past few weeks. I do need to bring it in pretty soon so I don't wreck it worse should I bite the bullet on the cat(s) or should I bring it in to a shop to have them check out possible missfire reasons other than the catcons

PS airjer can you shoot me an e-mail? jksutton@gmail

[Capt. Don*]

I strongly doubt the two codes are connected, IF the cat was causing a misfire, it wouldn't pick on one cylinder, it would be the whole bank. A P0420 means that the catalyst monitor has not seen a significant difference in the oxygen content of the exhaust after it has passed through the cat. The PCM compares voltage readings from the O2 sensor ahead of the cat with the voltage readings from the manitor after the cat. O2 voltage is switching above and below .5VDC rapidly as the PCM is trying to maintain a 13.7/1 air fuel ratio based on the input from the O2 sensor. After going through the cat the voltage should remain somewhat stable, indicating to the PCM that the cat has done its job reducing emissions. Your cat is worn out, it will have no effect on the drivability of your pickup, just the enviroment.

[united jigsticker]

How did you detirmine a cylinder 5 misfire?

An O2 sensor will richen up a whole bank. A CAT will cause power loss, and a foul "rotten egg" smell.

A single cylinder misfire can come anywhere from the cap, wire, plug, to injector, knock sensor, yadda yadda.

Have you noticed any difference in fuel mileage?

[Capt. Don*]

P0305 is cylinder 5 misfire, the PCM determines this, it measures time between firing of cylinders by way of crankshaft position sensor. If a cylinder is misfiring it takes longer for the crank to travel to the next cylinder to fire. All part of OBDII. P0420 is not a code used to determine physical condition of a cat, it measures its efficiency, ie. how it is cleaning and processing exhaust gasses. There are no codes to determine a physical condition failure of a cat, although other codes are generated when a cat is restricted, these codes have to be properly interpreted as the codes are results of a restricted cat, not the cause. Cat symptoms you decribe are true of a restricted or damaged cat, efficiency monitoring was mandated by the government with the onset of OBDII as it was not possible with OBD GenI.

[Jeremy airjer W]

I wouldn't worry about the cats until the misfire is taken care of. With a cylinder not working the cat has to work harder to process the extra amount of unburned fuel. That can lead to premature cat failure but there's no sense

replacing it, if it needs to be replaced, until the misfire is fixed.

Have you checked any thing yet?

Sutty is that a com. or net. email.

[MattWtech]

airjer is right that extra unburned stuff is abnormal for the 02 sensors and could possibly solve the cat code

whats the code? if not...

what type of missfire is it A or B? A missing every stroke, B missing every couple of strokes. that might be able to get us on the right track

and yea those spider systems are known for gettin all messed up and restricted.

[Sutty]

I thought I would post an update in case anyone was curious. It turned out that I had a cracked plastic housing that the distributer was sitting in. Moisture started getting in there and it started to arc.

Truck wouldn't start a couple of weaks ago so had to start tearing it appart in the garage...

My cat/con check engine code has not come back yet but I suspect it will after a while if it does than I'll look at replacing that or those..

Thanks for the advice