Where to start diagnosing this one - P0420 + rough start + overheating

[Curtis]

Will try to explain in detail - hopefully you wont loose interest

My CEL has been on for a while now without any symptoms or loss of MPG.

I stopped for lunch today, truck was off for about 30 minutes
When I start the truck to leave it dies almost instantly
Try again, and by feathering the throttle it stayed going but very rough. The truck was shaking.

I was 5 mins from home so started to drive and it smoothed out and I drove home without any problems.

Shut it off and started it again. It was rough and just at the point where the engine catches there was a clank sound.. just one... sounded like in the exhaust (you'll see where Im going with this in a bit)

Shut it off and started it several times and is started fine except for that clank sound.

I left it for about 30 mins and decided to go to Advance auto parts to get the code read.

Again very rough start but I could keep it going and after a minute it smoothed out.

Advance is 10 mins away and by the time I got there the temp gauge was maxed!!! Although the motor didn't seem "that" hot.

Advance pulled one code - P0420 - Catalyst efficiency below threshold bank 1

So I drove it home, temp gauge maxed out, but ran fine.
Checked the electric fan and its working.
Didn't check antifreeze yet - too hot.

So where do I go from here?

How can I test the catalytic converter - just wack it with a rubber mallet?
Could that clank I hear upon start up be the converter? Seems like good pressure coming out of the tail pipe.

I'll see in the morning if its a rough start when cold.

Headgasket? - compression test could tell me this right?

Thanks in advance

 

[Curtis]

I just checked the antifreeze and its full. Overflow container has some in it too.

started her again, still rough
I was thinking about the catalytic converter and thought Id try to plug the exhaust so I got a cloth and tried to hold it on the end of the tail pipe.

I could not stop the flow of exhaust and she didn't cut out either.

 

[robertoa1a]

The clanking could be a small fuel explosion in the exhaust caused by un-burned fuel.

you provably are correct in being safe and doing a compression check. That might help you avoid any catastrophic damage. Take note of the color of the deposits on the spark plugs. Check the coolant and oil for a mixture of the two.

 

[garyk1970]

Sounds like a classic mass air flow sensor (MAF) going south.....they go bad without codes,very common.
when it (MAF) goes bad,usually the engine runs very rich (and runs rough) causing the cat to overheat.......hook up to a scan tool and monitor the MAF count.should be around 4-6 g/s.....HTH

 

[Curtis]

I dont have a scan tool... although its on the list

I just did a compression test.

removed all plugs, disconnected the coil pack, held the throttle plate wide open

All cylinders were exactly 180 psi. (+/- 1 psi) So while not a real help, it points away from a blown head gasket.

I plan to pick up a couple O2 sensors at the junk yard and replace the one before and after the cat (although I have not gotten under the truck to look where they are)

I wonder can Autozone test the MAF sensor if I bring it there?

Will post back what I find.

 

[garyk1970]

I dont have a scan tool... although its on the list

I just did a compression test.

removed all plugs, disconnected the coil pack, held the throttle plate wide open

All cylinders were exactly 180 psi. (+/- 1 psi) So while not a real help, it points away from a blown head gasket.

I plan to pick up a couple O2 sensors at the junk yard and replace the one before and after the cat (although I have not gotten under the truck to look where they are)

I wonder can Autozone test the MAF sensor if I bring it there?

Will post back what I find.

i dont think they (autozone) can test the maf.......start engine and do the good ol "tap" test....tap on maf and see if the idle stumbles/changes any...

 

[Curtis]

i dont think they (autozone) can test the maf.......start engine and do the good ol "tap" test....tap on maf and see if the idle stumbles/changes any...

Cool, I'll try that
I put the plugs back in their original place
Started the truck and it idled ok for a min then got a little rougher
I watched the temp gauge and in the matter of 8-10 minutes was about 3/4 way to max so I shut her off. I dont get it.

 

[gijoecam]

Well, to start with, change the thermostat to help with the overheating issue. ou should never have driven it with the temp pegged in the first place.

The O2 sensor may not be bad. It's very possible that it's reading correctly, and the catalyst is, in fact, not as efficient as it should be. Only a good sniff-test would be able to tell that for sure.

 

[robertoa1a]

The maf has two little filaments that are sensitive and delicate. When a little dirt gets on them then things go wacky. There is not a really good way of testing the maf because you would have to simulate driving conditions.

Just buy a $3 can of maf cleaner and spray it off real good.

 

[Curtis]

The maf has two little filaments that are sensitive and delicate. When a little dirt gets on them then things go wacky. There is not a really good way of testing the maf because you would have to simulate driving conditions.

Just buy a $3 can of maf cleaner and spray it off real good.

funny you should say that

Just yesterday picked up a new thermostat, some throttle body cleaner and some maf cleaner and plan to give that a try today.

If I can I may try to take out the 1st O2 sensor and see if I can somehow get a pressure gauge on it.

 

[Curtis]

Today I replaced the thermostat
Cleaned the MAF - from outside the housing as I did not have that strange torx bit to get it out. It was dirty, nice and silver looking after.
Also sprayed and cleaned the throttle body.

She started nicely and idled much better
Ran her for 30 minutes in the driveway and the temp gauge never went above 1/4.

So where does that leave me?
CEL is still on so Ill see there are any new threads.

Question:
Should I notice a difference in idle by unplugging the MAF sensor and plugging it back in? I could not detect any difference. Maybe a new one is in order?

 

[robertoa1a]

You could reset it and see if the code comes back.