Rough Idle - Suspect Throttle System

[Scottydoggs]

its dirty in the pics there, and that was 2 years ago, so its more dirty now.

but thats not the cause if the flashing cel. thats gonna cause hard starts poor idle.

spark plugs and wires do or a bad coil or two. or mouse chewed injector harness wire.

being you have a scanner, see if you can make the cel flash again, then scan it truck running, dont turn it off. once turned off a pending code will clear, see what if any thing you come up with. a p0300 is my guess. random misfire code.

bad maf can also do some screwy things as well. sometimes they dont set a code right away. maf will also mess with the trans not shifting right, or feeling right.

 

[Doubeleive]

might be easier to try a back pressure test to rule the cat's in or out, I don't remember off hand what the cat temps should be, your scanner may show the the cat temps while you are driving I think mine were showing 700-800 deg on my tech2 data while driving on the freeway (live data), I think when checking them with a laser you want to see if one end of the cat has a lot of difference in temp than the other end.

 

[Justinj360]

its dirty in the pics there, and that was 2 years ago, so its more dirty now.

but thats not the cause if the flashing cel. thats gonna cause hard starts poor idle.

spark plugs and wires do or a bad coil or two. or mouse chewed injector harness wire.

being you have a scanner, see if you can make the cel flash again, then scan it truck running, dont turn it off. once turned off a pending code will clear, see what if any thing you come up with. a p0300 is my guess. random misfire code.

bad maf can also do some screwy things as well. sometimes they dont set a code right away. maf will also mess with the trans not shifting right, or feeling right.

Alright I'll eliminate one thing at a time. I'll start off with cleaning the throttle body tonight to see if that helps. I might try cleaning the MAF sensor after that too. The real mystery to me here is why the throttle position sensor would be reading a minimum of 20% so that's why I'm starting with that. Could there really be that much carbon build up holding it that far open?

 

[Scottydoggs]

ive noticed my ss camaros commanded tp% is high as well. might be normal.

 

[HiHoeSilver]

Alright I'll eliminate one thing at a time. I'll start off with cleaning the throttle body tonight to see if that helps. I might try cleaning the MAF sensor after that too. The real mystery to me here is why the throttle position sensor would be reading a minimum of 20% so that's why I'm starting with that. Could there really be that much carbon build up holding it that far open?

Mine was running at 13%. I cleaned her up good, and it came down to 2.7%

 

[Justinj360]

Alright some interesting things happened today. I cleaned the TB and the MAF sensor last night and started it up today. It fired right up like nothing was ever wrong with it and like it hadn't just been sitting around for a few months. I have no idea what the actual fix was.
I took it for a test drive around the neighborhood and within the first 5 minutes I got a "tighten fuel cap" message. I've never gotten that message before and I always make sure the fuel cap clicks. I stopped and tightened it and I haven't gotten the message again. The message cleared itself before I had a chance to stop. The gas cap was also replaced with an ACDelco one a few years ago because the plastic string broke. I also had the fuel pump replaced by a shop a couple of years ago because it died.

The truck still isn't running at 100% but it is much better than before. At times when I tried to accelerate fairly hard it felt like there was something holding it back still. Almost like it wasn't getting enough fuel or something like that. 45 was about the top speed I got up to because I just stayed fairly close to home. I have been noticing lately that the engine and trans temps get warm faster than they used to but not up to any excessive temps. The engine doesn't exceed operating temperature.

There still aren't any stored or pending codes. I left my scanner plugged in while I took a couple test drives but I didn't record the information. According to the scanner, there still needs to be a test run on the catalyst and evap systems. I'm not sure how often that occurs. I also noticed that when the truck is idling the TP sensor drops down to around 7 percent but goes back to 20% when you turn it off. Seems odd to me.

 

[Scottydoggs]

sounds like the cats went bad on ya. engines are quite gutless when the exhaust cant get out.

you could pull the 2 up stream o2 sensors and drive it, if its got more power with two exhaust leaks the cats are bad for sure.

 

[wildcatgoal]

This sounds like a clogged cat-verter to me...

You can diagnose by looking at expected vs. actual O2 sensor readings in combination with expected vs. actual laser temp readings before, at, and behind the cat.

Look up South Main Auto on YouTube -- he diagnoses this stuff all the time.

 

[Rocket Man]

I suspected the cats when I first heard the symptoms. I’m curious if that’s the problem after all.

 

[Justinj360]

Alright I was finally able to do some more diagnosing today. I've been crazy busy at work for the last few days so sorry about the lack of updates.

I went to Harbor Freight and bought a laser thermometer. While I ran there I also left my scanner plugged in and set to record the whole data set. When the engine was at operating temperature I tried 2 medium-throttle runs from stops. Those both gave me the flashing CEL again and I had the same symptoms as the first time I saw the flashing CEL - Lack of power, feels like a misfire or not enough fuel, and jerky etc.). It then immediately gave me a pending code for P0308, cylinder 8 misfire. There was never a stored code though. After those pulls I babied it the rest of the way home and when I had to stop for a few stoplights, it would intermittently idle rough for a second. Before those pulls, it idled fine at stops.

When I got home I left the truck running and used my new thermometer on the cats. There wasn't a very obvious difference in their temps but I'm also not sure how big of a difference there should be between them if one went bad. I've listed the temps below. I don't remember the temps just before the cats but they were a little higher than the temps listed below and slightly higher on the drivers side again. I don't think the temps before the cats exceeded 500*. Same story after the cats but slightly cooler.
Cat temps:
drivers: ~450*
passenger:~415*

A quick search online tells me that cylinder 8 is closest to the firewall on the passenger side. Before I knew where number 8 was I was thinking maybe there was a correlation between the "misfiring" cylinder and which side of the exhaust was hotter but maybe that wasn't the case because they're on opposite sides.
Also, I suppose the cats being bad could possibly be the problem because I had the infamous cracked heads from Castech... That's why I had the engine swapped. I looked up the causes of cats failing and one was due to leaking coolant. I have no idea how long my heads were cracked on the old engine but this newer Magnaflow exhaust was coupled to the old engine for at least 3 years.

What do you guys think so far? I'm getting tempted to just take it to a shop soon and have the pros diagnose it.