Slight miss at idle, poor MPG, sooty exhaust pipe, No codes. 250K

[Tozan]

The old girl is getting ... old 250k miles... lol

This has started happening in the last month...
I have a slight miss at idle when cold, it goes away after 30 seconds to a minute and does not affect acceleration or steady throttle. It goes away before I start driving down the road. I am not concerned with it but, include it in case it might be relevant. Runs smooth at cruising speeds too.

Power does seem a little soft sometime.. I am guessing this is related to running rich.

My average MPG has dropped down by around 3 MPG normally it is consistently around 16 and lately is around 12.5 to 13 MPG I live outside of town so my driving is very constant unless overlanding, hunting or out shooting.

The exhaust pipe is very sooty.

There are No codes triggered..

Some time the gas cap seems a little loose compared to the original that I lost a year ago. Can that be related if it gets loose and leaks?


OK, Some history.
It had been running rough before so I replaced plugs and wires and cleaned the K&N air filter a few thousand miles ago that roughness turned out to be a torque converter and it has run fine until recently.

I use injector / fuel system cleaner every 10 or 15k. V V T was turned off last year. I returned it back to the stock tune keeping the VVT off with no change...

I need to do a complete compression check and look at plugs I also need to rebuild the front end... But, it is hunting season right now so this will have to wait a couple weeks... lol

 

[Marky Dissod]

Why'd you turn off the VVT?

 

[Tozan]

Why'd you turn off the VVT?

Most people on here seem to agree it can cause engine problems...

 

[donjetman]

Most people on here seem to agree it can cause engine problems...

That's not my observation here for the past 6 yrs.

 

[Tozan]

That's not my observation here for the past 6 yrs.

Well I wont debate the issue of VVT... I turned it off 50,000 miles ago so I don't think it is related to my problem..

If you have any ideas or things to look for relating to my problem that would be great...

 

[Fless]

Show us the short- and long-term fuel trims and what the MAF is reporting in g/s at hot idle.

 

[petethepug]

If you’ve got e85, run a few tanks through to clean out the deposits.

 

[j91z28d1]

sounds like a case of E% drifting high when running normal fuel.

 

[Fless]

The other thing to check is individual cylinder misfires with a scanner that can show them by cylinder. Might give us a clue.

As @j91z28d1 noted, if it's a Flex Fuel system then check the alcohol percentage with a scanner, to see if it matches or is close to the fuel you're using.

 

[Tozan]

I guess I need to get a hold of a scanner...

 

[Foggy]

Also think about cleaning your MAF.. Running oiled air filters is very common
for some of the oil to get onto the maf.. Clean it.
When was the last time you changed your upstream oxygen sensors????
They are pretty much a maint item anymore.. Good for 100-125K... They should
have been replaced at least once as reg maint since you have 250K. Get the good
ones: GM or ACdelco if possible. Just don't get the cheapo ones
Also would be a good idea to do a reset on fuel trims and ethanol % as mentioned.
The ethanol is calculated "virtually" on our rigs. So if oxygen sensors are crappy
you will get funky ethanol calculations

 

[j91z28d1]

yes, forgot about maf, I cleaned mine and could see it idling in a different part of the airmass vs rpm map and I don't believe it's ever had anything but a paper filter in it.


o2 are a good idea too, as they foul out over time, especially from how much oil these things suck thru the pvc system. they drift lean and the ecm throws fuel at it. also on these trucks that don't use physical ethanol sensors, fuel trims is what it uses to come up with E% content. so if sensors are reporting incorrectly, it can mess up the math and think it's running e when it's not.

 

[Tozan]

Thank you, I have never replaced those sensors I did clean and oil the air filter not to long ago. I cleaned the MAF when I did the filter I guess I will clean it again. I always say go back to the last thing you touched... That would be the air filter...

 

[Charlie207]

yes, forgot about maf, I cleaned mine and could see it idling in a different part of the airmass vs rpm map and I don't believe it's ever had anything but a paper filter in it.


o2 are a good idea too, as they foul out over time, especially from how much oil these things suck thru the pvc system. they drift lean and the ecm throws fuel at it. also on these trucks that don't use physical ethanol sensors, fuel trims is what it uses to come up with E% content. so if sensors are reporting incorrectly, it can mess up the math and think it's running e when it's not.

Replacing my two up steam O2 sensors made a huge difference after my engine swap, and got rid of several DTCs that had popped up related to fueling, which made no sense, but worked. Improved overall driveability as well.

I got the OEM upstream O2s from Rock Auto.

 

[Tozan]

So far I cleaned the MAF replaced the air filter and replaced the up stream O2 sensor. I just did a hunting trip this past week so after my return I filled the tank and will do some driving this week to see if that helped..

I got a 4x4 mule deer on my hunt...

 

[Tozan]

Up date.. Doing the above has improved it a lot but, I think there is a little more on the table to get back to what I was getting..