New Issue has Me Stumped-Running Rough & Fuel Trim Issues
 

This is a relatively new issue and only happens after 100+ mile drives at highway speeds. This is a '98 3.4l auto 4x4.
After driving for for a long period at highway speeds and the car has been running great I hit the offramp and have to stop for a red light and my long term fuel trim drops to +1.56%(my usual long term fuel trim at idle is +3.12%) the engine is almost stalling and the Short Term Fuel Trim is jumping around between +18% to +20%. The light goes green, I step on the gas and the car is running fine again, no lack of power and not rough(fuel trims quickly return to "normal"). I drive it the mile home and pull in to garage, leave it in gear and the fuel trim drops back down to +1.56% and the STFT starts bouncing around -17 to -19%.(180degree flip on STFT) and after checking the exhaust it's definitely running rich.

Plugs, wires(NGK) and O2 sensors(OEM) are less than a year old and under 20k miles. MAF has been cleaned within 5k. There are no codes or pending codes either.
Any ideas on where to start? I'm at a loss.

Edit: This all goes away when i shut the car off and let it sit for 15-20 minutes
My rig has 348k miles and federal emissions.

How about when the fuel filter was last changed and what the fuel pressure is at the fuel rail?
Along with the air filter?
How many miles on the rig and are your O2 sensors resistance in spec?
Spark plug wires and coils within reistance spec?

I was just thinking at starting at the beginning and working forward. :-)

Once the engine is warm your engine will run off the O2 sensors instead of the static mappings in the ECM.

Your O2 sensors are floating around. So they seem to be working I just wondering if anything is close to the resistance limits, thus causing this very odd dip from highway driving.

Also is your 98 a Federal or CA emissions system?

Thanks. It was already so long I tried to do the condensed version. 348k miles and federal emissions. Fuel Filter was replaced 40k ago and air filter about 3k ago. Haven't checked fuel pressure yet because this all goes away after it sits for 15 minutes and it's a b*tch on our rigs. Haven't checked O2 sensors either they're newish, OEM and this all goes away by the time I could shut the car down and check it out. Just worried one of these times it'll stall and won't start back up.

Ok..sounds like the basic done. Did you ever clean the throttle body?

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Yes, I cleaned the throttle body when I replaced the plugs less than 20k miles ago.

Random stupid idea but have you checked your catalic converter(s)?

Rap on it like you were pounding on table with a closed fist. If anything rattling in there you've got loose catalic material inside.

You'd think you'd already have a P0420 code but worth a quick check. Had a similar running issue on my '96. Great under throttle but want to stall out at idle.

Cheers!

With your milage I would do a compression check.

I would also buy two big bottles of Techron and spend the cash on shells or Sunoco premium and run that tank down to 1/8 before refilling.

With your milage it could be a failing or clogged injector. A cylinder or three low on compression.

Your long term fuel trim is normal. Mine range from -3 to +3. Idle is -3 ish for me. My Driving is 0 to 3 depending.


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Thanks. I've been wondering about my converter a little, it has a lot of miles on it. I'll give that a shot.
I'm pretty religious about my use of Techron but I do have an injector that's ticking... Been doing it for a couple years now. I think I'll pull some injectors from the junkyard, have them flow tested and put them in when I do the valve cover gaskets.
Compression checked out okay less than 50k miles ago, all were in the 150's to 160's and I live at 5000ft so the numbers are going to be a little low.

The thing that has me really perplexed is the fact that it goes from running real lean to real rich(at idle) once I get off the freeway and after a mile of driving it on the street but with my foot on the gas I wouldn't know there was an issue at all.

Man It all goes away in 10 or 15 mins .. No pending codes? I am sure you verified all the vaccuum lines good. Maybe your lines are cracking.

PCV good and not sticking?

I can see why your scratching your head. How long has it been doing this?

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Wouldn't hurt to test the tps

It's been doing this a little over a month but that's only 3 times. Replaced vacuum hoses with silicone ones about 6 months a go. PCV was just replaced with OEM a year ago and it still appears to be fine, doesn't stick and I replaced the gasket on the oil cap a few months ago so it seals great now.
I had though it might be related to the cold but Saturday, when it happened last, was one of the warmest days we've had this year.