Rough idle, stalling, wild STFT, p0171, 2RZ-FE and headaches

Hello people smarter than I,

My '97 2.4L Auto is causing me major headaches.

I've done a ton of work and replaced a ton of parts over the last four months, so I can't pin point precisely what I did that would cause my issue. So, I'm turning to you guys for some ideas.

While idling in park, idling at a traffic light, or even while driving at low RPMs my engine in stalling out. I have to turn the key off and then restart the truck.

While idling in park, the engine will erratically jerk and the engine idle is obviously rough just from looking at the engine. While in the cabin, the engine shakes the mortal shit out of my steering wheel.

The engine has thrown a P0171 code and I have tried everything I know to do, including replacing ALL of my vacuum hoses, cleaning my MAF sensor, removed, replaced, and tightened all of the fittings on my aftermarket cold air intake. Checked the engine's vacuum by basically soaking it in carburetor fluid (no change in RPMs or idle conditions), built a smoke machine out of a paint can and pumped smoke into the intake manifold (extremely minuscule leak near the MAF sensor which I have since fixed by orienting the MAF tube properly, nothing more), and a few other diagnostic procedures. At this point I am 100% confident that if my issue is a vacuum leak, it has to be in the most obscure, hard-to-find spot on the 16V DOHC 2.4 engine.

My short term fuel trim at idle was around -20.4 while my long term fuel trim was around +40. I'm not savvy enough to know what these mean, but I do know that at around 2500RPM, the short term fuel trim increases from -20 to around +1 to +6. I'm not sure what else to look for in the OBDII scan data output.

I'm losing my mind. Further research online has led me to believe that I have a rich fuel mixture ratio, i.e. too much fuel is my problem, rather than too much air, which was my original assumption.

I replaced my injectors back in February sometime with new Denso injectors for the 2RZ-FE models. I recently relocated my fuel filter and changed it, but the rough idling and stalling was happening long before that. I'm mentioning replacing the filter because I was impressed to see what kind of pressure there was on the fuel rail.

Any help or suggestions you guys can give me would be amazing and greatly appreciated.

EDIT: Also want to mention that I replaced my forward oxygen sensor and I have a brand new rear oxygen sensor, I just haven't installed it yet because the nuts are rusted on.

EDIT 2: Here's a video of the engine idling. You'll notice that the fuel trims are better than they were when I originally wrote this post. This was after I changed my spark plugs, left the negative battery cable off for 24 hours, and fixed a small vacuum leak on the filter side of the MAF.

 

I'm gonna stew on this for ya. Have a couple thoughts but I'll spare you until I can decide if they make logical sense

 

Understood. Thanks for chiming in. I'll be watching the thread over the next couple of days.

 

Bad ecu or bad MAF? Weird that it seems normal at 2500 rpm based on fuel trims.

 

What's weird is that I replaced my old ECU because it was the original from 1997 with a brand new one and nothing changed. Today, I replaced my spark plugs to see if that would make a difference. The plugs that were in it were about 4 months old and they came out black and sooty.

Engine is still stalling out. I can put the pedal to the floor and it just sputters. No acceleration.

 

I think it would be wise to check proper function if the EGR system. You'll go dead rich if it is stuck open

 

I thought about replacing the EGR valve, but the $150 price tag put me off. I may have to bite the bullet and do it. Is there an easy way to verify EGR function?

 

You can look up ways to actuate the diaphragm for it. But if it's stick open the easiest thing would be to unbolt it from the intake manifold and block the hole for it and go for a test drive. It isn't a necessary system for proper function, there are even kits to remove it permanently. So as long as you block the hole in the intake manifold you can at least test and see if she runs right. Then you can decide to delete or replace, or put it back if it isn't the problem.

 

Cool beans. I like the idea of removing the unnecessary...especially from a 20 year old truck. I uploaded a video to the original post that may help with diagnostics. There's some footage of my OBDII scans at idle and at 1500ish RPMs.

 

It's odd that it's throwing a lean code while the fuel trims are negative. Negative fuel trims mean the computer is reducing fuel to the engine. Only reason it would reduce fuel is if you're running rich. Now ignoring the code and looking just at the symptoms and fuel trims, I'd say go with Speedytechs suggestion and check EGR function. Also check for a restricted exhaust system...

 

Hadn't thought about that but a collapsed catalytic converter could produce these symptoms too, easy to test, just unplug the upstream o2 sensor and unbolt it and give it a run.

 

After leaving the battery cable off overnight and changing the spark plugs, the short term fuel trim is reading ~20 at idle. Hasn't stored any new long-term fuel trim data.

In the video I added above, the p0171 code is gone after clearing the ECU, but after a test drive shortly after that video was filmed, I could still hold the pedal to the floor and the engine would just sputter and spit. After letting up off the gas and building some RPMs over a few seconds, the engine would pick backup and I could get up to around 40-50mph.

 

I hadn't thought of that either. The exhaust from the bosal back to the end of the muffler isn't really all that bad for a 20 year old original exhaust. The o2 sensor housings are rusted to crap, but otherwise, there aren't any big holes. The cat could be bad, but it doesn't look damaged, at least...if that's any concern.

Last week I replaced the exhaust manifold gasket, painted the manifold, and wrapped it in fiberglass tape to cut back on heat. I also found and re-installed a stock heat shield for the manifold, too. I dummied up and put an eBay $50 manifold on it and it caused exhaust leaks and was louder than hell. After repairing and painting the original, I put it back on it and the exhaust sounds 100% better. I also replaced the bosal gaskets while I was in there.

 

my gut is strongly going with a FUBAR Cat converter or EGR flow issue. like dude mentioned. disconnect O2, undo exhaust at the flange before the cat, clear codes and give it a rip up and down the block. if that thing drives better then there's your sign. basically, eliminate things one at a time if possible.

 

Cool. That's within my abilities. I'll give that a try tomorrow. Thanks for your advice.

 

Update: Installed a new catalytic converter and new oxygen sensors. Reset the ECM, no change. Still running very rich. Spark plugs came out black. Replaced with new plugs, still stalling out. I can hold the pedal to the floor and it will just sputter and crawl.

I'm about to pull my hair out.

 

when was the last time you changed out the fuel filter or cleaned/swapped injectors. before you dump anymore money maybe spend the 100 bucks to get it diagnosed somewhere? only so much we can help you with over the internet without working on the truck.

 

Changed and relocated the fuel filter to a more convenient spot about 2 weeks ago. Put in new injectors about 2 months ago.

I'm beginning to believe the EGR valve is stuck open. It was full of carbon when I took it off, but I have yet to find a way to test if it is actuating. Since it's mechanical, I can't even check it with a volt meter.

 

hmm. check out the cost of a new EGR vs a block off plate. see if it runs better at all

 

Just tried to put vacuum on the EGR with a break bleed pump. Wouldn't even start to build vacuum. Must be stuck dead open.