Recently I noticed my "Check Engine" light on. I ran by the AutoZone to have the code pulled. Here's what they found:

P0171 (bank 1 lean condition)
P1131 (bank 1 o2 at its lean limit)


About 1.5 years ago, I replaced the three figure "8" shaped gaskets on the upper intake manifold when I got CEL P0174. I've had no other CEL until now.

I'm thinking I have some sort of vacuum leak again which is causing the lean conditions. I've noticed a higher idle when the engine is cold, but sometimes the idle seems a little erradic and/or the engine stumbles when the engine is at operating temp and idling in gear.

Any suggestions?

BTW- truck's a '99 4.0L with automatic trans.

 

Have you ever changed the O2 sensors? The problem could be a bad O2 sensor.

 

For what it's worth, my truck has ~120K on the clock. If it were an o2 sensor, how would I determine which one was at fault?

My OHV 4L has 3 o2 sensors I believe. Last time I looked, these sensors were about $50 each.

I can't seem to pinpoint a vacuum leak. I've tried brake cleaner around the manifold, and I haven't noticed a change in engine idle. Then again, this didn't reveal the vacuum leak in those little "8-shaped upper intake gaskets" a couple of years ago either.

Some research on this site says that those little "upper intake gaskets" and another intake gasket (perhaps below the fuel rail) are very prone to leak and cause lean conditions on OHV 4.0 engines.

Thanks again for the advice.

 

Bank 1 is the passenger's side, and I would say it's the front O2 sensor. You are correct, some of the earlier 4.0L OHV engines were known for the lower intake bolts to loosen up, and the gaskets would leak. The other symptom that went along with this is a slight coolant and oil loss.

 

Re-read his original post, both codes are bank 1

 

UPDATE--



So I replaced the pulled the upper intake and fuel rail this weekend to address the problem from post #1. I replaced the upper intake gaskets and the gasket below the fuel rail.

I cleaned the MAF while I was under the hood.

Unhooked the battery to clear the codes.

I restarted the engine, and noticed a smoother idle after the initial relearning of the computer.

I drove around in the pasture a lot to check fences and cattle and even do some playing around. I drive over 150 miles and no check engine light.

But today it comes back on!

I sprayed a fair amount of carb cleaner under the hood to check for vacuum leaks, but I can't find anything.

I had the code re-read by an idiot at AutoZone, and it says there's 2 codes there but he could only figure out how to see one: P1131 , one of the original codes.

I began looking for anything that might jack with a sensor's reading and discovered a very small exhaust leak.

When I had my transmission rebuilt over 30K miles ago, the shop never replaced the donut gasket after the cat and the rest of exhaust system. The entire mating surface is not leaking, just one section about an inch long has a gap the width of a penny which I can feel exhaust escaping when the engine is running.

I'm leary to point the finger at this, because this has been leaking many thounsands of miles before the Check Engine light ever came on. But it upstream of an O2 sensor...




If you can't tell, I could use a little direction.

The plugs were replaced about 15K ago with MotorCraft plugs. Wires are still original. Fuel filter replaced about 30K ago. The IAC was replaced about 10K ago. Again, the truck's got 120K on it.

Help?

 

The IAC will react to a drop in idle speed and try to correct it. That's why spraying carb cleaner or other hydrocarbon in the search for vacuum leaks is less effective on a fuel injected engine than on a carbureted one that has a fixed idle stop.

I would try disconnecting the IAC before leak checking. If it refuses to idle (and it may with the lean condition), I use a coin or other shim to keep the throttle open just far enough to keep it running. Then spray the suspect areas and look for an RPM change.

 

--- UPDATE ---

For anyone whose been plagued by a Check Engine Code P1131

For about 7,000 miles now, the Check Engine Light has not yet illuminated. I replaced the Upstream O2 Sensor, Bank 1. There seems to be a small increase in fuel economy since the new sensor. No more occasional stumbling at idle, and no more random loss of power.

-aside-
I hate it when someone automatically thinks an O2 sensor is at fault whenever the CEL comes on, so I ate my words this time. But, danggit, I'm finally free of that pesky Check Engine Light!