Code 41

[bronc]

My 91 4.0 is giving me a code now. This just started. Since the swap all was fine until now it throws a code. CEL comes on after you drive for a while and it hits operating temp. Goes away if you shut it off and restart and then eventually it will come on again. I read the codes and got a 41 (both KEO and engine running). That's the only code I get. Hayne's says it means HEGO sensor lean condition. Is that right? What can I do?

[MAKG]

Take it to a shop.

Lean codes are a VERY bad thing to learn on.

An hour of competent diagnosis will cost about the same as the oxygen sensor someone is going to tell you to buy, that you MAY need. But I doubt it, especially since you've been modifying the vehicle. Even without it, there are dozens of ways to trip a lean code.

[steiner_5000]

I have the same prolem with my swap. 91 4.0 into 85 B2. It runs terrible and I got the same code. My truck cant idle at all it will rev really high then sputter and chug and almost die then rev up again. I know its not an O2 sensor because I replaced it about 4 months ago. Do I really have to take it to a shop?

[MAKG]

No, you don't HAVE to.

But I think you'll be running in circles if you don't.

If you have a manual transmission, you need a neutral switch for a '91 4.0L to idle correctly. You never did for a 2.8L.

The fun part about that code is that any serious miss will trip it. Among a bunch of other things.

A common F-up on a 4.0L is screwing up the firing order by not understanding the coil pack correctly. The cylinders are labelled on pack and they are NOT arranged 123 and 456.

[steiner_5000]

But would the nuetral safe swith make the motor surge up an down and die at idle

[MAKG]

There is no neutral safety switch on a manual transmission. There is a neutral SENSING switch on some. Though you might think it's semantic, it makes a VERY big difference to how it is used.

Missing the switch won't CAUSE "surging." It will tell the computer that idle speed doesn't need to be controlled because the truck is in gear and it can't anyway.

[bronc]

Thought I'd update. My problem was as simple as the O2 sensor. Replaced it and problem went away. The little filament thing inside the sensor was broke off. It was rattling. Probably should not be like that. The sensor looked clean though. What does a normal sensor look like? It was like it was hot, a little white, and clean. I took a pic I can post later.

[MAKG]

That sounds normal to me.

It should look quite a lot like a normal spark plug.

Light deposits are not a big deal. Heavy deposits or any carbon, coolant or oil fouling are problematic.