Hey everyone,

I've have a problem with my 97' Jeep Cherokee sport. 6months back I drove through a deep puddle in storm and the jeep is running rich. I've been getting around 7mpg.

The engine seems to be running rougher and there is black smoke coming out of the tail pipe when I accelerate quickly. I can smell fumes with I open the window. The check engine light is giving the codes:

51
21
42

I've changed both front and back the o2 sensors out and the spark plugs but nothing seems to be working. Any feedback will help.

Thank you.

I'd be willing to bet it's your throttle position sensor. There's a way to test it. Hopefully someone can chime in on how to do that

All 3 codes indicate it's lean.


Possibly when you drove through the water it was hot, the cold water on the hot metal cracked the exhaust manifold or headpipe.
That allows air to get in to the O2 sensor, causing the system to think it's lean due to the excess O2.

Check closely for an exhaust leak.

Don't let me distract you from good advise^, just an easy check. The Map sensor up there on the firewall needs to see good vacuum from the TB via that little tube. If that line is cracked, clogged, off, the Map will think you have your foot in it and dump extra fuel.

I believe the 97 had the map sensor relocated to the throttle body

Thank-you Jakbob! I did a little searching and it looks as indeed you are correct. I did see a reference to a vacuum line to it there on the TB ...in any case, there is no doubt it needs vacuum

Correct, there is a rubber elbow that goes to the sensor to the TB port.

Thank you for responses everyone. I'll see if this helps and get back to you if it works.

21 Oxygen sensor or circuit. O2 sensor or wiring faulty.

42 Automatic shutdown relay. Check relay and circuit. Needs specialist or dealer diagnostics.

51 O2 detects lean mixture. Check sensor and circuit and for vacuum leaks.

I found those in "info threads".

I had a vacuum leak right at the back at #6. 6 was lean, but I think the low vacuum from the leak had the MAP dumping extra fuel.

If you do go around with a can of starting fluid, (short little bursts, if you hit one the sound will change), be sure to hit that rubber elbow underthe map there on the TB.

Check fuses. Check fuses. 11? Might not run if 11 is blown. 17? 02 heater. Swap 02 heater relay...(s)

I thought a 97 would have a 4 digit code.