How do I find the cause of P0172 (Rich bank 1)?

Howdy all,

I've got a 96 blazer with 4.3L 6 cylinder with about 60k miles. Over the last couple of weeks, the Service Engine Soon idiot light has come on intermittently with code P0172 (running rich, bank 1). Unfortunately there are no other codes that are helping me narrow down the cause. I'm puzzled because it idles smoothly and it doesn't hesitate at highway speed. The air cleaner is new and there haven't been any recent repairs that I can blame it on.

Is there an easy or straight-forward way to diagnose this? Everyone says to replace the 02 sensors, but I don't want to spend >$200 on parts I don't need. I've also noticed an odd not-very-loud constant-pitch noise that seems to come from the rear-center of the engine. Could a bad fuel injector or fuel pressure regulator make noise like that?

Thanks in advance for your help!
Ron

If your truck has over 50K miles on it it could be an O2 sensor but I feel by your complaint you have a leaking injector which costs alot more than 200 to replace because the injector for all cylinders is a one piece design. If one leaks they all get replaced.

Thanks for your reply, Flatrater. Is there a way to test the fuel injectors? Are there any tell-tale signs I can look for?

One a side note, would there be any benefit to replacing the O2 sensors, even if they aren't causing the P0172 error?

I consider O2 sensors a maintenance item that should get replaced every 50K miles. People will disagree with me but the first thing you have to suspect is a lazy O2 sensor. If you ewant try swapping the left and right O2 sensor to see if the problem changes sides if not then the sensors are good.

There is not easy way to check the injectors on this engine usually it ends up being a process of elimanation, if everything checks out OK then it as to be the CPI unit.

Just to give you a little heads up. I am trying to solve the same problem bank1+2 rich and more likely than not its not your O2's. When an O2 goes bad they go in the lean condition 99% of the time and usualy throw an 02 code. It more likely a mass air flow sensor or egr.

Chevyguy, do a fuel pressure test and see if the pressure drops after the fuel pump quits. If it drops do the test again with the return fuel line(section of hose near the fuel filter) crimped

Sorry I forgot to post since I fixed my blazer. It turned out that fuel pressure regulator was bad. I did a fuel pressure test and the pressure did drop very slowly after the fuel pump quit.

My mechanic wouldn't replace just the fuel pressure regulator and wanted to replace the entire CPI unit. I think he was quoting about $600, but my memory is poor. I was able to replace it myself for about $60 and an afternoon's worth of work with the help of the service manual.

By the way, the odd not-very-loud constant-pitch noise went away too.

i had the same problem when i was on a trip in oklahoma. the local auto zone helped me figure out what it was. you may have a leak somewhere between your maf, and your throttlebody. i tightened all of the clamps back down, and the problem went away.


hope this helps