Hi Guys,

I am having an issue with my Cali emissions 2001 2500 5.9L V8 4x4. I initially noticed the LTFT was around -17% when checking for codes when buying this truck a couple of months ago. No check engine lights were on at the time. Since then, code P0432 has come on. It starts and runs fine (as far as I know), but seems a little rough above 3000 rpm. It will still keep going above 3k rpm but just seems rough. So far I have done the following:

• Compression check (all between 155-170 psi)
• Changed spark plugs (a couple looked a little rich)
• Checked O2 sensors on a computer (all seem to be operating properly)
• Checked fuel pressure (~45psi on the rail so within specs)
• Tried to check the temps of the exhaust on both sides of the cats, but was difficult with my thermocouple. Will try with an IR temp gun next. (inconclusive)

A Cat exhaust replacement for this truck is about $1400 just for the part. My buddy from day one was saying bad injectors (leaking maybe). Before I buy a set, which is ~$350 shipped for new ones, I wanted to see if you guys had any thoughts about pinpointing the issue?

FYI, it is an ex-police truck used for towing boats and going down trails. Maintenance seems like it was pretty good. There are 143,000 miles on it. Passed smog a couple of months ago before the code got triggered.

Thanks in advance for the help.

 

"Checked O2 sensors on a computer (all seem to be operating properly)" is pretty vague. Do the upstreams switch between 0.1V and 0.9V in closed loop ? What do the downstreams do ? Can you get graphs of all 4 sensors ?

 

I can try to get graphs tonight if my computer will graph them. I pretty much just viewed the voltage of all of them and they seemed like they were changing in a range close to 0.1V - 1.0V. My neighbor is a factory mechanic for Honda and told me if they are changing voltage they should be good. He said when they die, the voltage is more constant as you rev the engine. All of mine changed voltage so I figured they were good.

 

Right, *but* they are basically a simple AFR meter that might tell you why the LTFT are going negative. Working O2 sensors help with troubleshooting.

 

What are short term fuel trims doing? Are the O2 sensors original? Or were they replaced at some point? Just because they are switching, doesn't necessarily imply they are accurate..... or timely. Slow or failing sensors can cause all sorts of problems, that code being one of them. (it's just whining about cat efficiency.)

 

These are the graphs I was able to get.

Stationary: idle to 2k rpm:


Driving: ~30 mph:


All 4 O2 sensors: idle to 2k rpm:

 

I can map the STFT tomorrow if you think that will help. I am not sure if the O2 sensors are original. I have only owned it for a couple of months and have not replaced them myself. Looking at them physically, they don't look new, but I can't determine if they look 20 years old or not. They seem to change pretty quickly. Not sure if they are accurate, but I have no reason to believe that they are reading off at the moment.

 

The downstream O2 sensor graphs should not swing as much as the upstream graphs, if the cats are working. From your description you were revving the engine while capturing the graphs, it would be better to get a "stationary operating point" (read: constant RPM). When the engine RPM is changing the PCM enriches or leans out the fuel amount. Just let it run at idle in closed loop operation (warmed up) and maybe keep the engine at 2k RPM as well.

Seeing the voltage change on the O2 sensors I'd say they are working fine. What are the intake air and coolant temperature sensors reading ? Any chance there is a vacuum leak throwing off the MAP sensor reading ?

 

Here are new images at stationary RPM. Idle and 2k RMP. Included coolant and intake temps.

At Idle:


At Idle:


~2000 RPM:


~2000 RPM:

 

There are at Idle and 2k RPM respectively. Included coolant temp and intake temp.

Idle:


Idle:


2K RPM:


2k RPM: