"But the Code came back the very next day..."

[springoflife]

"...we thought he was a goner,
But the code came back; it just couldn't stay away"

I have a 2000 Venture with a 3.4L motor and 100K miles. It runs well 90% of the time. However, a symptom that I've been having for almost a year now is with the "Service Engine Soon" light. I took the van down to my local Advance auto, and they read the code (P0131) which is for an O2 sensor. So I replaced it, and had the codes cleared. About two days later, the SES light goes back on. Again, I take the van down to my local Advance auto, and they read the code (P0137) which is for the other O2 sensor. I replace it and have the codes cleared. But like the song goes, "the [code] came back it just wouldn't stay away!" Everytime I have the codes checked, P0131 and P0137 (both O2 sensors) are listed as "Low Voltage". I have the codes cleared, disconnect the battery, etc and give it about two days and the codes come back. I have a hard time believing that both new O2 sensors are bad, so I'm thinking that the sensors are picking-up a malfunction elsewhere in the system. I've been taking a hard look at the wiring and vacuum lines, but don't see anything unusual. I replaced the Plugs & Wires, filters, etc but that didn't yield any improvement. So I determined to live with it and just ignore the SES light. (I would check the codes occasionally to make sure that some other new problem wasn't being masked by the existing O2 Sensor codes.)

But now, another symptom has appeared, and is getting worse with time:
When traveling about 50 MPH and trying to maintain speed uphill, increasing pressure on the gas pedal results in a sudden bucking/loss of power. At that time, I can completely floor it, but the van will not maintain speed. Easing-up on the gas pedal can result in the situation instantly reversing itself, with the power returning to normal. It acts like a dead spot in the gas pedal, so I replaced the TPS -- but this didn't change anything. Also, sometimes when the motor is good and hot (say, I've been traveling on the Highway for awhile), then require the motor to simply idle (say, at the end of the exit ramp), the idle speed will "hunt" for a few cycles, then die. It always restarts, and the idle usually smooths-out somewhat.

Yesterday, I was driving along (about 50 MPG) and when I tried to maintain speed up a hill, the engine started bucking and the SES light turned-on again. I took the van to have the codes read, and *you-guessed-it*, P0131 and P0137.

Based on all these symptoms, what would you try first? EGR Valve? MAF sensor? I'm planning on taking a camping trip this weekend, including pulling a pop-up camper, so I need to fix this ASAP! I can't afford to lose power with a camper behind me.

[roadrunner2]

I used to have a 2000 Silo and still have the Haynes manual but after reading through it I can't seem to nail down a cure.

I wouldn't plan on going anywhere until you get this problem fixed.

A faulty/dirty IAC valve and/or dirty throttle body MAY be contributing to a stall or lean condition due to a poor air flow.

You may have to 'bite the bullet' and take the van to a reputable independent shop.

Getting first a code from the 'upstream' o2 sensor then also getting one from the 'downstream' senor tells me the fault is getting worse.

[MR1972]

have you changed the fuel filter ? maybe it is cloaked filter, could result in enough fuel pressure but not enough fuel flow. other possibility could be a dying fuel pump.

[springoflife]

Thank you for your reply!
Yes, I have changed the Fuel filter and regulator.
I hope it's not the pump! I changed one on my previous minivan (a Mopar) and it was a nightmare! The retainer was rotted and replacements were not available, etc.

[MR1972]

I have exactly the same issue with my 2000 Transsport (European version of the Venture). I have ordered a new filter, if it won't help I will make a fuel pressure test and if necessary change the fuel pump.

Did your issue go away after replacing the fuel filter ?

[thisnametooktolong]

131 and 137 ar bank 1, up streem and down stream o2 sensores. 1st I would check the fuel pressure and scan those two sensors and see what they are doing at idle and running down the road ( up a hill) But it sounds like you may have to take a looksee at the Cat.

because it is on bank one only you may have a problem in the engine. Try doing a comprssion test also

good luck

[MR1972]

thanks for your reply,

there is just one sensor before and one after the catalytic converter, so I don't think it is a valve / compression issue.
H02 sensor values are ok, when driving with medium load / idling. I have checked them with my notebook and ScanXL / Elm.
The issue only occurs when putting high load on the engine, so assume there is not enough fuel goint to the port injectors, I have already searched for vacuum leaks, but I couldnt find any.
The emission values are ok, I had them checked in a store, even when 131 and 137 is thrown and the PCM remains in open loop mode.

I have read that 131 and 137 can only occur, if TPS value is below 60%, but this seems not to be correct. It is thrown anyway. I have also checked the TPS values with ScanXL with key on - engine off. TPS seems to work correctly.
So now I am waiting for my fuel filter to arrive and then I will see.

I will keep you informed on the issue and post the solution when found.

[thisnametooktolong]

Woops. I often forget what engine has two and four O2 sensors.

At this point I would do one more road check. If the thing does have proper fuel pressure, drive the thing down the road. If you are traveling at 30mph and you smash the gas pedal ½ way to the floor, and it stumbles then takes off? You probably have a plugged injector. If it does not have power at WOT and a heavy load you have an obstruction in the air, or an internal vacuum leak. A compression test and a vacuum test can confirm this. If it does not then it is an O scope problem and I cannot help you Hands Off.

[springoflife]

Update on my van:
I replaced the MAF sensor. I did not take the time to go and have the SES light cleared, but the van is already acting better. The idle no longer "hunts", and the engine stumbling seems subsided. I will keep you guys posted if symptoms come back (maybe I was just being optomistic?) or if the SES light goes off on it's own.

[MR1972]

Also changed the MAF sensor and the problem is gone.
Engine runs smoother, accelerates better and idle speeds are correct now.

[springoflife]

After about two weeks, the SES light was still on, so I had it manually cleared by my local Advance Auto. That was about a month ago, and it has NOT come back on! Woo-hoo! I think this problem is solved. The van is still running strong, excellent throttle response, transmission shift points are correct, idles smooth.

[henry10]

Did you actually get a trouble-code for a MAF malfunction?

[springoflife]

No!
The only codes I received (over and over again) were for the O2 sensors.
BTW, it's now been about 2-1/2 momths, and the SES light has not come back on.

Motor still runs strong and idles perfect, too.