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Discussion Starter #1
Hello. We have a 2001 Caravan (not Grand) SE with a 3.3l flex fuel engine. It has not been driving the best and has progressively gotten worse.

The issue is that the engine hesitates (not quite stalling), more of a bogging down at the top of the shift range during hard acceleration. It feels like it wants to shift (maybe around 3.8k?), but instead begins to hesitate and stays in gear. Backing off the pedal allows the shift to happen fine, once the RPMs back down to somewhere around 3k. It starts fine, idles fine and hasn't actually quit running. I did get a P0172 or P0174 (can't quite remember which) saying the fuel was lean. It also had 3 evap leak codes which haven't returned since I changed out the evap vent canister. (and cleared the codes). Also - it is generally REALLY BAD immediately after fueling up and I don't have to accelerate hard to get the hesitation. It smooths out in a few miles. Same thing when getting near a 1/4 tank.

Its definitely NOT the transmission slipping. I rebuilt a 4L60e last year and am quite familiar with a slipping tranny. My wife believes it started sometime after running out of gas (actually my fault) - so I started with the fuel system. Fuel pressure at the rail holds at 61 psi while running and quickly drops to 56/57 psi when shut off. Seems to hold this pressure fine, at least 20 minutes or better. I also replaced the fuel filter, added injector cleaner and tried new plugs and wires yesterday. I will say that the plugs looked like they had some excess carbon like the fuel was running rich (all 6) and gap was still good. Fuel economy seems to be less than it had been as well, although I have not confirmed this yet.

It was running better with the new plugs and wires, but the problem is still there. It tends to only happen in 2 and 3 gear, but I suspect that's just because I'm usually not accelerating that hard at takeoff. I haven't added any fuel to test that out again just yet, and it gets just as bad once in a blue moon without being topped off.

My next thought was replacing the o2 sensor to see if maybe the computer is compensating with too much fuel. Does this sound like a waste of money? I keep coming back to considering a new fuel pump because of the immediate change when filling up and I keep running into conflicting fuel pressure specs - my Chilton manual says it should be at 49 psi, but I seem to read online about it being 6-10 psi higher. Perhaps the fuel pressure regulator is pushing too hard?

Any thoughts?
 

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Probably a stupid question, but have you checked the air intake hose and filter to make sure some rodent hasn't built a nest in there? If that's okay, maybe a good cleaning of the throttle body would help. My 99 with the same drive train just rolled 190,000 miles Sunday and still runs great.
 

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Fuel pressure sounds high. That would drive the PCM lean and set a fault code. Was the fuel filter/pressure regulator an OEM part?
If it is worse after a fill-up, the evaporative system/ORVR may still have a problem.
 

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Discussion Starter #5
I think the fuel filter was aftermarket. It looked exactly like the one I put in which definitely wasn't OEM. The pressure regulator is in the tank on the pump. I'll see if the next fill up is any different. I'll have another look at the EVAP system too.

Oh, and I tried cleaning the TB out at the same time as I changed out the plugs and wires. It was due.
 

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Filled the tank today. Looks like its getting around 8.7 mpg right now - so its definitely flooding the engine. I talked to a dealer about the fuel pressure and he told me 58 psi +/- 5 was correct, so the pump should be working properly.

I'm going to look at the TPS tomorrow and look for any leaks in the EVAP system too.
 

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It would be helpful to see what the O2 sensors are seeing in the way of an air/fuel mixture. A rich mixture fault or cat fault may take a long time to set. The previous P0172 rich mixture fault was probably real.
Do you have access to an OBDII scan tool?
If the catalyst is plugging or the fuel system is over-rich, the vehicle shouldn't be driven like this. Are the plug tips black?

P0172-1/1 FUEL SYSTEM RICH
When Monitored: With the engine running in closed loop mode, the ambient/battery
temperature above 20 deg. F and altitude below 8000 ft.
Set Condition: If the PCM multiplies short term compensation by long term adaptive and
the result is below a certain value for two trips, a freeze frame is stored, the MIL
illuminates and a trouble code is stored.
POSSIBLE CAUSES
POSSIBLE CAUSES
INTERMITTENT CONDITION
O2 SENSOR HEATER OPERATION
O2 SENSOR
EVAP PURGE SOLENOID OPERATION
O2 SENSOR SIGNAL CIRCUIT SHORTED TO VOLTAGE
O2 SENSOR SIGNAL CIRCUIT OPEN
TPS VOLTAGE GREATER THAN 0.92 VOLTS WITH THROTTLE CLOSED
TP SENSOR SWEEP
MAP SENSOR OPERATION
ECT SENSOR OPERATION
ENGINE MECHANICAL PROBLEM
FUEL FILTER/PRESSURE REGULATOR (HIGH)
PCM





PO
TEST ACTION APPLICABILITY



 

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Discussion Starter #8
Thanks IC. I must have mis-typed in my first post - the dtc code was for a LEAN condition (I do remember the general description, just not the number), so it would have been a P0171 or P0174. I'm not sure of exactly which components what the PCM can affect, but my thinking is that the computer believes the engine isn't getting enough fuel and so tells the injectors to spit out more, flooding the engine under heavy acceleration.

Were you asking about the new plugs being black or the old ones? The old ones definitely were carbon blackened - all six of them. I have not pulled one of the new ones to check yet.

As far as the cat goes, I don't smell the rotten egg smell at all. I do have a neighbor with an OBD-II computer, but he is currently out of town. If he gets back sometime soon I will be sure to return with the fuel trim values.
 

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Discussion Starter #9
TPS turned out to be fine. I did discover a badly split hose leading to the EVAP purge canister. I replaced that section with new hose, but it apparently didn't solve the hesitation problem - I don't know if that would have affected the mileage or not. When it was running good it was around 24 mpg I believe. I might look at the fuel rail and injectors too. Is there any reason to not suspect a faulty O2 sensor?
 

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Discussion Starter #10
I went ahead and swapped the oxygen sensor - no more hesitation at all. The old one had some oily residue, probably due to the system running rich.

So now I'm curious which came first - a lazy O2 sensor or the rich system? I figure if the sensor was still fluctuating it's voltage it wouldn't trip the OBD code, but the range of flux could be limited to show it reading lean - thus the rich fuel mix.

Perhaps the bad EVAP hose introduced more air into the system causing the original rich mixture which eventually killed the oxygen sensor?

Either way, it appears to run normally again. I'll still have to confirm the mpg, but I strongly suspect it will be much higher now that the engine shouldn't be flooding any more.
 

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The O2 sensor probably failed first and did not set an O2 fault code, but went straight to the 'fuel system lean' code. The PCM's only clue to rich or lean mixtures is through the O2 sensors. If it saw the O2 telling it that it was lean, it would have dumped more fuel in to compensate. A scan tool lean O2 voltage reading would have probably shown the O2 signal voltage pegged low.
If the split evap hose had opened the vapor-management system to air, it too would have eventually set an 'evap leak' fault code.
If you have fixed the mixture problem, it might be a good idea to disconnect/reconnect the battery to reset the fuel trim adaptives to center (neutral) and to erase the fault code if you haven't done so already.
If the engine oil smells of fuel contamination from the rich running, it might be a good idea to change that as a precaution as well.
 

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Discussion Starter #12
I pulled the battery cable after changing the split hose, but that was before I replaced the O2 sensor. I will reset it again to be sure. Thanks for all your help!
 
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