P0421, P0431, hard to start cold

[jlb0305]

I have had P0421 and P0431 since last summer. I also had some other problems that I had fixed at the time. However my mechanic couldn't find problem causing these codes (he clear p0431 somehow but it returned) and suggested replacing the cats (gave me a "exempt" sticker for the my inspection for this year but said to replace the Cat converters before next year.).

I'm noticing the engine is still running cold (at about 1/4) temperature ever since I had a coolant flush last summer. Every couple months or so (sometimes mutlple times in one week then not for 2 months) the coolant is low (bottom of the resivoir w/ light coming on, not low like empty).

Fuel pump went two months ago and had to be replaced w/ filter and relay.

New issue the past few days is hard to start when cold. Battery is only 2-3 years old. It won't start first try then turn it off and try to start again and it starts right up.

Any ideas? I am noticing that I put in non-dexcool coolant last summer after flush (standard green, not the dexcool), as did all mechanics since then when they attempt to fix the cooling issue by replacing thermostat. Could this be causing P0431 & P0421?

[far2grumpy]

Originally Posted by jlb0305

I have had P0421 and P0431 since last summer. I also had some other problems that I had fixed at the time. However my mechanic couldn't find problem causing these codes (he clear p0431 somehow but it returned) and suggested replacing the cats (gave me a "exempt" sticker for the my inspection for this year but said to replace the Cat converters before next year.).

I'm noticing the engine is still running cold (at about 1/4) temperature ever since I had a coolant flush last summer. Every couple months or so (sometimes mutlple times in one week then not for 2 months) the coolant is low (bottom of the resivoir w/ light coming on, not low like empty).

Fuel pump went two months ago and had to be replaced w/ filter and relay.

New issue the past few days is hard to start when cold. Battery is only 2-3 years old. It won't start first try then turn it off and try to start again and it starts right up.

Any ideas? I am noticing that I put in non-dexcool coolant last summer after flush (standard green, not the dexcool), as did all mechanics since then when they attempt to fix the cooling issue by replacing thermostat. Could this be causing P0431 & P0421?

I'm going to dump a couple of files on you ... might give you something to ponder. One addresses the codes and other is for fuel system.

Next time you try to start a "cold" engine try turning the key to ON for five seconds - without going to START. Turn key OFF - then back on for two or three seconds - and then advance to START.

[fdryer]

Although this may be a small stretch and without a P0128 code, noticing the temperature gauge at the 1/4 mark may be the only giveaway of a cooler than normal operating temperature. I believe our 3.0L engines normally have the temperature needle at the 3/8ths point, between the 1/4 and 1/2 marks. A cooler running engine just forces a richer mixture, more fuel consumption, and possibly the reason for the catcon error codes. Overwhelming the catcons with rich mixtures instead of lean mixtures can conceiveably create the low efficiency cat error codes. It would seem unusual for both catcons to error simultaneously since only three cylinders feeds one cat making both cylinder banks feeding two separate catcons a mystery unless there are some things in common upstream - the maf sensor, throttle actuator control module, all six injectors being controlled by the ecm, the coolant sensor, map sensor, and lastly the thermostat. If you read past threads you'll see a pattern of the 3.0L engines having faulty thermostats with the P0128 code. I also have this code and am preparing for a spring thermostat replacement. Yours is the only 3.0L to date with the P0421/P0431 code. Replacing the thermostat is not easy since its buried under the main intake manifold. This opportunity to replace the thermostat may also reveal a possibility of a coolant leak in the area.

[jlb0305]

Thermostat has been replaced twice already without fix. Also replaced the temperature sending unit/ coolant temperature sensor. Didn't see any leaks when I did it and neither did the mechanic when they did. I am going to do a coolant flush again and put in dex cool see if that helps. Also prior to coolant flush and when I only had a P0421 and not the P0431, the engine temperature usually stayed at the 1/2 mark.

Also going to try cleaning Idle Air Control valve and look at a couple other suggestions from the repair manual.

As for starting, heres what I tried.
1)
Turn key - turns but doesnt start
Off
Turn Key - turns then starts

2)
Turn key - turns but doesnt start
leave on
look under hood
off then on - starts up as soon as ignition.

[fdryer]

In my haste I forgot to mention the 195F thermostat. Its also important that the two-wire coolant sensor connector actually connects correctly. The ecm counts heavily on the coolant sensor to gauge coolant temps in order to determine the right fuel/air mixtures during cold starts. Has the fuel pressure been measured before starting? Possible fuel pressure regulator?

It doesn't matter what coolant you use and would be a waste of money changing from one brand to another. The issue with poor starting is either a fuel or ignition related.

The iacv wouldn't be a cause of this random no-start problem. Its only used to supplement idle speed control - if it were broken you'd know it and if sluggish you'd know it. All it does is allow fine tuning the idle speed fom the cold start high idle to normal idle when the engine's warmed up. If the cold idle is around 1200rpm and gradually reduces to nomral idle after 10-minutes or so of driving, the iacv is operating correctly.

[far2grumpy]

Originally Posted by jlb0305

Every couple months or so (sometimes mutlple times in one week then not for 2 months) the coolant is low (bottom of the resivoir w/ light coming on, not low like empty).

One item in the DTC extract I sent earlier makes clear coolant will damage the cat ... please make sure your lost coolant is not a regular part of the combustible mix before replacing your cat.

[far2grumpy]

Originally Posted by jlb0305

As for starting, heres what I tried.
1) Turn key - turns but doesnt start
Off
Turn Key - turns then starts

2) Turn key - turns but doesnt start
leave on look under hood
off then on - starts up as soon as ignition.

Turning key to ON - without advancing to START runs the fuel pump for two seconds. If you turn the key OFF and then back ON you get two more seconds of fuel pump operation.

Turning key ON and leaving ON does no good - you get two seconds and no more (until PCM detects ignition reference pulses - engine crank or run).

A fuel system extract follows ... it appears you have a faulty fuel pressure regulator. The FPR fails to keep the fuel return line pressurized for long periods when fuel pump is OFF. This increases time to prime the fuel pump at key ON.

The engine control module (ECM) enables the fuel pump relay when the ignition switch is turned ON. The ECM will disable the fuel pump relay within 2 seconds unless the ECM detects ignition reference pulses. The ECM continues to enable the fuel pump relay as long as ignition reference pulses are detected. The ECM disables the fuel pump relay within 2 seconds if ignition reference pulses cease to be detected and the ignition remains ON.

The fuel tank stores the fuel supply. The electric fuel pump attached to the primary fuel tank module, supplies fuel through an in-line fuel filter to the fuel injection system. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator, a part of the secondary fuel tank module, maintains the correct fuel pressure to the fuel injection system. A fuel return pipe attaches between the in-line fuel filter and the secondary fuel tank module. When the fuel pressure exceeds the set point of the fuel pressure regulator, excess fuel flows to the low-pressure siphon jet pump on the secondary fuel tank module. The return fuel creates a venturi action inside the siphon jet pump. The venturi action causes the fuel to be drawn from the secondary side of the fuel tank though the fuel transfer tube to the primary side of the fuel tank. The electric fuel pump also supplies fuel to the high-pressure siphon jet pump, which is attached to the primary fuel tank module. The high-pressure siphon jet pump supplies fuel to the reservoir of the primary fuel tank module. The fuel pressure regulator also maintains fuel pressure in the fuel return pipe, which reduces the time to prime the low-pressure siphon jet pump.

[jlb0305]

Originally Posted by far2grumpy

A fuel system extract follows ... it appears you have a faulty fuel pressure regulator. The FPR fails to keep the fuel return line pressurized for long periods when fuel pump is OFF. This increases time to prime the fuel pump at key ON.
[/I]

I was thinking fuel pressure regulator too but then I can't find one for this model. No repair manual or parts store around here lists one for this model and troubleshooting a fuel pressure issue on the 02-03 only says its possibly a clog in the fuel line most likely at the injectors, fuel filter, or fuel pump. Pump and filter were just replaced end of January and had either a 1 month or 3 month warranty... I can't find my receipt though.

As for the P0431 and P0421... troubleshooting guide suggests possible the MAF which on the 03 contains the IAT/IACV.

For coolant leak wouldn't I see gas/oil in the coolant or coolant in the oil or have white/sweat smelling exhaust smoke from coolant in gas? With the temp reading it appears maybe the coolant I have in there can't hold as much heat as the Dexcool, causing system to run more (thermostat fully open) but lower temperature and lower boiling point, which is why occasionally the coolant is low from boiling away. I plan on still doing a flush and replacement with Dexcool. I will see after what the outcome is. There may still be an unseen leak somewhere.