P0420, P0400 and P0303 on 1999 Mistubishi Gallant ES

The Mitsubishi Gallant ES has 262,000 miles on it. Been getting the P0420 catalyst efficiency below threshold for about a year, year and a half. I know that's due to burning oil (old car, high mileage). Never replaced the O2 sensors or the catalytic converter so far. I've just been ignoring it.

I was running down the highway the other day and the car bogged down when I stepped on the gas to pass someone. My speed dropped off dramatically. I thought I had accidentally pushed the shifter into neutral. Then the car started vibrating (cylinder not firing) and couldn't get my speed past like 45 mph. I had to get a tow back home.

I checked the codes and now P0400 (EGR flow malfunction) and P0303 (cylinder 3 misfire) has tripped.

Should I replace the EGR valve? Or are the bad O2 sensors/and or/a messed up catylitic converter causing the EGR flow malfunction? I don't want to have to shell out the money for the EGR valve if it's just bad O2 sensors tripping the EGR code.

Before you look at these other codes like the P0420 and the P0400 you need to take a look at this P0303 code which is the cylinder #3 misfire and see what is causing that, the #3 misfire can be caused by things like the spark plugs and or all the above

I have gotten the P0303 code in the past and fixed it with a new spark plug wire set. Right now, the car's got fresh plugs and I just replaced the spark plug wires again for the second time just a few weeks ago when I got some stumbling on acceleration again. I have also replaced the coil pack for cylinder #3 in the past.

At this point, I'm thinking the specific misfire is not due to plugs, plug wires or the coil packs. I'm thinking it's related to the either the O2 sensors, catalytic converter or egr. What I don't no is if bad O2 sensors can affect egr function.

What I'll wind up doing is replacing the parts according to how inexpensive they are unless someone got some experience and definite advice. I thinking I'm gonna start off with replacing the EGR valve first. Then if that doesn't work, the upstream O2 sensor seeing how I've been getting the P0420 code for over a year now.

if it gives you a specific cylinder misfire fault then it's vey highly unlikely a sensor or an egr is causing that fault! The other thing to check is if the exhaust system is restrictted in anyway! Again it won't cause a specific cylinder misfire but it can cause a sluggish, hestitation etc!
Do a compression test, ohm the injector's resistance and try a plug or wire form another cylinder that isn't misifiring!

Update:

I replaced the EGR valve and still no good. After that I T'd a vacuum gauge in the line going from the PCV valve to the intake manifold to check for air intake leaks. It was reading rock steady around 18 to 19 inches of vacuum. That's supposed to be in the "healthy engine" area of 17-22 inches. I could have sworn though that when I put a vacuum gauge on it a couple years ago it was reading above 20 inches. Anyway, I have no reason to suspect an intake air leak I guess since it's in the healthy range.

The plug wires are only like a month or two old. It can't be that. I went and got a new coil pack from Autozone today and tried that. Still didn't fix it. I erased the codes before I test drove it and it tripped a new code, P0302-cylinder 2 misfire. WTF? I then began suspecting the connector at the coil pack since that coil pack controls cylinders 3 and 2. I have had several other connectors "loosen up" on this car and make trouble for me. The connectors going to the head lights and a connector for the window and lock controls in door panel. That one still loosens up occassionly. What I do is just bend the tines on the male end slightly so that the tines enter the female side at a slight angle thereby forcing contact to one side or the other in the female receptacle end. I tried that also (bending the 3 tines slightly in the coil pack male end of the connection). No go. Still stumbles on acceleration starting around 35mph and up. However, the car idles real good. The tach is rock steady at idle.

I think this is getting boiled down to the O2 sensor since the O2 sensor sends the information the PCM needs to correct the "on time" for the fuel injectors. Not to mention the P0420 code which could indicate a bad O2 sensor that I've been getting for over a year now.

I think I will check the Ohm-age at the cylinder 2 and 3 injectors before I buy the O2 sensor. And also do a fuel pressure check/fuel pressure regulator check.

Problem Solved!!! Glory Hallelujah! It was a clogged MDP sensor, Manifold Differential Pressure sensor. Also known on other cars as the MAP sensor, Manifold Absolute Pressure sensor. The MDP sensor sends information to the PCM/ECM to correct the fuel injectors and the ignition timing, hence the cylinder #2 and cylinder #3 misfires.

Check this Wikpedia link for more detailed information on what the MDP/MAP sensor does: http://en.wikipedia.org/wiki/MAP_sensor.

To jeff compton: Sorry man, you were wrong on this one. It WAS a sensor. A sensor that the ECM/PCM depended on for correct fuel flow and ignition timing. All I did was do a google search under "car stumbles on acceleration at 35mph and up" and I found a forum thread where this person had similar (not exact mind you), but similar symptoms as mine. He said he solved it by unclogging a MAP sensor port. I tried it and it solved my problem too.
Link here: http://www.fixya.com/cars/t1189615-98_c ... mbles_2000

I pulled the MDP sensor out and checked it out and the little tiny port had some kind of crusty light beige color film down inside it spanning the entire diameter of the hole. I just pushed a paper clip through it and tried to ream the rest of the crust out. At first I thought that film was part of the internal sensor and I was afraid I may have damaged it. After I cleared it out I could see a silver colored terminal down inside it. There was also a little pool of oil down inside the sensor well on the intake manifold where the sensor sets, which I cleaned out.

I believe I got the EGR flow malfunction code from the enormous amount of extra unspent fuel and so forth coming out of the misfiring cylinder. The P0420 code is still probably a bad cat converter. I did replace the upstream O2 sensor and I have not incurred the P0420 code again yet. It remains to be seen if the new O2 sensor fixes that code trip.

More info for those who have these symptoms:
a) Car idles perfect. Tach steady at idle.
b) Car does not stumble on acceleration until it reaches 35mph or more. Smooth acceleration until it reaches 35mph or more.
c) You step on the gas hard when it stumbles and it may smooth back out (or not).
d) You may be getting cylinder misfire codes tripped.

Here's the solutions I tried that DIDN'T WORK:
1. Replaced the EGR valve. Did not work. ($75)
2. Replaced the upstream O2 sensor. Did not work. (pulling my hair out at this point)($70)
3. Replaced the coil pack that controlled cylinder's #2 and #3. (serious depression setting in at that point. Felt better later though because I was able to return the coil pack to Autozone for a refund, $100. .)

I also:
1. Checked all the fuel injector's resistance with a volt meter according to the Haynes repair manual. The injectors were reading like 1/2 ohm to 1.5 ohms high out of spec. That didn't tell me anything.
2. Checked for an intake air leak with a vacuum gauge. No help. Vacuum within specs.
3. Checked battery voltage to see if the voltage regulator was bad (the voltage regulator is incorporated into the alternator on the Mitsubishi). Got weird reading there. I had a standing voltage of 17.5v and a charging voltage of 21volts. Spec according to Haynes is 12.5v standing and 14.5v charging. I thought I might have a problem there.
4. Checked the vacuum at the fuel pump regulator. Good there

I never checked the cylinder pressure or the fuel pressure. I was getting ready to take the throttle body out and clean it but never did. I discovered the MDP sensor clog before that. Would never hurt for it to be cleaned though.

One more note. On my first test drive after I cleared out the MDP sensor, I did get some slight stumbling going up a big hill at round 35-45 mph. But, on my second time around the block and back up that same hill, the stumbling had disappeared. The overall performance was drastically improved on the first run up until the big hill though. Maybe it took some running for some of that microscopic debris still up inside the sensor to completely cleared itself out by the second time around. No problems whatsoever now though.

So there you have it! Clogged MDP sensor.

The reason for the EGR flow code is probably because Mistubishi uses the MAP to detect EGR flow.

Really. Yeah, couldn't have known that. Hind sight's a son-of-a-gun. That little bit o' knowledge would have led me to the MDP sensor a lot sooner. Hopefully this thread will help someone in the future. That's why I try to put as much detail as possible into these kind of forum threads. I sure appreciate it when I happen to find information like this when I'm in need. (or more like, when my car's in need). ~

If we had some data from a scan tool and not just codes and parts you replaced maybe we could of been of more help. There is more to diagnosing and repairing cars besides changing parts and reading codes.

It sounds as if you thought I was dissing you about your reply. I wasn't. I think it's good information to know for the future and for anyone else who happens on to this thread searching for an answer to solve the same problem I was having with my car. It's information in hind sight, but it's information. The more information the better for anyone out there trying to solve this particular problem.

I didn't really expect a tech expert to be roving these threads giving out iron-clad solutions to these OBD codes. I just expected shade tree mechanics like myself to be offering friendly advice, which I got. And which I'm grateful for.

I want what was written in this thread to help someone in the future who's desperate for a solution like I was. Our cars equal freedom. Freedom to go anywhere, anytime we want. When our cars go down, we loose our freedom. And for some of us, our paycheck too, if we also use our cars for our jobs like I do. I use the Mitsubishi for pizza delivery. I can't afford to pay some mechanic to check out my car every time something goes wrong with it. I'm not trying to deny the professional auto mechanics their lively hood. I'm just trying to preserve MY livelihood, and keep more money in my pocket. And why not? If I have the tools and capability to accomplish it. There's more than plenty of people who don't. And these people provide the pro mechanics their livelihood. That's good for them.

I'm grateful for the time anyone took to offer advice in my thread. Thanks and Cheers!

SORRY

>asecmt - No Problem.

Quick Update:

I've been driving the car hard at work delivering pizzas for the past couple of weeks and the P0420 code has not come back. The new upstream O2 sensor must have fixed that. It was a non-OEM BOSCH O2 sensor. So that was not a waste of money after all. I have to have a clean fault sensor status to pass inspection anyway. The last inspection I had to fake them out and errase the code with my scanner in the parking lot just before I gave it to them and hope it wouldn't trip when they started the car up to bring it into the inspection bay.

All is well on the stumbling. Smooth acceleration and no problems. And I didn't even have to buy a new MDP sensor. Just cleaned out the old one. Good as new.