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I have a 2004 Mazda 3 with a 2.0. Approx. 3 months ago it would not start. It turned over but wouldn't fire. I replaced the fuel pump and it started right up and ran fine for 3 days. Then it left my son stranded about 3 miles from the house where he said it just wouldn't go and died. I went and started it and it sputtered home barely. Could barely give it gas. It sputtered from the engine area and not really from the back exhaust. I started thinking it was possibly a O2 sensor or the catalytic converter. I replaced both o2 sensors and it didn't help. I replaced the Front Catalytic converter that has the header pipes attached and it seemed ok briefly during the test drive then it started sputtering again. When it sputters and you give it gas it just boggs down and you have to let off the gas then lightly press it to make it want to go. I still have a back catalytic converter (I think), that would require me removing the whole back exhaust system because a new rear converter would need to be welded in. But is this even the problem? It threw a code initially before I replaced the fuel pump and it was a fuel sending sensor. But its not throwing any codes now and it runs like crap. The sputtering comes from the front in the engine area. Its like the exhaust is clogged which is why I replaced that front converter. But the last thing I want to do is put more $ in it in replacing the back converter and still have the problem. IF YOU HAVE ANY IDEAS OR SUGGESTIONS I WOULD REALLY APPRECIATE IT. The car's been sitting for almost 3 months and I miss driving it. Thanks.
 

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Lots of things it could be, most of which can't really be diagnosed over the internet. Check fuel pressure, injectors, plugs, wires, etc. If there is a flange on the exhaust pipe before the cat disconnect it and see what happens.
Best bet is take it to someone who can properly diagnose the problem.
 

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It sounds like you have a scan tool, and if your do what are your fuel trims (long & short) at idle and 2,000 RPM.
They are OK if the short term and long term add up to no more than 10% or so.

What was the exact code from before?

Things to try:
• Clean the MAF
• With the engine off, unplug the upstream / pre-cat O2 and see if the car dives any different. (This will force a slightly rich fuel mixture)

I look forward to your reply. :)
 

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aka Goodwrench
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Also, pull the plugs and have a look at their condition. Carbon black plugs indicate a rich mixture and a wet plug indicated a leaking fuel injector.
 

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I had roughly the same thing just happen on my 05. It ran awful, not power, wouldn't go above 4k rpms most of the time. I replaced the MAF and didn't help then I replaced the header/upper cat and o2 sensors like you and it still ran like shit and disconnected the exhaust at the flange of the lower cat and downpipe and didn't help at all. I took it to the mechanic and they found a huge vacuum leak and figured out it was from a crack in the intake manifold. They patched it up and it ran fine but then I connected back the exhaust at the flange and the car had no power at all. I cut out the lower cat and it was clogged almost completely, I could only see a tiny pinhole of light when looking through it. I just replaced it with a pipe and the car runs fine now. I think the codes I got before I took it to the mechanic were cylinder misfiring and o2 sensor fuel running lean.

I'd say disconnect the exhaust at the flange and see if that does anything, if it doesn't get some MAF sensor cleaner and clean it and if that doesn't work take it to the mechanic and have them check the intake and vacuum lines.
 

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Discussion Starter #6
LeadV - Thanks for that. I'll try disconnecting the exhaust at the flange and see if it is the lower cat which I suspect it is. I'll let you know what happens.
 

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I'd say disconnect the exhaust at the flange and see if that does anything, if it doesn't get some MAF sensor cleaner and clean it and if that doesn't work take it to the mechanic and have them check the intake and vacuum lines.
Always start by doing free stuff before replacing parts. These suggestions from LeadVitamin and mazada are good. Sprayclean the MAF sensor with dedicated MAF cleaner. You can also remove the MAP sensor (located in the IM) and clean it.

You can do smoke tests to see if there is a vacuum leak. I also like the idea of checking for plugged up exhaust / cats by simply disconnecting the exhaust to see what happens!

Always be thinking that the engine needs air, fuel, and spark. OK, love too. If it isn't running well, then one of those first three is off. Checking the condition of the spark plugs is an excellent idea.
 

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Update. plugs were in bad shape, changed them, still had same problem. I also tried disconnecting the rear cat, eliminating it from equation to see if it helped, it didn't. Looking at it closer with a buddy, one weird thing happening when power gets lost is the popping sound I hear is blow back of exhaust coming back forward through the MAF and the air filter. One of the loud pops is the cover of the air filter popping up when the blow back occurs and slamming back down. I'm lost. Any idea why would exhaust come back forward through the front air intake?
 

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I don't have a lot of experience with the Mazda3 exhaust system other than disconnecting & reconnecting it when I had to replace an engine. The problem you describe sounds like your exhaust is plugged and the engine is being choked. So either your exhaust valves aren't opening or your exhaust is clogged.

Is it possible to disconnect the exhaust system from the exhaust manifold just to see if the thing breathes properly and doesn't give the blow back?

I have a 2.0 engine carcass in the garage right now that I just looked at, and the timing chain controls both intake and exhaust cams at the same time. And if the exhaust cam isn't turning, then I'd imagine the valves would be crushed. So, if it is turning but the timing is way off (worn timing components?) then the engine would run pretty crappy. Maybe the ECU is sensing something incorrectly and adjusting the timing to compensate?

Can your scanner check timing? I remember something like 9 degrees BTDC, but I could be wrong. Let me know if you want me to check my son's car. His is an '07 2.3L, but I can hook up the diagnostic tool for you and check whatever numbers you might need.
 

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Datsun 1600 - Just got off the phone with a very knowledgeable mechanic, I described the symptoms and he stated my timing chain jumped. That makes sense why exhaust is coming out the air filter/front. So I think you hit on it referring to the timing. Looking at $600-$700 minimum to repair and with 240k miles and still on original transmission, its not worth it. Its a shame, car is really nice and clean. Guess I'll be selling it for parts or parting out myself. It also sucks I bought and installed a new front cat converter thinking that was the problem. Thanks for the info, advise and effort in trying to figure this out.
 

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The blowback through the inlet sounds like valve or ignition timing. If you want to check for damaged valves you could try a compression test- if the timing chain is only slightly out the engine can misfire but the valves may have still missed hitting the pistons.
 

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Patrick '67,

A long time ago my brother's Datsun 200SX had a timing chain jump when he missed a shift and the engine RPMs went well past the redline. Car ran like crap pretty instantaneously. I declined mentioning "timing chain jump" because I didn't know if it was possible on these twin cam engines, but I guess with enough slack, anything can happen. Bummer!

It's quite possible that you didn't do damage to the valves. I have not looked up the cost of timing components, but they can't be that expensive. It does require removing the oil pan and timing cover and valve cover to replace the timing components and then begin resealing (with Permatex grey) the timing cover, oil pan, and then putting some new gaskets on the valve cover.

I have a junk 2.0 engine from an '07 that blew a head gasket. Actually, the engine isn't really junk since the head gasket can be replaced, engine cleaned out, etc. But it had 155k miles on it and to me there isn't value left is resurrecting it. If you need any spare parts, just let me know and I can send them to you.

OK, I looked up the parts from oemautopartz.com. About $220 total for timing components. Might as well replace the front main seal too for $8.

LF01-12-201 Timing chain $92
LF02-12-500 Tensioner $65
LF01-12-671 Tensioner guide $39
LF01-12-614 Chain guide $26
L3G6-10-602 Front main seal $8

The choice is yours, obviously. And I understand if you don't want to spend more money on the car. But think about that new front cat you installed....
 
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