99 Pro DX almost stalling/stuttering @ idle, any ideas welcome :D

[a61688]

Last week my pro threw a P0171 System Too Lean code. I looked on the interwebs and found that cleaning the MAF sensor can sometimes cure this. It was blacker than a piece of burnt meat. After this, I cleared the code and went on my merry way. This week I noticed something different. At idle after warm up, my engine seems to idle dangerously close to stalling rpms. It wants to stall and ramps back to life.....stuttering back to normal. Non-idle speeds are perfectly normal, however. I'm looking for causes of this and keep seeing two big things: O2 sensors and fuel filter. Car has 99 1/2 k miles and to my knowledge none of those have ever had to be replaced. I read that the fuel filter in the 3rd gen pro's are not serviceable and are actually a type of strainer. Would messed up O2 sensors cause this? I've checked for vacuum leaks and torn hoses but didn't find any. Penny for your thoughts!

Car in question:
1999 Mazda Protege 1.6L DX w/ automatic.
99.5k miles and bone stock if you don't count the tokico hp blue shocks and springs and EBC dimpled/slotted rotors and redstuff pads.

 

[Brains]

Does your 1.6 have an EGR valve? I had a similar problem and attributed it to a slightly stuck open EGR valve, so i took it off and cleaned it.
Ya know sometimes it's the EGR that'll cause the weird idle problem, but for me it didn't fix it completely. Mine's a little better, but i've got the 2.0 FS-DE so i've got an EGR valve... you might not even have one...
...but you probably have an idle air control valve that you might try cleaning. That could help, or it's at least something to look into. I don't stand 100% by my advice ever, i just suggest stuff to look into.
Ya know, I was supposed to clean my IAC, but i forgot about it several months ago and i've just now remembered it. I still refuse to do it.

 

[a61688]

Does your 1.6 have an EGR valve? I had a similar problem and attributed it to a slightly stuck open EGR valve, so i took it off and cleaned it.
Ya know sometimes it's the EGR that'll cause the weird idle problem, but for me it didn't fix it completely. Mine's a little better, but i've got the 2.0 FS-DE so i've got an EGR valve... you might not even have one...
...but you probably have an idle air control valve that you might try cleaning. That could help, or it's at least something to look into. I don't stand 100% by my advice ever, i just suggest stuff to look into.

I believe it does. I looked under the hood after reading a thread with instructions and pictures and I found the darn thing. Do you think it's really running lean in this situation or is it running extra rich to compensate for the code?

Ya know, I was supposed to clean my IAC, but i forgot about it several months ago and i've just now remembered it. I still refuse to do it.

LOL

 

[elooto524]

I had a similar problem awhile back, turns out an intake pipe was dry rotted

 

[a61688]

I had a similar problem awhile back, turns out an intake pipe was dry rotted

The tube you speak of is fairly new, having been replaced a few years ago after I found a huge crack in it. Checked it for similar problems but it's fine. Wishing it would've been damaged

 

[elooto524]

Ah that's upsetting. You always wish it was the easy stuff.

 

[a61688]

Seriously. And the worst part is that all the possibilities I can think of are pretty worst case scenario and expensive to fix at a mechanic. I know the first thing a mechanic is going to do is replace parts and probably start with the highest priced one.

 

[a61688]

I wanted to let everyone know that the problem has been repaired. My buddy and I took the throttle body and egr valve out during the weekend and gave them a good cleaning. While were testing the idle, my friend heard a 'sucking' sound coming from one of the tubes. (I swear I checked them! lol). We found a tube coming from the intake manifold going towards the firewall into some bulge. Don't know what it is but upon revving, the pressure rises and it was sucking in more air, which forced my engine's air/fuel mixture to be imbalanced. It has a sort of mold to it and a plastic tube on the inside to keep the tube molded and prevent collapsing on itself. Anyone know what it is? I'll try taking a picture of it. Anyway, we went to Advance Auto Parts and replaced it with tubing of the same diameter, cut to length to prevent flex or collapsing. Sea foamed the car after all this to make sure we were done and since then, car has regained lost power! Woohoo!

 

[a61688]

Here is an image I got from searching.

If you look at the intake manifold, there is a vacuum tube just on the end, next to the throttle body and it connects to a black bulge on the firewall behind the brake fluid reservoir. Not sure what it is......

 

[a61688]

Better angle on this picture. Note the tube with the clamps that look like you'd need to use a plier to press on the two tips

That's the tube I replaced.

 

[a61688]

Just found out, this is the brake booster vacuum line! Of all things, sheesh. I'm not sure this thread would need to be up but it might help somebody out there who somehow still has a protege.