Severe hesitation, missing, erratic idle, occaisonal stalling

[varg]

It's had this problem since I got it, and I figured it would be an easy fix, but not so far.

Symptoms:
- Erratic idle, appears when warm, worst after a hot/warm start
- Severe hesitation and missing, appears when warm, worst after a hot/warm start
- Occasional stalling after warm/hot start.

MIL is constantly on, sometimes flashes. Codes are for random misfire, cyl 3 misfire and "fuel trim" (research showed this is really a "boost sensor" code). All codes eventually return if reset.

I have read up on the usual suspect - the EGR valve. I cleaned it and noticed no difference. Unplugged it and noticed no difference. Blocked it off and noticed no difference.

I have also tried removing the vac lines to and unplugging the boost sensor, no difference there either. (Aside: "Boost sensor?" on an NA car? Silly name for it). I've cleaned the MAF and checked for vacuum/intake leaks.

The car has new plugs, wires, cap, rotor, air filter, and I've replaced the fuel injectors (had a spare set). The plugs themselves look fine - though they read a bit rich (fuel economy around 27mpg so it's not running too rich).

So what's up here? I don't want to throw money at this car because I don't have money to throw at it. What's the next suspect now that I've eliminated the usuals?

Thanks.

[smokstac]

The main thing that changes with regards to fuel injection when the engine is warm is the use of the primary O2 sensor to regulate fuel injection...i.e. when the engine is cold the PCM is using predetermined values to run the engine. The primary O2 sensor is located in your exhaust manifold and a bad one can cause all kinds of problem even though a code isn't thrown (I had a bad O2 sensor on mine and absolutely no codes....to really find out you need a laptop, USB->OBDII connector, and some software to graph the O2 sensor's output). The O2 sensor also controls fuel trim.

Boost sensor = MAP sensor. Boost is just another way of saying less vacuum.

[Boosted Escort]

Have you checked timing?

[varg]

The main thing that changes with regards to fuel injection when the engine is warm is the use of the primary O2 sensor to regulate fuel injection...i.e. when the engine is cold the PCM is using predetermined values to run the engine. The primary O2 sensor is located in your exhaust manifold and a bad one can cause all kinds of problem even though a code isn't thrown (I had a bad O2 sensor on mine and absolutely no codes....to really find out you need a laptop, USB->OBDII connector, and some software to graph the O2 sensor's output). The O2 sensor also controls fuel trim.

Yeah. The O2 sensor was my next guess since the symptoms seem to be present when the ecu should be in closed loop operation. Is it common for a bad O2 sensor to cause the symptoms I described above? I'll pull the sensor and inspect it.

Honestly, I wish I could afford to rip the stock stuff out and megasquirt it.

Have you checked timing?

No, I haven't checked it. The symptoms don't exactly hint at ignition timing being off, unless the ecu advances timing enough when cold (when it drives fine) to nullify the effect of it being retarded at the distributor. I've got nothing to lose by checking it though.

[smokstac]

You can't look at an O2 sensor and know if it's bad or not.



This is a graph from one of my previous vehicles. A properly working O2 sensor will create a graph like one of the ones on the left. The O2 sensor should create a fluctuating voltage as the ECU tries to keep the air/fuel ratio near an ideal value. There's no way to look at one and know if this is what's happening.

Your symptoms are similar to what I experienced. Usually a bad O2 sensor results in severe loss of gas mileage. In my case it would idle sort of OK, accelerate under light throttle OK, but, if I tried to accelerate decently the car would bog down and try to die...as if it were starving. Only did it when the engine was warm; never when it was cold. The car even backfired if I put the throttle open too much. Took forever to get up to 35-45MPH (literally 0-60 in 10 minutes).

[irishkev90]

Sounds like he exact problems I'm having. Sub'n to see if/how this gets fixed.

Sent from my HTC Droid Incredible using Tapatalk Pro

[bloodyspirit058]

if you had access to a scope or a really good obd2 scanner i would look at air intake temp (just to see what its saying and when you had the maf out, did you make sure that little wire in there wasn't broken? i might be thinking of a different style of maf.

a good obd2 scanner can graph your o2 sensor, but a scope works better, would look at that also. it really sucks cause the equipment i usually use is really expensive and i get to use it at school, and i know most people don't have access to it

actually i would probably start with the 02 sensor cause you already took care of the first one (egr) that could be causing like symptoms.

one last thing... how the hell are you getting 27 mpg with all the misfires.
just another thought came to me, when was the last time the fuel filter was replaced? if you know someone that has a scope you might be able to use a fused jumper wire on the fuel pump and watch what the amperage is doing, if the pump is going (low fuel pressure, vapor locking fuel ect) the pattern will look somewhat normal with tons of interference.

[1Mazda323Fan]

Stupid question, but are you 100% sure you got the wires in the right order on the disty cap? That could cause pretty severe problems not unlike what you're experiencing. Check the timing to, as was suggested.

[bloodyspirit058]

Stupid question, but are you 100% sure you got the wires in the right order on the disty cap? That could cause pretty severe problems not unlike what you're experiencing. Check the timing to, as was suggested.

i don't think this is stupid at all, always check the basics, one thing we always forget, from what he said it sounds like it is running ok cold though but i like the thinking :D

[irishkev90]

Just bought an upstream O2 sensor (replaced the downstream sensor less than a year ago) and it should be here tomorrow morning. I'll get it in as soon as I can and see if that solves my problems.

[smokstac]

Upstream sensors can cause drivability issues but the downstream one will only throw a code if it's bad.

[smokstac]

Here is my Protege's O2 sensors graphed:

[irishkev90]

Just replaced my upstream sensor this morning. So far no bouncy idle or random stalling. I was getting 28.7mpg before the new sensor so I'll see if that changes at all as well. It usually acts up after work also, so I'll update when I get home from work if anything has changed.

[irishkev90]

The new O2 sensor didn't do anything. It died on me at least 6 times this morning dropping my girlfriend off at school. Ugh, this thing is angering me.

My dad thinks it might be that the timing belt is stretched and causing the idle to go funky. Any opinions on this?

[Mazda_Powered]

doubtful on the timing belt, but you can always pull your cam cover and check the cam gear marks with a manual/diagram.

I'm thinking it's either your coolant temp sensor or TPS to be honest.