98 Mazda 626 GF 2L ATX zero ignition timing at idle?

Question

My 1998 Mazda 626 GF 2L ATX has zero ignition timing at idle when fully warmed up causing a rough idle condition. I've replaced the faulty O2 sensor with no effect on the retarded timing and replaced the Neutral inhibitor switch due to testing identifying poor continuity. Have also replaced the PCM with another from an identical model but it still has retarded ignition (zero) timing at idle when warmed up. Car always has 10-12 degrees of ignition timing at idle on cold start then progressively drops to 0 degrees ignition timing at idle once reaching full operating temperature and engine fan cuts in. If anyone has successfully managed to resolved this issue with the same model car please let me know.

Answer 1

So there are a few things that need answering here:

1. Why the car has a rough idle
2. Why your fuel trims are so negative
3. Why your timing is retarded

The rough idle question is the most difficult, so I'll leave that to last. The easiest is probably why the timing is retarded.

First of all, the timing on this car is controlled completely by the ECU, so the ECU must be intentionally retarding the timing. In general, there are two reasons the ECU will retard the timing. The first and most common is because of engine knock. However, I'm fairly certain that is not the reason in this case. The other reason the ECU will intentionally retard the timing at idle is if the engine is idling higher than it should be.

The 97 EU WSM says that idle with all loads turned on should be between 550-750 rpm. The only other relevant thing it says is that at idle with no load the MAF should be reading approximately 2.2g/s and should read approximately 8.1-8.3g/s at 2500rpm with no loads.

I found some other data in the 98 US WSM; it said that at 650-750 rpm, MAF should be reading between 2.2 to 3.1 g/s, and load should be between 18% to 23%. I'm assuming 2.2/18 is at 650rpm and 3.1/23 would be at 750rpm.

Based on your scan data, at 749 RPM your engine is drawing 4.1 g/s and load is 18.8%. That MAF reading is suspiciously high, and I would bet that if you take a look at your IAC percent open, it will read 28.5%, which is fully closed. I'm also guessing that either you have opened up the idle air screw to try and raise the RPM and smooth out the idle, or that you've got carbon build up in the IAC preventing it from fully closing.

Under these circumstances, the ECU will retard the timing ( and close the IAC ) to bring down the idle speed to what it's target is.

So close the idle air screw some and I bet you'll see the timing get more positive.

The second thing to deal with is the negative fuel trims. It's a little confusing that the short term would be at -100% while the long term is only -7%. Usually the ECU will adjust the long term and try to keep the short term as close to zero as possible. That aside, you need to find out what's going on. The first thing I would check is your fuel pressure regulator. Pull the vacuum hose off of it while running and see if fuel leaks out. Also watch the fuel trims on your scan tool while doing this. If the fuel trims don't change when you pull the FPR vacuum hose, then that's likely your problem. If they do change, then you most likely have a badly leaking injector or one which is stuck open. One way to check this is simply to unscrew the whole fuel rail and energize the fuel pump by putting the key to the on position without starting the engine. You'll need to tie each injector to the rail with a zip tie or something similar so they don't pop out due to the fuel pressure. You can see allot of these tests on DJDEVON's youtube page.

As far as the rough idle, there can be allot of reasons for that, and you should deal with the two above issues first, then ask a new question about the rough idle I think.