05 Mazda MPV 3.0 V6 Poor Driveability

I recently bought the subject van (cheap) that came with issues. Biggest one is a problem that feels like fuel. Two main symtoms:

When blipping the throttle in park, from idle to say 2500 RPM you can feel a bit of a stumble and then as the revs drop they go past idle and it dies as if the ignition was turned off. Every time. You can catch it and keep running with the throttle. If you keep revs above about 3000 you can blip the throttle and it responds smoothly.

On the road it will not respond to increased throttle. It just bogs badly. If you back out of the throttle it will accelerate (slowly). If you keep the trans in 2 or 3 it will accelerate reasonably if it has enough RPMs.

It always starts and idles okay but slightly rough. No MIL or codes, which is surprising to me considering how bad it is to drive.

I've found a couple things in my troubleshooting that may be pertinent:

Fuel trims are negative with both banks roughly equal, -15 to -20 at about 2500 RPM. Negative numbers increase with eng. speed.

MAF may be indicating high, never lower than 4.2 g/s at ~750 RPM. I've cleaned it twice which improved it (lowered) a bit.

MAP sensor (known as 'EGR Boost Sensor' with this Mazda eng): With engine running at various speeds the signal output from the boost (MAP) sensor is constant, 4.3V (measured manually with a DVM). With a manual vacuum input the signal from the boost sensor varies as it should (~2-4.3V). The boost sensor is not getting vacuum from the boost sensor solenoid. The boost sensor solenoid is getting vacuum from the EGR valve. When powered manually the boost solenoid works as it should. The boost solenoid is getting system voltage from its connector. The other wire should be a ground from the PCM to energize the solenoid. I don't have an o-scope, but when I put my DVM probe on the ground side of the connector I get a constantly changing value, so it is getting something from the PCM.

I understand that the BS solenoid's job is to alternate between atmospheric and manifold pressure. I don't know when/how often it does this. Can anyone clarify this for me?
___________

I suspect coils, since they are know to be a problem with this engine, but I haven't replaced them yet. I would have expected a misfire code if they were the problem, but I don't really have the experience with that.

Thanks in advance

what are the o2 sensors doing? i wonder if there fixed rich and the pcm is taking away fuel with the negative fuel trim numbers. also have you checked exhaust back pressure?? try doing a volumetric efficiency test you need maf g/s intake air temp rpm. and engine liter size.

I would try exhaust pressure test as cheryl stated sounds like possible plugged cat

Thanks, I'll see if I can do something with backpressure tomorrow.

Rule out a problem with the O2 by creating a vacuum leak and see if the O2 / fuel trims go lean in response.

Did a bit more playing:

O2 sensors are regularly fluctuating between ~ .08 and .85V so that looks okay.

I disconnected a vacuum line and fuel trims went from neg to pos so that looks okay.

I haven't done a positive exhaust pressure test, but I can hold it at 4000 RPM and manifold vacuum does not drop a bit. I don't have any first hand experience with a restricted exhaust, but this one doesn't feel like how I've seen/read that described (no power, like towing a very heavy trailer).

I've noticed what seems more like dropping cylinders, so I'm going to see what happens when the coils are replaced.

My MAF signal has gone back up to just under 6 g/s at idle, which is more than what I've been led to expect. I'd also like to understand why my boost sensor o/p is full scale on the atmospheric end of the range and unchanging.

Thanks

Don't just replace the coils without verifying there is a problem first. Get yourself a cheap scan tool that can check misfire data, an 05 should probably support that.

Another option is a cylinder drop test. Basically, unplug either the injectors or coils one at a time and listen for a change in engine rpms. No change means you hit the missing cylinder. Once that's done, use a test light to check for spark and COP control, then for injector control. if you find a problem, swap either coils or injectors to see if the problem follows.

Is the MAF o/p of 5.8 g/s @ idle indicating a problem? That's nearly double the rule of thumb 1/litre displacement that I've read. And it fits with the neg fuel trims?

Fixed. Coils.

What test did you do to verify that it was the coil/s?

I don't have the means to test them. I could tell that it was dropping cylinders under load so I took a bit of a gamble. Car runs/drives fine now, but I still have the neg. fuel trims at low RPM so I think there's still something going on. Suspect the MAF sensor, but it doesn't seem that far off and several cleanings haven't really improved it.

You could either have leaking injectors or a bad fuel pressure regulator, both would cause too much fuel and negative fuel trims.