how bad is it if my pressure sensor is dead? I have an aftermarket boost gauge and it registers full boost, but i don't get any movement in the stock gauge. Thanks

 

No sweat, unless it just bugs you.

Are you getting an ECU check light?

 

Without the pressure sensor, the ECU will not know how much boost you are running. I see this as bad.

 

It is responsible for timing retard under boost. Without this, you may be running too much advance and under the right conditions you could set yourself up for pinging, which can kill your engine in short order.

I would fix it asap or not boost much more than 3 or 4psi.

 

I assumed he had checked his AFR gauge to see that there was enough fuel.

I should ask:

Jack - does your A/F gauge stay green/yellow under full
boost?

Oops - I forgot about the timing retard issue.

 

no check lights, the stock gauge just doesn't budge at all. I haven't hooked up the A/F part of my timer yet... i guess i should hop to that

 

It doesnt affect fuel, only timing. Fuel delivery is controled by the airflow meter and intake air temp sensor. Together, those 2 calculate intake air volume and match it with fuel accordingly. The boost sensor is used to tell the ecu how much to retard timing.

 

Err.. have you checked the gauge cluster connection? It's flaky in these old cars.. before you change the pressure sensor out I'd check it with a multimeter per factory service manual

 

I thought S5 pressure sensors were involved in adjusting fuel also.

 

can anyone confirm whether it affects fuel?

 

Why would you need a pressure sensor to adjust fuel? As i said, the afm and intake air temp sensor do this for you. Air flow meters do a good job of measurig intake volume, but corrections must be made for air density according to temperature. IN other words if the afm measures x amount of air volume at 70* ambient, you need a certain amount of fuel for that. IF the afm measures that same x amount of air volume at 30*ambient, there is actually more air then there was at 70* due to the density of cold air. This is why correction from teh intake air temp sensor is necessary. BEtween the two of them, they measure air, and fuel gets matched per a preset table in the ecu.

Why then would you need a boost sensor to fool with fuel even more? You dont...but you do need a way to know how much advance you can/cannot run. To do this, you need to know the boost level, and thus the boost sensor was born. That is all it does.

IF you doubt this, unplug the thing and go for a drive. The car will not run lean or rich any more than it did before, but it wil lfeel slightly more powerful because you are running more advance then you should stock with it plugged in.

You can take what I say as truth, or you can debate it further, but I feel I have explained the situation as much as it needs to be.

 

Actually the S5 MAP sensor does affect fuelling, so if it doesn't work then in is very bad.

So far all we know is that the gauge is dead. The sensor needs to be checked using a voltmeter and the data in the FSM.

 

What proof do you have to back this up?

My info comes from Ted, who once did testing of a stock car on a scope, and confirmed that regardless of boost sensor input, fuel delivery did not change.

 

Ted tested an S4 and the MAP sensor had no effect on fuelling. Another member datalogged his S5's ECU and it clearly showed the MAP sensor affecting fuelling.

 

Any link to this info? Personal experience? Not that I don't trust you, but I like to see stuff for myself. Ted *showed* proof of what he said.

Besides, why would they change their complete philosophy of fuel system between s4 and s5? There are minor differences between the two systems, but there would have to be a significant reason for them to affect this change IMO.

Can you give any reason/speculation as to why they would need to do this? As explained above, an afm and intake temp sensor are all that is needed to accurately measure intake volume and thus match fuel to it.

 

Buy yourself a wideband 02 meter. Go drive down the hwy. Look at the meters reading. Stop. Get out. Remove the vac hose from the pressure sensor and plug the line. Go drive down the hwy again. Notice the difference in the wideband reading now vs before? Sorry....but I do. I'm not saying the pressure sensor is the end all for fuel delivery, but it does effect fuel delivery.

 

doesn't timing effect a/f readings too?

 

Exactly. Timing can affect a/f ratios. We know the boost sensor should affect timing, so it could affect a/f ratio in the exhaust. Not saying that it affects fuel deliver INTO the engine, but timing changes how much fuel the engine burns.

The only way to know FOR SURE if the boost sensor affects fuel delivery is to measure injector pulsewidth directly. This is how Ted got his results on the s4 test.

until someone does an identical test for an s5, we will not know for sure. However, I still have a hard time believeing that they changed the very basis of their fuel injection system between s4 and s5. IT wouldnt make any sense.

I will continue to believe that the boost sensor only changes timing until someone conducts a proper test to prove otherwise. If I had access to a scope and a dyno I would do it with my own car.

 

****so it could affect a/f ratio in the exhaust****

It's evidently been a long day for one of us or someone is celebrating New Years early.

 

Well, let me rephrase that.

It could affect the reading as taken in the exhaust, versus injector pulsewidth directly.

In other words:

you dump in X amount of fuel to the engine. it burns Y percent, and Z is left in the exhaust and is measured as rich or lean.

Now, you retard the timing. You still dump in X amount of fuel, but the engine burns a different amount, A percent of that fuel, because you change when the combustion occurs in relation to the ports. So a different amount, B, is then present in the exhaust, and will therefore show a different rich/lean reading. But you didn't change X, fuel delivered into the engine.

 

I searched for the thread but couldn't find it. I can't remember the guy's name but his avatar was a S5 boost gauge. He datalogged several ECU inputs and outputs including MAP sensor, AFM and injector pulsewidth. From memory, the AFM signal maxed out surprisingly early and then the shape of the injector pulsewidth reading followed the shape of the MAP sensor output exactly. Based on Ted's test I didn't believe it either until I saw his data. The only obvious difference was that Ted tested an S4 and this guy tested an S5.
Who knows why Mazda does half of what they do? Compared to the many other significant changes Mazda made for the S5 it's no big thing. I can't explain it, I'm just passing on the info I've seen. Take it or leave it.

Even if I'm wrong about the S5, a non-functional MAP sensor is still a bad thing. Ignition timing that's too advanced will cause engine damage just as easily as a lean mixture.

 

sooo, verify this plan of action? tomorrow i will attach the a/f meter (all i have)... and see how it's doing. Then I check the pressure sensor.If both check out, then the gauge is busted and i don't care. If mixture is lean, then i go on a fixing rampage.

 

also, sometimes my stock gauge registers like 10psi... no fuel cut before i put in FCD... is this another sign of the pressure sensor going bad?

 

Err... checking it with an A/F meter won't really help unless you have a wideband and a wideband monitor. In fact it could be dangerous.
I'd say that your best bet would be to get a multimeter and check the pressure sensor as indicated in the Factory service manual. If it's the gauge that's faulty it's normally a simple fix.... remove the gauge cluster clean the terminals and your good to go

 

alright, voltmetered the pressure sensor. Seems to be ok. What's the best way to get to the ecu to install the a/f?