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EGR system...Back Pressure Transducer


rc_cola_j
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Hi all,

 

I'm trying to understand the EGR system on my R50 a little better. I understand how most of the basics work, but I don't fully understand what the Back Pressure Transducer does, how it works in line with the EGR valve and the rest of the system. What does the BPT do exactly?

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Hi all,

 

I'm trying to understand the EGR system on my R50 a little better. I understand how most of the basics work, but I don't fully understand what the Back Pressure Transducer does, how it works in line with the EGR valve and the rest of the system. What does the BPT do exactly?

The BPT controls the amount of vacuum to the EGR Valve according to exhaust pressure.

 

Sent from my SM-G530T using Tapatalk

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The BPT controls the amount of vacuum to the EGR Valve according to exhaust pressure.

 

Sent from my SM-G530T using Tapatalk

So more backpressure causes more or less vacuum application to the EGR valve?

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So more backpressure causes more or less vacuum application to the EGR valve?

That I have no clue. I just know what it's function is. You can look in the FSM factory service manuals and that might be able to give you a better idea or a better clue on what it actually does and its function

 

Sent from my SM-G530T using Tapatalk

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My understanding from the WD21 manual is that exhaust backpressure makes the BPT reduce vacuum applied to the EGR valve, causing it to close. I assume this is meant to regulate the flow rate of exhaust gas through the EGR valve, preventing high backpressure from pushing more gas past the EGR valve than the engine and ECU are expecting.

 

But yeah, FSM is your friend here! It doesn't always explain things as well as I'd like but it gets you most of the way there.

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My understanding from the WD21 manual is that exhaust backpressure makes the BPT reduce vacuum applied to the EGR valve, causing it to close. I assume this is meant to regulate the flow rate of exhaust gas through the EGR valve, preventing high backpressure from pushing more gas past the EGR valve than the engine and ECU are expecting.

 

But yeah, FSM is your friend here! It doesn't always explain things as well as I'd like but it gets you most of the way there.

 

 

That makes a lot of sense. so, If I had a small exhaust leak (which I now know I do) it could cause the EGR to read too high flow because there is not enough backpressure in the system?

 

Could this cause the EGR to stay open at idle after coming off of the throttle?

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Keep in mind that these don't really read anything--they're just pressure diaphragms and valves. The BPT is actuated by pressure, not flow. If I've got this right, and the BPT is acting as a regulator for the EGR, it should ensure that the EGR passes about the same amount of gas regardless of exhaust pressure. Pressure rises, the BPT closes the EGR a little to compensate (higher pressure but smaller opening = no change in flow). If manifold pressure drops, whether there's a leak or not, I'd expect the EGR to open a little (lower pressure but bigger opening = no change in flow). That's simplifying things, of course; I can't imagine they're perfectly matched, and besides, the vacuum source for the whole mess is the intake, so the flow rate is changing with manifold vacuum anyway. The BPT's presumably just making the relationship between manifold vacuum and exhaust recirculation a little more predictable.

 

But you're asking about idle, and the EGR's not even supposed to be on at idle--and the BPT has nothing to do with that. There's an EGR-C (cut) solenoid that the ECU kicks on at high and low engine speeds that disconnects the EGR valve from manifold vacuum and lets the air back in, causing the EGR to close up. If your EGR valve is staying open at idle, my guess is that either your EGR-C is hooped or your EGR valve is hanging up and sticking open.

 

Of course, this is all based on my hazy understanding of the EGR system described in the WD21 FSM, and my '93 doesn't even have a BPT valve (it was a California-only thing until late '93 or '94 IIRC). If you're trying to diagnose an idle issue on your R50, consult the EF&EC section of the R50 FSM before taking my word for anything.

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Keep in mind that these don't really read anything--they're just pressure diaphragms and valves. The BPT is actuated by pressure, not flow. If I've got this right, and the BPT is acting as a regulator for the EGR, it should ensure that the EGR passes about the same amount of gas regardless of exhaust pressure. Pressure rises, the BPT closes the EGR a little to compensate (higher pressure but smaller opening = no change in flow). If manifold pressure drops, whether there's a leak or not, I'd expect the EGR to open a little (lower pressure but bigger opening = no change in flow). That's simplifying things, of course; I can't imagine they're perfectly matched, and besides, the vacuum source for the whole mess is the intake, so the flow rate is changing with manifold vacuum anyway. The BPT's presumably just making the relationship between manifold vacuum and exhaust recirculation a little more predictable.

 

But you're asking about idle, and the EGR's not even supposed to be on at idle--and the BPT has nothing to do with that. There's an EGR-C (cut) solenoid that the ECU kicks on at high and low engine speeds that disconnects the EGR valve from manifold vacuum and lets the air back in, causing the EGR to close up. If your EGR valve is staying open at idle, my guess is that either your EGR-C is hooped or your EGR valve is hanging up and sticking open.

 

Of course, this is all based on my hazy understanding of the EGR system described in the WD21 FSM, and my '93 doesn't even have a BPT valve (it was a California-only thing until late '93 or '94 IIRC). If you're trying to diagnose an idle issue on your R50, consult the EF&EC section of the R50 FSM before taking my word for anything.

 

 

I had this similar issue a while back. A bad idle after coming to a stop, just very lumpy...almost like a miss. I replaced my EGR valve and the problem went away.

 

Well its back now. But, it yet again goes away when I disconnect the EGR from the vacuum source.

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The cut solenoid. Again, not sure if the R50 is rigged the same, but on the WD21, it's down under the throttle body, has one plug and three vac lines, and it's how the computer shuts off the EGR when it wants to.

 

 

I'll have a look.

 

I just took the vacuum line off of the EGR again and hooked it up to a vacuum gauge. Its reading nothing.....maybe 1/4 to a 1/2 psi..maybe....but I don't think its pulling vacuum through the line at idle

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So, I left the vacuum line between the BPT and the EGR disconnected and plugged with a golf tee. The rough idle continued for a bit, then went away. The car drove very well and started with no issues.

 

When I re-connected the EGR, the problems came back after about a day's normal driving. Rough idle, hard start when warm....and for some reason the oil pressure light stays on for about 5 seconds after start.

 

When I disconnect things again....this goes away.

 

It's the EGR valve itself? No?

 

How is oil pressure related to this?

 

 

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