Sudden Drop in Boost.

[MCH86GN]

I'm running into an issue where for a reason yet TBD when under boost I hear a pop, decompression and in an instance boost drops to 0. I'm only observing it at boost levels above 15PSI. I'm not seeing any vacuum lines pop off and I'm not seeing any spark retard.
I'm going to take my car again and get some PL data and see if there is any knock, that I'm not seeing on the Scan Master. Also it seems to happen after my I have laid into a few times and at full boost a couple of times.

I have checked all my turbo and IC plumming, all hoses are on securely. Any ideas? I know it is usually a good idea to start with basics. Just wondering if anyone has ran into this issue and what was the cause. Try to narrow down my what it could be.

 

[GNRick]

When that happened to me it was a dead spot on the tps. I've also read that a bad crank sensor can do it too.

 

[Nascar83Fan]

I'd inspect the crank position sensor to see if it has any physical damage to any of the ears on it, had a crank position sensor that got alittle beat up and car ran ok then would above 8psi boost would get very upset. Replaced the crank position sensor and she was happy again. But your case also could be the throttle position sensor, one way to rule out the Tps ( as long as you have a scanmaster which you do )would be key on engine off toggle over to the tps display and slowly put pressure on the gas peddle to see if it jumps around which would show a possible bad spot.



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[MCH86GN]

When that happened to me it was a dead spot on the tps. I've also read that a bad crank sensor can do it too.

Like I said, I need to get some data. I have both a Scan Master and a Power Logger. I can easily test the TPS and I can also get a data trace and see what is happening when the boost suddenly drops. The other times this happened, I had a vacuum line come pop off and I have had the PCV pop out of the grommet in the lower intake. However, I replaced the vacuum line and zip tied it.

I have a question about the vacuum lines that are used to HVAC controls, under high boost is there a check value or something that keeps those lines from experiencing the same boost pressure.

 

[MCH86GN]

I'd inspect the crank position sensor to see if it has any physical damage to any of the ears on it, had a crank position sensor that got alittle beat up and car ran ok then would above 8psi boost would get very upset. Replaced the crank position sensor and she was happy again. But your case also could be the throttle position sensor, one way to rule out the Tps ( as long as you have a scanmaster which you do )would be key on engine off toggle over to the tps display and slowly put pressure on the gas peddle to see if it jumps around which would show a possible bad spot.



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I will take a look at it. Yes I both a Scan Master and a Power Logger. I plan on capturing the data with PL the next time the weather permits. I have a 109 block slated to be rebuilt next year, can't have any sensors failing or any electrical problems. If I rebuild and something happens to the engine, my wife will probably divorce me. I better start going over things with a fine tooth comb.

 

[MCH86GN]

It appears that my rail fuel pressure isn't increasing as the boost increases. Does the ECM use the boost level to command more fuel pressure or the TPS or what? Any ideas. I didn't get a chance to look at my TPS yet.

 

[mikestertwo]

If your fuel pressure isn't rising with boost...that's your problem. Your getting a "lean pop". At WOT the computer is using the fuel curves programmed into the chip based on load. The computer has no idea what the fuel pressure is.

 

[MCH86GN]

If your fuel pressure isn't rising with boost...that's your problem. Your getting a "lean pop". At WOT the computer is using the fuel curves programmed into the chip based on load. The computer has no idea what the fuel pressure is.

I didn't say the ECM knows what the FRP is, that's obvious as there is no FRP sensor. Whether the ECM knows the FRP or not, something causing it to command a higher DC to increase FRP under boost or is the fuel pump not under ECM control once the vehicle is cranking/started? Anyway, I have someone way more knowledgeable than me taking a look it now. Once they figure out the issue, then I update this thread.

 

[87-WE2]

What causes your fuel pressure to increase is the fuel pressure regulator. The regulator has a "vacuum line" plumbed to it. As boost pressure increases, it pushes on a diaphragm that mechanically causes the fuel pressure to climb.

The ECM does not control fuel pressure (as you stated). The fuel pump is not controlled by the ECM, although the ECM does command that it be supplied with electrical power. As for what the fuel injectors are doing; they ARE commanded by the ECM. I am not 100% certain of how the ECM figures the pulsewidth to command to the injectors, but it probably has to do with TPS signal voltage and AF figures (and I'm sure a few other things).

The ECM does NOT use boost reference at all in our cars. We do have a MAP sensor, but it is only for the factory boost gauge or your alcohol injection system or your Powerlogger.

 

[Raeban]

Like I said, I need to get some data. I have both a Scan Master and a Power Logger. I can easily test the TPS and I can also get a data trace and see what is happening when the boost suddenly drops. The other times this happened, I had a vacuum line come pop off and I have had the PCV pop out of the grommet in the lower intake. However, I replaced the vacuum line and zip tied it.

I have a question about the vacuum lines that are used to HVAC controls, under high boost is there a check value or something that keeps those lines from experiencing the same boost pressure.

Yes there is a check valve that keeps the HVAC controls from seeing high pressure. Should be the three port check valve near the firewall by the drivers side head.

 

[MCH86GN]

What causes your fuel pressure to increase is the fuel pressure regulator. The regulator has a "vacuum line" plumbed to it. As boost pressure increases, it pushes on a diaphragm that mechanically causes the fuel pressure to climb.

The ECM does not control fuel pressure (as you stated). The fuel pump is not controlled by the ECM, although the ECM does command that it be supplied with electrical power. As for what the fuel injectors are doing; they ARE commanded by the ECM. I am not 100% certain of how the ECM figures the pulsewidth to command to the injectors, but it probably has to do with TPS signal voltage and AF figures (and I'm sure a few other things).

The ECM does NOT use boost reference at all in our cars. We do have a MAP sensor, but it is only for the factory boost gauge or your alcohol injection system or your Powerlogger.

At the end of the day, it was a bad fuel pump. It has been replaced. I also had to add a volt booster, because even with the new pump the fuel pressure wasn't increasing proportionally to the boost. Once the volt booster was added success. It also needed tuned, now it hauls ass according to the person who replaced the pump and tuned it. Sorry if I came across as rude, I didn't mean to.

 

[87-WE2]

Glad you got it fixed.

 

[GNRick]

If you "needed" a volt booster I suspect you still have a problem. Without going back and reading the thread, do you have a hotwire from the alternator to the fuel pump? Is your alternator good?

 

[MCH86GN]

If you "needed" a volt booster I suspect you still have a problem. Without going back and reading the thread, do you have a hotwire from the alternator to the fuel pump? Is your alternator good?

Yes, I do have a hotwire. I suspect there is an issue with the alternator also. Even though I have the voltage booster, I'm still going to pull the alternator and have it tested. Also, I noticed that the pulley on the alternator is larger than stock. Meaning it doesn't spin as fast, but when I initially installed the alternator I wasn't having any issues with my fuel pump. That was 3 or 4 years ago. The alternator that I bought was from Autozone, just a Duralast. I believe it comes with a limited lifetime warranty, so if there is an issue I think they will replace it. However, I was considering buying a 200A alternator. People always talk about voltage, but if there isn't sufficient current that will cause issues as well.

 

[MCH86GN]

If you "needed" a volt booster I suspect you still have a problem. Without going back and reading the thread, do you have a hotwire from the alternator to the fuel pump? Is your alternator good?

Everything is working fine with the volt booster. I'm only going to be cruising this summer, I won't be beating on the car. So when it comes time to put it up for the winter, I will pull the alternator and have it tested. If it is bad, it will be replaced. If not, I will replace it with a 200A alternator.

 

[MCH86GN]

If you "needed" a volt booster I suspect you still have a problem. Without going back and reading the thread, do you have a hotwire from the alternator to the fuel pump? Is your alternator good?

I went ahead and pulled the alternator and had it tested and it wasn't putting out the specified output under load. It was replaced for free. Voltage is now where it should be, 13.4v to 14.0v, depending on the temperature.

 

[GNRick]

Glad it's fixed.