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Now, Emissions, was: Conundrum: Pressurized air in fuel rail. How?

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Old 12-15-2006, 05:22 PM
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worf928
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Originally Posted by mark kibort
Thats an independant system. leaks in the diaphram could cause fuel to leak out , but not air to go in.
Yeah. Implausible. But I wanted to get the fact out. You could get air into the system that way IF the vacuum line was pressurized and the diaphragm was toast though.
Old 12-15-2006, 05:28 PM
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One more thing: FP was at ~48 PSI idle, blip to ~58 PSI with throttle. (Pretty normal.) ~54 PSI with engine off and dropping to ~44 quickly, but holding there for at least 10 minutes.
Old 12-15-2006, 05:32 PM
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John V
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Dave - Wild guess or theory.


The air wouldn't have to be pressurized.. at least not initially.

Could the injectors be leaky, allowing residual fuel to drain out via gravity into the intake. The resulting fuel occupied space would be replaced by unpressurized ambient air. The next time fuel enters the rail, the air ends up above the fuel which pressurizes the sytem and compresses the air. Since the air would tend to stay above the fuel, it may not be flowing back to the tank or get drawn into the combustion chamber. Possible?

Might injector spray pattern also be contributing to emmisions issues?


Lastly - you've probably already considered this but faulty MAF can effect emmisions this way... this artical is not directly about the 928 but I think it applies:
http://www.gatewaycleanair.com/mecha...4/dirtymaf.htm

Last edited by John V; 12-15-2006 at 05:49 PM.
Old 12-15-2006, 06:15 PM
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the flyin' scotsman
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Originally Posted by worf928
One more thing: FP was at ~48 PSI idle, blip to ~58 PSI with throttle. (Pretty normal.) ~54 PSI with engine off and dropping to ~44 quickly, but holding there for at least 10 minutes.
Pressures are good but it should hold min. pressure after 20 mins of 3.0 bar. Is there an issue with the FP check valve?
Old 12-15-2006, 06:30 PM
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a4sfed928
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My guess is excessive combustion temperatures from the indicator of high NOX, low HC and CO cause excessive heat soak temperatures this in turn caused the fuel in the rail to boil after setting several minutes.
Old 12-15-2006, 06:32 PM
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Garth S
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Cavitation is generally the destructive collapse of vapour ( bubbles) of the same liquid that is being pumped - and this occurs at the pump impeller ( for a variety of reasons), not downstream in pressurized fluid.: cavitation can influence the discharge pressure, but the pressures in this case are 'normal'
The observation is one of a two phase mixture - air and fuel, and IIRC, air is not terribly soluble in pressurized gasoline.
The point I made of an intank pump ( not in an '87) would also apply to a blocked tank/pump suction strainer.
Would a suction hose adequately porous to admit N2 and O2 when under "vacuum" leak gasoline to atmosphere when the pump was not running? Maybe not, for there is a huge difference in molecular sizes ... and only ~ 14" of positive water head from the gas tank verses ~3 bar of negative pressure when the pump runs.
Old 12-15-2006, 07:08 PM
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Larry928GTS
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Pressurized air, or fuel vapor from heat soak escaping? Think along the lines of "vapor lock". In this case there wouldn't be any "lock", because the fuel back by the pump would be cooler and liquid instead of heat soaked and vapor, and as soon as the pump was started it would purge any vapor out of the system.

A lot of the original foam fuel rail insulation on these cars isn't exactly in the best of condition anymore, which adds to the heat soaking of the fuel in the rails, especially after a fully warmed engine is shut off with the hood closed.

When the end cap on the fuel rail was loosened the pressure inside dropped, allowing for fuel vaporizaion at lower temperatures than if it's pressurized. Think about the bubble formation that occurs when a carbonated beverage is opened and the pressure in the container is reduced.

Winter gas is blended to vaporize at a lower temperature than summer gas, so that makes it even more likely to occur with the winter blend that's in use at this time of the year in many areas of the country.
Old 12-15-2006, 09:47 PM
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mark kibort
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this is just an indication of a lean condition (extra lean) very lean actually creates lower egts and chts. 14.7:1 fuel to air is the most complete combustion, but doesnt make a lot of heat overall in idle conditions.
this is why you can change the NOX levels and CO with the idle mixture screw on the AFM or MAF.
fuel boiling could happen, but it would only happen if here was a serious leak, as the pressure would raise the boiling point.

mk

Originally Posted by a4sfed928
My guess is excessive combustion temperatures from the indicator of high NOX, low HC and CO cause excessive heat soak temperatures this in turn caused the fuel in the rail to boil after setting several minutes.
Old 12-15-2006, 09:48 PM
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Originally Posted by Larry928GTS
Pressurized air, or fuel vapor from heat soak escaping? Think along the lines of "vapor lock".
Of Course! The FP results indicate that there's a small transient leak right after shut down. So, with the heat-soaked rails (no insulation in the covers) and the pressure drop some of the fuel vaporized. The vapor is compressed and when the cap nut is cracked-open the fuel vapor escapes and that makes the 'air' noise.

I'd completely forgotten about the 'vapor lock' phenomenon that can occur when the fuel rails empty quickly - can lead to hard or no start situations.

Looks like Larry928GTS is winner with the simplest explanation. (Although Garth's less-obvious explanation just might be operative too.)

Originally Posted by Garth S
Cavitation is generally the destructive collapse of vapour ( bubbles) of the same liquid that is being pumped - and this occurs at the pump impeller ( for a variety of reasons),
Yup. I'm familar with cavitation in water and air (and what 5 pounds of brass can do to a water tunnel )

air is not terribly soluble in pressurized gasoline.
That's what I was thinking but didn't know for sure. Thanks Garth.

Would a suction hose adequately porous to admit N2 and O2 when under "vacuum" leak gasoline to atmosphere when the pump was not running? Maybe not, for there is a huge difference in molecular sizes ... and only ~ 14" of positive water head from the gas tank verses ~3 bar of negative pressure when the pump runs.
Seems logical also. Pressure 'up stream' of the pump in the outlet hose is not much. The pump could suck in a bit of air and the fuel rails are the highest point in the system, so the air would accumulate there once flow stops. Yes?

I'm still left wondering why this car is failing emissions with what seems like a lean condition.

The car's been sniffed three times. HC has been 15 ppm (100 limit), CO 0.01% (.32% limit) on a rolling dyno for all three tests. The first failure was NOx at 1300 ppm (700 limit). The owner installed a new oxygen sensor and drove the car for a few hours prior to the re-test to allow the LH to adapt the 02 loop.

With the new O2 sensor the car tested with the same HC and CO numbers, NOx decreased to 1200 - still failing. I loaned him my 928SP cross-over with high-flow cats and the NOx jumped to 1970 with the same HC and CO numbers.

But, the engine temp is at worst just a 'little' warm. I suppose the cooling system could just be doing a great job at removing the lean combustion heat.


Originally Posted by John V
Might injector spray pattern also be contributing to emmisions issues?
Yes. But, all other things operating, one or two leaky injectors - that result in one or two cylinders exhibiting poor combustion and high CO and NOx - usually result in the LH richening the mixture with the result that HC, CO, and NOx numbers all go through the roof. With the very low HC and CO numbers I wasn't inclined to put injectors at the top of the list.

Lastly - you've probably already considered this but faulty MAF can effect emmisions this way...
Yes. I swapped his well-aged MAF for a less-well aged MAF I have on a shelf.

With the fuel vapor system hooked up and a probably much-better MAF in the car, along with new cats and new O2 sensor the car better pass emissions. Or at least the numbers better change in some way that provides more clues.
Old 12-15-2006, 10:08 PM
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the flyin' scotsman
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Garth must have some brain encouragement juice today..........great explanation; bravo.
Old 12-16-2006, 09:18 AM
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Originally Posted by worf928
............. and when the cap nut is cracked-open the fuel vapor escapes and that makes the 'air' noise.

I'd completely forgotten about the 'vapor lock' phenomenon that can occur when the fuel rails empty quickly - can lead to hard or no start situations......

.
Sheesh - and I entrusted youse guys to be able to distinguish between 'passing air' and 'passing gas' ... perhaps I should say, 'passing gasoline fumes' .

Vapour lock is the obvious first thought, but that is normally accompanied by hard starting: as to cause, well there is no vapour lock with the oven like underhood conditions of a 928 when the fuel pump check valve is operative, the pump pressure is at spec, injector pintels seal tightly, diaphrams are intact, no pinholes in the lines and the firewall mounted fuel line cooler is operative ..... even on a well heat soaked, shut down engine.
Were the rails emmitting 'gas' ( vs. liquid) on a cold engine?
Old 12-16-2006, 10:19 AM
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If there's a mixture of air and gas in the fuel rails, then it follows that either air is being injected into the intake along with the gas, or the air is preventing/blocking the correct amount of gas from being injected during any given injector cycle. This would creat a lean condition/high NOx reading. The computer that runs the engine can compensate for this to some degree but there's only so much these old brains can do compared to todays systems...I think that when you find the reason for the air in the fuel rail you'll have solved the problem.

Mark
Old 12-16-2006, 03:07 PM
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Bill Ball
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How are you getting heat soak of the fuel rails with the hood up? I would expect you have the hood up during all this testing.

I guess I'm getting lost here. How is air/vapor staying in the rails while the car is running? Any air gets quickly bled out the injectors.

I don't see what this has to do with the failed NOX.
Old 12-16-2006, 03:27 PM
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Not actually having seen the condition one can only speculate. Quite honestly I have seen the condition described numerous times and never thought twice about it. This vehicle just was just driven for 3 hours on highway with no apparent driveability concerns, unless I missed something. Fuel vaporizes well below the boiling point of water. Sounds normal to me.
Old 12-16-2006, 06:25 PM
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Originally Posted by ZEUS+
... This vehicle just was just driven for 3 hours on highway with no apparent driveability concerns, unless I missed something. Fuel vaporizes well below the boiling point of water. Sounds normal to me.
It is possible that I've never cracked open a warm just-running-a-few-minutes-ago fuel rail on a 928 with a leaky check valve.

What struck me as odd was that it took a good 10 seconds or more for the gas pressure in the rail to bleed off after the cap nut was cracked. If it had been a one or two second spurt I probably would have dismissed it.

With a full-operational check valve and good injectors the contents of the rails should remain mostly in liquid form for 30 minutes at least.

Originally Posted by Bill Ball
How are you getting heat soak of the fuel rails with the hood up? I would expect you have the hood up during all this testing.
The car was driven for 3 hours and then parked in my garage. We shut it off, poked around the engine bay for 20 minutes or so - in the process reconnecting the disconnected vacuum line to the fuel vapor system - and then cracked the rail to install my FP gauge. Subsequent FP readings indicated that the check valve (or something else) bleeds off a few PSI of pressure immediately on shutdown. I suspect that the check valve is 'slow' - just a bit sticky. We didn't drive the car around with the FP attached; it was just a static FP test. The car cooled for at least another 30 minutes while we R&R'd the MAF. No gas pressure was observed when we swapped the FP gauge for the test port cap.

I guess I'm getting lost here. How is air/vapor staying in the rails while the car is running? Any air gets quickly bled out the injectors.

I don't see what this has to do with the failed NOX.
That's the crux of it. I was hyper-sensitive yesterday to any observations that could point to lean running. IF air is being constantly introduced to the fuel system while the car is running then it will likely go out through the injectors and that is unmeasured air. Unmeasured air will lead to lean running if the LH is unable to compensate fully. I SHOULD have hooked up the DMM to the 02 sensor - like John suggests above - to confirm lean running. The car's three hours away now so that's not going to happen. Damn it.

It's all very tenuous. Garth's explanation is the only plausible way for air to get into the pressure side of the system while the car's running. However, I'd think that extra air getting injected into the cylinders directly from the fuel system would result in a crappy spray pattern, unburned fuel, and high HC readings. The other variable is the unhooked fuel vapor valve.

The simplest explanation is that:

- the gas pressure in the rails is due to a leaky check valve
- the high NOx is due to a MAF about the fall off the cliff
- poor gas mileage is the result of the fuel vapor system not connected

Contrary indications are:

- the engine temp doesn't seem too hot (inconsistent with lean running)
- the car does not exhibit very-poor idle problems or off-throttle stalling (almost-sure sign of a necrotic MAF)


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