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FarPoint
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Joined: May 04, 2016
Posts: 58
Location: Santa Clara
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 Posted: Thu Oct 11, 2018 9:34 pm Post subject: fuel pressure regulator |
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After replacing the FPR on my 2.l L, I checked the fuel pressure (at the T-junction). At idle the pressure reads 24 psi and with vacuum hose remove on the FPR, it reads 30 psi.
This seems low since according to Bentley it should be approx. 29 and 36 psi respectively.
Is the low reading normal for these aftermarket FPRs? |
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MarkWard
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Joined: February 09, 2005
Posts: 17124
Location: Retired South Florida
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| Posted: Fri Oct 12, 2018 4:24 am Post subject: Re: fuel pressure regulator |
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Is the gauge accurate?
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RicoS
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Joined: December 02, 2006
Posts: 583
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| Posted: Fri Oct 12, 2018 5:52 am Post subject: Re: fuel pressure regulator |
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| FarPoint wrote: |
. . . This seems low since according to Bentley it should be approx. 29 and 36 psi respectively.
Is the low reading normal for these aftermarket FPRs? |
Years ago, I fell victim to some Vanagon "expert" stating that the fuel pressures HAD to be regulated within 1 PSI of the spec or else the sky was going to fall. As a result, I went to the trouble, and expense, of buying an OE VW regulator.
Whenever I installed the new regulator, it read a little low, but exactly the same as the one I was told needed replaced. Whenever I tried a couple of used regulators, they too read the same.
The gauge I used was of the highest quality and although a few years had past, it had been initially calibrated in the instrumentation lab at a nuclear power plant, so, I trusted what it was telling me.
My conclusion: If a fuel pressure regulator changes the fuel pressure when the vacuum is removed and it does not smell from fuel leaking through a queefed out diaphragm, use it and put it out of your mind.
Forget about splitting hairs with the exact pressures; if it is shot, you'll know about it right away.
Richie (near The Burgh) |
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DanHoug
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Joined: December 05, 2016
Posts: 4777
Location: Bemidji, MN
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| Posted: Fri Oct 12, 2018 6:56 am Post subject: Re: fuel pressure regulator |
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when i was bench cleaning my OEM pintle injectors, i could observe the effect of fuel pressure on spray patterns. these injectors DO like high fuel pressure but only because the discharge spray more forcefully atomizes in finer droplets. as the pressure drops, the injector changes from a Windex bottle like spray to a stream.
these injectors do NOT require any minimum pressure to force a valve open as there is none. the pintle is a simple solenoid that lifts the armature off the seat. they will emit a single pee-like stream at just a few psi. the quantity and atomization at that pressure tho is not adequate for proper running.
24-30, 29-36, these pressure will all spray with a decent pattern. sure, the higher pressure creates a finer mist. is it significant? dunno. slightly i'd say. but i'd also question the accuracy of the gauge, and if the gauge is accurate, the source of the regulator. was it a true Bosch? a counterfeit? off-shore knock off?
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-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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DanHoug
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Location: Bemidji, MN
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| Posted: Fri Oct 12, 2018 7:39 am Post subject: Re: fuel pressure regulator |
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| RicoS wrote: |
Years ago, I fell victim to some Vanagon "expert" stating that the fuel pressures HAD to be regulated within 1 PSI of the spec or else the sky was going to fall.
Richie (near The Burgh) |
yeah, not so much on the Digifant/Jet systems. this is more true for the GM FI system used on the Vortec V8 & V6 engines. they really DO have a narrow pressure spec and use 'poppet' injector that require a minimum of psi to open and is tightly spec'd for 55-57 psi. this is so critical, one of the diagnostic tests on the Vortec FI systems is to sample the current draw of the in-tank fuel pump with a Digital Storage Oscilloscope and to look for a dead commutator bar on the wvaveform.
i did that chasing a persistent miss in the engine of my truck, everything checked out good but i replaced the regulator anyway, necessitating removal of the intake galley. after obsessive reading, i concluded that the distributor "Cam Offset" spec was wrong and that can only be set with a special GM scan tool. i took it to the dealer, told the service manager my symptoms and that i wanted the "distributor cam offset" checked and adjusted. he looked at me sorta slack jawed and asked if i wanted to work there! it made my day! and it WAS the problem as someone had re-installed the distributor 1 tooth off after replacing a leaking intake manifold gasket.
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-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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MarkWard
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Joined: February 09, 2005
Posts: 17124
Location: Retired South Florida
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| Posted: Fri Oct 12, 2018 8:00 am Post subject: Re: fuel pressure regulator |
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| FarPoint wrote: |
After replacing the FPR on my 2.l L, I checked the fuel pressure (at the T-junction). At idle the pressure reads 24 psi and with vacuum hose remove on the FPR, it reads 30 psi.
This seems low since according to Bentley it should be approx. 29 and 36 psi respectively.
Is the low reading normal for these aftermarket FPRs? |
A 5-6 psi difference high or low is going to affect how much fuel is sprayed per injector pulse. Enough to cause the engine to run richer or leaner. 1-2 psi I would not sweat. That's why I asked about the gauge accuracy. A simple test of the fuel supply would be to pinch the return and watch the gauge.
I have used aeromotive adjustable fuel regulators on a couple engines. The 1.9's seem to like a slightly higher pressure for moving off from a dead stop.
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DanHoug
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Location: Bemidji, MN
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| Posted: Fri Oct 12, 2018 8:13 am Post subject: Re: fuel pressure regulator |
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Mark- do you know of any adjustable regulators with the vacuum boost port?
_________________
-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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FarPoint
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Joined: May 04, 2016
Posts: 58
Location: Santa Clara
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| Posted: Fri Oct 12, 2018 8:36 am Post subject: Re: fuel pressure regulator |
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I'll check the gauge. But with the new FPR, my symptom is still there - engine missing when moving from dead stop. So low fuel pressure means it's running lean which would explain the lack of power during acceleration.
What about the fuel pump? This could also cause the lower pressure? |
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crazyvwvanman
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| Posted: Fri Oct 12, 2018 8:42 am Post subject: Re: fuel pressure regulator |
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Not really, but there is a simple test. With the engine running and gauge hooked up, pinch the fuel return hose that comes out of the regulator and goes back to the tank. That should cause the pressure to shoot up high and show that the pump can easily deliver more than the needed pressure. Only pinch it for a moment while watching the gauge as it may approach 100 psi with a strong pump.
Mark
| FarPoint wrote: |
....
What about the fuel pump? This could also cause the lower pressure? |
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FarPoint
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Posts: 58
Location: Santa Clara
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| Posted: Fri Oct 12, 2018 8:46 am Post subject: Re: fuel pressure regulator |
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| Ok I'll try that. I thought removing the vacuum hose on FPR does the same thing? |
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MarkWard
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Joined: February 09, 2005
Posts: 17124
Location: Retired South Florida
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| Posted: Fri Oct 12, 2018 8:48 am Post subject: Re: fuel pressure regulator |
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| DanHoug wrote: |
| Mark- do you know of any adjustable regulators with the vacuum boost port? |
https://www.summitracing.com/parts/aei-13301/overview/
These are what we use. I would not go about trying to Band-Aid a drivability problem with an adjustable regulator. We use these when all else fails. They are not cheap and do not include adapters.
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crazyvwvanman
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| Posted: Fri Oct 12, 2018 8:54 am Post subject: Re: fuel pressure regulator |
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Not at all, the vacuum line is so the fuel pressure automatically adjusts slightly for changing engine vacuum.
Mark
| FarPoint wrote: |
| Ok I'll try that. I thought removing the vacuum hose on FPR does the same thing? |
Last edited by crazyvwvanman on Fri Oct 12, 2018 8:55 am; edited 1 time in total |
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FarPoint
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Posts: 58
Location: Santa Clara
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| Posted: Fri Oct 12, 2018 8:54 am Post subject: Re: fuel pressure regulator |
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| crazyvwvanman wrote: |
Not really, but there is a simple test. With the engine running and gauge hooked up, pinch the fuel return hose that comes out of the regulator and goes back to the tank. That should cause the pressure to shoot up high and show that the pump can easily deliver more than the needed pressure. Only pinch it for a moment while watching the gauge as it may approach 100 psi with a strong pump.
Mark
| FarPoint wrote: |
....
What about the fuel pump? This could also cause the lower pressure? |
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Do you mean with engine off? Turn ignition ON and pinch the return line? |
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crazyvwvanman
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Joined: January 28, 2008
Posts: 9923
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| Posted: Fri Oct 12, 2018 8:56 am Post subject: Re: fuel pressure regulator |
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Watch and briefly pinch with the engine running.
Mark |
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DanHoug
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Location: Bemidji, MN
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| Posted: Fri Oct 12, 2018 9:10 am Post subject: Re: fuel pressure regulator |
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thanks for the application. nice to find one with the compensation port and the pressure ratio appears about right, 1:1. if an engine is idling at 15" mercury vacuum and throttle is cracked open to 2", it's about a 6psi increase in fuel pressure. sweet.
_________________
-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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tencentlife
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Joined: May 02, 2006
Posts: 10078
Location: Abiquiu, NM, USA
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| Posted: Fri Oct 12, 2018 9:23 am Post subject: Re: fuel pressure regulator |
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| DanHoug wrote: |
| Mark- do you know of any adjustable regulators with the vacuum boost port? |
Most of the ones I see on the market for injection have that, it's basically essential for a regulator to be useful in an injection system, where the differential pressure at the nozzle needs to be held constant. Wouldn't matter at all if feeding a carb, but the two types tend to be differentiated because the range of needed pressures is so far apart (single digit psi's for carbs, 30psi and up for FI).
With injectors, as you've seen, too low a pressure and it streams, so delivery isn't consistent, but once the feed pressure is into the atomisation range for the nozzle, mass delivery changes in direct proportion to feed pressure. Lambda can correct to an unlimited extent but it's a per-engine cycle iterative process so the farther the delivery is from ideal the more time it takes to bring it into range, which can account for stumbling on tip-in, poor throttle response, failure to reach a stable idle, etc.
The ECU lookup tables are based on the rated 2.5 bar (36psi) differential pressure at the nozzle. At high manifold vacuum the FPR draws down ~7psi, because the absolute air pressure at the intake ports is also effectively ~15"Hg. (~7psi) under ambient, to maintain the same 2.5 bar differential pressure. That change isn't for high load enrichment, as many claim, it's for normalising the injection rate.
A change in 1 psi is about 3% change in delivery. That's not too bad, but one more is another ~3%, so pretty soon it's pretty far off the mark. So having exact test pressure's isn't critical, but for the system to perform as it should, I would reject an FPR with 3psi off, with the 7psi spread being even more important.
All that said, I rarely see one of these being that far off. They are remarkably consistent, even very old ones. When they're off they're way off, or go to complete failure. But it is the basic value upon which the whole system operates and the ECU has no way of knowing fuel pressure. That's not a VW thing, even many of the newest systems regulate fuel pressure mechanically and don't monitor it. It's assumed to be normal, so it's really the first, fundamental value you have to establish when troubleshooting most FI systems.
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DanHoug
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Location: Bemidji, MN
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| Posted: Fri Oct 12, 2018 9:44 am Post subject: Re: fuel pressure regulator |
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| tencentlife wrote: |
That change isn't for high load enrichment, as many claim, it's for normalising the injection rate.
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okay. that statement ties together something that has bothered me. i'd always assumed the vacuum compensation was for load enrichment but with Lambda control, it would get negated in about 1/2 second. that it is for normalizing the injection rate in a Lambda controlled system makes far more sense!!
my injector bench system uses an old vanagon FPR, so it runs about 7psi less than with the nozzle in the manifold vacuum. i forgot to play around with the MityVac to see the compensation effect while squirting into a jar, but as you've just pointed out, with the nozzle in the manifold under varying vacuum, it wouldn't make much difference anyway!
i love it when a nagging incongruity finally comes together!
_________________
-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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FarPoint
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Posts: 58
Location: Santa Clara
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| Posted: Fri Oct 12, 2018 10:35 am Post subject: Re: fuel pressure regulator |
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| What I'm seeing is that the injector spray is in the form of 4 narrow streams in a square pattern. When I spray it into a aluminum cookie pan, for example, you can see the splashing fuel. This is not considered atomized spray correct? |
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tencentlife
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Location: Abiquiu, NM, USA
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| Posted: Fri Oct 12, 2018 10:38 am Post subject: Re: fuel pressure regulator |
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It must be a cone of microscopic droplets, almost a fog. Even small identifiable streams within the cone is unacceptable.
| Quote: |
| i love it when a nagging incongruity finally comes together! |
Glad that helped clear that up for you Dan.
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DanHoug
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| Posted: Fri Oct 12, 2018 10:42 am Post subject: Re: fuel pressure regulator |
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i didn't take a picture of my patterns but after cleaning 8 of the OEM injectors, i can say that i COULD detect a similar pattern to 4 mist streams within the spray they produced. this was observed with the injector mounted in a 5" diameter clear cylinder about 16" tall. mind you, there was pretty nice atomization overall but i could see the surface of the liquid gasoline get 'dimpled' in a very slight pattern of 4. it was very subtle but i expected a perfectly concentric cone.
_________________
-dan
60% of what you find on the internet is wrong, including this post.
'87 Westy & '89 Westy both 2.1 4spd
Past projects can be found at--
www.thefixitworkshop.com |
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