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With those results your fpr is fine and pressures are within spec too.
In order to find out the source of the pressure drop you could clamp the pressure hose shut between pump and fuel rail when pressure is 54psi. If pressure still drops, then one or more injectors are leaking which could cause fuel dilution.
Clamping the pressure hose that goes from one fuel rail to the other at the same time allows you to identify which rail or whether both rails are affected by leaky injector(s) fitted to them.
If not, then the check valve within the fuel pump itself is bad or you have a (albeit tiny) leak in the pressure hose inside the tank. Neither of those causes fuel dilution.
With those results your fpr is fine and pressures are within spec too.
In order to find out the source of the pressure drop you should clamp the pressure hose shut between pump and fuel rail when pressure is 54psi. If pressure still drops, then one or more injectors are leaking which could cause fuel dilution.
Clamping the pressure hose that goes from one fuel rail to the other at the same time allows you to identify which rail or whether both rails are affected by leaky injector(s) fitted to them.
If not, then the check valve within the fuel pump itself is bad or you have a ( albeit tiny) leak in the pressure hose inside the tank. Neither of those causes fuel dilution.
I’ll work on that but I’m still going to replace the FPR since it appears to be slightly weeping gas from the vacuum port. But isn’t some amount of system leak down acceptable? Isn’t the system only designed to hold pressure just long enough to allow the fuel time to cool and not boil in the line?
The weeping of fuel will at least contribute or cause it in its entirety depending on how much fuel weeps out so replacing it is a good idea.
Perhaps the valve inside the fpr between inlet and return is compromised too and a new fpr would solve that in one go. I don't know tbh if that is actually a failure mode of an fpr in real life but you never know.
Rerun the test after you have fitted the new one to keep a tab on what action resulted in which change to the symptoms. It will help in keeping on overview on the problem solving as chances are you'll be doing more than just replacing one component.
True that some pressure loss is acceptable as long as the fuel bleeds to the tank and nowhere else.
Worthwhile reading in this thread too wrt fuel pressure and the fpr https://rennlist.com/forums/996-foru...d-start-2.html
Last edited by hardtailer; 10-14-2022 at 05:15 AM.
Lack of cylinder wall/ piston ring seal is a mechanical reason for fuel dilution. If the rings don't seal, unburned fuel and carbon goes straight into the oil. This is why mechanical inspection is required prior to carrying out other corrective measures to engine subsystems. All too often we are seeing engines with new injectors, FPRs and other items that were replaced rather than proper diagnosis being carried out first. This is the absolute most incorrect way of attacking these issues. Rule out mechanicals first, then you can start chasing the issues by firing the parts cannon at it.
People often say "the engine runs fine" so they avoid the proper chain of diagnosis. Don't do that.
^^^BRS is correct and is why I posted the "blow-by Test for Engine Health" thread.....too much blow-by will cause fuel dilution, BUT takes quite a few miles to show up or effect "fuel trims"..
Your fuel trims are out of spec...
It's like the "chicken or the egg" but we KNOW which one comes first.........with fresh oil too much blow-by will not cause fuel trims to be out of spec, once the oil gets diluted so badly that "fuel fumes" are being drawn off the oil and through the crankcase ventilation system can it have any effect on fuel trims...
So if fuel trims remain out of spec with fresh oil, then blow-by is not the cause......
Also your fuel pressure is in spec , so fuel pressure is not the cause....( the wetness you saw in the vacuum hose is probably due to oil vapor condensation..)
My money is on the MAF Sensor, it takes 45 seconds to install a known good one...and a few minutes to re-test to see if the fuel trims go back to near 0 ....I would just borrow one from someone/somewhere since it is so easy to install before spending the $$$$.....go to a local PCA Cars and Coffee and make a friend with a 99 996,,,, or I can loan you one, I have one sitting not being used....
^^^BRS is correct and is why I posted the "blow-by Test for Engine Health" thread.....too much blow-by will cause fuel dilution, BUT takes quite a few miles to show up or effect "fuel trims"..
Your fuel trims are out of spec...
It's like the "chicken or the egg" but we KNOW which one comes first.........with fresh oil too much blow-by will not cause fuel trims to be out of spec, once the oil gets diluted so badly that "fuel fumes" are being drawn off the oil and through the crankcase ventilation system can it have any effect on fuel trims...
So if fuel trims remain out of spec with fresh oil, then blow-by is not the cause......
Also your fuel pressure is in spec , so fuel pressure is not the cause....( the wetness you saw in the vacuum hose is probably due to oil vapor condensation..)
My money is on the MAF Sensor, it takes 45 seconds to install a known good one...and a few minutes to re-test to see if the fuel trims go back to near 0 ....I would just borrow one from someone/somewhere since it is so easy to install before spending the $$$$.....go to a local PCA Cars and Coffee and make a friend with a 99 996,,,, or I can loan you one, I have one sitting not being used....
You’re probably right about the FPR but it had a very faint smell of gas to the liquid so I figure it’s a cheap and easy item to knock out.
The oil was changed in mid-August (that oil sample is the one that came back with a fuel dilution warning). The fresh oil only has about 500 miles on it at the moment.
Yesterday I disconnected the battery for 30+ minutes and connected the positive and negative battery cables to each other in order to reset the fuel trims. I drove to work today with no issues or CEL, though the idle began getting a little rougher the longer I drove. I will drive home this evening and check the trims again.
Fwiw, apparently Bosch has a lifetime warranty on MAFs and they are working to get a new one sent to me. Hopefully I can get this all resolved before my shiny new UAOS arrives.
I think that table is just an "random" example and to show that it's important to group those certain values when doing diagnosis. 0.96 in that table is below acceptable range for example for fuel trims (0.98) and MAF is just at the low level. In that example it's easy to see that adding air affects only on TRA and not FRA because that's is measured above 1700rpm+ range.
That 18.00 at 680rpm is actually within acceptable range as it's at 680rpm 17.00 +-2.5
However I agree with Porschetech3 that the issue is likely a faulty MAF since both of those trims are way above accepted range and if fuel pressure is ok.
In order to find out the source of the pressure drop you could clamp the pressure hose shut between pump and fuel rail when pressure is 54psi. If pressure still drops, then one or more injectors are leaking which could cause fuel dilution.
Clamping the pressure hose that goes from one fuel rail to the other at the same time allows you to identify which rail or whether both rails are affected by leaky injector(s) fitted to them.
If not, then the check valve within the fuel pump itself is bad or you have a (albeit tiny) leak in the pressure hose inside the tank. Neither of those causes fuel dilution.
@hardtailer
This may be a dumb question but how do I clamp off the fuel line when they all appear to be made of hard plastic?
I have an interesting development (and I’m still somewhat skeptical of the results below). I replaced the FPR yesterday, took the car for a 14 mile drive to get her warmed up, and then hooked up the Durametric to see if anything had changed. It’s important to note that I did NOT reset the fuel trims before doing this nor did I make any other changes to the car.
My MAF readings are still problematic but take a look at my short term fuel trims. One is now zeroed out (as it was previously -.38) and the other has ever-so-slightly improved from -.30 down to -.29. 🤷🤔🤷🤔
My MAF readings are still problematic but take a look at my short term fuel trims. One is now zeroed out (as it was previously -.38) and the other has ever-so-slightly improved from -.30 down to -.29. 🤷🤔🤷🤔
Small correction - both TRA and FRA are long term fuel trims - Porsche/Durametric doesn't publish real short term fuel trim values.
10-13-2022, 05:44 PM
