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Old 10-20-2015, 12:56 PM   #1
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How do I interpret fuel pressure readings?

My 2001 viggen takes a lot of cranking to start and sputters/misfires at WOT, both intermittently/unpredictably.

I worked through the usual suspects, swapping with a working car.
- crank position sensor (surprised it wasn't that)
- DI cassette
- evap check valve and hard line nonsense (one of the valves was sticky but works fine after blowing through it both directions)

Since it wasn't any of that, and air metering and boost control stuff shouldn't cause long cranking, I hooked up a fuel pressure gauge. I've got a 3.5 bar fuel pressure regulator on this rail, so I'm looking for around 50 psi. Trouble is I don't know what else I'm looking for.

When the key is turned to on and the pump primes, pressure immediately jumps to 50 psi but then slowly bleeds down to zero over the course of about 10 seconds. This seems wrong to me, the regulator should hold pressure on one end and the supply line check valve should hold it on the other.

When I start the car, pressure rests at 40 psi while idling, or 50 if I pull the vacuum line off the regulator. I assume this is normal.

While driving the car pressure goes higher than 40 psi and spikes up to about 60 briefly when shifting from WOT. I suspect this is also normal, the regulator diaphragm can't open instantly when the injectors stop flowing and rail pressure spikes. I couldn't get it to misfire while testing that night, unfortunately.

I'm going to try swapping regulators first since that's easier to do, but I suspect bad check valves because of the pressure leakdown at startup. Anyone want to confirm my diagnosis or offer another?
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Old 10-20-2015, 01:30 PM   #2
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Do you have a tune for a 3.5 BAR regulator?

With no pressure applied to the vacuum line the pressure should read whatever the regulated value is, I.E. 3.5 BAR (~50-51 psi) +/- a psi or two. 5 psi of vacuum on the line should drop the fuel pressure 5 psi, 5 psi of pressure on the line should raise it 5 psi.

A bad air mass meter or torn or loose intake boot or damaged hose after the AMM will cause both of those things.
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Old 10-20-2015, 06:39 PM   #3
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Do you have a tune for a 3.5 BAR regulator
Same nordic stage 3 I've had for the last 8 or so years. The tune is definitely not the problem.

I can try swapping more parts, sure, but what about the fuel drainback from 50 to 0 psi? Isn't that what the check valve in the feed line is there to prevent?
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Old 10-21-2015, 12:24 PM   #4
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Yes, that or the FPR.

Or there is a leak in the pressure side of the line between the check valve at the pump housing and the rail...

It could also be an injector leaking. If you pull the rail up and prime it you'd see it dribbling from the pintle.
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Old 10-21-2015, 05:41 PM   #5
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Good call, I didn't think about leaking injectors. I just had them rebuilt and flow matched by RC less than a year ago so I hope it's not that.

I'll try things in order of accessibility and report back.
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Old 10-22-2015, 06:04 AM   #6
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It's most likely the check-valves. It happens all too often.
Mine does it, too. seemingly more in the cooler days.


I'll usually let it crank for a sec, let off, and it fires right up the 2nd time.
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Old 11-12-2015, 02:23 PM   #7
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Fuel pressure regulator had hardened o-rings and one was damaged at the edges. Installing new FPR with fresh o rings seems to have fixed leakdown after prime according to the gauge.

The car is still hard to start, especially when hot. I tried a backup MAF. Then tried pulling the hose between charcoal can and evap purge in case it was vapor locked. Still no luck. It's so symptomatic of crank sensor, but that's been thoroughly ruled out between new part and known working part off another car.

Any other ideas? Does the workshop manual have a debugging workflow for this symptom?
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Old 11-12-2015, 02:27 PM   #8
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Other than intake hoses, which DrewP suggested earlier. I'm going to pressurize the intake next and see what happens.
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Old 11-19-2015, 03:22 PM   #9
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I dropped the fuel tank and replaced the inlet line and fuel filter. Turned out to be a good thing, because the inlet line had several big kinks in it, presumably from two owners ago. The car was crashed and repaired, apparently not thoroughly enough, before I bought it and had other artifacts near the fuel system like cracked filler neck internals that I dealt with earlier. Should have just replaced everything the first time around.

I felt good for finding and fixing that kinked line, but it didn't help the worsening hot start problem. Finally it started misfiring while cruising at steady speed and threw a code. Crankshaft position sensor, which is where I started in the first place.



Heres my current theory. I had cold start hard cranking and wot misfire from a some combination of the regulator o-ring, the kinked line, and a gradually failing crank position sensor. When replacing the failing crank sensor with new didn't fix all the symptoms authoritatively, I pulled it out and tried the "known good" part from a running car. When the known good part didn't help either, I left it in there rather than swap again. Then during the time it took to find and fix the other stuff in the fuel system, the used crank sensor failed. It's a stretch to assume my luck is that bad, but I suppose stranger things have happened.

So far so good for the three times I've driven the car since putting the new in box part back on.

If the crank sensor circuit error returns again, it's down to a small number of options. Most likely electrical: too much resistance in the wiring harness back to the ecu due to corroded pins on either side or pin broken off ecu and merely resting on the surface trace on the board, or cracked insulation in the wiring harness, something like that.

If it's not electrical, the mechanical options are all wild guesses. Trigger wheel could be bent and not rotating perfectly on axis (unlikely), flat on the block casting was not milled deep enough for close gap between trigger wheel and sensor (unlikely), car could be running hotter than intended at steady state due to sticky thermostat and thereby increasing resistance out of acceptable range of the sensor (unlikely).
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Old 11-19-2015, 07:09 PM   #10
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I just diagnosed a similar issue to the sputtering/start issues on a 2003 Aero w/190k miles and it turned out to be fuel injectors--I know you had them rebuilt recently but it sounds really similar. For me I don't know if it was just one or multiple because I swapped them all, Cylinders 1 and 2 were both acting bad for me. Before getting to the fuel injectors, a new fuel filter made it better, and a new FPR made it more better too, but didn't solve it.

Anyway, something to think about.

On the CPS, is the guard in place and is the correct (short) bolt installed and the new sensor seated properly? I took a car apart one time that had an extra, old o-ring under the sensor that slipped off and was still in the block. So it was in there acting like a spacer, whoever did it had to use a longer bolt and left off the guard... because the sensor was further out from the trigger wheel with a larger gap than intended in between.

Since you had the car already, it's hard to think of scenario that would bend the trigger wheel that you didn't know about, it would take something catastrophic to do that. If it were machining, you'd have known that already too. For the temp scenario, you'd have to have coordinated failures of the thermostat as well as the temp sensor. I have had one sensor read artificially low on the high end, where the actual temp was enough to boil over because it thought the temp was below 105C (or whatever the setpoint is, I don't remember the exact number) and never turned the fans on in traffic. You can look at the temp on the TechII, stock thermostats are 89C.

Is the wiring too close to the exhaust? Those DP's get hot for sure.
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Last edited by Drew in Houston; 11-19-2015 at 07:11 PM.
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Old 12-02-2015, 12:30 AM   #11
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Fuel pressure/leakdown is good, starts right up whether cold or hot, and no misfire at WOT. Three unrelated bad parts on the same car (o ring, kinked line, crank sensor). Thanks for everyone's input. On to the next project!
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