Fuel pressure woes.


Fuel pressure woes. With FPR attached to vac line idle is 38 BUT if you give it a little throttle it drops to 34-36. It recovers as engine returns to idle. WOTis only 41-42 barely and it should be over 43. Without vac line it’s 42 across the board. Just replaced brake booster, master cylinder and all vac lines and backflow between booster and throttle. Replaced fuel injectors with cleaned ones. FPR is new last year but does this with other ones. Interesting note: pumping brakes lowers oil pressure and idle rhythm gets choppy. Fix this for me. It’s killing my engine because it’s running 15+ air fuel over 5000. Also it does this with replacement fuel pumps. And fuel pressure mods to get pressure up fix the problem with air fuel so i know the AFM is working right. Voltage is good to both pumps.


On my way to work, I was thinking about your problems and this is what I think. You have several interrelated issues that you need to isolate from one another to know for sure.

  1. You need to start by confirming fuel delivery is not your primary issue. There could be a problem with your fuel pumps, a restricted fuel line, plugged filter or a leak somewhere in your fuel system. To rule out fuel delivery as a source of your problems, skip your fuel delivery system entirely by running a long hose from Oscar’s high pressure fuel pump to inlet side of your fuel rail and see if this raises your fuel pressure. If fuel pressure is suddenly is normal you have a fuel delivery problem.

  2. Regardless of number (1), you still might have a fuel regulation problem which could include a faulty regulator or a leak somewhere in the fuel rail. With your dad’s car pumping fuel, check another regulator with the engine off; if fuel pressure jumps to 43 PSI, you had a faulty regulator. In the event that the fuel pressure did not change, you must have leak in the fuel rail somewhere that is bleeding off fuel pressure. Pull the fuel rail, with injectors attached from the intake manifold and remove from the car so you can closely examine while it’s pressurized with fuel pumping through it from Oscar’s car. You could have fuel leaking around or through an injector that is bleeding off pressure, causing inaccurate O2 sensor readings, which would influence the DME to lean out the mixture and explain your AFR issues.

  3. This is unlikely considering precision and care that your world class engine builder puts into your engines, but your engine might be suffering from valve guides or seals. Put a vacuum gauge on your intake manifold and see what could of vacuum your engine builds. The Bentley outlines the procedures on what how to do this test.

  4. You could have a vacuum leak. When I install manifolds, I always replace flattened wave washers with new ones and use fresh nuts. Hardware is cheap insurance to help avoid vacuum leaks along with using a torque wrench to make sure things are tightened down evenly. You could have a vacuum leak at the intake manifold, dipstick tube, any one of the rubber hoses, brake booster, master cylinder (check the o-ring), valve cover cap, throttle body ports, and the FPR vacuum port on the intake manifold. Our engines don’t run well with vacuum leaks.

Given your symptoms, my bet is your problem is with (2).


I still think that the FP issue is small potatoes. The DME should be able to happily adjust to a couple psi low. It’s the vac leak that I would put as first priority.


I disagree; fuel pressure is a big deal. In my experience fuel pressure doesn’t change the shape of the AFR curve, but shifts it up and down depending on the adjustment. You can legally make similar AFR adjustments with the AFM, but if not tuned properly and the fuel pressure drops, I would expect the car to run lean. In my testing, I never have seen much change from long term fuel trim.


Well, then our results are at odds. Not a big surprise. The DME is kinda a black mystery box.


I have no idea what technically happens inside the DME box, but believe long term fuel trim is like Valentines Day and thought up by the German equivalent of Don Draper. If some Boasch marketing guy claims that the DME learns over time, it may be the case, but I have never seen AFR curves drift after adjustments and repeated testing. Maybe it has something to do with WOT.


I’m with fishman. ECU runs off the O2 sensor up to contact by the full throttle switch. Then it goes to an internal fueling/timing map, all of which are supposed to be the same.

I would try: first…install a fuel gauge before the fuel rail and see what the maximum pump reading is…should be 100ish psi.

Next, install gauge after prv and crimp line to check for leaks and max psi.

Those should uncover the problem.


Update. HUGE vac leak from rear passenger side camshaft blank plug. Pretty sure that’s the cause of braking and FPR losing vacuum and not functioning properly. Have parts coming tomorrow. I have a feeling that this will solve the fuel pressure problem because the leak was so bad I think it was affecting the FPR. At least that’s what I’m hoping. MIR next weekend will tell the tale. Weird that there was no idle issue though. Guess that only happens if it is intake or before.


Any updates on this? Mine is idling at 38 give it a little gas and goes down to 33. Haven’t tried WOT. We just replaced the tank due to a clogged return line and pressure was 92 PSI. New FPR will be here Monday.


Not really. Engine is sealed up, weird rev issues were bad TPS, ran great at Daytona after TPS replaced. Update us after you replace FPR.


Hi, I am reviving and adding to this thread.

My son, Justin, runs his 87 SE30 in Norcal. Primarily at Sonoma, Thunderhill and occasionally Laguna Seca. For some time we have suspected a loss of power at 4500 rpm and higher. Like a hesitation or lag when the pedal is down. This was confirmed by a veteran we asked to drive the car for a few laps at Sonoma. So, we replaced the in-tank pump. Later installed a new 3 bar FPR. Note: Also, car does have the AIM MXL2 with fuel pressure sensor and sampling port installed. At idle we saw ~37 psi and when we mashed the throttle it dropped to 33-35. Now, we just replaced the in-line pump with a new Bosch unit. Judging by the looks of what we removed I’d say it was the original, so 30 years old. Buttoned everything up, started it up and pressure at idle was ~39, more important, mashing throttle now it spikes to 44. Looking forward to what this means on the track. Will report back how things go next trip to Sonoma. Finders crossed.

PS: In off season injectors are coming out for professional bench cleaning and testing. I use a guy in Sacramento (Dr Injector) who does great work and yes, he can check them for leaks in the body and tip.


You made two references to FP changing at WOT. In both cases the engine had a load on it, correct? I mean as opposed to a car being in neutral and someone playing with the gas pedal and watching FP.

Imo, everyone should have a fuel pressure sensor on their dash. This is hugely helpful when one has an engine management problem because it rules out FP as a possible cause.

Don’t get your hopes up. The DME will happily change injector duration to account for low FP. 10psi should be within it’s range to adjust to.

The other gauge you need is a fuel to air ratio meter. That will tell you if something funny is going on with your mixture. For example, biggest engine management culprit is the AFM and it can cause the engine to run lean. When my AFM punked me years ago, mostly because well meaning folks felt that they knew how to adjust the AFM spring, my engine would often start missing between 4500-5200rpm at WOT.

My then brand new F/A meter was telling me that the engine was running very lean, but because the meter was new, I didn’t trust it. I’d have spared myself a lot of pain had I trusted what it was trying to tell me.


Hello Ranger!

Thank you for responding. Both FP readings I mentioned were under static conditions. Car at the shop. in neutral, engine not under load. No doubt The true test is on track.
We need to pull up past FP data from Sonoma runs. Then go back out to the track and collect similar data (now with new fuel pump) and overlay/compare to see what it reveals. Should have that completed this month.

You’re spot on recommending installing Air/Fuel meter. AIM does have an add on module for $380 (LCU-One) for that purpose. Will consider it.

I take it you eventually ended up replacing your AFM?

Reason I ask is our AFM was also “adjusted/tweaked/tuned” at a shop.

Our dynojet data from 3 pulls in April 2017 showed A/F at 4642 RPM as 12.29,12.36 and 12.44.


A no load test isn’t very meaningful. With no load, fuel demand is low, air demand is low, and vacuum for both idle and WOT is high. If engine was under load, vac would be low at WOT and FPR would react to that by increasing FP. As a result of low fuel demand, nothing that has anything to do with fuel is really tested in a low engine load test. As a result of low air demand, the AFM isn’t tested. Because no load means no vac change no matter what you do with the throttle, the FPR’s reaction to changing vac isn’t tested.

$380 is an awful lot for a F/A meter. Prob get a gauge on your dash and a wideband O2 sensor for 1/2 that cost. Have to weld a bung on to your exhaust. Note that you should connect the gauge’s data out to your AIM.


Reporting back after a day at Sonoma. Changing the FPR and external pump made a HUGE difference. The AIM data (FP before vs after) plots overlaid (real time at the track) clearly showed it. Most important, he set a new best lap time 2 seconds faster. PUMPED…literally