Bad fuel pressure regulator maybe?

Mike Puckett

SCCoA Member
Well, SuperdadSC and I swapped out his FMIC to a double IC to see if it eliminated a lack of power problem when in boost. Thinking that we had a leak somewhere we wanted to eliminate that as a possibility. It took all afternoon but we got it all changed over and took it for a test drive. Now instead of falling flat at 3psi of boost we were able to get it up to 8psi at about 1/2 throttle. The sc is overdriven 15%. The regulator is an adjustable and the fuel pressure gauge we had showed 40 psi at idle which seems just fine. The exact problem is that the car runs great until the boost comes up to about the 8 psi mark and the power just falls to nothing after that. It doesn't misfire or run rough it just smoothly noses over. Back off on the throttle so the boost comes down and it picks right back up again. This is the same regulator that acted like it wasn't delivering any fuel at first. After cycling the ignition switch several times when pressing the shrader valve, nothing came out. Eventually we got the pressure up and it seemed to be Ok. Unfortunately we weren't able to test the fuel pressure while under load. It's been driving fine except for this. I've suggested that we swap the sc pulley out to a standard size and see if lowering the boost helps. It's got a 10% overdrive jackshaft pulley and a 5% sc pulley right now. I'm wondering if the regulator is the problem though. This wasn't a problem before the old motor blew up. Anybody run into this before or got any ideas?
 

dave377

Registered User
May be a weak fuel pump that can not supply enough fuel when the boost comes up. What happen to the old engine could it went lean and blew it up. Yes the fuel pump could be screw up when boost presser go to it.
 

Mike8675309

Registered User
Do you know the brand of Adjustable FPR? Back years ago the adjustable FPR was a thing for folks to change to when trying to fool the computer with mods (back before it was under $1000 to tune your car). A number of different adjustable FPR were found to be complete junk back then. Idle FPR of 40 is a tich high but if the car was tuned for it then it should be fine. The EEC can deal with it in closed loop.

My experience with lack of fuel pressure would be pre-ignition along with the power loss as the engine leans out from the loss of fuel pressure. Fuel pressure is so critical which is why on my car I added a digital FPR that I could datalog. Answers lots of questions easily. If you think it may be the fpr, you can buy or borrow a large fuel pressure gauge with a long hose and a schrader fitting. Something long enough you can run it out under the hood and tape it to the windshield to watch fuel pressure when driving.

I assume we are unable to datalog this car to see if anything odd is going on? It is always possible the overdrive is causing enough air mass to be read that it gets into ranges of the tune that pulls timing all over the place as well.
 

Mike Puckett

SCCoA Member
The regulator is the same type I've got on mine, a Kirban I think? Looks just like it at least. It really acted funny while i was trying to get it going. At one point the gauge was showing 70psi while other times there was nothing. The old motor blew up because it dropped a valve. The story of it is in another thread. I've wondered about the fuel pump, too. I've got a fuel pressure gauge but the hose isn't very long. Maybe if we block the front tires we can put enough load on it so I can watch the gauge. It's a 5 spd so it's not so easy to do. I've got an extra stock FPR but not sure if it's up to the task since he's running 58# injectors. This motor has a smaller cam than the old one but everything else was swapped over and was running good before it let go suddenly.
 

David Neibert

SCCoA Admin
The regulator is the same type I've got on mine, a Kirban I think? Looks just like it at least. It really acted funny while i was trying to get it going. At one point the gauge was showing 70psi while other times there was nothing. The old motor blew up because it dropped a valve. The story of it is in another thread. I've wondered about the fuel pump, too. I've got a fuel pressure gauge but the hose isn't very long. Maybe if we block the front tires we can put enough load on it so I can watch the gauge. It's a 5 spd so it's not so easy to do. I've got an extra stock FPR but not sure if it's up to the task since he's running 58# injectors. This motor has a smaller cam than the old one but everything else was swapped over and was running good before it let go suddenly.

If your getting goofy pressure readings like that, I would absolutely change the regulator. I think SCP sells the new billet Kirban for around $100 and it's probably the best quality you are going to find for a bolt on regulator.

David
 

Mike Puckett

SCCoA Member
David, I'm inclined to agree. It's such a pain to replace I only want to do it once so I'd rather not try a stock unit for testing purposes.

Stephen, Do you want to go ahead and order one?
We can still test the one that's on there on Sunday afternoon if you'd like.
 

David Neibert

SCCoA Admin
David, I'm inclined to agree. It's such a pain to replace I only want to do it once so I'd rather not try a stock unit for testing purposes.

Stephen, Do you want to go ahead and order one?
We can still test the one that's on there on Sunday afternoon if you'd like.

Mike,

The early model 3 bolt regulator is a pain to change because the bolts that attach it to the rail are on the bottom side, but I've done it several times and if the supercharger and return plenum are removed it's pretty easy. If it's the 94/95 late model 2 bolt unit, it's a lot easier because it bolts down from the top and can be changed without removing anything except maybe the wiper cowling.

David
 

superdadsc

Registered User
That is the FPR that I bought. I bought it from Bill at SCP. It is basically new. I think billet aluminum. I put it on the old motor that I blew as the stock one wasn't working right. So it has 3 to 4k of miles on it. That doesn't mean that the unit didn't poop the bed:D

Car is starting to run rough. The check engine light came on and I was going to bring it to run a scan at the auto store. Went to buy some food beforehand and when I got back in the car and the check engine light was no more. Also I am able to add less and less of the gas pedal as it is falling on it's face faster now. Also up my hill the car registered 5lbs of boost but was moving like a snail?
 

Mike8675309

Registered User
Also up my hill the car registered 5lbs of boost but was moving like a snail?

That means almost nothing unless you were at wide open throttle. Partial throttle depending on the load can cause the bypass to close and result in some boost.

It is not unusual for new things to be faulty. I would really want to track the fuel pressure when experiencing problems. Until you can verify fuel pressure you still have either fuel or spark to blame. If you can eliminate fuel as the problem then you can turn your focus to spark.

I ran a stock fuel pressure regulator with 80lb/hr fuel injectors running E85 and 19psi of boost. In that combination the stock fuel pressure regulator worked like a champ and it wasn't the weak link.
 

David Neibert

SCCoA Admin
That is the FPR that I bought. I bought it from Bill at SCP. It is basically new. I think billet aluminum. I put it on the old motor that I blew as the stock one wasn't working right. So it has 3 to 4k of miles on it. That doesn't mean that the unit didn't poop the bed:D

Car is starting to run rough. The check engine light came on and I was going to bring it to run a scan at the auto store. Went to buy some food beforehand and when I got back in the car and the check engine light was no more. Also I am able to add less and less of the gas pedal as it is falling on it's face faster now. Also up my hill the car registered 5lbs of boost but was moving like a snail?

Still sounds like fuel starvation to me. You need to monitor fuel pressure when driving and especially under boost. Have you already eliminated the fuel filter as a possible problem ? If the car was recently loaded or unloaded from a trailer or up on a lift, I would inspect the fuel line for a crushed or kinked portion. Have you confirmed that the regulator is not leaking fuel out of the vacuum/boost hose port ? Hard to spot because it dumps the fuel straight into the intake manifold.

David
 

RSC'91

Registered User
I just went through what you are explaining and I did just what has been suggested. I replaced the FPR first only to experience the starvation again, I replaced the filter to no avail. I was hesitant to replace the fuel pump as I had just installed a Walbro. After a lot of troubleshooting and finding a way home more than once I replaced the pump with a stock unit and its been trouble free since. Pump was definitely easier than the FPR.
 

Mike Puckett

SCCoA Member
RSC'91
I just went through what you are explaining and I did just what has been suggested. I replaced the FPR first only to experience the starvation again, I replaced the filter to no avail. I was hesitant to replace the fuel pump as I had just installed a Walbro. After a lot of troubleshooting and finding a way home more than once I replaced the pump with a stock unit and its been trouble free since. Pump was definitely easier than the FPR.

That's good to know. This car has a new 255 lph Walbro as well. I haven't ruled it out as being bad, either. One thing to realize, just because a part is new doesn't mean it can't be bad.
 

Mike Puckett

SCCoA Member
It appears that the fuel pump is the culprit. Apparently it's not that new and the fuel pressure regulator was changed to the one that's on there due to fuel pressure problems. Right after Stephen arrived yesterday I pressed the shrader valve and nothing came out. I hooked up the fuel pressure gauge I had and we started the engine up and had 20 psi of pressure. We blocked the front wheels and I spun the tires but the fuel pressure didn't go up. I tried to adjust the regulator but the pressure only went down. I pressed the shrader valve while the engine was running it just gave a weak squirt out to confirm the pressure was down to only about 10psi or so. I'm surprised the engine would even crank up and run at all. So a new pump has been ordered and we'll get it installed and see how it does.
 

gr8ghost

Registered User
I had a similar problem with the car missing under 3-5# boost. Guess what wires and plugs to blame. Test the fuel pressure as described and be sure it's not or is the fuel pressure. Also have the codes read or buy a code reader. There is a code in there if the light was on for even a bit. Otherwise you'll be through parts at it like pissing in the wind and wondering why your pants get wet every time. you can also use a laeser guide thermometer to see if you have a cold missing cylinder. We all should have one. Only about 30 bucks at Harbor Freight and Tool.
 

Mike8675309

Registered User
It may be worth it to try a static fuel flow test before swapping out the pump. Did you try taking the gas cap off after running the car? A blocked vent line could cause low to zero fuel flow. poor fuel pump performance can be wrapped in so many other things that are not actually the fuel pump.

What are the horsepower expectations for that vehicle? Does it have the right capacity fuel pump right now? 20psi at 4500rpm and up is going to take quite a bit of fuel.
 

superdadsc

Registered User
The car's target rwhp was 350. Now with the baby cam and mild porting my goal is in the 270 range or lower.
The car has 58lbs injectors, MPI, DIC, 75 tb , 76 maf and quarter horse chip w/ base tune from Dave.
 

Mike Puckett

SCCoA Member
When I had the fuel return line disconnected I could blow into it and hear gurgling in the tank at the other end so I don't think it's blocked. It's not shown a check engine light nor has it misfired. It just smoothly noses over when the boost comes up past about 5 psi or so, almost like you putting on the brakes. It definitely sounds like fuel starvation. Also we saw on the old motor's dyno sheet where the power falls off rather abruptly when the boost comes up. Since it doesn't misfire and the plugs were just installed along with the wires I'm discounting that these are the culprit. I've had broken plugs and bad wires before so I know how it feels when that happens. Additionally, mine has a 255lph pump also and it is much louder and smoother running. This one when jumpered on sounds weak and it stumbles a little bit when running.
 

XR7 Dave

Registered User
Just FYI, Walbro is one of the most counterfeited products on the market today. If you don't buy from a genuine Walbro dealer, you don't know what you are getting.
 
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