LarryD

So, is there such a thing as fuel pressure too high.??.. Bought a new gauge and mine reads 51psi running at idle and drops about 3psi when stopped.. Doing a leak down test right now but its been about 20 minutes and hasn't moved from where it was when I stopped it (holding steady at 48psi). UPDATE: At one hour 15 minutes the pressure has dropped to 12psi.

Every once in a while it acts like it has to build up fuel pressure before it starts. Bought a new fuel pressure regulator (yeah $80) and decided to check pressure before and after.. I haven't put in the new one yet and wondering if I should.. The existing one is factory original so 20 years old and 250,000 miles.

I just changed the fuel filter and have recently had the injectors cleaned, rebuilt and tested. I have been getting nervous about the fuel pressure regulator and fuel pump since they both came factory original. Any thoughts on whether I should go ahead and change the regulator and/or fuel pump? How long do the factory parts usually last?

LarryD

Ok, so been reading on the internet (and testing).. Sounds like the fuel pressure regulator can be bad with either low or high pressure (which makes sense).. Does the Schraeder valve tap pressure at the fuel pump side of the regulator or the injector side of the regulator? I am reading 50psi at that point so either that is the fuel pump pressure directly from the fuel pump or the regulator is not working and passing full fuel pump pressure to the injectors.....

Regulators should maintain an outlet pressure slightly lower than the inlet pressure. That difference between outlet and inlet is how they actually regulate. So, if my theory here is correct, the fuel pump pressure on mine is 50psi, the regulator should drop that to what I have found here as a range of 43 to 47psi. If the Schraeder valve is on the outlet side (injector side), then it appears my regulator is not working and is simply passing the full fuel pump pressure to the injectors. That would cause it to use more gas and it does appear to be lower on fuel economy than it used to be. I hate to go ahead and install a new regulator but that may be the only way to know for sure unless somebody has more info (like what is the normal fuel pump pressure).

Any thoughts or input?? Thanks...

jiyasa

iTrader: (1)

I'm by no means a mechanic but you've already spent the $80 and installing it, you can't be any worse off with the new one in than having the existing one you've got. I have a 3.1L too and my idle is so erratic, some warmer days it idles as high 2500 and some days at a stop light it wants to die. If it was me, I'd go ahead and install it.

LarryD

Well, I did go ahead and install the new one.. Figured I might as well get some answers that way... The Schraeder valve is on the injector side... So, the fuel pressure is now at 40psi key on but not running (was 50psi with the old one). Running it drops to about 38 or 39psi (didn't drop when running with the old one)... If I rev the engine a little it goes up to about 43 or 44psi (didn't change with rev with the old one)... It does seem that the old one was bad.. Driveability didn't seem to change much so hoping that mileage increases a little since it isn't forcing quite as much fuel through when the injectors open...

Can't say I have had an idle problem like you described... A surging idle sounds like a bad or intermittent sensor.. Maybe a poor connection from a sensor to the ECM...

project89

iTrader: (1)

ur old regulator was bad, with the new one at idle u should see 43psi with the vacum line unhooked form the regulator

with it hookied up the fpr will drop fuel presure at idle , and return it to 43psi when u go wot

this is done to help with the fueling of the motor since at part throttle and idle were the injectors spray tip is is in a vacum it dosent take as much fuel presure for them to flow what they would at 43 psi when there is 0 vacum

this is the same reason we add fuel presur eon boosted cars if u have +7 psi in the intake manifold and dont raise fuel presure the injector will only spray the amount of fuel it would at 36psi wit6h no vacum


hopefully i explained that so u could understand it

LarryD

Thanks for confirming my numbers and the explanation. I had searched high and low trying to find specs and description on how it was supposed to work without much luck.. I did see some numbers on here but they said it should drop to 10psi when running and I pretty much knew that would be too low for the injectors.. Hopefully this thread will help the next person having fuel pressure and injector issues..

Now, I need to find the fuel pump pressure specs.. I am planning to change mine in the near future since it is getting up there in age and I am getting a little nervous that it could quit at any time.. Just haven't decided whether to do the full monte and take it completely apart or do the "hatch" modification.

Thanks for your help.

Maverick H1L

iTrader: (8)

It's not the fuel pump pressure you need to be worried about as much as the flow rate...

Fuel pressure for a stock engine should be between 37 and 47 PSI running. That 10 PSI number may have been for a TBI system of some sort (they use about half the pressure a port injection system does).

LarryD

So, I now have about 1000 miles on it since the regulator change.. Mileage has remained about the same (21 around town and 25 to 26 long freeway drives at 70).. I have noticed a bit of a stumble in the lower gears and lower speeds but don't know if that is related to the now lower fuel pressure.. Think I will probably pull the plugs to see what they look like.. Wondering if one of the rebuilt injectors is acting up a bit or if now that the fuel pressure is lower and separate problem is showing up....

LarryD

Ok, so I pulled the plugs and they all look normal (maybe a touch rich) and are all consistently the same... So, guessing I don't have an injector causing a problem (they were just bench tested, cleaned, and rebuilt).. It has a new fuel filter... Re-checked the fuel pressure with the new regulator and it is 38 psi at idle goes to about 40 when I accelerate it but drops to 38 again when I hold it at high idle (was 50 all the time with the old one). It also bleeds down pretty quickly (starts falling about 3 to 4 minutes after turning off the key).. Mileage didn't seem to improve with the new regulator.. Now, it has started an intermittent miss during steady acceleration from start to 50 or 60mph... I also notice that when starting it after it has sat for a while that it takes longer to fire (5 to 6 seconds) where it use to fire as soon as the starter kicked it over... After the first start, it fires immediately without the delay until it has been left to sit for a while...

There are no error codes. I did notice while running some Seafoam through it that I have a little exhaust leak at the exhaust manifold to header pipe donut connection. The exhaust is all new (header pipe, cat, exhaust pipe, muffler, tail pipes and O2 sensor)..

Anybody have any thoughts on why it is bleeding down so quickly, stumbles sometimes and has this miss?

Thanks...

Maverick H1L

iTrader: (8)

If you my posts, I have outlined how to do a fuel pressure loss test any number of times...

LarryD

Searched your posts and found 4. The only new information beyond what I have already done testing is to check the pressure with the fuel pump relay disconnected.

Originally, it had 51psi running with the old regulator and rebuilt injectors. Turn it off it dropped immediately to 48psi and held steady past 20 minutes, bled down to 12psi at one hour 15 minutes. New regulator and rebuilt injectors it has 38psi running. Turn it off it doesn't drop immediately but steadily drops after only a couple of minutes. Admittedly, I didn't wait to see how far it would go after the three minutes because everyone has indicated that it shouldn't drop at all for 20 minutes.

So, my only thought was that either the new regulator is bad or one or more of the injectors went bad after the rebuild unless I am missing something. There is only one remaining guess on my part and that is the fuel pump is leaking back into the tank now and I am losing pressure between the fuel pump and the regulator.

So, I will run it up to check the running pressure. Shut it down and check pressure again after 20 minutes then again at about an hour to see what the drop off curve looks like. Then, I will bring it up to pressure again turn it off and pull the fuel pump relay to see what it does without the relay (or under this test I would think the key should be turned back on).

A couple of questions come to mind. Should the fuel rail hold pressure if you disconnect the fuel line back at the fuel filter? In other words with everything turned off, can fuel back bleed in reverse flow through the regulator for a normal good regulator? If fuel pressure is to be maintained for a long period of time, is it held there by the regulator or by the fuel pump? These questions assume the regulator is between the fuel pump and injectors. Or, do I have it backwards, Is the regulator on the fuel return side such that it regulates pressure from the outflow point of view allowing the pump to directly feed the injectors and opening flow to the return lines to keep pressure from going above 40 or so psi.???.

LarryD

So, last night I checked the pressure again. Turned the key on (engine not running) pressure went to 41psi. Turned the key off and waited. Bleed off started right away and was down to 20psi in 15 minutes. Then, I engaged my brain a bit more and here is what follows:

So that’s what I get for being an electrical engineer instead of a mechanical engineer.. A voltage regulator goes between the source of power and the component.. Not so for the Fuel Pressure Regulator… After Googling theory of operation I found I did have my thinking backwards.. Fuel goes from the fuel pump to the filter to the fuel rail to injectors. Excess fuel from the rail goes through the pressure regulator back to the tank.. As pressure increases, the regulator opens to bypass more full to the tank and maintain the regulated pressure in the fuel rail and to the injectors.

So, tonight I will block the return line and see if it holds pressure. If it holds pressure then I assume the injectors and fuel pump are not bleeding it down so therefore it would be a bad (although new) FPR. If it drops pressure, I will pull the plenum and fuel rail, pressurize it again with the return line blocked and look for a leaking injector. If no leaking injector, then it would have to be the fuel pump leaking back into the tank…

A side benefit is that I can check the injectors when I have them out to see if they are pintle or disk... Re: thread on injector change.

LarryD

Tonight's results.

1) I disconnected the fuel return line and turned the key on. Pressure went to about 41psi and fuel came out the return line. Fuel pump stopped, turned key off and pressure was at 40psi. The pressure bled off right away as before but no fuel came out of the return line so assume the regulator is not leaking to the return line.

2) I took the plenum off and pulled up the fuel rail with fuel lines connected, vacuum to regulator disconnected, injector wires disconnected. Turned the key on, pressure to 41psi, turned key off, pressure bled down as before, no leakage from fuel injectors or fuel rail and no leakage from the regulator vacuum port.

So, assuming the above tests are conclusive that the regulator and injectors are not leaking then I guess the only thing left is a fuel line leak or failing fuel pump. Does the fuel pump have a separate external check valve to prevent backflow to the tank or an internal check valve or no check valve at all...????...

Maverick H1L

iTrader: (8)

1. KOEO, note fuel pressure. Between 37-47 PSI and holding for 20 minutes, good. You already know it's not, so it's on to step 2.
2. Use a clamp of some sort or locking pliers with a folded rag between the jaws to clamp off the fuel pressure hose in the engine compartment after fuel pump shuts off. Pressure holding means either the check valve in the fuel pump is bad or there is a leak between the tank and the engine. Pressure not holding, step 3.
3. Remove clamp from pressure hose and put it on the return hose. Cycle pump again. Pressure holding means leaking FPR. Not holding, step 4.
4. Cycle pump again after finding second clamp for pressure hose. Put clamp on pressure hose LEAVING THE CLAMP ON THE RETURN HOSE. Pressure still drops, leaking injectors. And yes, I've had injectors leak without a noticeable drip (the set of 3.4 injectors I had).

Dezarov_17

I need to try this on a 3.1 that I have here. Great info Maverick!

LarryD

I did a quick test at lunch time today but I am not certain that the vise grips were closing the line completely.. Got bleed down with both lines clamped.. Somewhere I have some other flat clamps that I can maybe make sure the lines are completely closed off so will try again later tonight..

LarryD

Ok, so final result I guess is that the injector(s) have a leak(s).. I used a couple of my body working vice grip clamps so had a big wide surface to close off both feed and return fuel lines. I am pretty certain they were closed tight. I still get the bleed down. I noticed that if I clamp the feed line from the fuel pump and then release the clamp right away, the pressure immediately drops 20psi. So, clamping the line is squirting fuel somewhere. I hate to have to replace all the injectors after just spending $100 to have them rebuilt but looks like that is the next step.

The only remaining item that could have any effect is that fuel expansion tank in the middle of the hard line running to the fuel rail. It is supposed to reduce surging in the feed line when the injectors all open and close at the same time. I assume it is just an empty "can" that holds fuel and I don't see any fuel leaking from (it sits right below the throttle linkage bolted to the front of the intake manifold.

LarryD

It's driveability is going down hill fast, power loss and acceleration miss, some hesitation/stumble... Interestingly, no real loss in gas mileage... So, emailed the guy that rebuilt the injectors and he said they will retest for free up to a year and will pay shipping... Pulled the injectors tonight (have it down to 15 minutes flat now) and they are headed back tomorrow for re-test. Will post what I find out...

LarryD

So, one for the book... The injectors were tested today.. Initial leak down showed no leaks.. They ran the injectors on the bench at an equivalent rate and load to simulate a low to medium road speed.. Spray from one injector looked incorrect and when they stopped them, the pressure (and that injector) leaked down right away... They ran them again at what would be a high rpm and load, spray still looked a little off on the one but when they shut it down, it did not leak.. Repeated the low speed test and it again leaked.. Looked like the injector was sticking open. They again cleaned and backflushed the one injector and it appears to be running normally.. Injectors are on their way back to me in priority mail so I can try them again... They marked the one that they were having trouble with an X so I can track it.. If I still get leak down, then I will need to replace that one injector...

By the way, they received my injectors at 10am and had them on their way back to me by 2pm... If the injector again fails, they will refund my cost for rebuild of that one... They only charge for rebuild of the one's they can fix... That's service...