Jump to content

fuel pressure regulator


cessna
 Share

Recommended Posts

Anyone have to replace the fpr value on there 3.4 I think mine is on the way out, looks to be buried under upper plenum I guess there is no short cuts for this one if it turns out to be the problem .

Link to comment
Share on other sites

Yes I pulled the intake about 2 years ago to replace intake gaskets and fix oil pump seal, wasn't really paying attention and  realized they tucked the fpr under the upper ....thanks Gm . I was just wondering how many of you guys have had to replace the fpr valve and which brand you went with. Also what kind of pressure are you guys getting at the rail while running 

Edited by cessna
Link to comment
Share on other sites

 Hello 55trucker,

It just started with hard starting recently 2 or 3 cranks before it fires off i'm looking at 40 lbs for pressure when running stays consistent, when key is turned on but not running pressure is very low 10 to 15 lbs then turn off and back on pressure climbs a bit to around 25 drops very quickly when key is turned off , no smell of gas. I haven't pulled the uim yet to really give the fpr a test but wanted to have one handy to put back in instead of waiting for 3 or 4 days seems they are out of stock in my area had to order one today. I just changed plugs about a week ago they all looked fine for having 60,000 miles fuel pump seems to run fine I would think that if the injector or injectors were leaking that much it would be noticeable in the way it runs, seems to do fine very slight shake in idle at times that just started also but on the road its fine no noticeable smoke , doesn't smell like its running rich either, so just wondering what type of pressure others are running and if they had similar problems.      

Link to comment
Share on other sites

I take it the engine is the LQ1?.......pre 94 or post 93........makes a difference, post 93 has no map sensor, just the vacuum port for the FPR & trans modulator

generally pressure at the rail when running at idle is just above 40lbs., it would seem that you're in the ballpark, when the FPR goes it usually is the diaphragm that tears, little pressure & the plenum will suck the fuel in thru the map port. Without removing the plenum one can remove the rubber hose for the fpr at the map & you will instantly get fuel odour, now are the rubber hoses at either end of that tube that feeds the FPR are not rotten & leaking vacuum? If the internal spring goes the fuel rail goes lean but there is no leakage into the plenum.

Engine running/wide open throttle/low vacuum/full spring pressure/max fuel pressure. Engine at idle/full vacuum/vacuum pulls on the diaphragm against the internal spring to lower fuel pressure. 

If you have a vacuum pump/gauge on hand test the FPR for vacuum at the MAP port, if you can't pull anything then either the tubing is leaking or the FPR has failed. Normal vacuum at engine idle is approx 16-17 inches vacuum. The only way to test the FPR itself is with the plenum removed. 

There are 2 ports at the MAP..... 1 is the FPR feed & the other is the vacuum for the trans modulator, if either are leaking engine idle will be upset.

Edited by 55trucker
Link to comment
Share on other sites

55trucker ,

Thank you for the info, yes it is a 1993 3.4 sorry I should have mention that in the post, The reason for not pulling the uim I just wanted to wait until I got the fpr in hand before I decide to tear it down seems as tho there is none to be found locally right now I have it on order. I never thought about checking at the map sensor I will need to find my vacuum gauge and do some testing. We are going threw a major storm right now suppose to be all day long with tornadoes  hopefully I wont be in the dark .... Thanks 55trucker for your input greatly appreciate it.

Link to comment
Share on other sites

Hey 55trucker, did a vacuum test at the map sensor seems to have good vacuum, also when i pull the line while running it dose raise 3 to 4 lbs on the pressure gauge .Now i'm starting to think with the bleed off so fast when shut down that maybe check valve in fuel pump is open and not allowing the pressure to stay in the rail, that would account for the prolonged start .

Link to comment
Share on other sites

If you suspect that the fuel leakage is in the lines pinch the return line off at the engine before you shut down the engine. Note what happens to pressure when you do this. If the pressure doesn't hold then your leakage is in the pressure side. 

Now with the return side still pinched off do the same procedure with the pressure side, if the injectors are not leaking the rail will hold the pressure indefinitely, if the pump is draining back into the tank this should find it.

Edited by 55trucker
Link to comment
Share on other sites

19 hours ago, cessna said:

 It just started with hard starting recently 2 or 3 cranks before it fires off i'm looking at 40 lbs for pressure when running stays consistent, when key is turned on but not running pressure is very low 10 to 15 lbs then turn off and back on pressure climbs a bit to around 25 drops very quickly when key is turned off , no smell of gas.

Losing fuel pressure after shutting engine off could certainly be a bad FPR.  Could also be leaking injectors, or a rupured fuel hose (or damper assembly) between pump and the metal tube of the sending unit assembly.  Could be a failed check-valve in the fuel pump,

Low pressure with the key on, engine off, and "normal" pressure with the engine running sounds like a failed fuel pump relay--perhaps the contacts are burned, so the pump gets low voltage and low amperage, can only manage a few PSI.  When the engine is running the oil pressure switch supplies normal power to the pump, so the pump runs fine.  This would also cause hard starting--long crank time.

Link to comment
Share on other sites

Did some more troubleshooting today pinched off the return line turned key on  and the rail held pressure, released the clamp to let the fuel out and reattached the line, turned key back on and and crazy enough it built great pressure and has been holding it fine . I left the gauge hooked up for about a half hour and the pressure was still good. Before the pressure would drop instantly to zero when the key got turned off, now the Car starts on first crank every time, pressure is good ,thinking maybe the spring got stuck open ? i'm at a lost with this one but if it holds up i'm happy. Car still has a very slight shake to it i'm going to pull plugs again and see how they look after checking the vacuum lines. I'm not going to pull the uim until I see the problem return then I will do a thorough check on the fpr and lines going to it. What do you guys think ?   Hey schurkey how you been man good to here from you yes I was thinking maybe fuel pump relay in the beginning so I checked that , 55trucker has been a big help with this one, gave me some great ideas for troubleshooting from the map sensor  really appreciate you guys stepping in. 

Edited by cessna
Link to comment
Share on other sites

Well its been about a week now and pressure is fine and starts on first crank like it should, really don't know what caused the problem with the lost pressure before and the hard starting but can only think there was some foreign matter in the system at the fuel filter or the line Hopefully it doesn't leave me stranded somewhere due to faulty pump or FPR valve time will tell I guess ....Thanks for all the help guys 

Link to comment
Share on other sites

It's quite possible that some foreign material was lodged in the FPR that was keeping the port slightly open. Hopefully your problem is behind you.

Do you have a fresh filter under the car?

Link to comment
Share on other sites

55Trucker, I put a new fuel filter on the car about a week before this started happening and now I'm wondering if something got sent up the line to the FPR from that change, seems to be running real well now. I would like to Thank you for the great suggestions for troubleshooting, that saved me from pulling the UIM for now anyway ......but I have all parts on hand if it comes back to haunt me again. Thanks again 55trucker and Schurkey  hope I can return the favor one day. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...