Slowly Losing Fuel Pressure

[dgoodhue]

Re: Slowly Losing Fuel Pressure

The problem with my situation is I have a brand new Accufab AFPR and when I put the key in on position the fuel pump does prim and goes to the desired level immediately starts dropping in fuel pressure down to about 20lbs than stops.. Its the reason I started this post. someone resonded saying that they had the same exact problem with there new Accufab AFPR and had a new one sent to them and it did the same exact thing again. apparently that's what Accufab regulators are prone to do. when the truck is runnin it holds pressure though..

Dropping fuel pressure is because of 4 things. Fuel pump check valve, leaking fuel injectors, fuel pressure regulator, and fuel leak I put this in order of how common I think these problems are. You should obviously will be able to smell a fuel leak. You are not going to be able to use the factory service manual adjustment of the fuel pressure with the system leaking down, plus I am not sure how accurate this is with an aftermarket full pumps, since it priming the system.

 

[dgoodhue]

Re: Slowly Losing Fuel Pressure

In DataMaster, Setup, Display Units and switch it over. Switch everything else over to English while you're there. I don't know where you're getting all your info but...with all due respect, dgoodhue is mistaken. Per the manual:

Install fuel pressure gauge
disconnect vac hose
ign off for min 10 secs and AC off
ign on. Fuel pump should run for 2 secs.
Note pres after pump stops.
Pressure should be 35-38 psi and holding steady

Don, I responded to your private message, but I think you are mistaken about the relative pressure. What I wrote I believe is correct except that I used bar (14.5psi) instead of Atmosphere (14.7psi) for the conversion. Stupid me, the A in KPA is for atmosphere.

The quote above is from the service manual, but if you look at the details this is not with the engine running. It is the priming of the fuel pump, which i have found is different than the engine running with the fuel pressure disconnected (at least it is with my Walbro fuel pump that I installed shortly after purchasing my Syclone 15 years ago). To use the data master vacuum setting, and have it match a vacuum/boost gauge and the expected value a 1:1 fuel pressure regulator, you likely need to subtract 14.7psi the value. [I have not opened datamaster in about 10 years, so they may alter the value] The ECM output on the ALDL line the boost/vacuum transition as 100kPA or 14.7psi.

 

[Z SKI]

Re: Slowly Losing Fuel Pressure

Friend of mine sent me this in a PM:

Quote:
The "pressure drop test" is to assist in determining whether an injector is leaking or hanging open. As long as the pressure drop is NOT an injector problem, the pump=off pressure drop doesn't mean squat as to how the engine will run or even start.

It doesn't matter what the pressure is when the pump is NOT running. It can go to zero, and it doesn't matter because the engine is NOT running either so it doesn't need fuel pressure.

If the pressure goes to appx 40psi as soon as the pump runs, and maintains appx 40psi during cranking, there is nothing wrong with "fuel pressure". Again, it does not matter what the pressure does when the pump and engine is not running as long as you have ruled out injector leakage.

If the pressure is 36-42psi with the engine running, hose off, the OP's stalling problem is NOT related to fuel pressure. Note that the allowable pressure is a RANGE, not an absolute specific pressure. If the BLM in Closed Loop is 128 +/- 6 or so, the pressure is close enough. One can adjust pressure to get the BLM centered on 128, but it isn't necessary. Or you can adjust pressure to get 14.7 AFR on a Wideband at idle, or whatever AF delivers the best idle quality, as long as pressure remains in the acceptable range.

Fuel pressure affects the amount of fuel injected for a specific Injector Pulse Width. The "chip" or program may not be correct for all conditions of throttle, temperature, RPM, etc. Actually, the chip is NEVER "perfect". That's what "tuning" IS. To get the fuel delivery (IPW) as optimal as possible under all conditions, but it will never be "perfect", that's why "tuning" is an ongoing process. You're never done "tuning", it can always be better. Having the fuel pressure stable is important to have consistently optimum fuel delivery so that the tune delivers the correct amount of fuel for the demanded IPW.

While the acceptable pressure is in a RANGE, it is important that it always be the same value every time you check it. If you check it and find 41 psi, that's fine, BUT if you check it again a week later under the same test conditions (hose off) and find it's now 36psi, something is not right. The engine will run differently with the two pressures. Something is causing the different readings. (I would begin with system voltage at the pump and then FPR problems).

So check that you have steady fuel pressure between 36-42 psi when the engine is running, hose NOT installed. (Hose plugged or not, it doesn't matter for the fuel pressure check). If you do, move on. Your fuel pressure is adequate. Regardless of what it is when the engine or pump is NOT running, or if it falls or not, or falls quickly or not at all when the pump is shut down.

 

[Don W.]

Re: Slowly Losing Fuel Pressure

Dave, the next set of instruction refers to checking for a "slight drop" in FP when the vac line is reinstalled with the engine running.

Paul Z's friend nailed it very well. I was trying to avoid that much typing...:rotf:

Because of the cam my truck only pulls about 16" of vac at idle. If memory serves after I set FP then reinstall the vac line I loose 2-3 PSI. If a stock truck pulls 20" vac then I would expect to see 4-5 PSI drop but I'd have to actually check that.

 

[MyTy]

Re: Slowly Losing Fuel Pressure

thanks for all the help and info guys, I'm trying to look up how to check for a leaking or stuck fuel injector. when watching my fuel injector info on DM the BPW and inj DC sky rocket but only for a blip then go back to normal... does that make any of you suspect something that could be making that happen? I notice it happing at idle and normal speeds..

 

[Don W.]

Re: Slowly Losing Fuel Pressure

Extremely brief spikes in DM are usually data errors. Your truck is probably not actually doing what you're seeing. Why look for a leaking inj when you know the FPR leaks? If I recall you said FP held with the stock FPR and doesn't with the new one. For all the time and trouble just order a new AFPR from Sportmachines and be done with it.

If you wish there are just two steps to checking the fuel system for a leaking FP, FPR or injs.

Run FP using the test term and gradually block (IE pinch off) fuel pressure line. Remove test lead. If pressure holds then the FP is faulty or you have a leak.

If pressure doesn't hold then run FP using the test term and block the fuel return line by pinching the flex hose. Remove power to test term and see If pressure holds. If it does the injs are not leaking and the FPR is leaking (faulty). If pressure doesn't hold you have a leaking inj.

 

[MyTy]

Re: Slowly Losing Fuel Pressure

Extremely brief spikes in DM are usually data errors. Your truck is probably not actually doing what you're seeing. Why look for a leaking inj when you know the FPR leaks? If I recall you said FP held with the stock FPR and doesn't with the new one. For all the time and trouble just order a new AFPR from Sportmachines and be done with it.

If you wish there are just two steps to checking the fuel system for a leaking FP, FPR or injs.

Run FP using the test term and gradually block (IE pinch off) fuel pressure line. Remove test lead. If pressure holds then the FP is faulty or you have a leak.

If pressure doesn't hold then run FP using the test term and block the fuel return line by pinching the flex hose. Remove power to test term and see If pressure holds. If it does the injs are not leaking and the FPR is leaking (faulty). If pressure doesn't hold you have a leaking inj.

K I'm gonna go on your first piece of advice and go through SportMachines, wish I would have done that in the beginning.. thought Accufab was a good way to go. My old regulator help pressure from what I remember but when I tried adjusting the pressure somting scary happened either smoke or fine mist of fuel shot out from somewhere near the regulator was hard to tell cause of rushing to turn off the truck. I was sort of suspecting it was hardwear issues with the occasional high numbers as well..