Help me diagnose low fuel pressure
01-03-2017, 12:00 PM
#1
Help me diagnose low fuel pressure I have a stock LT1 out of a 1996 Caprice in a Jaguar. I keep having issues with low fuel pressure at the rails. The engine chokes and stumbles like its starved for fuel. I've been throwing parts at it in a disorganized fashion, and it seems to fix it for a while, but then I get fuel pressure issues again. So I finally decided to take an organized approach.
The Jaguar has an external gravity fed sump tank. I cleaned out the sump tank and replaced the fuel pickup sock. The sump has a gravity feed to a new Summit electric fuel pump http://www.summitracing.com/parts/sum-g3138/overview. I put the gauge after the fuel pump, and it pegged the gauge and held pressure. After the fuel pump is the Jaguar fuel filter. I replaced the fuel filter and put the gauge on after the fuel filter, and still pegged the gauge. After the fuel filter is a hard line to the engine compartment. I put the gauge on the engine end of the hard line, and still pegged the gauge. I have the stock fuel rails. I put the gauge on the Schrader valve, and got 20 psi. I previously replaced the fuel pressure regulator. I admit did buy the cheapest no-name regulator possible.
I don't mind buying a decent quality regulator if that will fix this issue. From searching here it looks like the stock regulator is best. Before I do, does anybody have a different suggestion? Basically everything in the fuel system is new except for the tank and the lines, but I still have low fuel pressure and a stumbling engine. Whenever I drain fuel while doing a pressure test the fuel looks clean, with no sediment. Also the fuel is fresh.
The Jaguar has an external gravity fed sump tank. I cleaned out the sump tank and replaced the fuel pickup sock. The sump has a gravity feed to a new Summit electric fuel pump http://www.summitracing.com/parts/sum-g3138/overview. I put the gauge after the fuel pump, and it pegged the gauge and held pressure. After the fuel pump is the Jaguar fuel filter. I replaced the fuel filter and put the gauge on after the fuel filter, and still pegged the gauge. After the fuel filter is a hard line to the engine compartment. I put the gauge on the engine end of the hard line, and still pegged the gauge. I have the stock fuel rails. I put the gauge on the Schrader valve, and got 20 psi. I previously replaced the fuel pressure regulator. I admit did buy the cheapest no-name regulator possible.
I don't mind buying a decent quality regulator if that will fix this issue. From searching here it looks like the stock regulator is best. Before I do, does anybody have a different suggestion? Basically everything in the fuel system is new except for the tank and the lines, but I still have low fuel pressure and a stumbling engine. Whenever I drain fuel while doing a pressure test the fuel looks clean, with no sediment. Also the fuel is fresh.
01-03-2017, 12:44 PM
#2
20PSI at the schrader is very low. 43.5 is what you want (vac line off/plugged)
Given your test sequence "after" FPR is where it drops that low it would appear the FPR is bad.....weird a new one albeit a no name brand still is the same but maybe bad out of box or just not the right part
a stock AC Delco unit should be fine or you could get a adjustable to see if for some reason your pre regulator FP is not as high from that EFP you have inline.
A plug & play adjustable that is a direct fit for stock one is BWD (Borg Warner) #23065. I got mine at local auto part store. If FP does not increase as you adjust FPR using gauge at Schrader....IDK why
You have a return fuel line to tank...right?
Given your test sequence "after" FPR is where it drops that low it would appear the FPR is bad.....weird a new one albeit a no name brand still is the same but maybe bad out of box or just not the right part
a stock AC Delco unit should be fine or you could get a adjustable to see if for some reason your pre regulator FP is not as high from that EFP you have inline.
A plug & play adjustable that is a direct fit for stock one is BWD (Borg Warner) #23065. I got mine at local auto part store. If FP does not increase as you adjust FPR using gauge at Schrader....IDK why
You have a return fuel line to tank...right?
01-03-2017, 02:08 PM
#4
^^
yeah I saw that to when looking at the specs on the summit link. it says 43 flow but the description also says 40..and free flow pressure is lower than a stock FP, by almost 25 PSI
That is why I suggested trying a adjustable FPR but it may not "adjust" enough to = FP & volume of a pump more = to the output of a stock LT1 fuel pump
yeah I saw that to when looking at the specs on the summit link. it says 43 flow but the description also says 40..and free flow pressure is lower than a stock FP, by almost 25 PSI
That is why I suggested trying a adjustable FPR but it may not "adjust" enough to = FP & volume of a pump more = to the output of a stock LT1 fuel pump
01-03-2017, 10:44 PM
#5
Thanks for al the inputs. I looked for a while for that adjustable FPR, but couldn't find one in stock anywhere. I ended up ordering the AC Delco from Amazon. If I still can't get 43.5 psi I'll look into changing the pump, or buying one of the more expensive adjustable FPRs.
Yes there is a return line. The Jaguar originally had an EFI 5.3L V12 in it, so it is set up for fuel injection. I had rubber lines made up with quick disconnects on one end, and hose clamped the other ends to the Jaguar steel feed and return lines.
I'll report back with results.
Yes there is a return line. The Jaguar originally had an EFI 5.3L V12 in it, so it is set up for fuel injection. I had rubber lines made up with quick disconnects on one end, and hose clamped the other ends to the Jaguar steel feed and return lines.
I'll report back with results.
01-04-2017, 10:15 AM
#6
I got mine at O'Rielys
http://www.oreillyauto.com/site/c/de...2494&ppt=C0024
Given the output of your inline FP, I don't think a stock, pre-set, FPR will resolve your low FP
An adjustable may it it "opens" up enough
The free flow rating of your inline pump is only 1.5 psi above optimum fuel pressure at the schrader (after FPR)
http://www.oreillyauto.com/site/c/de...2494&ppt=C0024
Given the output of your inline FP, I don't think a stock, pre-set, FPR will resolve your low FP
An adjustable may it it "opens" up enough
The free flow rating of your inline pump is only 1.5 psi above optimum fuel pressure at the schrader (after FPR)
Trending Topics
01-06-2017, 08:07 AM
#8
D'oh So the short answer is I'm an idiot: there was not enough fuel in the sump tank.
I don't like to keep a lot of fuel in the car while it's a project, because sometimes it sits for a while, and also I never know when I'm going to have to go into the fuel system. And also old wooden garage and fire. This explains why I had intermittent fuel pressure issues, because I would run the engine and pressure would be good, and then the car would sit for a while and I would run it again and the pressure would be bad, I guess because the fuel would evaporate. The tank has a hinged cap and the gasket looks old, so maybe I'm losing fuel that way.
With the adjustable FPR I got the pressure right at 43.5, but the pump is really (unacceptably) loud. With the stock FPR the pump is quiet, but I only get 30 psi. The engine runs fine, but this is probably not acceptable? Do I need a better pump? Can someone suggest something with about the same form factor of the Summit pump, to make mounting easy? How about this one: https://www.summitracing.com/parts/sum-251000
I don't like to keep a lot of fuel in the car while it's a project, because sometimes it sits for a while, and also I never know when I'm going to have to go into the fuel system. And also old wooden garage and fire. This explains why I had intermittent fuel pressure issues, because I would run the engine and pressure would be good, and then the car would sit for a while and I would run it again and the pressure would be bad, I guess because the fuel would evaporate. The tank has a hinged cap and the gasket looks old, so maybe I'm losing fuel that way.
With the adjustable FPR I got the pressure right at 43.5, but the pump is really (unacceptably) loud. With the stock FPR the pump is quiet, but I only get 30 psi. The engine runs fine, but this is probably not acceptable? Do I need a better pump? Can someone suggest something with about the same form factor of the Summit pump, to make mounting easy? How about this one: https://www.summitracing.com/parts/sum-251000
01-06-2017, 11:11 AM
#9
IDK if the summit brand is a re-boxed Walbro....but if not I would go with the Walbro
https://www.summitracing.com/parts/v...92bx/overview/
it puts out closer to what a stock pump would in terms of PSI
you can use the adjustable FPR to dial it in
https://www.summitracing.com/parts/v...92bx/overview/
it puts out closer to what a stock pump would in terms of PSI
you can use the adjustable FPR to dial it in
11-29-2017, 01:08 PM
#10
I'm reviving this thread because I just pulled the car out again today, and it's doing it again. I've been letting it sit because I'm mad at it, but I thought I'd give it another chance. It drove fine around town this morning, then I parked it for a while, and when I drove it again it started, and then began stumbling and stalling like before, like it has low fuel pressure. After I crank it for a while it fires back up like nothing every happened.
My thought is to install an electric fuel pressure gauge on the dash, so that when this happens I can know for sure if the fuel pressure is low. This is so intermittent that by the time I get out and pop the hood so I can see the underhood fuel pressure gauge it has fixed itself. Also I'm often in traffic, or can't easily get out of the car.
Any other suggestions?
My thought is to install an electric fuel pressure gauge on the dash, so that when this happens I can know for sure if the fuel pressure is low. This is so intermittent that by the time I get out and pop the hood so I can see the underhood fuel pressure gauge it has fixed itself. Also I'm often in traffic, or can't easily get out of the car.
Any other suggestions?