92RSFivePointSlow

I posted about this over the weekend, but I think it was too wordy and unclear, so I'm just going to keep it simple.

Does anyone know why the fuel pressure on a TBI won't drop below 15 psi? It used to. Now it doesn't.

The pump is original and I've had a JET adjustable regulator on it for years. It just seems to be "stuck" up at 15psi.

If you want more info then check out the original post
"Cant lower fuel pressure even with an AFPR...HELP... " I really need the help so please let me know your thoughts, even if it's just an idea you have.

PaulD

Only thing that comes to mind is a blockage in the return line.

92RSFivePointSlow

Paul, I was able to blow through the return line with my mouth, so it allows pressure to get back into the tank, but it did have a bit of resistance, do you know how to make sure it's completly clear?

MrDude_1

i would take a air hose, and blast some air power down that sucker.

then pull the TBI, yank the FPR off, and clean it with some carb cleaner... perhaps some varnish gunk is making it stick....

then put it back together and see what she does.

92RSFivePointSlow

I sprayed some carb cleaner down the return line, and blew 45psi through it, do you think I should use more pressure than that?
I took the regulator off and made sure it's spotless, and I also swapped the top cover, diaphram, and stock regulator from another vehicle and the pressure didn't drop, then I inspected the original diaphram and it seemed ok so I put the original top cover, diaphram, and JET regulator back on, making sure the little post that sticks out to keep it from spinning was at the lowest posible position once installed on the car.
So you guys are thinking the problem's still in the return line, eh? I have spent some serious time thinking about this thing and in my own opinion, that is the only thing it could be, but I don't know how to make sure it's 100% clear. Do you think there's anything else that could be the cause of this? I could imagine replacing the return line isn't fun.

ShiftyCapone

iTrader: (7)

If I remember from your ogional post you said that you have a fuel pressure gage on your feed line correct? If you are reading the pressure from there than the 15PSI is what it should read at any RPM. Adjusting the fuel presure will affect what goes into the TBI and not the line pressure. The point of the gage that you have is to make sure that your pump is keeping the necessary line pressure. Adjusting the regualtor will not change the readout on your gage no matter what you do.

92RSFivePointSlow

Shifty, I know you're trying to help given only the information I provide you with about my setup, and I appriciate your help. I'm not sure you are picturing my guage in the same place I have it. It is mounted in-line on the length of rubber hose by the driver side fenderwell that is between the hard-routed line from under the car and the hard routed line that goes along the valve cover. Where the guage is mounted, it most certainly does change when the regulator is adjusted. I have had the guage read as low as 8 psi when I had the regulator bottomed out, and I was able to bring the guage up to 15 psi by simply turning the bolt on the regulator, raising the post that sticks out of it. I'm not disputing that the location I chose for the guage may not be the ideal location for it, but it is in fact dependant on the adjustment of the regulator, and the gauge's readings were consistant.
But now I am unable to achieve the lower readings by making the same adjustments to the regulator that I did before. This is my problem.

ShiftyCapone

iTrader: (7)

Yea I see what you are saying. When you lower your fuel pressure via the regulator you will in esscence drop the line pressure because you are increasing the flow. The stock pump is so weak that you will see those kinds of changes. If you have stock heads and cam then your car will run with the regulator bottomed out. You will be running low PSI but high flow instead. If you had a walbro or TPI pump, you would see zero change on your gage because the pump is much stronger than stock.

Dewey316

not entirely true shifty, the FPR regulates the entire line to a set pressure, it does this by allowing flow past the regulator. it lets just enough by to mainain the desired presure in front of the regulator. where he has the guage is where it should it be. the only time he should see a pressure drop, is if his pump in not up to snuf. as to the holding 15psi, i really have no idea. if you have a FPR that is vacume effected, you might check that route.

MrDude_1

return line is all i can think of too man


and yea, 45PSI should be enough.....

92RSFivePointSlow

Yeah, I heard the TBI pump is real weak, and 175k on it doesnt help any. I'm pretty sure the diaphram is not vacuum operated, because there's the return line port behind it. I am about to get the hammer out and beat this thing into submission! All signs point to the return line but I thought it checked out ok.
Shifty, going back to what you were saying about fuel pumps, whats your opinion on me splicing in one of those in-line regulators just before the gauge, setting the JET one to the lowest setting, and setting the in-line one to about 11psi, and using the JET one for fine tuning? Do you think that would be a band-aid for getting the pressure to where it was for the sniffer test?

johnjm22

Why would you want your less than 15psi? I thought the more fuel pressure the more horespower, like say 46 psi.

Dewey316

TBI was designed to run at a lower pressure. more pressure does not equal more HP. proper air:fuel ratio = more power. you get the right AFR by getting your fuel right for you engine. tuning fuel pressure is just one part of the equation.

ShiftyCapone

iTrader: (7)

Yea that is my point. The stocker is so weak though that if you increase the flow demand with a low PSI or just have it wide open with a lot of power the stock unit will not maintain and you will get wiered FP readings if you do not know how your regulator is set-up or what condition your pump is in. It does regulate to a point its just that the stock pump is so bad it never exceeds those limits.

Edit: For got to mention about the in line pump. This is usualy not looked at as an answer because it is usually easier to just go with a high flow unit to start out with. Cheaper and easier in the long run. It is easy to just have an all out fuel pump and adjust the regulator and fuel tables accodingly. Than just let the rest go through the return line.

92RSFivePointSlow

what about using an in-line regulator, like those made by Holley for a carbed application? The setup would look like this:

stock fuel pump->inline fuel pressure regulator->inline fuel gauge->TBI assembly->JET AFPR->bleedoff goes to return line->back to fuel supply(if return is working)

steve8586iroc

92RS, I would double check the tbi between the regulator and the return line connection. This could also be an area for a retsriction and be a little harder to diagnose. Get some pipe cleaners and some carb cleaner then clean it as best as posible. I would not run two regulators in line with each other, seems it would be more of a restriction in fuel flow. The stock pump shouldn't be overpowering the regulator anyway.

Steve

92RSFivePointSlow

the part of the TBI that is between the regulator and the return line "looks" to me like it's contained in the meter cover, which I replaced with another one for comparison purposes. That didn't change anything, unfourtunatly. Good point about the flow restriction of another regulator, although I'd only need it for passing emissions, then I wouldn't mind the 15psi as much. What sucks is that I almost passed one day, then I went home and tried to lower the FP only to find it was stuck, and I went back and now all my results are double what they should be. This is why I'm thinking the problem is in the area of dirt/gasket material somehow getting somewhere that it shouldn't be.
Keep the ideas coming, eventually something's got to work.

ShiftyCapone

iTrader: (7)

I agree. The time when you say 7PSI could have been the pump acting up or something else. You should only see pressure that low when the engine is under load. Put the car in gear, hold the brake and give it a little gas. Have someone look at the gage. It will plumit.

You should be able to pass with the stock setting though. Is your car out of tune or have you done some serious mods where a drastic cut in fuel could help the smog ***** pass you?

steve8586iroc

There is a passage that goes between that top plate and the return line connection at the bottom of the injector housing base. It's just a thought but you never know. If you know the return line is clear from the tb to the tank then this is the only other place a restriction could be. The only other thing that comes to mind is a bad fp gauge. I don't remember if you stated that you did some data logging or not but if you haven't it might help to do so. This would show you if you are running really rich or not.

Steve

92RSFivePointSlow

well, most of my bolt-ons should not negatively affect emissions, in fact, I got better readings with the JET stage II chip than the stock one, even with the 160* thermo and fan temp sensor. When I passed inspection two years ago the only internal mods were 1.6 rockers. I did have to lower my fuel pressure then, but that was probably to compensate for a high mileage engine and a high-flow catalytic converter.
Since then, the only thing I added was an LT1 camshaft.
I came damn close to passing emissions two weeks ago, I passed two of the three tests, I think it was because of the high fuel pressure and the low engine temp. So thats when I messed with the fuel pressure regulator, as well as changed the air filter, dropped the timing down to -4*, new oxygen sensor, and put in a 195* stat. My fuel pressure went to hell after that, and so did my emissions readings. I blame myself for screwing something up with the fuel system, I just don't know what.

ShiftyCapone

iTrader: (7)

A little data logging and a custom chip will allow you to pass and to get the most out of that cam. You will have a hard time passing with a cam swap on the stock tune. That is your problem right there.

92RSFivePointSlow

Shifty I know my cam certainly doesn't help from an emissions standpoint, but it really bothers me that two weeks ago I was almost passed and then something in my fuel system took a dump. I would love to be able to get it to just be where it was when I almost passed, and then fine tune it. Dude if I get it to that point I could probably just ad a bottle of dry gas and call it a day. But I really don't want to turn this into an emissions thread at the moment. First things first, whats wrong with my poor car?!?!
lol, I hope everyone understands my humor here. I see passing emissions as a game that I always eventually win. I mean, if I take more then the "allowed" time are they going to suspend my registration like the papers say they will- No, I've never seen it happen. I just don't like driving around with a 195* thermo and a fan that comes on at 180* and stays on, so I'd like to pass and go back to my preferred settings. Anyone know a better way to clean a fuel return line?
Steve, is the passage you're talking about the one on the rear passenger side of the TBI and is the port that the "smaller" gasket goes around when you take the top cover off? That's the one I shot carb cleaner into and blew air through. I also blew through the port that connects to it while I had the top cover on the workbench. Maybe I should use some harsher chemicals or a higher pressure from my air compresser and do it again. I'm also going to buy a new gauge this weekend, or hopefully find a way to test my own. But it zeros, so I doubt thats the problem.

ShiftyCapone

iTrader: (7)

Well I would maybe back the timing out a bit, leave the regulator as low as it will go and put the stock air cleaner assembly back on. You may also want to get a bottle of that always pass alcohol stuff that seems to work like a charm.

steve8586iroc

I'm a little curious as to what you meant when you said you set your timing to -4*. Did you mean you took four degrees out of your base timing or did you set it four degrees after tdc? I also concur with Shifty about the Always Pass stuff you can get at the auto parts store.

Steve

92RSFivePointSlow

-4* after tdc. I normally like to run +8* advance but I've gotten better emissions with it at -4*.
I prefer to get the car running right before adding any "in tank help". Like I said before, this isn't just about passing emissions anymore, it's personal. I'm sure you all hate it too when your efforts to correct something seem to get you nowhere.

steve8586iroc

Still if you can pass emissions and get your registation and not have to add another worry to the list you will be that much ahead of the game. And yes that was the passage I was talking about.
One more thing I just thought of you might try. Take a peice of fuel line and connect the output or pressure line directly to the return line completely bypassing the tb and see if you are able to get any readings off the fp gauge, if so you know where you restiction is or is not. I would think it would show little or no pressure if everything is ok. Just a thought.

Steve

92RSFivePointSlow

damn good idea, and even if there's an obstruction, the fuel flowing through it might clear it out. I had similar thoughts with an air compressor- I turn down the pressure on the air tank to a low number like 10 psi, and seal the air nozzle against the return line and blow air through. While doing this, have someone slowly lower the pressure on the air tank and see if air stops blowing through and find out how much it's restricting. But I like your way better because it seems like it would flush the return line at the same time.

steve8586iroc

Hope it works for you.

Steve