With the FPR vacuum line connected, what should the fuel pressure be at WOT? I think my regulator is failing because I only have 30 PSI at WOT. My car is driving slow.

 

At WOT there is not a lot of vacuum. At idle there is because the butterflies are closed. So I don't think the vacuum line is a primary focus.

I think 38 PSI minimum is normal (this is for my 1987 - not sure about 1985). It could be that your pump can't keep up with the fuel usage. Do a fuel pump flow check by disconnecting your fuel line and seeing how long it takes to fill a container.

 

I replaced the fuel pump already. The car is bad pickup, if i floor it it doesn't even take off very fast.

The car never surpasses 30 PSI except before the car is started, fuel pressure is about 35 psi, after car is started it goes down

 

Before the car starts mine is 48 psi. 38 psi when running. Spikes up when revved. Sounds like FPR then. Higher psi generally means more hp potential as more fuel is available through the injectors.

 

Fuel pressure regulators operate opposite to the way many people would think. The LOWER the vacuum the HIGHER the fuel pressure.

GM pressures differ depending on the year of the car. Even some car manuals give different pressures as "normal". My 96 LT1 calls for 41-47 psi with the vacuum hose DISCONNECTED. It will be lower with the hose connected but by how much depends on how much vacuum the engine pulls.

As I recall it's on the order of 1 psi in FP reduction for each 2" of vacuum. You can do the math.

Some drive all the time with their FP vacuum hose disconnected and the vacuum port plugged in order to increase their FP.

Jake

 

I don't know the year of your car, but 30 psi is much too low. You've got a problem somewhere and you could easily burn a piston/valve, etc. from being too lean at WOT.

I've burned a dime size hole in the top of a forged piston from being too lean. You've got to get your problem addressed before it's too late.

Jake

 

The 85 uses 24# injectors vs. the 22# found in later years. They also run at a lower pressure. 30 psi is about right when idling. Do you have the original injectors?

 

My injectors are original, but i still think that my pressure is too low because I've tried everything else.

 

It sounds like a new FPR diaphragm is in order, or even a AFPR.

 

Just pulled the plenum off of the Superram (i'm getting good at this, its the third time i've done it in two months) and the inside of it was covered in gas.

I have a question, I have a Holley AFPR that I had on the car before but it leaked, does it used a stock diaphragm? the stock diaphragm has a spring connected to it where the Holley doesn't, is there a replacement i can get from Holley for this specific AFPR?

http://www.summitracing.com/parts/HLY-512-501/ This is the AFPR I have.

Otherwise I'd just buy a new AFPR.

 

I replaced the stock injectors on my 85 with Bosch III's supplied by FIC. My fuel pressure at idle is 31psi with the vacuum hose connected. I have to replace the fuel pump because it does not hold pressure with the car off (it drops to 10 in about 4 minutes). Per Jon at FIC, for the injectors he sold me and I installed, I need to be at 36psi with the vacuum hose. If you are running stock injectors, I believe you are OK with 30, perhaps someone else with a stock 85 can correct me if I am wrong.

With regards to your adjustable fuel pressure regulator, it looks like the kit includes it all. Most other AFPR's use a stock diaphragm. FIC recommends those sold by TPiS. The "stock" diaphragm for the 85 FPR is no longer available, it is not the same diaphragm for other years of the TPI however. If you can find a NOS one, it generally runs $150. If I was to use one of the "stock" replacements they currently have listed for my 85, I would have to use it in conjunction with the adjustable housing to get the correct pressure.

After I get my fuel pump done and I am sure she's holding pressure, I will swap out the stock FPR for an adjustable one to get the most out of the new Bocsh III's.

 

Quite a few years ago I sat and timed the fuel pressure drop rate on my 86 Vette because this issue of how long pressure should hold continued to come up. I posted the results here and there were some responses from members who did the same thing, but maybe not in the small time increments I used. IIRC, took me around 30-40 minutes as I sat there watching my ACCEL fuel pressure gage and wrist watch.

Anyway, the bottom line is there was no "standard" for how long or at what rate the pressure should decrease once the engine is shut down. I came way with the conclusion that as long as there was no sudden, dramatic drop in pressure , like going from, say 40 psi to 20 psi instantly, the drop rate didn't really matter.

On top of that, I've never come across any GM spec on that, or in any other manual for that matter. If anyone knows of one I'm sure we'd all like to see it.

Jake

 

I don't have a GM FSM for the 85, so I can't take it straight from the General's mouth but TPIS, in it's INSIDER HINTS book, has a chart of the GM recommended fuel pressures for years 1985 through the 1993 LT1.

It lists the GM recommended fuel pressure for the 1985 as 34-40 psi.

In 86/87 it went to 37-43; 88 thru 90 41-47; LT5 48-55; 91 thru 93 also 41-47.

Hope this helps.

Jake

 

If you have original injectors and your pressure is at 30 psi, I don't think your pump is the problem. You could have clogged injectors.

 

I have the TPiS hints book. I think the way they stated it was that those are the pressures TPiS recommends. Remember that part of their hints book was to sell their adjustable fuel pressure regulator. The 85 had larger injectors. Therefore, GM reduced the fuel pressure. Just like TPiS, GM found that by increasing the pressure, the fuel was better atomized. So in later years they reduced the injector size and increased the pressure. Better atomized fuel burns more efficiently which means the O2 sensor will see a leaner condition. The leaner condition increases the injector pulse width and is supposed to increase HP.

I installed the TPiS regulator on my 85 and I set my pressure to approx 40psi. I replaced the original injectors with Bosche III from FIC. I also run the SR intake. The increased pressure and better atomization from the Bosche III's did give me an increase in performance.

 

How would I tell if my injectors are clogged?

I don't think its a bad fuel pump, I replaced it less than a week ago. If I clamp my return line on the gas tank the pressure goes up to 60-65psi and holds.

I want to get new injectors but if I don't have to, then I won't.

Is the difference between 30psi and 40psi that big of a difference performance wise? My guess is that it is all the difference because before I tore into this whole SuperRam ordeal my pressure was fine and I was running the ASM big tube runners and all was well.

 

No, the chart specifically says "GM's Recommended Fuel Pressures"

Check Chart "A", page #1, lower right corner of the page, and again on PAGE #26, Chart "Q", lower left corner of the page. Both charts show exactly the same thing.

I try to be as accurate as I can when post stuff like this. Some guys shoot from the hip, but I try, as best I can, not to give out incorrect or unsupported information. But that's just me.

Also, I disagree, too with your conclusion as to the purpose, as to why that section in the book was written. Sorta cynical don't you think, especially when TPIS supported it with their dyno results?

I've been seeing this more and more lately, evidence of cynicism. Guys will read something then post that the reason the author wrote it is because the company paid them to or is a BIG advertiser, etc. That's text-book cynicism.

Jake

 

Jake, You're being a little harsh here. I don't think I was one bit cynical. I know you are a long time forum member so I won't say any more than that.

I happen to believe what TPiS published. To deny that they use their book to help sell parts is unrealistic. I don't have their book in front of me as you do, but as I remember they published dyno results for a stock 85 motor and one with an adjustable regulator. The increased fuel pressure produced more HP. I got the same results on my motor when I installed their AFPR and set it to 40psi.

Now back to the OP's original issue. I'm trying to convey to this young man who doesn't have a lot of money that the pressure he is seeing on the rail on his 85 is very close to the stock fixed pressure my 85 used to have. I doubt his performance issues are related to the fuel pressure at the rail. However, if he is still running the original injectors there is a good possibility they are part of the problem. When I removed my original injectors I found tips broken off. And I definitely got better performance from new ones.

Unfortunately, there is no way to test for clogged injectors. You can ohm test but that doesn't tell you if fuel is flowing.

 

I did not intend any personal attack, only to indicate my disagreement with your conclusion. I'm sorry if you took it in a different way.

It's basically because I've been reading such comments so often lately on all the 8 or 10 other Forums I regularly visit. Seems like an epidemic starting, attributing ulterior, less than honest, genuine motives to claims and representations made by the author. Perhaps if you had the same degree of exposure as I have you'd better see and understand my concerns.

"Remember that part of their hints book was to sell their adjustable fuel pressure regulator". I'll just leave it to others to decide for themselves if that is the purpose or not for the information and extensive testing included in the book.

I took it as a scientific, engineering approach to discover and share the results of altering fuel pressure, not as a marketing strategy. Here again each has to make his own call.

One guy posts it, unsupported. Then others pick it up and pass it along and before you'd know, it takes on the semblance of truth and accuracy. I believe this is how so much "bad" information gets spread around the web.

Anyway, that's enough on that, so let's just put it behind us and move on. I'm Good.

Jake

 

Some of the confusion is coming from the charts in the TPiS book. They are not consistent with the 85 FSM. The 85 is unique. I learned that from Jon at FIC. TPiS states that GM recommends 34-40 for the 85 but the 85 FSM troubleshooting chart shows that a pressure of 30-40 psi is acceptable. When my 85 was stock it ran at 30psi with no driveability issues. That is why I believe the OP needs to look elsewhere to solve his driveability problem. If he was looking for an inexpensive way to boost performance, I would recommend going to an adjustable reg and increasing the pressure as TPiS documented.