the flyin' scotsman

As posted before I fitted a fuel pressure guage to my otherwise stock fuel system and observed the WSM settings of 3.8 bar without vaccum..........so everything is good.


Now I've removed the stock FPR and installed a 928SP adjustable unit. I set the pressure at the recommended 4-6 psi above stock and alls good 'till I take it for a run get in nice and warm and the pressure falls.

So, should I adjsu to the higher fuel pressure with a warm engine OR is there an issue with the new adjustable regulator OR is there a vacuum issue that as I run the car for a while the vac reservoir fills and affects the fuel pressure?

Louie928

How is the cold/hot fuel pressure with no vacuum. That is with the vacuum line disconnected from the fuel pressure regulator. If you have the vacuum line connected, I believe there is a difference in vacuum between cold and hot and that will affect the fuel pressure.

the flyin' scotsman

Good point Louie but as you know to get at the vac line to the regulator means taking the air box completely off. Its fully assembled now and with the engine off (no vac) the guage still reads lower than where the AFPR was set.

The AFPR was set cold at 55psi idle/ 60psi engine off.

the flyin' scotsman

Another thought; the WSM states to check pressure with engine off the and the fuel pump jumpered...............but this is not what the instructions state with the AFPR. They call for 4-6psi above stock at idle with the vac line off. Another question may be should the vac line be plugged if adjusting to this method?

Louie928

Engine off and fuel pump jumpered should do the trick. I have no idea why the instructions would say to arbitrarily set the pressure to 4 - 6 psi higher than stock. Unless the MAF is way out of specs, S4s normally run very rich on the top end and there is no reason to make it richer. Plugging the vac line would be the "best" way, but I doubt that the small vacuum leak due to the hose being off would do anything to affect the fuel pressure.

BrianG

Malcolm, just get the LH unit in and the fault codes checked, and get your butt up here and hook up to the Shark Tuner! You might as well get the A/F map right, as set some arbitrary presssure and end up with some random fueling result!

the flyin' scotsman

Brian, I'm waiting for the LH to arrive through Louie. Assuming that it does before the weather turns I'll be right up; its only 12 hrs round trip.

I even have a chip to blast or we can save it; John Speake GT eprom!

Louie928

Your LH is on the way. Probably be here in less than a week. I'll need your old one....

the flyin' scotsman

Thx Louie..........pls send a PM when it arrives; I'll want to keep a hold of the 'old' one for another week hopefully; freezing weather forecast next week.

Back on topic; I have a email into 928 Tech and shall share the results ASAP.

the flyin' scotsman

Spoke w/DR today who advised setting the AFPR to the 4-6psi higher setting than stock (per the instructions) with the engine warm...........I did, 60psi with no vac.

So I go back out for another run come back and the pressure has dropped again. I realise this is not the most accurate way to set up the AFPR (DR gave other advice) but I would expect the pressure to have consistant readings.

Any thoughts?

For reference:

stock FPR gave 55psi with no vac (equates to the 3.8 bar WSM)
AFPR set to 60 psi no vac which drops to 55 psi engine running.
Pressure readings drop to 55psi engine off w/AFPR after run.

Same pressure guage, mounted on passenger fuel rail, no other changes.

PorKen

Any new insights on setting the fuel pressure with an AFPR, with the fuel pump relay jumpered?

Set cold or hot? I'm seeing the same drop in FP when hot with the Kirban Performance AFPR (KPP#4500, 928 Specialists, et al).

What happens with the stock 2.5 or 3.8 bar fixed regulators? Are they overpressure cold, or under hot?

PorKen

Oops! I found one issue that may be most of my problem.

I went to swap the gauge over to my '85, to check what a stock '87 S4 regulator (matching LH chip) puts out when cold. I noticed the gauge was reading 10 psi as I was carrying it! When I installed the glycerin filled gauge, the rubber stopper popped off and was lost on the first drive. (I guess you are supposed to poke a hole in it.) I covered the hole with a piece of metal duct tape, when the engine was hot. When I pealed back the tape now with a cold engine, the gauge went to zero.

Anyhu, with hole poked in the tape, fuel pump jumpered, engine off and cold, the S4 regulator registered 58 psi, within the range of the 3.8±0.2 bar spec (55±3 psi).

PorKen

Ahem. Never mind. Move along, move along...

Jumpered fuel pressure with the adjustable is the same now, cold and hot (63 psi in this case).

the flyin' scotsman

Ken..............looks like you've answered your own questions.

From when I first posted re. this topic I've found the FP to much better and steady since I did the intake R&R (including having the injectors cleaned/rebuilt) all new rubber fuel lines and new pressure valve.

PorKen

So far all the '87 S4 FPRs I've checked measure 58psi with the engine off, and fuel pump relay jumpered (elev. 500ft).

I found that the 3 psi difference makes almost a point AFR difference at WOT! EG. -3psi, 12.25 -> 13.25!


So if you are using an AFPR on a S4, and set it to 4psi over 'stock', then you may actually only be 1 psi over.




(Note: if you are using an AFPR with a LH2²r/55 chip, it should be reset to 58psi!)