Fuel pressure too high at idle, causes?

[onnaj]

Hello,

I have the problem that with a cold engine my engine hestitates a bit and with a warm engine i hear backfire in the intake box.

Because of that i've mounted a fuel pressure meter and checked the fuel pressure. At idle with the vacuum hose disconnected it idles at about 40 psi, which is perfect (at least that's what's in the manual) . When connect the vacuum hose again the fuel pressure is about 35 psi, which is too high. The manual says it should be around 28 psi. I've changed the fuel pressure regulator for another one but that's also too high, it reads about 33-34 psi. With that other fuel pressure regulator the backfires are even worse.

Any ideas on causes?

How long should it take for the fuel pressure to drop to zero after shutting down the car? Maybe that way i can check for leaking injectors or so? I have no problems though with starting the car. Only sometimes after a long quiet drive i have brown/black smoke when stepping on the gas.

Thanks!

[jondee86]

AFAIK the 20V uses a standard 1:1 rate vacuum compensated FPR to maintain
a constant pressure differential between the fuel rail and the inlet manifold. That
is to say, with the engine off and the pump running, the rail pressure should be in
the range of 34-40 psig. With the vacuum compensation hose disconnected and
the engine idling, the rail pressure should also be in the range 34-40psig (no change).
But with the compensation hose connected and the engine idling, the rail pressure
should drop by an amount equal to the vacuum existing in the intake manifold
(measured as psig below atmospheric).

Now I don't know what manifold vacuum a BT 20V 4AGE idles at, but being a multi-
throttle engine, I'd guess around 40-50kPa(absolute), so lets say 45kPa(absolute)
or 55kPa(gauge) which is 8psig of vacuum. Thus, if your rail pressure was on an
average value of 37psig without compensation, it would drop to 29psig when the
compensation hose was connected and the engine idling. If you had 40psig in the
rail, you would get 32psig.

So if you only get a 5psig drop in rail pressure, the chances are that you do not have
a lot of manifold vacuum. And if you don't have a lot of manifold vacuum, the engine
will be running rich, and you will get a bit of unburned fuel in the exhaust.

Cheers... jondee86

[onnaj]

How do i exactly measure the vacuum? Just grab a gauge and connect it with the vacuum hose that goes to the map sensor?

Thanks

[jondee86]

What I did was buy an old 100 dia industrial vacuum gauge, and
adapt the fitting to accept the same size hose as connects to the
MAP sensor. Then I went to a garden centre and bought a small
3-way barbed "Tee" connector that fitted the hose. These fittings
are usually found on garden irrigation systems.

Then I took another short length of vacuum hose from a spare
MAP sensor and hooked the hoses up so that the vacuum gauge
was tee'd into the map sensor hose. That way the engine runs
as usual. and you are reading exactly the same vacuum as the
MAP sensor. Let the engine idle and read the gauge

I'd like to know what you get, as I have not been able to figure
out any easy way of increasing the manifold vacuum on my 16V
engine with ST ITB's. Currently, fully warmed up it idles at 78kPa
(abs) or a bit less than 3.5psig of vacuum. That is not enough
vacuum to get a usable MAP sensor signal, so I have to run the
engine on a TPS only fuel map.

Cheers... jondee86

[yoshimitsuspeed]

Onnaj do you have a stock fuel pump or aftermarket?

Yes you just hook a vacuum gauge up to one of the ports that recievs full vacuum.

Jondee
Do you have bigger cams?
Manifold pressure is set by how much air needs to go into the motor to keep it idling. You cannot increase vacuum without the idle dropping. The only exception could be if the ignition timing was so far off it required a lot more air to keep it running.
I have a very hard time believing the manifold is only seeing 3.5PSIG. The motor would be racing like crazy. Is it possible you are using a port that only sees partial vacuum or something?

[onnaj]

Onnaj do you have a stock fuel pump or aftermarket?

Yes you just hook a vacuum gauge up to one of the ports that recievs full vacuum.

Jondee
Do you have bigger cams?
Manifold pressure is set by how much air needs to go into the motor to keep it idling. You cannot increase vacuum without the idle dropping. The only exception could be if the ignition timing was so far off it required a lot more air to keep it running.
I have a very hard time believing the manifold is only seeing 3.5PSIG. The motor would be racing like crazy. Is it possible you are using a port that only sees partial vacuum or something?

I have a stock Celica fuel pump (it's also used in the stock 3SGE and beams as well, so enough power to feed the blacktop )

[yoshimitsuspeed]

Onnaj do you have a stock fuel pump or aftermarket?

Yes you just hook a vacuum gauge up to one of the ports that recievs full vacuum.

Jondee
Do you have bigger cams?
Manifold pressure is set by how much air needs to go into the motor to keep it idling. You cannot increase vacuum without the idle dropping. The only exception could be if the ignition timing was so far off it required a lot more air to keep it running.
I have a very hard time believing the manifold is only seeing 3.5PSIG. The motor would be racing like crazy. Is it possible you are using a port that only sees partial vacuum or something?

I have a stock Celica fuel pump (it's also used in the stock 3SGE and beams as well, so enough power to feed the blacktop )

I was more concerned that you might have too large of a fuel pump.
Many people switch to the Walbro 255 which is known to overwhelm the stock FPR and cause high fuel pressure at idle.
Pretty sure that should not be an issue with the Celica pump though.

[jondee86]

Jondee
Do you have bigger cams?

272/268 running 100/100 lobe centres. MAP reading tapped off all four
manifold runners with small accummulator and pulsation damper in the
MAP sensor line. Reading is rock steady and fast responding. Running the
ST ISCV feeding separately into each intake runner.

I can get a bit more vacuum if I go to 110/110 lobe centres. It seems
that ITB's just inherently have lower vacuum at idle than a single throttle
setup... but I have never seen any scientific explanation why ?

Cheers... jondee86

[onnaj]

At last i had time to check the vacuum (with a boost pressure gauge . Don't know how accurate it is, but on my brothers 4agze he's giving the exact ammount of pressure as with his new guage.

I measured about -0.63 bar (-63Kpa / 18.6 InHg) with a cold and half warm engine. As soon as i give some throttle the vacuum rises to about 0.00 which is normal i guess?

Hope you guys can help me out here. Otherwise the only things i can imagine to do is either replacing the injectors or get them cleaned or replace the o2 sensor.

[jondee86]

Nominal fuel pressure at idle, hose off 4A-GE (AE101, AE111)... 235-275 kPa
Nominal fuel pressure at idle, hose on 4A-GE (AE101, AE111)...196 kPa

Your pressure at idle with hose off is 275 kPa
Your manifold vacuum at idle is (-)63 kPa
Your pressure at idle, hose on should be 275-63=212 kPa
But you measure around 235 kPa

So your rail pressure is higher than recommended. Under closed loop conditions
the ECU should be able to compensate for some variation in rail pressure by
adjusting the injector open time. Maybe it can't cope with 40 kPa variation ?

I'd suggest cleaning/refreshing the O2 sensor first. They can usually be given
a new lease of life by heating with a butane torch. You will find the procedure
on the interweb

Cheers... jondee86

[yoshimitsuspeed]

Though I don't have experience with the 20v in my experience with other 4A ECUs they can handle about 15% changes in fuel flow rate without issue. 40 KPA or 5 PSI should fall within that.

onnaj my BT pulls about 17 in/HG at 6000 feet so I'd say you are pretty normal.

You are right about giving it gas. As you open the throttle the vacuum drops. It doesn't take much throttle at idle to get it pretty close to zero.

[onnaj]

Yesterday i remeasured it with a completely warmed up engine. Now it was almost 70 Kpa. When throttling it goes too almost zero and when releasing the throttle it falls a bit below 70 kpa and then directly back to almost 70. So that seems to be perfect

Still have that fuel pressure being a bit too high at idle. So next week i'm going to try to mount the new o2 sensor and otherwise i'm going to test and clean my injectors somewhere. Just have to find out where i can exactly do this. Most companies don't have the filters in stock and i don't know where to get those