What should the WOT fuel pressure be on an Aerostar 4.0L? I'm getting about 38-39psi. The manual says the pump is rated at 16.9g/hr @ 45 psi. I can't find the actual pressure specification anywhere in the Ford service manual. They talk about using the fuel pressure test gage, but don't give any specifications on what the pressure should be.

 

With engine running: 30-45psi

With engine off, key on/off a few times to build up pressure: 35-45psi.

Maximum should be at WOT or with vacuum line removed from regulator.

 

under Service Fuel Sec 10-1 Fuel tanks, pumps, lines
Test A1
Check for adequate fuel supply.

l Key off.

l Install fuel pressure gauge.

l Install test lead to Fuel Pump (FP) lead of VIP test connector.



l Turn key to RUN position.

l Ground test lead to run fuel pump.

l Check if pressure is within acceptable limits, 240-310 kPa (35-45 psi).

A18 HIGH SPEED TEST


l With engine running at idle and vacuum line disconnected, note fuel rail pressure.

l Rapidly accelerate the engine and note the fuel pressure.

l Does the pressure remain within 5 psi of the starting pressure?

NOTE:
Road testing the vehicle while monitoring the pressure may give a better test under load conditions.

Yes
Fuel pump is OK.
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your WOT fuel pressure is in specs

 

10 psi (35-45 psi) is a large range for having the vacuum line removed from the regulator. I understand that the pressure would vary when the vacuum line is connected, because a certain pressure drop is always maintained across the injectors. With the engine at idle, the gage pressure is lowest. At WOT there is little or no vacuum, so the gage pressure is the highest. What I am looking for is the value of the constant pressure drop across the injectors. I've worked with GM systems and it was always 42.5 psi (Tuned Port Injection). It appears that this specification is 39 psi for this engine, but I have not seen a specific number spelled out anywhere.

 

39 is a good pressure. It does not need to be specific, because Ford's EEC-IV and EEC-V computer are more advanced than anything GM has even now. These computers have many hidden potentials that Ford never even used. Did you know that the EEC-IV can support up to 400 HP per liter?

 

Aerostar1:

All I can tell you is that the numbers I quoted came directly from the "fuel delivery troubleshooting" section of the Ford Service manual.

 

The amount of power generated is not the EEC-IV computer so much as the engine it's controlling. All the EEC is doing is reading sensors and activating actuators. The EEC's limitation is the speed of its CPU, which is an old Intel 8061 processor running at a mere 15 MHz. In typical street trim, it can handle running your engine at up to about 6300 rpm before it runs out of steam. But unencumbered of emission control duties, it has been used to control Ford's IMSA GTP engines that run at over 9000 rpm and produce many hundreds of HP.

 

I have always been impressed with Ford fuel injection versus GM. I took GM forever to catch up with Ford's tuned runner length, port fuel injection systems. I had several vehicles with GM's Throttle Body systems. They were an improvement over a carb, but not nearly as good as the tuned runner length, port fuel injection systems that Ford had. This was back in the 80's and 90's.

I don't mean to be particular about this fuel pressure, but it is important that it is correct to within 1-2 psi in the open loop modes. I have messed around with adjustable pressure regulators, and 1-2 psi makes a big difference. The injectors don't control the fuel flow regardless of pressure (like carbs), it is a function of BOTH the pulse width received from the ECM AND the fuel pressure. The ECM depends on the fuel pressure being a constant pressure drop across the injector at all times. I just want to know what is the constant pressure drop that the system is designed to operate with. It appears to be 39 psi, but I have not seen a specific value in the manual anywhere. In the open loop modes, if the pressure is running low, I could run the chance of the engine running lean. I just put new heads on this motor and I don’t want to burn the valves or anything.

 

Aerostar1:

I see why you are concerned. I have gone back to look at the manual more closely, but still couldn't find any thing that could help.

However, since the pump maxes out at 45psi, I doubt you are going to be running lean at 39psi in open loop. If any, my nose tells me that all my cars run a bit rich in open loop.

 

I think the variant in the fuel pressure at WOT is a result of increased voltage to the electric pump. The regulator works within a range and does not just cut off the pressure abruptly at a preset level. So, the ECM senses the fuel pressure increase at the 02 sensor (not 14.7) and corrects for it at the injector.

I do not -know- for certain, but, I -think- this is how it works knowing what sensors it uses. Never really thought about it before this post :-)

 

no chance of burning the valves in open loop. the ECU/PCM dumps in so much fuel that it's super rich, more than 10:1...once the engine is firing, the PCM will do anything to keep the engine running including increasing the injectors' pulse width until raw fuel will flow out the exhaust pipe...that's why Ford had to include an algorithim in the PCM for flooded engines, while cranking hold WOT, shuts off injectors and flushes engine with air only

i've never heard of burning the valves in open loop...if locked in open loop=worn cyl's and rings from oil wash down and possible damaged crank bearings and cam from oil dilution by fuel

with the hardened valves and seats we have now with the O2 controlled engines PCMs always spending part of their search closes loop cycle in lean burn, there is little chance of burned valves

a1 scribed " I don't mean to be particular about this fuel pressure, but it is important that it is correct to within 1-2 psi in the open loop modes."

not in Ford OBDI EEC-IV and OBDII EEC-V systems, they will fluctuate 5>10 psi while driving as fuel demand changes and vacuum changes

 

The fuel pressure regulator takes all the other variables (i.e. pump voltage) out of the equation. The fuel pressure is in no way set by the fuel pump. It is only set by the resistance that the fuel pressure regulator provides at the end of the pressurized fuel line. The fuel pressure regulator DOES have a preset exact base level that is the design pressure across the injectors. As the manifold pressure changes, the regulator senses the manifold pressure (vacuum) and adjusts the fuel pressure to maintain this exact pressure differential from the input end of the injector to the output end of the injector. Since the output end of the injector is subject to manifold vacuum, the fuel line pressure must change (decrease) to maintain the exact constant pressure drop that the injectors are designed to operate with.

The only way the fuel pump will control the pressure is if it is beginning to fail and can't keep up. If it can only produce 30 psi, then the (39 psi) pressure regulator will never be pushed open and the fuel line pressure will only be 30 psi. This is my current concern. Is my fuel pump in good enough condition to provide the flow and pressure necessary to maintain at least 39 psi at WOT. If not, the engine will run lean and burn valves. I have done a WOT road test with a fuel pressure gage attached to the fuel rail and it did maintain 39 psi. I think my fuel pump is fine based on this test.

 

Aerostar1,

The simple test of the fuel pressure regulator is to pull its vacuum line at idle, and the fuel pressure should go up to whatever the regulator is designed for. This should be around 40 psig, but I doubt 1-2 psi variation at idle will make much of a difference. As others have said, the EEC maintains a pretty rich mixture in open loop, mainly to keep the engine running while cold, and to help with cold driveability. And you can certainly smell it in the exhaust.

A better test of the fuel pump is to run WOT under load and watch the fuel pressure, as then the engine will use more fuel. If your pump is weak, it may not be able to maintain the required fuel flow rate and pressure under load. This can show up as knocking as well, but it's better for the engine if you can monitor the fuel pressure before knocking starts.

I'm not sure if they're still run this way, but the old EEC-2 and EEC-3 systems only applied full 12v to the fuel pump when cold starting and when the engine is under heavy load. The rest of the time it was more like 9 volts. So to test the switch over, we had to floor the gas pedal.

 

The open loop condition I'm concerned with is when the engine has already warmed up. Closed loop only kicks in at a cruise and maintains a 14.7 to 1 air fuel ratio. When you kick the engine down to pass or accelerate, it jumps out of closed loop and goes to a much richer mixture (about 11 to 1). This open loop fuel ratio is not determined by feed back from the 02 sensor. The only way it is created is through the preprogrammed pulse width tables in the ECM. If something is wrong, like low fuel pressure, the ECM has no way to know to compensate for it. 02 sensors only tell you when you are at 14.7 to 1. It would take a wideband 02 sensor to monitor the fuel ratio under full throttle. To my knowledge, they are not used in production vehicles.

My original question has been covered. I'm just continuing this for the sake of discussion.

 

Wideband O2 sensors are used in production vehicles. Almost all Nissans, Toyotas, Hondas and Ford Made after 2003 use wideband O2 sensors, also known as fuel/air sensors. Our trusty Aerostar will not support wideband O2 sensors, the computer would need to be reflashed. BTW, the computer does make its decisions based upon preprogrammed pulse width, but they are part of a chart that uses data from the MAF sensor. So while the computer cannot monitor the actual mixture, it is not totally blind. Also, it does still read the sensor, and tries to prevent an overly lean mixture. This is how lean codes are set. If the sensor reads a lean condition during WOT, it sets a code. So long as you are getting at least 38 psi under a similated WOT, you should be fine.