Fuel Pressure Readings

[mickey_vista 0]

Hi, I'm new on the forum and this is my very first post!

 

My problem is that after hooking up a fuel pressure gauge between the fuel filter and fuel rail on my 1991 Gti 1.6 I'm not sure how to interpret the readings. The car is completely standard spec. apart from the OEM fuel pressure regulator having been replaced by an FSE adjustable one. The idle speed can go up and down a bit, especially when the engine is hot, and the car is prone to the occasional 'kangaroo' at low engine speeds, but otherwise it drives pretty well and I really just wanted to check that the fuel pressure was set correctly. Now I'm not sure if I have a fueling problem or not.

 

This is what I'm getting on the fuel pressure gauge:

 

First off, there is no fuel pressure at all until the engine is cranked. Just switching on the ignition does nothing and it takes several turns of the engine before it fires, presumably because of the lack of fuel at the injectors. Is this normal or indicative of a fault, maybe in the fuel pump relay?

 

With the engine idling at 950 rpm, and the vacuum pipe connected between the inlet manifold and regulator, the pressure gauge reading is 3 Bar, which is what the manual says it should be. However, as the engine speed is increased the pressure gradually drops until it has fallen to 2.5 Bar by 3000 rpm.

 

With the vacuum pipe disconnected the reading goes up to 3.5 Bar at idle. After adjusting the regulator to bring the pressure back down to 3 Bar and then reconnecting the vacuum pipe, the pressure immediately drops to 2.3 Bar and doesn't go any higher or lower no matter what the engine speed is.

 

Does this mean that the correct fuel pressure is not being maintained at higher engine speeds? The fuel lines are all in good condition with no signs of any leaks, the fuel filter was renewed less than 5000 miles ago and the FSE fuel regulator is fairly new with no signs of leakage. If the pressure readings indicate a problem should I suspect a weak fuel pump or is there something else which could be the cause?

 

I know that fueling issues, including faulty fuel pumps, have been discussed in previous posts but I can't find anything which directly answers my questions. I hope someone can either put my mind at rest or help me to pinpoint where the problem(s) lies.

 

Cheers, Mike

[Simes 248 2 Cars]

Welcome to the forum.

What an informative first post!!

 

These engines hunt on idle, they did when new and also kangaroo, this just happens to be a trait of the XU unit allied to the Jetronic.

Sounds to me like it is running as intended.

 

 

If your fuel pressure gauge is accurate then keep the FSE valve, although it needs to be set right to avoid over fueling. Otherwise get an OE one.

 

FSE they were all the rage in 2001!

Edited January 13, 2012 by Simes

[mickey_vista 0]

Welcome to the forum.

What an informative first post!!

 

These engines hunt on idle, they did when new and also kangaroo, this just happens to be a trait of the XU unit allied to the Jetronic.

Sounds to me like it is running as intended.

 

 

If your fuel pressure gauge is accurate then keep the FSE valve, although it needs to be set right to avoid over fueling. Otherwise get an OE one.

 

FSE they were all the rage in 2001!

 

Thanks, Simon. I know a lot of people have reported the idling and kangarooing problems on this forum but I didn't realise they were inherent even when the cars were new. I'd assumed it was the result of aging components and could be cured. I guess replacing the whole induction/injection/engine management system with something more modern would do it but then the car wouldn't be original any more. I've seen a number of Gtis which have been 'modded' in this way but to my mind it detracts from their status as the iconic hot hatch of the eighties.

 

As regards setting the FSE valve correctly, and assuming my gauge is accurate (also an FSE item), should I do this with the vacuum pipe connected or disconnected? The difference between the two readings seems quite large.

[kyepan 291]

I would say more interestingly, what happens to the pressure when you run it then switch off. Reason I ask, is my old pump had a faulty non return valve, and as a result would not maintain pressure in the rail.. This led to long cranking times. From what I recall, vac advance connected you should see a solid three bar of pressure.

 

Good that your taking an analytical approach, welcome to the forum.

 

Cheers

 

J

[petert 554]

The fuel pump doesn't start until the motor is actually spinning.

 

Fuel pressure is relative to the manifold vacuum. So if you had a 3 Bar regulator and you had 0.5 Bar of manifold vacuum at idle, the pressure should be 2.5 Bar. At wide open throttle, when there is no vacuum, you will have 3 Bar.

 

Thus the fuel pressure could vary from 2.5 Bar to 3.0 Bar depending on the manifold vacuum/load.

 

So disconnecting the vacuum tube at idle should see an increase in fuel pressure.

Edited January 14, 2012 by petert

[welshpug 1,626]

a well setup Jetronic system with good condition components will not hunt, nor will it kangaroo (badly...)

 

personally I'd ditch the FSE and fit a standard FPR, you're pretty close to Baz so he may be able to sort you out one

 

as Peter said, it should be 3 bar with no vacuum, dropping further as more vacuum is applied.

Edited January 14, 2012 by welshpug

[mickey_vista 0]

I would say more interestingly, what happens to the pressure when you run it then switch off. Reason I ask, is my old pump had a faulty non return valve, and as a result would not maintain pressure in the rail.. This led to long cranking times. From what I recall, vac advance connected you should see a solid three bar of pressure.

 

Good that your taking an analytical approach, welcome to the forum.

 

Cheers

 

J

 

Hi J, thanks for the welcome.

 

On switching the engine off the pressure starts to drop, quite quickly at first then more slowly. It takes about 15 minutes for it to drop back to zero.

 

If I set the pressure at 3 Bar with the vacuum pipe DISCONNECTED it drops to 2.3 Bar at idle when I reconnect the pipe. The pressure stays at about that level as the engine is revved, but as the gauge is spliced into the fuel line next to the rail I can't see it from the driver's seat to take readings out on the road under real driving conditions.

[bensonmi 4 2 Cars]

my fuel pump comes on when ignition turned on, i take it the tachy relay is at fault there then!

[welshpug 1,626]

only if it stays on, most do "prime" for a few seconds.

[bensonmi 4 2 Cars]

it stays on, could be something to do with it being full of water once!

[mickey_vista 0]

The fuel pump doesn't start until the motor is actually spinning.

 

Fuel pressure is relative to the manifold vacuum. So if you had a 3 Bar regulator and you had 0.5 Bar of manifold vacuum at idle, the pressure should be 2.5 Bar. At wide open throttle, when there is no vacuum, you will have 3 Bar.

 

Thus the fuel pressure could vary from 2.5 Bar to 3.0 Bar depending on the manifold vacuum/load.

 

So disconnecting the vacuum tube at idle should see an increase in fuel pressure.

 

Thanks for this clarification. I will set the pressure at 3 Bar with the vacuum pipe DISCONNECTED and see how the car behaves under real driving conditions. Unfortunately, I can't see the fuel pressure gauge from the driver's seat, but presumably any problems arising from insufficient fuel pressure (e.g. due to a weak fuel pump or faulty return valve) will manifest themselves in terms of poor throttle response and/or flat spots.