fuel pressure bleeding off causing hard start, help!

[Obin Robinson]

ok,

here's the deal. this problem didn't exist much during the summer because it was warm, now that it's cooler, it has returned.

here's all the details:

car is hard to start when cold. it WILL start if i stand on the gas a ton, crank it forever and ever, but the startup is really rough. lots of smoke and it runs really bad. but as the car warms up everything smooths out and it runs normal.

i installed a fuel rail pressure guage and i have found that the fuel rail is not holding pressure after a few hours.

KNOW that the ignition system is working properly, and it has been checked. so has the DME, the reference sensors, etc. all are new or verified as working fine. all relays, fuses, etc are fine.

the only thing is the low fuel pressure, it seems like it takes a few tries for the pressure to build up enough for the car to start. i figure that the warm starts were not a problem because cars don't require as much fuel to start when warm, only when cold.

my theory:

the fuel pump check valve is bad. i KNOW that the pump is original from 1984. the system is not holding pressure. the fuel pressure regulator and damper haven't been checked, how do i test them?

(also, if you're wondering where i've been, i've been working on my GTI race car )

thanks!

obin

[951carter]

Leaking injector(s).

It runs rough at first because the cylinder(s) is flooded.

Check to see if your oil smells like gas =0

[Makis]

Remove the spark plugs before you are about to start the engine and see if they are clean. If there is a lot of wet petrol there you know the injectors are leaking. This way also will identify which of the injectors is leaking. Also the temperature sensor can cause starting problems from cold.

[Scott R]

Thats a bad thing. I once had a leaky injector on a 4.0L Heep motor and it filled the chamber, I cranked it over and bent the connecting rod.

Fluid does not compress.

[macreel]

Obin... did you ever have cold start problems with your 924?
Hot start issue is usually loss of fuel pressure by leaking injec-
tors, fuel accumulator, lines, regulator, fuel pump check valve,etc

What you've briefly described sounds like that and is commonly
ascribed to the Cold Start Valve or associated pieces, eg Thermo
Time switch. 'Tho I don't believe you have one of those. Say, what
year wheels are we working here? Do you have the L-Jetronic?
It has a cold start valve in the fuel circuit.

I had classic cold start prob on '79 924 that I got in winter; didn't run her much 'til Spring. Hard starting lil machine. After chasing
the fuel system bits & pieces, I found the Thermo time switch plug
on but not making contact; no juice, no cold start valve working.

Reconnected and haven't had a bug for a year. Not that bug at
any rate. Just a couple others. Press, jiggle and tug all wires, etc.

G'luck
macreel

[Obin Robinson]

Originally posted by Makis
Remove the spark plugs before you are about to start the engine and see if they are clean. If there is a lot of wet petrol there you know the injectors are leaking. This way also will identify which of the injectors is leaking. Also the temperature sensor can cause starting problems from cold.

plugs are dry. also the oil doens't smell like gas.

so is this a dead fuel pump check valve? the pump IS almost 19 years old. also, i checked the temperature switch and it measures properly.

thanks!

obin

[ae1969]

Did you get pressure build up when you turned the ignition (cycling it)?

If the pressure builds...the pressure should hold. If the pressure drops quickly, there may be a leaking fuel injector, a bad fuel pressure regulator, or a bad check valve on the fuel pump.

Fuel regulators rarely fail, but if they do, it usually results in low pressure. I was told you can verify this by pinching off the fuel return line to see if the pressure rises.

You have ruled out the plugs.....and the fuel pump is next

[Makis]

Originally posted by Obin Robinson


, i checked the temperature switch and it measures properly.

obin

Do you mean temperature sensor? if so how did you check it.
It should be about 2-3KOhms cold engine and about 100-200 Ohms hot enginge. Also when you measure the resistance of the sensor make sure that there is no resistance between any of the sensor pins and a ground. If there is a resistance then the sensor must be replaced. A faulty sensor like that will definetelly cause starting problem. If you cannot measure the sensor resistance then you could attempt starting the engine by just disconnecting it. If the sensor is faulty the engine will almost certainly start.

Once your car is started and warmed up. does it have good power? If so then you can ruled out the fuel pressure regulator and the fuel pump.

[mjadair]

pull off the vacuum tubes to the dampner and the fuel pressure regulator. have someone crank the engine over. Make sure you don't have fuel leaking from these ports. The raw fuel ends up back into the intake but makes the car extremely hard to start.

[Obin Robinson]

Originally posted by Makis



Once your car is started and warmed up. does it have good power? If so then you can ruled out the fuel pressure regulator and the fuel pump.

yeah, once the car starts it feels ok, but it idles pretty low for a minute or two and i have to keep my fuel on the gas. could that be the fuel dampener?


also, won't a bad check valve in the fuel pump cause this pressure to bleed off after a few hours?

thanks!

obin

[Makis]

Originally posted by Obin Robinson

also, won't a bad check valve in the fuel pump cause this pressure to bleed off after a few hours?

I don't quite know about the the check valve in the fuel pump but your assumpion that there is a problem with the fuel line is quite possible for the symptoms you are having. The fuel lines somehow loose pressure but also get air in. So candidates are as follows:

1. Fuel pump check valve.
2. Fuel pressure regulator.
3. Dumper.
4. Injectors
5. Corroded leaking fuel lines.

I think 4 you checked OK. 2 & 3 are also OK but check to see when you remove the vacuum lines if you can smell any petrol. 5 should be quite easy to check. So you may be right it could be 1.

Just to make sure, you may try to pressurise the fuel lines before cranking the engine by setting the ignition on and bridging the Fuel pump relay. Once you have pressure in the system try to see if the engine starts OK. Also try to check if the fuel pump delivery is adequate. (1liter/30seconds I think)

Another reason for poor starting is the ECU not providing correct starting mixture for given engine and air temperature. That is why I suggested to check the coolant sensor. Also an engine that suffers from low compression may have problems starting.

[drew1]

It seems that a car is harder to start when warm with a bad check valve & it makes little difference when sitting overnight.


drew1