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  #1  
Old 10-12-2022, 04:10 AM
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W108 4.5: Best way to test fuel pressure (hot start issue)

Evening all - I'm working on my '72 280 SEL 4.5 and the last remaining issue of note is a hot start issue. To keep it short, the car starts perfectly when cold, perfectly when hot provided you restart within minutes of shutdown but has a terrible time starting and running if you try to start hot after its sat for 10 - 15 minutes. It'll basically crank, try start, run in three cylinders and die. You can repeat this a few times, put your foot into it and slowly bring it around to normal - but its no fun.



It sounds like a vapor lock and or flooding issue to me - something like that. I already tried wrapping the fuel rails to keep the heat off - but no changes. So my next guess is perhaps a leaky injector(s) that is allowing both flooding and for vapor to build in the rail as fuel leaks out. I figure before I go tearing out injectors, its worth a diagnostic test and someone also suggested a bad fuel check valve might cause this - I guess fuel leaking back the other way, rather than out the injector.



SoI want to check the fuel pressure, but I am not sure of the most efficient / convenient way to do that on this engine. I have various fuel pressure gauges but would like to know the best place to tie in for a reading. Also - what pressures should I be looking for?


Thanks!

Allan.

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Old 10-12-2022, 01:03 PM
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test it at the fuel rail. You should be able to find a fitting on the fuel pressure test kit that can plug in at the rail and test pressure. If it's off, you need to check the fuel pressure to the rail
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Old 10-12-2022, 08:35 PM
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I ran a pressure test with the following results:


-Key On , engine off: Pump primes pressure to 20 PSI


- Key on, engine running: Pressure maintains at a steady 30 - 31 PSI


Upon shut down however, pressure immediately drops by half to 15 PSI, and within another 35 seconds or so reads essentially zero (probably 4 -5 PSI on the gauge).
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Old 10-12-2022, 08:52 PM
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Quote:
Originally Posted by Treozen View Post
Upon shut down however, pressure immediately drops by half to 15 PSI, and within another 35 seconds or so reads essentially zero (probably 4 -5 PSI on the gauge).
Consider this as a test:
1) Install a small ball valve in the return line downstream of the pressure regulator.
2) Install the pressure gauge.
3) With the aid of an assistant, simultaneously shut off the engine and close the valve.
4) Does the system hold pressure?

If yes, the regulator may be leaking.
If no, the check valve in the fuel pump may be leaking
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Old 10-12-2022, 09:47 PM
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I'm guessing that either the regulator of the dampener is failing.
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Old 10-13-2022, 01:23 AM
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Quote:
Originally Posted by Frank Reiner View Post
Consider this as a test:
1) Install a small ball valve in the return line downstream of the pressure regulator.
2) Install the pressure gauge.
3) With the aid of an assistant, simultaneously shut off the engine and close the valve.
4) Does the system hold pressure?

If yes, the regulator may be leaking.
If no, the check valve in the fuel pump may be leaking

I actually have a new regulator, so perhaps I should just install it. I bought it for my 300SEL but never got around to putting it in. If the check valve in the fuel pump is leaking, is that replaceable separate from the pump itself, and or can I simply install a stand alone check valve close to, but upstream of the pump?
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Old 10-13-2022, 12:20 PM
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Quote:
Originally Posted by Treozen View Post
If the check valve in the fuel pump is leaking, is that replaceable separate from the pump itself, and or can I simply install a stand alone check valve close to, but upstream of the pump?
If memory serves, the original D-jet fuel pump has an internal pressure relief valve, and an external check valve. In any event a check valve will be installed downstream (discharge/pressure side) of the pump.
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Old 10-15-2022, 12:01 AM
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Quote:
Originally Posted by Frank Reiner View Post
If memory serves, the original D-jet fuel pump has an internal pressure relief valve, and an external check valve. In any event a check valve will be installed downstream (discharge/pressure side) of the pump.

Yes - It looks like the check valve is where the banjo bolt / barbed line fitting connects - at least on the Bosch replacement pumps. I ordered a new one - not idea how old the one on the car is anyway. We'll see if that fixes it. Given how fast the pressure bleeds off, it doesn't seem like its leaky injectors.
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  #9  
Old 10-15-2022, 10:52 PM
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Quote:
Originally Posted by Treozen View Post
Yes - It looks like the check valve is where the banjo bolt / barbed line fitting connects - at least on the Bosch replacement pumps. I ordered a new one - not idea how old the one on the car is anyway. We'll see if that fixes it. Given how fast the pressure bleeds off, it doesn't seem like its leaky injectors.
The FSM explains how to test the fuel system. You are doing most of it. https://www.tonk.ca/Index/117.4/07.4MechElec.htm

Easy first step is to clamp the return hose closed after it leaves the damper which is downstream of the regulator. Turn the key on and off a couple of times and see if the pressure gets to 30psig and holds or still leaks down.
- If not, then the regulator is leaking.
- If it still leaks, it is either leaking back through the pump check valve. Or it is leaking through the cold start valve or injectors.
- You can clamp the hose to the cold start valve closed and see if the leak is there.
- If not, then with engine running and pressure at 30 psig, clamp the supply hose closed and turn engine off. If pressure does not drop, then you know it is the pump check valve.

You said you had a new Bosch pump? Is it a new style Djet pump like the one sold here? If so, the check valve is in the outlet nozzle. No reason not to add a second one (if you can find room) or to replace the existing one (if they are available) Hard to see a newish check valve failing though. https://www.pelicanparts.com/catalog/SuperCat/12140/MBZ_12140_FULFUL_pg2_fuel-pumps.htm

Good luck with it.
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Old 11-03-2022, 01:20 AM
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Why would a leaky check valve cause these symptoms? When the ignition is turned on the pump runs for a few seconds and that should re-pressurize the system. FWIW I had the same problem on a '84 volvo, but that car had K-jet injection. I can't remember now, but I don't think I was ever able to resolve the issue, but it was exactly the same, I had to keep it floored and then it would run on just a few cylinders for a while, then eventually clearing up.
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1972 280SEL 4.5, silver
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Old 11-03-2022, 02:11 PM
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When the check valve leaks, it allows the pressure in the rail to fall below the vapor pressure of the hot fuel in the rail. It all flashes to vapor and the brief 2 second run of the pump is not enough to flush all the vapor from the system. When you try to start it, no fuel to atomize...hard to start. You can test this by repeatedly turning the key to run, allowing the pump to cycle 2 seconds then turn the key off. If you do this enough times, you will flush out the vapor and it should start.
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Old 11-03-2022, 11:06 PM
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Makes sense, thank you for the explanation!

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