Warm start diagnostics

prebordao · 05-01-2011, 11:05 AM

For diagnosing warm start issues, what is the pressure drop that should be measured - system or control pressure ??
And with a cold or warm engine ?

Thx

don gilbert · don gilbert's Garage

you measure "residual press" or "at rest press." just after engine shut down. System needs to hold press. for at least 15 min. for next warm start. Loss of press. means hard starts, and fuel vapor locking in your cis lines, esp. if they are metal. usually a check valve or bad accum. is the prob. bently book has exact specs you need.

prebordao · 05-01-2011, 12:43 PM

But should I measure system press or control press ?

ossiblue · 04:23 PM

Follow Don's advice. The drop in pressure that is important for warm start issues is residual pressure, though it is possible warm control pressure is a factor but it would show up during running.

There are four pressures that are important: system pressure, cold control pressure, warm control pressure and residual pressure. You should know all four for a functioning system. However, you have a warm start issue and we assume your system pressure is in spec. Most likely it is loss residual pressure that is your problem--check that first.

You might follow this plan when setting up for the first time:

Check system pressure. This tells you if the pump is suspect or if a blockage is present.

Check residual pressure. This tells you if the system holds pressure as it should for warm starts.

Set warm control pressure. This is the regulated pressure for a warm, running engine. It must be set when the wur is fully warmed and the engine is fully warmed. It is adjusted by moving the round housing for the diaphragm up or down in the wur body.

Set cold control pressure. Must be checked/set when the engine is stone cold (ambient temp.) and done after warm pressures have been set.

prebordao · 05-04-2011, 04:24 AM

Ok, Yesterday I finally got to measure pressures, and this is what I got:

System - 4.8
Cold control - 1.2

Then I started the engine. After about 5 mins the control pressure hadn't yet started to rise, so I guess the heating element is kaputt.
As I was in some hurry, I didn't wait for the engine to get warm, so I still don't have the warm control pressure.

I switched off and watched the control pressure. In the first 10 secs it dropped a little, but in the next 30 secs it went almost all the way to zero... So no residual pressure.

I guess it explains my problems with warm starts. In the recent past I changed the fuel accum, pump check valve and there are no obvious leaks, so what should I look for now ?

prebordao · 05-04-2011, 06:07 AM

bump

ossiblue · 06:54 AM

First, double check your connections on the gauge set. Next, retest the accumulator and check valve by isolating them and see if they hold pressure--just to be sure they are good and eliminate them as suspects.

Have you tried shutting the valve to the wur and seeing if the system holds pressure? Does it? If not, there may be a leak in one or more of your injectors or even the CSV.

If the residual pressure holds when shutting the valve to the wur, then you know the problem is after the fuel distributor/injectors. Your 74 has a throttle positioner branching off before the wur, and that could be leaking but I'm not familiar with that component. It's possible that that is leaking. If you can cap off the line to the positioner, or hook your gauge set up between the wur and the T to the positioner and retest, that will confirm if there is a leak there.

If the positioner checks out, that leaves only the wur. The 74 wur is not repairable so if yours is stock, that may be the end of your hunt. Later models have internal O-rings that can be replaced but I believe you cannot access the internals of the early 73/74 wurs.

prebordao · 05-04-2011, 07:22 AM

I'll try to follow those steps. Besides all those possible causes what about the pressure regulator inside the FD ?
I've read in a post that dirt inside it may prevent it from totally closing, thus letting fuel return. Does it make sense ?

ossiblue · 05-04-2011, 07:34 AM

Yes, that is a possibility also and would show up in the test which isolates the wur from the FD.

prebordao · 05-04-2011, 10:37 AM

Just did a test with the valve to the Wur shut. When the pump stopped, system press dropped to 3 bars, and after 15mins was at 2.1
So maybe the circuit is ok until the Wur ?

ossiblue · 05-04-2011, 04:18 PM

Quote:

Originally Posted by prebordao

Just did a test with the valve to the Wur shut. When the pump stopped, system press dropped to 3 bars, and after 15mins was at 2.1
So maybe the circuit is ok until the Wur ?

That would be my assumption also as everything was pressurized, and remained so, up to the valve on the gauge set. Do you still have the original 74 wur?

prebordao · 05-05-2011, 01:14 AM

Yes, it's still the old one. Type 0 438 140 001
I don't see any leaks there, but...

ossiblue · 05-05-2011, 06:42 AM

Quote:

Originally Posted by prebordao

Yes, it's still the old one. Type 0 438 140 001
I don't see any leaks there, but...

I've never seen the inside of the 73/74 wur but it likely has a similar configuration as the later ones, thus there may be seals inside that allow pressure to escape. You would not necessarily see any outside evidence of a leak. I suspect the pressure is escaping through the return line.

prebordao · 05-05-2011, 08:01 AM

i've seen the inside of both and the difference is mainly in the bimetal and heating assembly. The diaphragm appears very similar.
I can check for a leak here by disconnecting its return line and dropping it into a bottle.

prebordao · 05-05-2011, 12:14 PM

Also, if I understand correctly, on a leak-less system, with a warm engine, the source for the slowly decreasing pressure after shut-off is the gradual cooling of the WUR, and its consequent decrease in pressure... right ?