78 SC Hot Start / WUR failure?

[DSH1]

Chronic hot start problem revealed itself once again today. Since full rebuild, I've been chasing idle and stall issues and thought I had it sorted. But a short, high speed run in the heat of the day and I couldn't get it re-started. Engine turned but not a hint of firing. Let it cool for three hours - still nothing. Had a spare accumulator and swapped it. No luck.

Solution found by disconnecting the vacuum line between the thermo valve and WUR. Cranked it and it fired briefly, then stalled. Re-connected the vacuum line and it started perfectly.

So is this likely an internal issue within the WUR, something upstream in the vacuum lines, or on the CP side of things?

Apologies if this is the billionth CIS posting regarding WURs and hot starts. I couldn't find one similar.

[T77911S]

removing the vac line to the WUR lowered the CP making it richer.

I would go thru and verify no air leaks, start with the intake bolts. they can loosen.
verify fuel pressures. you have 4.
system pressure
cold pressure
warm pressure with no vacuum
warm pressure with vacuum

fuel pump should run in the start mode. verify that it does
check valve at fuel pump.
I would also try removing the connector on the top rear of the AFM.
when hot and no start, turn the key to on and let the FP run for a few, then start.


sounds like it could be too lean.

[DSH1]

No air leaks for certain.
Fuel pump runs in start and check valve is good.

I do have the vac line to the decel valve disconnected and plugged (which fixed the stall issue) as I've got SSIs and no cat system. But the hot start was a problem both before and after this.

Disconnecting the AFM seems like it would get me out of a jam, but not sure the diagnostic value?
I'll get into the fuel pressure readings this weekend.
Many thanks -

[al lkosmal]

Difficult warm starting can sometimes be related to a faulty fuel pump check valve. When the engine is cold, the cold start valve will provide enough fuel to fire up the engine....but when warm, the cold start valve is not actuated and you are relying on residual fuel pressure/fuel to start....if the check valve is toast, there will be no/low fuel in the lines and you will have to wait while the fuel pump pressurizes the system and gets fuel to the fuel dizzy etc.

regards,
al

[kenikh]

To Al's point, there are two, one on the passenger side of the fuel head and one on the fuel pump. Either can be a culprit.

Sent from my Nexus 6P using Tapatalk

[ossiblue]

Originally Posted by DSH1

No air leaks for certain.
Fuel pump runs in start and check valve is good.

I do have the vac line to the decel valve disconnected and plugged (which fixed the stall issue) as I've got SSIs and no cat system. But the hot start was a problem both before and after this.

Disconnecting the AFM seems like it would get me out of a jam, but not sure the diagnostic value?
I'll get into the fuel pressure readings this weekend.
Many thanks -

Originally Posted by al lkosmal

Difficult warm starting can sometimes be related to a faulty fuel pump check valve. When the engine is cold, the cold start valve will provide enough fuel to fire up the engine....but when warm, the cold start valve is not actuated and you are relying on residual fuel pressure/fuel to start....if the check valve is toast, there will be no/low fuel in the lines and you will have to wait while the fuel pump pressurizes the system and gets fuel to the fuel dizzy etc.

regards,
al

Since you haven't done any fuel pressure readings, how can you determine that the check valve at the fuel pump is good? Further, there is also a check valve at the FD. Not trying to be picky, but when trying to diagnose CIS issues it is not a good idea to assume anything, especially when that "anything" is a component that could cause the problem.

Al has posted the consequences of a faulty check valve which would result in an issue like yours. Unless you have another way to verify the check valve that you haven't disclosed, it is worth your while not to consider it "good" until your pressure tests have been run.

We can only offer advice based on information received. It would help greatly if you could add information as to how you determined that there are no vacuum leaks and that the check valves are good.

[boyt911sc]

Originally Posted by DSH1

No air leaks for certain.
Fuel pump runs in start and check valve is good.

I do have the vac line to the decel valve disconnected and plugged (which fixed the stall issue) as I've got SSIs and no cat system. But the hot start was a problem both before and after this.

Disconnecting the AFM seems like it would get me out of a jam, but not sure the diagnostic value?
I'll get into the fuel pressure readings this weekend.
Many thanks -

DSH,

Have you done a residual fuel test? Or test the ignition signal when you were having a hot start problem? How did you know that the FP check valve was good if you did not have a gauge? Keep us posted.

Tony

[DSH1]

Pump and valve are both new and, though that doesn't guarantee functional, a flow rate test, downstream from the FD, checked out. I know pressure and flow aren't the same, but assume good flow rate rules out bad check valves?

Ignition signal is good and pump always active upon start position.

Can't help but feel this is a vacuum issue as a simple reconnect of the line to the WUR was the fix, but as all have noted, I need to get into pressure tests.
Thanks all -