[s-cars] Hot Start: Episode III, Revenge of the Sbitch (longish)

[Vincent Frégeac]

Summary of the last episodes
The Patient: A '96 S-Bitch
The Symptoms: Do not start with warm engine, idle has a hard time to keep up
load (A/C or power steering).
The cure so far: New CPS, ISV marinated one night in carb cleaner then
pressure cleaned.
The results: None, as you cold expect from an S-Bitch.

After waiting 6 weeks to finally get my fuel tester on the third shipment
attempt, I was able to diagnose the fuel system, expecting that the tester
will be worth the wait. In a way, yes. It's an Equuus Pro Series Fuel
Injection Tester I got for 40$ on eBay and the nice feature is it includes a
12x1.5 banjo bolt adapter that fit perfectly on the front of the fuel
filter, so no need for a home made adapter. Air box removal help a lot for
the installation but can be reinstalled once the fuel pressure tester is in
place.

Once installed, I run the tests as per Bentley using a half-fuse for VAG
1348/3 A remote control, first on a cold engine:
- Fuel pump running: 58 psi (spec 58-61psi)
- Fuel pump stopped: 54 psi (spec 49-55psi)
- After 10 min.: 52 psi (spec no more than 7 psi drop)
- After 60 min.: 45 psi (no spec, I just run out of cigarettes. Yes, I know:
gas+cigarettes=BOOM)
Then I warmed up the engine until coolant and oil reach 90C and re-run the
tests:
- Fuel pump running: 57 psi (spec 58-61psi)
- Fuel pump stopped: 55 psi (spec 49-55psi)
- After 10 min.: 55 psi (spec no more than 7 psi drop)
The I runned a flow test:
- 470ml @ 12.45V;15 sec. (spec 700ml at 12.45V;15sec.)

So the check valve works even better when the engine is hot (with no reason
as it's a bit far from the engine) and the FPR does its job perfectly. The
fuel pump is on the low side but this should affect high rpms, not the
start.

Then I run a few non-Bentley test. I replugged the fuse 17 and tried to
start the engine to make sure it was in no-start condition. And it was,
pressure was at 57 psi but no start. First, I poured water from the fridge
(3-4C) on the fuel rail because I'm still suspecting some vapor problem due
to the 10% ethanol we have in summer gas. The pressure drop to 40 psi. Some
more water on the fuel filter made it drop to 25 psi. Then, the engine start
after 3-4 turns, as it does when it's cold. So, at least I know the problem
is related to gas temperature.

With the engine idling, the pressure is steady at 55 psi. At 3-4Krpm, it
drop to 50psi and stay steady there. When I release the gas pedal, it
doesn't raise as fast as the rpm drop, so the engine fall at 6-700rpm where
the pressure rise above 55 psi then it surge as usual after an rpm drop
below 700rpm. The most noticeable phenomena happens when I stop the engine.
The pressure fall at 40psi the rise slowly to 55psi in 30-45sec. I suspect
the pressure is rising on vapor as the fuel pump is stopped at this time.

But, at the end, I still have the same conclusions:
- FPR is OK.
- Check valve is OK.
- Fuel pump is on the low side but has no problem keeping up with the
2-300rpm of the starter as expected.
So what?

I know I could go the dealer way, replace every fuel related part until it
works but I already tried this way on a few other problems and each time it
just lightened the wallet without any improbvement of the patient condition.

So, if you had the courage and perseverance to read so far, any idea of a
logic-backed solution with these numbers?

TIA,






Vincent.
'96 S-Bitch silent when hot
'81 Fiat Spider noisy whatever the temp but at least running

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