At 02:40 PM 8/5/02, Abe Reinhartz wrote:
>This might be a stupid question....but....sometimes the 928 GTS has a lot of problem starting when hot. Anybody have any ideas what this is? Once it lights, it's fine.
There are several possibilities. Starting with the most likely - If an injector, pressure regulator, or pressure dampener leaks into the intake system, the added fuel will cause the fuel/air ratio to be too rich, causing hot-starting problems. If the hot engine starts more easily with the throttle half-way down, this is the primary suspect.
You can pull the vacuum line from the pressure regulator and see if there is any trace of fuel in the regulator port or the line, and then do the same for both pressure dampeners. Any trace of fuel in the line is cause for replacement.
If the problem is a leaking injector, Techron or another good fuel system cleaner might help. Remember that cleaners do more good on daily, short-trip driving than they do on a road trip. If the injector doesn't clear up in a month or two, you might want to pull all of the injectors and have them cleaned.
There is another fuel-related possibility - there is a check valve in the end of the fuel pump that is supposed to hold system pressure to prevent fuel boiling in the lines while the engine is hot, and to make cold starting more positive. If this valve leaks, the pressure loss will allow hot fuel to boil in the injection lines, causing a hard-start condition similar to vapor lock. Only cure is replacement - not too expensive (we sell the check valve for $15.87 plus shipping) and not too difficult a task (except that the fuel pump/filter fittings are usually really tight).
Some folks have installed a fuel pressure gauge in the end of the fuel rail to check on such things. The gauge (VDO, either 0-60 psi or 0-100 psi) is $29.95 plus shipping.
Call Jeannie at (828) 766-9280 if you need anything.
Wally Plumley
928 Specialists
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Vapor lock is usually used to describe problems with carbureted cars whose
fuel line pressure is only 3-6 lbs or so and often have mechanical engine
driven fuel pumps . The vapor recovery charcoal canister and fuel tank one
way valve all deal with capturing fuel vapors from the fuel tank in the back
and have nothing to do with any fuel "vapor lock" on the engine but to
let
the fuel tank inhale but not exhale . Very rarely there is a problem with
the one way valve and the fuel tank is collapsed by the vacuum that is
created as fuel is consumed . In addition the way a 928 engine is supplied
fuel is by using a pump which moves more fuel than the engine can ever use .
The excess is bypassed and returned to the fuel tank on the top of the tank
into the fuel level sending unit . The pressure of the fuel pump is at 30-50
lbs which increases the boiling point of the fuel considerably .
All 1980 and new engines (USA) the excess fuel is returned after the fuel rails by the fuel pressure regulator so the fuel rails are being cooled by the excess fuel from the tank or the gas in the tank is being heated depending on how you look at it . The fuel line for models 84 and newer passes through a fuel chiller mounted on the firewall at the back of the engine which uses the cold A/C refrigerant to cool the fuel , assumes the A/C is working . With this as background information it is difficult to for me to see "vapor lock" being much of an issue with the 1980 or newer 928 because as soon as the pump turns on large amounts of fuel rush toward the engine as the fuel pressure regulator is blowing off the excess pressure . This flood of fuel would displace any vapor / or cool it to the point it liquefies . Wally has already commented on the flooding which sometimes occurs with leaking injectors requiring that the foot pedal be depressed to get enough air into the engine for the fuel to be able to burn .
Jim Bailey
928 International