Explain hot start versus cold start

Your engine computers inject more fuel to charge up your fuel rails so that you can crank your engine up when cold.

But when your engine is hot, your fuel pressure regulators (FPRs) maintain pressure in your fuel rails. Of course, over time this pressure slowly bleeds off.

When a fuel pressure regulator fails (or fuel non-return valve, or fuel injector), you won't maintain fuel pressure in your fuel rails while the engine is hot (but turned off). Plus, since your engine computers can tell that your engine is hot, they aren't adding the extra fuel needed to start your motor when the fuel rails have no pressure (i.e. a cold start).

That combination makes it difficult to start up your 348 when the motor is hot. But let the motor cool and then your engine computers will add the extra fuel for a cold start...so a 348 with a failed FPR will sometimes start up just fine when cold, but not when hot.



One free test for a bad FPR is to pull off the little black vacuum hose that goes to each one (348's have 2). Fuel in that vacuum hose indicates a bad FPR.

Easy to replace. Use the cheap generic FPR instead of the OEM Bosch unit (fewer failures in the clone).

 

In order to really make the proper test for a bad fuel pressure regulator, you need to hook a special gauge into the fuel line, and get a reading to see if the pressure drops once the engine has stopped and the fuel pump has stopped running. The FPR is supposed to maintain pressure in the rails - a bad FPR bleeds the pressure back into the tank. Would a bad FPR cause a hot start problem, but NOT a cold start problem?

I think you may have a flywheel problem. As stated many times before, do you have either of these symptoms:

1) A distinctive "rattling" sound from the engine compartment when the key is shut off and the engine stops?

2) Evidence of fluid leakage on the bottom of the clutch housing, through the perforated panel, at the very rear of the drivetrain, just in front of the license plate?

These are indications that the grease in the flywheel has become diluted and is being slung out, causing a flywheel imbalance that can prevent the engine from starting. It's worse when the engine is hot and the grease is thinner.

Edit: The relay installation is for when the engine fails to crank over on the starter when you turn the key. If the engine cranks over OK but still will not fire, then the relay won't help you.

 

I have EXACTLY these symptoms. Starting a major this week.... was planning to check the flywheel anyway (along with the transaxle, etc. etc.). Glad I saw your input!!!
Rick

 

If the engine won't crank over with the key on hot start attempts, then the relay modification should fix you right up. It gives full battery voltage through to the starter solenoid, instead of a drop in the voltage caused by excessive resistance in the ignition switch.

This isn't the same thing as the flywheel problem.

 

Yup. Not a fuel pressure regulator problem, either. The starter relay is the way to go, kudos to your mechanic.



...but, in the meantime before you get that installed, if you encounter a no crank situation (rather than an engine that will turn over but not catch/start), flip on your headlights briefly, then back off and then try your ignition key again.

Putting a brief load onto car batteries can sometimes prime them to give a bit extra juice on the next attempt. No need to try it more than once, though (i.e. don't run down the battery trying this trick multiple times). If the first "load attempt" doesn't work, then that's that.

 

Thats great to hear Eddy.

If the box is waterproof, then I wouldnt worry about it mate.