Runs Nice for a Few Min. Then Rough, Then Kills...

[OldSkoolFool]

Hey All,

My father and I have brought back to its former glory a 62k, prior one owner, 280Z 5 speed. We did some concours/close to concours bodywork/paintwork and are working on the finishing touches. One of those finishing touches is getting it running real nice. Here is some background:

-Sat for 20 years or so (prior owner passed away)

-Prior owner's brother this year drained the old fuel, put on new fuel filter, new fuel pump (not sure what brand), and put on a fuel pressure gauge

-Tank has at least 4+ gallons of good fuel in it right now.

So here is what happens: When we first start the car, it runs real sweet! Nice and smooth, revs up all the way real easy, etc. It will run real sweet for about 3 minutes the first time from cold. Fuel pressure shows about 10-20 psi (I am not familiar with what it should be at) right after the fuel filter. After about three minutes, it starts to run rougher, then even rougher, then to the point pushing the accelerator pedal causes some stalling, then to the point it pops/backfires through the intake, to the point it basically is not able to idle any longer. The mass airflow meter seems great. Ignition system seems great and adjusting the distributor not really affecting this.

So what I am wondering if a sensor for running when cold is working fine but when the engine warms up a little the air/fuel mixture becomes way off (lean?). So what should I do/look for?

Thanks,

Shane

[Zed Head]

Fuel pressure should be ~30 psi when idling, and 36 psi if you test without the engine running, but with power to the fuel pump. 10 - 20 psi is way too low. You may have other problems, and it may run rich if you fix the fuel pressure but it sounds like the engine might be getting by on low pressure when cold because the ECU adds fuel when cold. When the engine warms up the low fuel pressure makes the mixture too lean. Popping through the intake is a typical sign of that. Clogged fuel lines and inlet screen in the gas tank are common 280Z problems. It's also possible that the new filter is already clogged with rust from the tank.

Read the Engine Fuel chapter for a good description of how things should work.

Index of /FSM/280z

[Hodgimus-maximus]

It is definitely lean because your fuel pressure should be about 36 psi not 10-20. You should check to see if that fuel pump is bad. You should also check your fuel pressure regulator to make sure it's operating properly. Also check if you have a bunch of rust in your fuel tank that will clog your fuel filter and make the pressure drop.

Edited September 30, 2013 by Hodgimus-maximus

[Virto]

Sounds like tank rust for sure, clogging the pump inlet or the fuel filter/lines. Been there, done that.

Pop the bolt out of the bottom of the tank and see if you get any rust. Depending on how long the old gas was in the tank, it could be so thickly varnished that all the lines need to be replaced.

[SteveJ]

For more information on the fuel injection system, use the FSM (link below) and/or the fuel injection reference guide (aka Fuel Injection Bible) at XenonS130 - S130 Reference.

[Wade Nelson]

Try running it with the gas cap off and see if you have the same problem.

[240260280z]

Do you have fouled plugs?

if so then it is probably the Bosch L-Jetronic EFI electrical connections in the engine compartment that need a bit of cleaning. It is easy to do.

Try this... it can't hurt: http://www.atlanticz.ca/zclub/techtips/electricalconnections/index.html

The pigtails to the temp sensor up at the thermostat housing are usually the biggest culprit.

Edited October 1, 2013 by Blue

[superlen]

Shane,

All the others had great info, but start with your pressure first. You must have 36psi on the fuel rail at ALL times. Idle, part throttle, WOT, under load, loafing, ect. If you don't have 36psi on the rail then you'll never get it to run right. As others posted, you're going to have to check your fuel system starting at the tank and moving forward.

It's a smelly grungy annoying weekend.

Lenny

[Zed Head]

You must have 36psi on the fuel rail at ALL times. Idle, part throttle, WOT, under load, loafing, ect.

Lenny

Sorry Len, but that is not how the vacuum adjusted fuel pressure regulators (FPR's) used on the L-Jetronic system work. The hose that runs from the FPR to the intake manifold adjusts pressure to the FPR's internal diaphragm as manifold pressure drops (intake vacuum). So at high intake vacuum (low intake pressure) conditions the fuel pressure in the rail will drop. Typically to the 27 - 30 psi range. When the throttle is opened and manifold vacuum matches outside conditions, the pressure will then rise to ~36 psi.

Some of the older Jetronic systems (D-Jet maybe) apparently ran a constant pressure regulator with no vacuum adjustment. Maybe you're thinking of those systems.

[superlen]

Yes. You're right, the 36psi is always referenced to the inside pressure of the intake manifold.

[OldSkoolFool]

Gents,

Thanks for all of the replies and resources. I will be out working on the car again this weekend and will do what has been suggested, and will make sure to keep matches away from the garage! Where exactly on the tank is the "inlet screen" located? Also, I know which one is the fuel pressure regulator but how do I check if the pressure regulator is good? I have yet to dive deeper into the provided resources so apologize if those answers are in there. Read through my Hayne's manual prior to posting too and it didn't have logical suggestions like provided here.

I will post up what the issue was once determined. Just for fun, here's a pic from a couple weeks ago. Car is on the ground now, new emblems, etc. Just itching to take it on its first drive in 20 years (yes, all other fluids were changed & other recommended tasks).

Thanks for the help,

Shane

[superlen]

Car looks great. You'll need a new parking sign...unless you want to make your Z 4x4.

I'm not sure about a screen in the tank, but there is a small screen on the inlet to the fuel pump that most likely will be gunkified. Pull the fuel pump and look in the inlet and you should see it.

Lenny