Intake & Exhaust Manifold Gasket Job

[Ben's Z]

I know I have a donut gasket leak on my 280z. This car runs like crap cold and isn't making any power. Previous owner did remove all the EGR stuff not sure if this is it. Have some smoke under hard accel, smells rich. I think I might have a vacuum leak causing it to run rich.

I was going to do an intake and exhaust manifold gasket. Any words of advice before I try this? Are the exhaust manifolds bolts going to be a bitch to remove?

[Zed Head]

The Zs don't have a donut gasket like chevy's do, they just have a plain old flat two port gasket at the exhaust header/manifold junction. A leaky gasket really shouldn't affect cold running or crappy power-making. Changing the intake/exhaust gasket probably won't help your problem.

If you do decide to do it anyway, you'll probably break a bolt or two on the thermostat housing and a stud or three on the intake/exhaust system. Search ez-out, tapping, broken stud, etc. before you start to know what you're heading for. From my own experience and what I've read, you will break at least one stud in the head and have to put extra effort in to getting it our before you'll be able to finish the job.

A vacuum leak would cause it to run lean. Extra resistance in your water temperature sensor circuit would cause it to run rich, one among several rich-running possibilities.

[Ben's Z]

Hmm, I thought that a intake leak would cause the ecu to fatten up the mixture causing it to run rich.

[Zed Head]

The ECU holds the injectors open longer as the AFM vane opens farther. With a vacuum leak less air flows past the AFM, the vane doesn't open as far, so the injector open time is shorter, giving a leaner mixture. The ECU also takes information from the temperature sensors and the tachometer. The Engine Fuel section in the FSM describes it all, with graphics.

Fuel pressure can affect the mixture also. Have you checked your fuel pressure and fuel pressure regulator? When mine went bad, the engine ran very rich.

[Ben's Z]

I have not checked either. I do own a fuel pressure gauge. Should I remove the filter and put in the my Tee fitting and check the pressure there? What should be the pressure?

The ECU holds the injectors open longer as the AFM vane opens farther. With a vacuum leak less air flows past the AFM, the vane doesn't open as far, so the injector open time is shorter, giving a leaner mixture. The ECU also takes information from the temperature sensors and the tachometer. The Engine Fuel section in the FSM describes it all, with graphics.

Fuel pressure can affect the mixture also. Have you checked your fuel pressure and fuel pressure regulator? When mine went bad, the engine ran very rich.

[FastWoman]

Ben, you need to measure the pressure with a T-fitting inserted between the fuel filter and the fuel rail. (Just add another little length of hose.) Normal pressure should be 36.3 psi with the vacuum hose removed from your fuel pressure regulator. Then with the vacuum hose reconnected, the pressure should drop about 7 psi to 29 psi (at idle). When you rev the engine hard, you should see the pressure surge to 36.3 and then drop into the low 20's as the RPM comes back down.

In addition to Zed's suggestions, a stuck cold start injector can also richen your mix.

BTW, I just finished installing a "permanent" gauge under my hood, fashioned from fittings from True Value hardware. I used a cheap, generic gauge and a brass T-fitting -- total cost about $12. The gauge and two 5/16" hose barbs both screwed into the T via 1/4" NPT threads. I came up from the fuel filter to the first barb at the base of the "T". Then one side of the T went to the fuel rail, and the other side went to the gauge. No brackets were necessary, as the gauge is small and is held nicely in place just by the hoses.