Exhaust stinks when throttle closed

[JeffGarcia77]

Hey everyone,

Every Z car (L-series engine) I've ever owned (and I've had a few, none with a new/rebuilt engine), has produced a bad stench from the tailpipe under the following conditions:

 

1. Throttle shut

2. Trans in gear

3. Moving down the road

 

And of course, as we all know, the Z will draw in exhaust through an open window. So under the above conditions, I can really tell a huge difference in the smell when the door windows are open. Makes me want to ralph!

 

My question is: WHAT'S causing this? Is it valve seals in most cases? My current '77 burns almost no oil (about 1 quart in 3000 mi.), considering the engine has 208,000+ miles on it. As far as I know, it's the original engine, and has not been rebuilt (no major engine work ever). The car has been in our family since 1985, so I know what's been done since then.

 

If it's valve seals, I can buy the spring compressor tool and replace the Datsun valve seals with the Ford part number listed here on Hybridz somewhere.

 

Let me know if you've experienced the same exhaust phenomenon, and what you have found the cure to be.

 

Jeff

[D1CNAR]

is it automatic

if it is there is a diaphram in the carb that is hooked to a vacum line to the tranny if it goes bad it sucks tranny fluid into your carbs prolly not thou

[ktm]

Jeff, try extending the tail pipe past the rear bumper. The aerodynamics of the Z create a low pressure area right behind the car and the exhaust is trapped in this area.

 

Other things you can do is to check the condition of your tail light gaskets, rear hatch gasket, vapor tank and fuel vapor hoses gaskets, etc. in the rear of the car. Check your rear hatch striker and make sure that the hatch is pressing down all around the gasket. Check your floor pan grommets/plugs as well.

 

This topic has been extensively discussed over at http://www.classiczcars.com

[JeffGarcia77]

Thanks for the ideas D1CNAR and ktm (Bo).

 

D1CNAR: My '77 has a manual transmission and stock fuel injection.

 

Bo, my car still has it's ORIGINAL exhaust system, as amazing as that sounds. So yes, the tailpipe terminates under the bumper. I need to do something about that. Also, there are a couple of holes in the bottom side of the muffler (where water tends to collect on short trips), and there is black soot accumualted around these holes. There is also a gap between the juncture of the muffler and the rear tailpipe, where it has rusted out. The gap goes for about 150 deg. of the circumference of the tailpipe, and is about 1/16th" wide, so it is a significant leak. FYI, I found a factory style rear exhaust replacement (from after the pre-muffler to the end) for $98. It includes the tail pipe, the muffler, and the rear tail pipe. It's made by Bosal and is sold by various retailers. Sure beats $221 for just an original muffler from Motorsport.

 

I have not checked my tail light gaskets yet, but I have resealed this area on other Zs, with great results.

 

I also need to do the old "petroleum jelly" test on the hatch gasket. What I do is apply a layer of Vasaline to the top of the gasket (the surface which touches the hatch), close the hatch normally, then open the hatch and check to see if there is Vasaline on the entire hatch contact surface. If not, there's a leak. Then it's either adjust the hatch catch and recheck, or buy a new gasket. Mine is the original gasket, by the way.

 

Actually, the reason I presented the original question was because I not only don't want to smell this weird odor, I am wondering what causes it and how to get rid of it, at the source. It seems D1CNAR was thinking along the right lines when he suggested that something besides gasoline was getting into the fuel system. That's what I was thinking too, when I asked about valve seals (something I can replace fairly cheaply). BTW, I did a compression test a couple of days ago (dry), and it showed 1. 118 2. 115 3. 115 4. 116 5. 115 6. 115 p.s.i.

 

I should try testing it wet, for information's sake.

 

What seems to work pretty well is keeping the windows closed and the fan pulling in fresh air. This seems to pressurize the cabin, keeping the nasty stuff out. But when the weather is hot, that's the hard way to go.

 

Jeff