Fuel or Exhaust issue - not sure

[bhermes]

I have the FSM. What is the EFI bible (attachment above)? I have a multimeter and know how to use pretty well. I will get the other items. I will try the fume window test.

I really appreciate the help. I am extremely interested in learning how to do this work myslef.

This is what I get growing up a jock and never really ever working on cars.

[FastWoman]

It doesn't make sense to go replacing parts on these old cars without knowing whether they're bad or out of adjustment. Parts are in short supply, so trashing a perfectly good part that's not in aftermarket production would be a sin. The BCDD is not in aftermarket production, and many/most of them do not work. If you have a working one (which it sounds like), please enjoy it, and don't trash it! It merely needs some adjustment (see the FSM) for the idle to kick down a bit faster. Without the BCDD, the idle just plummets as the throttle slams shut, and the idle speed will overshoot, possibly causing stalling. The BCDD gives you a more refined drop in idle that prevents idle overshoot and smooths your shifting.

And if you don't want your BCDD, please send it to me! I'll gladly pay the shipping!

Your black plugs could mean either that you're running rich or running really lean (which can cause incomplete combustion or a partial misfire). You can distinguish between the two conditions by the character of the exhaust:

Rich: Sooty exhaust with a flow rate similar to that of a normally running engine. The fumes will burn your eyes a bit.

Extremely lean: Exhaust doesn't look as sooty. It blows hard, much faster than that of a normally running engine. There will be random misfires (hard exhaust puffs) that spit small droplets of sooty water.

FAIW, there seems to be an issue with the 77/78 (particularly 78?) ECU, such that it drifts leaner over the decades. This has happened to several '78 models on this forum, including mine. It's possible the '76 models share this problem. I believe the '75 system is made by Bosch, and the '77/'78 is made by Hitachi. I don't know about the '76, though. Anyway, if yours has drifted, there's a remedy for that.

Let's first find out whether you're running rich or lean:

Can you describe the exhaust?

What's your engine vacuum? (Do you have a gauge? They're cheap and very useful.)

If you pull off a small vacuum line, does your idle rise or drop? (You can also crack your oil filler cap, but just THE TINIEST bit).

What happens if you pull the vacuum line off your fuel pressure regulator and cover the end of the hose with your thumb?

--------------------------

As a general approach for getting these engines right, you should first look for the most obvious stuff that plagues most of our engines:

1. Rotted vacuum lines (You'd be smart to replace them all -- very easy)

2. Split boot between AFM and throttle body.

3. Other vacuum leaks (e.g. intake gasket). I've described a "yogurt cup" test that works pretty well.

4. Corroded electrical connections

5. Frozen breaker plate in distributor. (Remove cap and rotor, and try rotating the "guts" of the distributor CW by hand. Does it bind? Put a tube on the vacuum advance, and suck on it. Is it air tight?)

6. With the vacuum advance disconnected (and something inserted into the tube to plug the vacuum leak), check the timing. The specification in on a sticker on the underside of your hood. (On the '78, it's 10 deg BTDC, but many of us run a few degrees more advanced. Mine is at 13.)

7. Check/adjust your valve lash.

I've listed these in order of ease and importance. Fortunately the most import ones (listed first) are generally also the easiest.

Edited June 29, 2012 by FastWoman

[bhermes]

Wow. Thanks. I will go check out my exhaust. Just a reminder that I am a novice at best. Alot of the parts referecned it takes me a while to find them and figure out what parts everyone is talking about. I know that most of this is gerneral car knowledge which I have little of. I do learn quick and not afraid to try stuff. I never really thought about getting rid of good parts as you describerd. I was more interested in getting the car running great. Very good point and I will subscribe to this in the future.

I will check out the BCDD in the FSM.

Back with you guys soon.

[bhermes]

Just looked at the exhaust and really did not see anything worth mentioning. Seemed to have consistant flow. No burning of the eyes. No soot. I did notice before some small amounts of black water and there is some black residue in the tail pipe. Could not even see the smoke coming out of the tail pipe. Very little oder.

Should I need to run the car longer to check after the car heats up or does that matter?

[FastWoman]

Yes, the engine needs to warm up.

Try this: With your car warmed up and idling, hold a clean paper napkin behind the exhaust pipe for a couple of minutes. Does it get speckled with sooty little water droplets? If so, your mix might be too lean. Also, does the exhaust seem to be blowing rather hard, compared to a properly running car's exhaust? This, to me, is the most telling feature of a lean mixture. A lean-running engine will blow like the exhaust of a vacuum cleaner.

[bhermes]

OK, no sooty water droplets. However, not sure if this is a good comparison, but when compared to my 2000 Navigator the exhaust in my Z is blowing probably 3 times as hard as the Navigator.

[FastWoman]

It's a very good comparison. Based on the exhaust flow rate, it sounds like you're running lean, not rich. It might be there are no water droplets because it's so HOT. Perhaps in cooler weather your exhaust would be spitting.

The best way to diagnose your mixture is to remove the black plastic access cover from the side of your AFM and to lightly finger the counterweight to slightly open or close the AFM vane. Lightly opening it (by turning it counterclockwise) results in a richer mixture, and lightly closing it results in a leaner mixture. If your mixture is right, fingering in either direction will drop the engine RPMs. I would bet that if you finger it in the CCW direction (richer), your engine RPMs will pick up quite a lot, indicating that you're currently running quite lean.

[bhermes]

A couple question. I have not looked that closely to the black cover but how does it remove? Can I move the counterweight with the engine running?

Thanks.

[grantf]

cover is held on by silicone. Just jently pry it off. start the engine you can see it move on its own when you revv it.

At idle just jently move it to the left and right. If idle increases when you move it counter clockwise, you are lean.

[olzed]

You are probably sucking fumes into the cabin through the rear of the car. As Grant suggests in post 6.

I found mainly when a window is partly down.

[bhermes]

So I have played with the AFM counter weight a little this morning. In the current position I am idling around 1000 rpms. I move the weight about 2 screw driver widths CCW and the rpms increase to about 1100 rpms. I move the weight the same amount CW and the rpms reduce to about 900 rpms. I get simialr things happening at the exhaust. The car seems to run a little better moved a little CCW. Do the rpms idling at 1100 seem high?

I will look at doing this at higher rpms to see what happens.

[FastWoman]

Ok, you're definitely running a bit lean, then, at least at idle. And yes, 1100 is a bit high, but you would simply adjust it down with the idle adjustment screw on the top of the throttle body. That's not really an issue.

You might try a little experiment now. Pop open the little black plastic cover on your throttle position switch. You'll be able to see the contacts inside. When you open the throttle all the way, there will be two contacts that close together. Figure out which two pins those go to on the connector that connects to the TPS, and insert a wire to short between those two pins (i.e. with the connector disconnected). This will give you full throttle enrichment. Dial down your idle speed as needed. Now take your car for a short drive, and see how it feels. If it runs better, that would be further confirmation of a lean condition.

The most common cause of a lean-running condition is vacuum leaks (usually at least a few of them). Also carefully check the boot between the AFM and throttle for splits. Try my "yogurt cup test" to trouble-check the entire intake all at once.

http://www.classiczcars.com/forums/thread44365.html

... around post 6?