'86 Z24s - Sensor light and running rich

[csliva]

Hi all, 

 

Just got a new (old) 1986 carbureted 720 with the Z24 engine. It's my first and only old car. It came with a couple of problems that may or may not be related. The engine was misfiring and had a loud tick so I pulled off the valve cover. One of the threaded studs was mangled and it allowed a rocker to walk its way up. I fixed that set the correct gap for the rest. It sounded better and some of the misfire went away, but the motor is still shaking.

 

I noted that the timing chain was slightly loose and the guides were worn down. 

I did a compression test to be safe, but luckily the PSI was even all the way around. I think that addresses all the typical problems I've seen mentioned on these engines.

 

What's got me lost is it's blowing white smoke out the tailpipe like it's running extremely rich. It has a sensor light on so I was guessing some kind of 02 sensor, or whatever computer controls the fuel ratio, since it isn't just a screw on the carburetor.

 

I replaced the spark plugs which were black from rich fuel. Not oily. Just a dark rich black. After running for 15 minutes or so I pulled out the new plug and it was immediately black. I'm curious about a couple of things.

 

* How do I manage the fuel ratio?

* How do I figure out what sensor needs to be replaced?

* Do you think the misfire is related to the fuel or the timing chain?

* Anything I'm missing since I'm new to this?

 

Thanks! Any help is massively appreciated. I love this truck and I want to drive it 100k miles.

 

[datzenmike]

Just to be sure one or two wingnuts holding the air filter on??? One is carb, two is TBI.

 

There is an electrical plug at the rear of the carb. If round and 6 wire, you have the ECC. (electronically controlled carb) When there is a failure it resorts to an over rich "limp home' mode. There are a variety of sensors for the ECC. But first is the temp gauge reading normally at about half way up the gauge. A cold engine will run richer by design. If not running at proper temp perhaps the thermostat is stuck open. There is also a coolant sensor for the ECC so it can calculate the mixture based on if the engine is cold or not. If it fails the ECC will be fooled into thinking the engine is too cold and force a rich mixture. The ECC carb is regulated by closed loop feedback from the O₂ sensor in the exhaust, when the engine is warmed up. Failure of the O₂ would probably default to the 'limp home' mode. 

 

It's cooler out and the exhaust is more noticeable. Is the coolant level always dropping? Is this a huge steamy exhaust and more than the other cars around you? Head gasket is possibly blown. Again, likely coolant is getting into the combustion chamber and you're seeing steam. A compression test might not show this unless blown between cylinders. The Z24 is well known for blowing the HG every 100K.

[csliva]

One wingnut on the air filter. 

 

Coolant level is consistently full so I'm not too worried about the gasket. It's much more white smoke than other cars on the road when starting cold. White Smoke dissipates after the engine reaches operating temperature. I should also mention the engine struggles to hunt for a good RPM on cold start. Idle starts slow, revs up to maybe 2500, then drops to normal after 5 minutes. It does eventually reach a good operating temperature with the gauge reading midway up. 

 

From your input, O₂ sensor or coolant sensor makes the most sense to me.