280z runs rough and loses power after driving for 10-15 minutes

[Darksteel]

So I have a bizarre problem and I'm not sure what to check. Let me start at the beginning.

 

I was driving my 1976 280z and noticed the temp gage all the way to the right so I pull over soon as I can. When I look under the hood there is coolant squirting out a hole in the front of the radiator. After I tow the car home, I drain the oil and it doesn't look like any coolant has gotten in. I drain the coolant and it doesn't look there is any oil in the coolant. Then I install the 3 row radiator from MSA and add fresh coolant and oil. (I accidentally put in too much oil, so the level is at the left "leg" of the H.)

 

I have only driven it a couple of times since then, because it has been acting strange. Here are my observations:

 

-When I start the car cold, it runs fine

-After driving for 10-15 minutes, it starts running rough and loses power

      -Here is what I mean by running rough:

            -The exhaust makes a "chugging" noise; puffs of air come out of the exhaust

            -The car shakes a lot more that it has before

-When I shut the car off, and start it a short time (30-60 mins) after that, it will still run rough

 

It seems like it runs rough only after it has reached operating temperature, but I don't know for sure.

 

Here is what I have done:

 

-Checked the ignition to make sure the spark plugs are firing. They are firing.

-Checked the vaccum advance on the distributor. It still advances the timing.

-Checked the compression in each cylinder. The compressions were decent, 160-175 for all cylinders.

      -When I checked this, I noticed that all of the spark plugs have wet oil around the outside ring of the spark plug.

 

I haven't check the fuel pressure yet.

 

At this point I'm not really sure what is wrong. Anyone have any ideas of what could be causing this? Any help would be apprciated.

 

[Zed Head]

Did the engine get wet from the coolant that came from the radiator?  Did you jostle any wires when you were changing the radiator?  Or the hoses?  

 

Running good when cold, then bad when warm is a sign of a disconnected coolant temperature sensor.  Check that first, maybe you'll be lucky.  Make sure you didn't swap it with the thermotime switch.

 

Download the Fuel Injection Guide and run the tests at the ECU connector if you don't find an easy fix.

[Darksteel]

Oh, I forgot to mention that. I did check the water temp sensor from the ECU plug; although I only checked it when it was intalled in the car with ambient air temperature. The resistance reading corresponded to the ambient air temperature from the EFI manual. I'll go through the rest of the EFI manual next.

 

Could the overheating have damaged the water temp sensor such that it reads incorrectly at higher temperatures? When the car is cold the water temp sensor reads correctly, but when the coolant reaches a certain temperature, the sensor isn't reading it correctly? Just a thought I had when reading your comment.

Edited October 12, 2015 by Darksteel

[siteunseen]

I think you should clean all your connections and ground points.  It's explained here, http://atlanticz.ca/zclub/techtips/electricalconnections/index.html

[Chickenman]

 

-When I checked this, I noticed that all of the spark plugs have wet oil around the outside ring of the spark plug.

 

That's not a good thing and likely due to overfilling the oil pan as you mentioned in your first post.

 

Overfilling the oil pan can cause more issues than people realize. Can cause extreme amounts of oil to be throw on the cylinder bores, over whelming the oil rings and causing deposits on plugs. Just like you have.

 

First things I would do is drain some oil out to the proper level. And put a fresh set of plugs in. New plugs are cheap. 

 

In fact, maybe pull a couple of plugs and take a picture of the oily firing ends. Use a good High Res digital Camera with the flash turned off. Flash on changes the color saturation of the picture.

 

Post the pictures up here so we can get a good look at them.

 

BTW, overheating would not have damaged the Temperature sensor.

 

Cleaning all connections and ground points as Cliff mentioned is always a good idea on these old cars.

Edited October 12, 2015 by Chickenman

[Darksteel]

Did the engine get wet from the coolant that came from the radiator?

 

I forgot to respond to this, but I'm not sure what you mean; could you explain it a little more?

 

I have already checked a lot of the electrical connections under the hood, but I'll go through them again.

 

Hmmm, I didn't know overfilling the oil pan could cause those problems. I'll drain the oil soon. And I'll take some pictures of the spark plugs tonight when I get home from work.

[Zed Head]

I forgot to respond to this, but I'm not sure what you mean; could you explain it a little more?

Coolant is conductive.  If you sprayed it all over the engine's electrical parts you might have some new electrical problems.

[Chickenman]

hmmm, I didn't know overfilling the oil pan could cause those problems. I'll drain the oil soon. And I'll take some pictures of the spark plugs tonight when I get home from work.

What happens is that the crank can dip into the oil, scooping it up and throwing too much against the cylinder walls. It will also aerate the oil, turning it into a froth. Frothy oil contains a lot of air bubbles and air is not a good lubricant at all!! Very bad for motor, especially the bottom end bearings.

[Darksteel]

I have attached the pictures of the oily spark plugs (they are a pretty nasty). Those are from cylinder 1 and 2 but the rest look about the same.

 

I also my have found the problem... I randomly unplugged the AFM plug to check the contacts and noticed it was a little loose. After looking at the contacts, the socket that goes to pin 27 on the AFM was pushed down inside the plastic housing. So I'm not sure if it have been making contact. Pin 27 is part of the air temp sensor circuit. I think the plug was jostled when I removed the air filter to get to the last bolt holding the old radiator on.

 

We'll see how it runs after I take care of the last few things.

Edited October 13, 2015 by Darksteel

[Chickenman]

Yep..toss those plugs in the Garbage. Unless you have a old abrasive Plug blaster those will never clean up. Fixing Pin #27 and draining some oil will hopefully fix the issue.

[madkaw]

You need to look at the possibility that the head gasket has been compromised by overheating .

So now your temps are steady when running?

Might borrow or use a pressure tester that mounts on the radiator. Put it on and run the car and see if pressure rises too high. Start pulling spark plug wires while pressure is rising and see if pressure drops.

[Darksteel]

You need to look at the possibility that the head gasket has been compromised by overheating .

So now your temps are steady when running?

Might borrow or use a pressure tester that mounts on the radiator. Put it on and run the car and see if pressure rises too high. Start pulling spark plug wires while pressure is rising and see if pressure drops.

I thought checking the compression would show if the gasket is good?

 

Yeah, the temp has been steady and the same as before it overheated during the few times I have driven it.