Spark Plug question

[MEZZZ]

Hey guys, after taking a break from my car for the past few weeks, I am back at playing around with it. Last week I replaced the distributor with a rebuilt unit and still cannot get it to run quite right. When I start it cold, it sounds smooth and revs nicely. Once it warms up it degrades, the idle seems off and when I try to drive it it misses and I can hear an occasional backfire thru the air cleaner area. I could not get the timing set before, but now with the new distributor, I can dial it where it should be.

Too date, this is what I have done. Replaced, ignition coil, wires, distributor, cap, rotor, plugs and battery. Also put in new injectors and wires and fuel regulator. New air and fuel filter, all new vaccum and f.i. hoses.

Replaced the dual wires with the bullet connectors going to the thermotime and temp switch. Cleaned every connection I could find and tightened down every clamp as well, even putting some clamps on hoses that did not have them before. The A.F.M. boots are soft and crack free.

I have performed most of the test in the FSM and all have checked out, from voltage to the battery, fuel pressure, good spark to all plugs, juice to all injectors. I've run just about every test at the E.C.U. per the F.S.M. as well.

It seems like once it warms up, it runs rougher, doesnt idle as well. I can hear a bit of chatter from the valve cover and have not attempted to adjust the valves yet. I went out and picked up a new Fel-Pro gasket last night.

Here is my question...

I discovered a few months back that I have an N42 block with a P79 head on my '77. I bought plugs for a '77. I was out in the gargage this morning and all of a sudden it dawned on me that maybe the p79 head takes a different plug!

I came in and searched and it seems a '79 280ZX takes one type and an '82 280ZX takes another. Gap still seems to be .044 regardless of year. So my dilemna is this, is there a way to figure out what year head I have?

Im not sure if this is the problem or not, just trying to narrow it down. The car ran great when I got it, I drove it several days (maybe 100 miles total) before it started to run rough and miss.

Thanks

[MEZZZ]

I "googled" the p79 question to death and finally found a link that reads that the P79 head was available on the '81 to '83 year models of 280ZX. Can anyone verify the year that the p79 head was introduced?

In looking up what plug goes with what year, this would make sense. A '77 280Z and a '79 280ZX are listed as using the same plug.

I would like to make sure what year the P79 head came off of so I can get the correct plug for it.

Thanks

[Walter Moore]

Personally, I doubt that you need different plugs, but it does appear that the P79 head made it's appearance in 1981. See this link for more information:

http://datsunzgarage.com/heads/

I suspect that you need to adjust the valves. The backfiring with a hot engine sounds like the gap may be too small. But the plug question is interesting. Let us all know what you find out.

[MEZZZ]

Personally, I doubt that you need different plugs, but it does appear that the P79 head made it's appearance in 1981. See this link for more information:

http://datsunzgarage.com/heads/

I suspect that you need to adjust the valves. The backfiring with a hot engine sounds like the gap may be too small. But the plug question is interesting. Let us all know what you find out.

Thanks Walter, your assumptions are correct about the plugs. I went out and bought a new set that the P79 head calls for. Gapped them, installed them, started it up and she ran like a sewing machine when cold. Smooth, no hesitation. I let it warm up, adjusted the idle and turned it off. Then I pulled the vacuum line off the distributor, plugged it, set the timing to 10 BTDC.

Started it up, backed down the drive, put it in gear and sputter, cough again.

I guess my next step is to pull the valve cover and take a look at adjusting the valves. Thanks for the help.

[sblake01]

You say you've cleaned all of the connections. Have you checked the functionality of the temperature sensor? If it has a bad connection or the sensor itself is bad it will cause the car to run rough when it warms up because the fuel mix is still where it should be for a cold engine. As the engine warms, the temp sensor allows the fuel mix to start out richer and gradually lean to the point it shoud be for proper running at operating temp.

[MEZZZ]

You say you've cleaned all of the connections. Have you checked the functionality of the temperature sensor? If it has a bad connection or the sensor itself is bad it will cause the car to run rough when it warms up because the fuel mix is still where it should be for a cold engine. As the engine warms, the temp sensor allows the fuel mix to start out richer and gradually lean to the point it shoud be for proper running at operating temp.

Hi Steven, I pulled this from a post in which you were helping me back in August....

I pulled out the FSM and ran a few test. Used a multimeter on the water temp sensor, cold 2.25 ohms, ran motor 10 minutes, retested, .085. Checked the thermotime switch, it was 74.8, between the guidlines in the FSM.

I was thinking the same thing earlier, that it may be the water temp sensor.

I have the smoking issue solved, it no longer is smoking at all. I replaced the wires going to the temp sensor and thermotime switch as they were pretty well deteriorated. Not sure if these numbers make sense to me now. I pulled out the F.I. Bible and went to page 54 for the water temp sensor check and my numbers do not seem to coincide.

Checked page EF54 of the FSM and it seems more in line.

[sblake01]

Your thermotime reading is within spec but your wts reading is high at .85 which would make it run rich. At full operating temp. the reading at the water temp switchshould be in the .29-.36 range. The charts in the EFI Book and the FSM are the same except the EFI book gives the reading in ohms instead of k/ohms. And as far as those lower readings on the charts.....if the coolant temp is -30 or -10 degrees, you can throw those out. I don't know about you but I'm not working on the car if it's that cold!

Edited November 3, 2008 by sblake01

[MEZZZ]

Your thermotime reading is within spec but your wts reading is high at .85 which would make it run rich. At full operating temp. the reading at the water temp switchshould be in the .29-.36 range. The charts in the EFI Book and the FSM are the same except the EFI book gives the reading in ohms instead of k/ohms. And as far as those lower readings on the charts.....if the coolant temp is -30 or -10 degrees, you can throw those out. I don't know about you but I'm not working on the car if it's that cold!

, I dont think I would be either. Its usually too hot or too cold to work on it but today was perfect...65 and sunny here in Cincy, not bad for Nov 2nd.

I will go ahead and replace the water temp switch and see if that corrects it. Its an inexpensive part. As always, thanks for your help.