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1976 280Z Replacement ECU Issues

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Just joined. My first post. I have a 76 280Z with manual transmission and have owned it for about 24 years. a year ago I ran into a leaking injector hose (original) which has cascaded into a replacement intake manifold, new injectors, all new wire ends, a couple new sensors, etc.. When completed it sounded fine but smoked like a crop duster from the exhaust and there was a lot of carbon buildup. I've gone through every test for every component as shown in the bible and the FSM and now it sounds great and I'm getting a lot less smoke though it's still not right. At this point, as it's also clear it's been running somewhat rich for a long time, I figured it would be worth trying a different ECU to see if it would make a difference. I have the A11-600-000 with only pins at 26, 27 and 30-35 on the lower row. I got one on Ebay, pulled from a working car, with the same part number though it had all 35 pins. I installed it and the car would crank and crank and not start. I put the original back in and the car started right up. I then figured I'd have better odds to try one from a good parts company and got a remanufactured one, boxed as compatible with the same part number, same 35 pins, same car cranks and nothing. I put the original back in and it starts right up. I did take the case off the Ebay one and there's only solder connections on the same pins as my original. The others are all just dummies. Obviously, I don't want to mess with the other if I plan to return it. My hope for expert advice, as this is the best forum I've seen on FI, is am I overlooking something, are there some real year to year to year ECU issues, or did I just get a couple that aren't going to run in this car? If that's the case is running through a few to get a good one not uncommon? Thanks very much in advance. I appreciate it.


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I tried all of the years frm 75 to 78 on my 76 and they all started and ran.  The 78 is the only one that caused problems, it died after about 15 minutes.  I think it was just a bad ECU though, not the model of ECU.  Where did you get the reman?  

I'd be more inclined to look at other things for a rich conditon, than the ECU.  Corroded connections on the coolant temperature sensor for example.  You said that things tested well but didn't say if you just did continuity checks or measured actual resistance.  

Are there any other modifications to the car?  Ignition system, tachometer, etc.  The ECU uses the pulse on Pin 1 from the coil to determine when to open the injectors.  I had a problem where the ECU would not work when the tachometer was removed with the stock igntion.  That was on two different cars.  But it did work with a different igntion setup later.  The ECU seemed sensitive to that Pin 1 voltage.

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1 minute ago, Patcon said:

If you're trying to deal with an over rich condition why don't you add resistance on the water temp wiring?

It's the other way Mr. Patcon.  More R = more fuel.  That's why I asked about the actual numbers.  Maybe he has some dirty connections on the coolant temp circuit.

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Thank you for your responses. Just an update and more detail about my situation.

I ran the FSM ECU test on the two new ECUs. I got a noid light response on the ECU I got from EBay, then used it to start the car and it ran fine. I let the car warm up to operating temp with no smoke and thought it had solved my issue. No luck, the smoke returned as I was taking it for a test drive. The smoking issue increases as the operating temp of the engine does. There's no smoke, or a negligible amount, until 1

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Sorry, pushed the submit button by accident.  There's no smoke, or a negligible amount, until the engine temp gets to 100 or so, and then gets increasingly more pronounced at operating temp. Because of this, in the testing process I've spent a lot of time on the two temp related sensors, air temp and water temp. I've tested both on and off the car including putting the air temp sensor in a box and raising the heat from 20 to 110 and immersing the water temp in a jar of water going from 40 to 180. In both cases, measuring along the way, the results for both sensors aligned perfectly with a curve I found a while back on this site. Yesterday was the first time it struck me to test these two at the 35 pin right after shutting it down at full operating temp. I got 1300 ohms on the air temp with an outside air temp, measured at the AFM, of 98-100 and got 280 ohms on the water temp at about 175-180. Both look to be right on. The water temp sensor is new. It also has a new connector and a new pigtail back to the bullet connectors which were replaced.

Generally, every 35 pin test in the bible has produced the results expected and every wire to the 35 pin has been checked for continuity as well as any leakage to ground. I suppose there could be some sort of abrasion inside the harness but I would think that would result in more inconsistent results. I've been through these tests 4 or 5 times and always get the same results. I've also been through the physical tests in the FSM with the exception of the relay and the dropping resistors. I couldn't imagine they were the problem and figured I'd only add to my problems in messing around with them. I did the noid tests on all injectors and and got brighter lights with the temp sensor disconnected.

Fuel pressure at idle is about 31 psi and vacuum of 16 inches. The dropping and rising when increasing and releasing the throttle matches results documents that I found on this site told me to expect. I've forgotten what they actually were.  If I pinch the return fuel line, the engine sputters. Fuel filter is new.  All new injectors (Standard) as well as all new fuel hoses everywhere in the system from the steel lines near the alternator forward. I've tried a number of times to detect an issue with the FPR that might be leaking fuel into the manifold. Can't find a trace of it. I've also tested the cold start valve by plugging the fuel hose to it and finding that the smoke continues. I reconnected the hoses to the carbon canister, including a large hose running from the manifold to the canister, a smaller one that t's to the distributor and the throttle body, a third to the fuel tank and a fourth that is capped off. The thermotime switch is new, as is its connector and pigtail. All wire connectors, excluding AFM, are new. Other than originally cleaning it, I've done nothing with the throttle body. It gives the results expected in the idle switch, full throttle switch resistance tests. The AAR works as expected.

The AFM gives all the correct resistance at the connector and at the 35 pin. When doing the test where you apply 12 volts to 2 pins while reading voltage on two other pins as the shutter is slowly opened, the voltage range is 7 (closed) to .2 (wide open) with voltage dropping quickly in the first 50% of the swing. It does have one issue where testing the voltage there is a moment, going below 6 volts at about 5.8, where it drops to 0.0 for just the tiniest amount of time then returns to 5.8 and continues on without a flaw.  Someone adjusted this in the past. I tried the fingering adjustment Atlantic Z describes, couldn't see a difference so I put it back where it was.

Outside of fuel injection, the engine has 150 compression in each cylinder. A leak down test showed 96 per cent retention in 2 cylinders, 93 in two and 90 in the other two. Any air loss seemed to be felt at the oil filler cap. Oil looks fine as does the coolant. No leaks anywhere. No overheating or oil pressure issues. I just removed BPR6ES plugs and replaced with BR6ES as suggested on the NGK site. The plugs I removed were all heavily coated in black soot.  The car is a California 76 model. It is stock with a couple exceptions. It has a 75 intake manifold, which has everything the same without the extension for EGR. Obviously, all EGR stuff is removed. There is a blue wire that went to a fourth sensor on the thermostat housing which has broken off. I believe it's part of EGR so I've ignored it. The car has MSA headers and a custom stainless exhaust. Other than removing the non functioning air conditioning components is about all that's been changed. If I've missed something, please let me know.

The car sounds great. Starts quickly, idles smooth and runs better than it has in years. It just blows an embarrassing amount of smoke. I apologize for the long post getting around to probably a stupid question. I assume, as there was such carbon buildup on the plugs, that there might be buildup inside the cylinders as well. Could it be that this stuff heats up and begins moving back through the system the warmer the engine gets?  If this actually makes sense, does anyone have a thought on how best to check for, and remedy, that? Again, thank you very much, in advance, for your time.



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Posted (edited)
On 7/4/2024 at 12:40 PM, DD76 said:

leaking injector hose (original) which has cascaded into a replacement intake manifold, new injectors,

What's the part number of the new injectors?  Maybe put the original ones back in.  The hose on the injector can be replaced easily.

One possibility is valve seals.  Maybe they finally gave up the ghost.  Gasoline smoke would smell terrible.  Rich conditions smell like raw gasoline before the smoking gets bad.  If you're not smelling gasoline I'd suspect oil.

p.s. if you're sure it's gasoline check your fuel pressure regulator.  The diaphragms are known to rupture.  Pull the vacuum hose, if it's blown the hose will have liquid fuel in it.  The extra fuel would have a bigger effect when the engine is hot.

Edited by Zed Head
caught by the effect/affect conundrum
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Thank you for the responses and imput. Finally, in the last two days, my smoke issue has been controlled enough that I could take the car for an actual drive. Went 25 miles yesterday and a good 50 miles today. I brilliantly deduced, as I lost half a quart of oil in today's 50 miles, that my smoke has nothing to do with the fuel injection. Attached is a photo of how the plugs looked at the end of today's run. They only have about 80 miles on them. Smoke was most evident starting from a stop but also some accelerating and decelerating. As an aside, the car ran extremely well.  I also did a compression test this afternoon and had 145 pounds in #5 and 150 in the others. I took the plugs to my neighborhood general repair shop. I've clearly got oil problems in cylinders 3 and 6 and they suggested I should be  looking at the valve area as the source because of the decent even compression. Noting Zed Head's comment above about valve seals and seeing those seals in a number of post, would that be a potential culprit in this scenario and is there a way to confirm that as the case.  They also mentioned trying an oil additive designed for valve trains. Has anyone had luck wit something like that. Thanks again.


280z plugs 7-9-24.jpg

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If you do a leakdown test on the cylinders and the valves are worn and letting air escape you will be able to hear it with an ear to the exhaust pipe and the opening of the throttle body.  If the rings are leaking, removing the hose from the blowby on the engine block should let you hear air escaping.

Did you do the compression test on a hot or cold engine?

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