1976 280z engine runs rough then dies, will not idle.

[Dave WM]

the oil cap off runs better confirms the rich fuel mix, unmetered air entering thru oil fill hole.

a couple things look for gas leaks (no engine fires) from ALL the rubber hoses, if they have been replaced I hope it was with FI rated hoses.

do the engine off fuel pressure (36 psi with fuel pump on engine NOT running). The process requires disconnecting the starter solenoid (spade connector small on starter) so the engine will not turn over, then key to START position until fuel pressure comes up, should be full pressure in a matter of seconds. should be 36psi (sea level). Release key and watch pressure.

It should hold for at least a few minutes, if it drops off instantly then you have a leaking injector or FPR or something else. Try that and report back.

[Zed Head]

The next simplest test is to check the vacuum hose from the FPR.  Give it a twist to break it loose before pulling it off.  If you only have a short time to mess around.  If it's wet with fuel, you'll need another FPR before continuing with your other tuning tests anyway.  Edit - forgot to say that you can still get proper pressure readings with a leaky FPR.  You just can't see the extra gasoline.

Edited March 20, 2019 by Zed Head

[Dave WM]

have some vacuum tube handy, if that is the OE piece it will be hard. I used some hemostats to get at it to remove, can be a bit of a bear. after confirming leakage from that you can just replace with some new stuff, much easier to slide into place if its the type with the real short piece.

Edited March 20, 2019 by Dave WM

[mbz]

12 hours ago, Dave WM said:

do the engine off fuel pressure (36 psi with fuel pump on engine NOT running). The process requires disconnecting the starter solenoid (spade connector small on starter) so the engine will not turn over, then key to START position until fuel pressure comes up, should be full pressure in a matter of seconds. should be 36psi (sea level). Release key and watch pressure.

It should hold for at least a few minutes, if it drops off instantly then you have a leaking injector or FPR or something else. Try that and report back.

Yes, will do.

12 hours ago, Zed Head said:

The next simplest test is to check the vacuum hose from the FPR.  Give it a twist to break it loose before pulling it off.  If you only have a short time to mess around.  If it's wet with fuel, you'll need another FPR before continuing with your other tuning tests anyway.

Agreed, great call.

Thanks again guys, will report back when I have more news.

One thing I forgot to mention at the start of this thread (and my apology for the oversight), this car is NOT a daily driver. Meaning there is no real urgency to get it fixed quickly. I'm more concerned about getting it fixed right as long as it takes.

Hopefully not too long!  I'd like to drive it some more!

[mbz]

11 hours ago, Dave WM said:

have some vacuum tube handy, if that is the OE piece it will be hard.

Yes will do. I'll pick some up this week along with a fuel pressure gauge.

I'm planning on replacing old / damaged hoses as they are found anyway just as a measure of good maintenance habit.

[Dave WM]

Once you get it sorted out the EFI works very well. The only odd thing that seems systemic is the hot restart issue. Its manageable by simply popping the hood up at filling stations. Seems both the carb and EFI had issues with this. Eventually Datsun came up with cooling vents and cooling fans that help keep the hot restart issue at bay. There is even mention in the owner manual about it, recommending holding full throttle while cranking when the conditions are very hot.

Other things to look into, the water pump, keep a close listen to it, check it for play in the shaft (try and shake the fan side to side). I have had two Z's both water pumps went out on me while on a long drive. Usual warnings about breaking bolts apply during replacement (there are two bolts that are very long and skinny, easy to break). Use what ever technique you like to prevent breakage. this applies to a LOT of the bolts on a Datsun of this vintage. Personally I like my induction flameless heater.

My 75 is not a DD either, but I do get out on the weekends to take it up to highway speeds, and have had a few long trips (over 400 miles) with no issues after the water pump replacement.

just use common sense, hoses, belts (not too tight), no water pressure cleaning under the hood etc... These cars are so simple to work on and not a lot to go wrong once you work out the EFI I find it to be very reliable. Back in the day Datsun was very well regarded for reliability and economy. I owned a 1972 Datsun 1200 for my college car, it was perfect for that. I miss that car it was the definition of "basis transportation" 35mpg back in the oil shortage days came in real handy.

 

[mbz]

23 hours ago, mbz said:

On 3/20/2019 at 9:32 AM, Zed Head said:

The next simplest test is to check the vacuum hose from the FPR.  Give it a twist to break it loose before pulling it off.  If you only have a short time to mess around.  If it's wet with fuel, you'll need another FPR before continuing with your other tuning tests anyway.

OK got a chance to check the vacuum hose from the FPR this evening. After idling the engine for about 10 minutes, the car finally died. I figure at this point the engine is saturated with enough fuel so time to check the vacuum hose.

Took the hose off and sliced it open length-wise to inspect inside (I had a new replacement hose handy).

The inside was BONE DRY... not a drop of fuel or anything. (see photo 1):

Also, to be sure there is no fuel coming from the vacuum line of the FPR, I started up the car and let it idle without the vacuum hose to see if any fuel was coming out. The car did not idle long, and there was no fuel coming from the vacuum line of the FPR. (see video).

Also, while installing the new FPR vacuum hose, I noticed that the air hose going to the rocker cover had a split at one end on the underside that was  patched with silicone sealer..... so I replaced that as well with a new hose. (see photos).

I am still planning on running diagnostics on the distributor and cap as well as the ignition coil this weekend.

I need to address the running rich problem however, as it has been confirmed that the engine is getting too much fuel. Seems like it would be the result of a vacuum leak somewhere, however cannot rule out Water Temperature Sensor.

Any thoughts on where to look next? (Besides the FI bible, which I am reading through).

As always, comments and input appreciated.

Thank you!

 

 

Edited March 22, 2019 by mbz
typo correction

[gwri8]

Don't forget to check the PCV valve and hose also. The one under the intake manifold.

[mbz]

1 hour ago, gwri8 said:

Don't forget to check the PCV valve and hose also. The one under the intake manifold

Thanks for the tip.... to confirm, it's item 14 in the diagram?

[Dave WM]

1st things 1st, check fuel pressure. rich running is a function of two things duty cycle (ECU) and fuel pressure (FPR).

Best practice is to check the pressure 1st. Duty cycle is controlled by the ECU with inputs from the AFM/water temp sensor/throttle position sensor.

Clearing internal fuel leaks into the intake will require leak testing of the fuel injectors. you have cleared the FPR of leaking fuel, that leaves the injectors and the cold start system.

if everything checks out, the ECU itself become suspect, but that is prob the least likely to be a problem. Its solid state, lives inside the cabin so has a easy life.

after rich running is resolved air leaks can be looked into, generally speaking they would allow unmetered air in a system that would account for rough idle but not rich mixtures.

The split hose on the valve cover would prob make it run BETTER since its allowing air in when something else is allowing too much fuel in. Sounds like a combination of problems exist keeping the FI from ideal, so you may have some set back in performance before it all gets better.

All the above being said the easiest thing to check and the most likely to be causing the issue is a defective water temp sensor. they live in a wet environment, can corrode and generally fail in a high resistance reading resulting in rich run.

HOPEFULLY no one has messed with the calibration of the AFM. There is really no reason for it to become "UN" calibrated. In  the electronics world we have tuned circuits that often become in operative due to "screw driver" drift... people can't seem to resist tweeking things in an attempt to resolve a problem. Tuned circuits have a very definite "peak" that can be ruined by the slightest turn. Now the AFM is not a tuned circuit like I have described but it can still be messed up I would think.

 

 

 

 

 

 

Edited March 22, 2019 by Dave WM

[mbz]

DaveWM, again solid advice.... much appreciated. ?

35 minutes ago, Dave WM said:

Sounds like a combination of problems exist keeping the FI from ideal, so you may have some set back in performance before it all gets better.

I agree with you and suspect the cause(s) of the running rich problem have existed for a while, hence the past smog failures.

I plan on keeping this car for a while, so it's best to get it sorted out now before moving on to other issues.

I'll stay on my original plan for this weekend of testing fuel pressure and distributor/ignition coil.

Stay tuned for updates (bad pun not intended).

Edited March 22, 2019 by mbz
grammatical; changed plurality for clarity.

[Zed Head]

35 minutes ago, Dave WM said:

HOPEFULLY no one has messed with the calibration of the AFM. 

Check your glue blobs.

If you have a meter, measuring resistance of the coolant temperature sensor is easy to do also.

Working on the EFI system is really just taking measurements and comparing them to a check list.  Measure-check-measure-check.... If you get a bad measurement, you find the cause and fix it.  When all of the measurements are in-spec. if the problem still persists then you go to the weird stuff.  Often-times people adjust things to out of spec. trying to fix other out of spec. things.

Forgot the glue blobs picture.  Post #29.

 

Edited March 22, 2019 by Zed Head