Hi, I have just bought my first z, but I have a problem which I'm not able to figure out. The car is a stock 280z with a l28. The engine runs good when it's cold but when it gets warm it starts to have a rough idle and it will not rev smoothly. It can run bad for some time and all of a sudden it will be able to rev smooth and without any backfire through the AFM/air filter. If i try to drive the car it will stumble on acceleration and it will eventually die out. But the engine will easily fire up again just after it has died. And it can run smooth again for several minutes before the same problem occurs again. Or it can still have a rough/unstable idle and have trouble revving.

When I got the car the fuel pump had been replaced and the fuel damper had been bypassed. I reconnected the fuel damper and for a few minutes I felt that the engine was behaving better, but once I tried to drive the car it was worse than ever and after some struggle at idle it died. Now I had a hard time starting the engine, not like before when it started right up again. I bypassed the fuel damper again and it fired up and without any hesitation, so I guess the previous owner bypassed it for a reason....

The plugs are sooted so it seems like the engine gets too much fuel/not enough air. I've tried to disconnect the cold start valve but that does not make any difference. Just before I bought the car the plugs had been replaced, but the gap was wrong so I have adjusted them according to spec. But can the plugs have gone bad already? My previous car was a 180sx with a ca18det, that engine was very sensitive with regards to the plugs, how is the l28? I have not checked the fuel pressure yet, but if there was a problem with the fuel pressure the problem should be there all the time, right?

I've gone throught the FSM and the EFI bible and after some troubleshooting it looks like there is a problem with the air temperature sensor. When I run the test where I move the flap the fuel pump does not start. The fuel pump is connected directly to the battery and it runs all the time when the ignition is on, so I guess it's a fix from the previous owner. Is the fuel pump supposed to be running all the time when ignition is on? On my other cars the fuel pump is running for a few seconds when ignition is on before it stops.

The lid on the AFM is glued so it seems like some has been fiddling with it at some point, but I have not tried to adjust the AFM yet.

I've used several hours searching the forums for any tips/clues but so far without any luck, I'm starting to get a bit frustrated:ermm:

Does anyone have any tips, I'm starting to run out of ideas...

AFM covers have clear silicone holding them from the factory, take it off and make sure the wiper and weight are moving freely. You really should check the fuel pressure as soon as you can. It'll save you money in the long run. Your TPS could be out of adjustment too. You should clean all the electrical connections associated with the EFI system, http://atlanticz.ca/zclub/techtips/electricalconnections/index.html. Here's the EFI Tech page, Z Tech Tips EFI & Fuel (AtlanticZ.ca) || |  |   |    |      |        |          |.

I've gone through the FSM and the EFI bible .....

Is the fuel pump supposed to be running all the time when ignition is on?

I just picked out this one thing because it shows that you might need to spend more time in the FSM. There is a lot of information in there and the Engine Fuel chapter is kind of like a college level EFI course book. The way your engine runs after you read it is your grade (kidding, kind of).

Since you've been in to some of the engine and EFI parts it would pay you to go back the Engine Fuel chapter and read from page one. Things will make more sense each time you go through, and you'll realize that there are primitive mechanisms that need to work right. In short though, most of the individual pieces need to work correctly, and they all work together to make the engine run right. There are a few items that are for cold start only, but the rest are important. One other key is that all intake air must have passed through the AFM at one time. And the PCV system is connected to the intake system so any leaks there are essentially intake system leaks.

To the question I quoted - no the fuel pump should not run all the time. As you noted, the PO has wired it that way. This is a sign that there was probably some other problem that he was trying to fix.

Fuel pressure is key. And once you figure out the Engine Fuel chapter, you can do a lot of testing from the ECU connector without removing parts. What the ECU "sees" is most important.

To the question I quoted - no the fuel pump should not run all the time. As you noted, the PO has wired it that way.

Agreed, the fuel pump should not run all the time, if the engine is off, key on or off. If it (pump) is actually direct wired to the battery, then it would run all the time. Literally. Not good.

HSL, you have a '77- which I had to find out by reading your profile, btw. The correct operation is: the fuel pump switch is located in the AFM and will energize the fuel relay when the flap counterweight is lifted by air flow, even as little as cranking the engine. As long as there is air flow through the AFM- cranking, running- the fuel pump runs. No air flow? Pump off.

Fuel pressure is key. And once you figure out the Engine Fuel chapter, you can do a lot of testing from the ECU connector without removing parts. What the ECU "sees" is most important.

I've done most of the test referred to in the EFI bible and the one that fails is the one with the air temperature sensor. Which is obvious since the fuel pump is connected differently.

I will get the fuel pressure tested and also measure the A/F ratio, just need to buy the equipment first:-)

Small update on my problem. After some more troubleshooting I think I was wrong saying the fuel pump was wired directly to the battery. From what I have figured out it is wired correctly. But it is still running all the time when the ignition is on. I did the "checking fuel pump contact points" from the EF-section and it fails, so I guess that's why the fuel pump runs continuously?

Also checked the fuel pressure and there is something funky about it. At first I had great pressure, 35-40 psi, decreasing on throttle. After running good on idle for a while I took it for a spin and the same problem occured, stumbling on acceleration. Checked the fuel pressure gauge and pressure had dropped to about 25 psi. As I kept letting the engine run fuel pressure decreased even more ending up at about 18-20 psi.

The fuel pump has recently been replaced, but I'm not sure if it's working as it should.

What is causing my fuel trouble, rust in the fuel tank clogging up the intake for the fuel pump? If I let the car rest for some time it will fire up and have good pressure.

Still having the same problem. I pulled out the tank and checked for rust etc but it looks very good and not any loose particles. The line from the tank to the fuel pump is not clogged.

Is there a good way to check if the fuel pressure regulator is broken? Or can it be partially broken, since I have good pressure the first minutes the engine is running.

I'm not positive about this but it seems like I've read that bad injectors will leak and flood the cylinder giving you the sooty plugs. I suppose they could cause the pressure to drop from leaking too, I know after you shut it off, not sure when running though. Google it. When I bought my '77 it had 3 original injectors and 3 replaced ones that were a mint green color, seems like the originals were brown. The ends that spray are plastic and crack after time. My new set are aluminium on the ends. I'm sure the FPR could go bad just never heard of one actually doing it. Captian Obvious, a member on here, has cut one in half. Maybe send him a private message, super nice guy.

I have a SWAG with the pressure dropping after time. If the tank isn't vented enough, causing a vacuum inside the tank, would that cause the pump to work harder and loose pressure?

Just a thought

--Justin

Checked the fuel pressure gauge and pressure had dropped to about 25 psi. As I kept letting the engine run fuel pressure decreased even more ending up at about 18-20 psi.

The fuel pump has recently been replaced, but I'm not sure if it's working as it should.

What is causing my fuel trouble, rust in the fuel tank clogging up the intake for the fuel pump? If I let the car rest for some time it will fire up and have good pressure.

IF you can keep the engine running while you see low pressure, one way to check the FPR and pump and/or the fuel tank is to squeeze the return line. If pressure increases the problem is the FPR. It's not doing it's job. If pressure does not increase then there's a problem with supply, either the tank or the pump. If you determine it's supply, you'll still have a dilemma since the problem only happens when things are warm.

But you can let your fuel pump run for hours with no issues as long as your battery holds up so you could follow-up by trying to recreate the problem with the engine not running, in the garage.

Just keep narrowing things down until you have one possibility.

IF you can keep the engine running while you see low pressure, one way to check the FPR and pump and/or the fuel tank is to squeeze the return line. If pressure increases the problem is the FPR. It's not doing it's job. If pressure does not increase then there's a problem with supply, either the tank or the pump. If you determine it's supply, you'll still have a dilemma since the problem only happens when things are warm.

But you can let your fuel pump run for hours with no issues as long as your battery holds up so you could follow-up by trying to recreate the problem with the engine not running, in the garage.

Just keep narrowing things down until you have one possibility.

There's no problem keeping the engine running on low pressure so I will try to pinch the return line. Need the l28 a specific kind of fuel pump? I haven't checked the part number on the pump but I know for sure that it is a replacement, not an original Datsun fuel pump.

I can't remember how much the pressure dropped after shutting of the engine so I will double check that as well.

Seems like I have a lot of investigation left to do:sick:

Do the fuel return line check that Zed Head mentioned. If the FPR checks out OK then I would be looking at the tank. It could be a lot of dirt and rust floating around in the tank and clinging to the fuel pick-up stainer and eventually blocking it enough to reduce the pressure. Your tank might be the cause of it all.

Chas