Not your typical hot start issue. Suggestions needed.

[cozye]

On my EFI 280z, I've replaced a leaky cold start valve, all injectors, fuel pressure regulator, and check valve. all hoses are good and I can't find a leak anywhere.

However, it takes about 6 hours for my fuel pressure to leak down and I still can't get rid of my "hot start" vapor lock issue. The car will start, but air in the fuel lines and missing on a few cylinders for a few minutes until it all smooths out.

I'd love to solve this issue, but short of injector fans and dry ice, I'm not sure whats left to do. The car runs at a normal temp, 180-190 or so. A little cooler maybe on the interstate. Stop for 10 minutes on the hot day, and it will be a rough start.

[LeonV]

Have you tried cooling down the fuel lines before you attempt a hot start, or when the symptoms show up?

[cozye]

Have you tried cooling down the fuel lines before you attempt a hot start, or when the symptoms show up?

No. What would you suggest doing that with? By the time a fan would cool them down, it would be cool enough to start anyway. I suppose I could use a block of ice.

If you let the car sit for 20 minutes with the hood open, it's fine.

[cozye]

I think the issue could be solved if I found the source of leaking fuel pressure, but since I've replaced every source of leaking fuel I can think of, I'm out of ideas.

[LeonV]

Spray cool water on the metal lines from a bottle. If it helps, the lines need insulation or some sort of heat shielding. Nissan installed a cooling fan on the 280ZX to alleviate this problem.

[buysell]

Just a SWAG, how old is the fuel pump? It may be leaking back to the tank. Also, are there any heat shields missing/damaged?

--Justin

[five&dime]

Does that car have an O2 sensor? I would check that as it takes a while for the O2 sensor to warm up.

[Zed Head]

When I got new injectors my hot start problem went away. But it came back months later a few times after some long drives in the summer. Then, after replacing my FPR with an Aeromotive unit (because my stock one went bad), it has gone away for good, I think because the Aeromotive FPRs leak down rapidly. The downside is that if I don't prime the fuel lines with a switch on my fuel pump circuit, it takes a lot longer to start, especially when cold. Aeromotive says that the leak-down is part of the high-flow design, but I think that it's a flaw.

I bought the relatively inexpensive BWD injectors. I think that they sealed well when new but probably leak occasionally when hot, after they got some wear and heat. With the Aeromotive FPR that doesn't hold pressure without flow, no more leaking injectors, no more hot start problem. But every start requires a prime. I will probably go with more expensive injectors and a better FPR in the future now that I know what know.

I'm not suggesting buying an FPR that doesn't hold pressure, just adding some perspective. I would guess that new factory injectors probably won't leak like the aftermarket ones do. Or maybe a better, modern injector design. My 95 Pathfinder doesn't leak down, even after weeks of sitting, and it has 200,000+ miles on original injectors.

If you wanted to experiment, you might put some sort of manual bypass valve around the FPR to the return line and depressurize the lines immediately to see if leaky injectors is the source of the problem.

[cozye]

Just a SWAG, how old is the fuel pump? It may be leaking back to the tank. Also, are there any heat shields missing/damaged?

--Justin

It's the original fuel pump, however the check valve is new. I just installed it hoping it would fix the problem. The check valve prevents back flow of fuel through the fuel pump, so that rules out the fuel pump

Does that car have an O2 sensor? I would check that as it takes a while for the O2 sensor to warm up.

no o2 sensor. It's a plain old 280z, with regular L-Jet fuel injection. As dumb as box of rocks.

When I got new injectors my hot start problem went away. But it came back months later a few times after some long drives in the summer. Then, after replacing my FPR with an Aeromotive unit (because my stock one went bad), it has gone away for good, I think because the Aeromotive FPRs leak down rapidly. The downside is that if I don't prime the fuel lines with a switch on my fuel pump circuit, it takes a lot longer to start, especially when cold. Aeromotive says that the leak-down is part of the high-flow design, but I think that it's a flaw.

I bought the relatively inexpensive BWD injectors. I think that they sealed well when new but probably leak occasionally when hot, after they got some wear and heat. With the Aeromotive FPR that doesn't hold pressure without flow, no more leaking injectors, no more hot start problem. But every start requires a prime. I will probably go with more expensive injectors and a better FPR in the future now that I know what know.

I'm not suggesting buying an FPR that doesn't hold pressure, just adding some perspective. I would guess that new factory injectors probably won't leak like the aftermarket ones do. Or maybe a better, modern injector design. My 95 Pathfinder doesn't leak down, even after weeks of sitting, and it has 200,000+ miles on original injectors.

If you wanted to experiment, you might put some sort of manual bypass valve around the FPR to the return line and depressurize the lines immediately to see if leaky injectors is the source of the problem.

Well, that's interesting that yours still doesn't hold pressure. I think you might have missed the first line of my post. I've replaced the injectors with brand new ones, from MSA. The expensive ones. No help. I replaced the cold start valve, it was leaking when hot, not now. I also replaced the fuel pressure regulator, one from rock auto. Don't remember the name.

Basically I've replaced the entire fuel system minus the actual rail, and the fuel pump itself.

Gas is definitely boiling on the rail. No doubt about it.

Where did you get this Aeromotive FPR? Where did you find out about their high flow leak down design ? I could put a mighty vac on the FPR after driving it and relieve the pressure quick. Thats a good idea.

I've searched and searched on this topic in the past trying to solve it. The injector replacement and the check valve replacement I just did yesterday was my last hope. I just hadn't had the chance to look into it

[Zed Head]

I did miss the injector comment. I wonder where MSA gets their injectors.

I bought my FPR from a local guy on Craigslist, brand new. An Aeromotive 13301 universal adjustable unit.

As for the comment about why they leak down, I sent Aeromotive an e-mail about it and Brett Clow of Aeromotive replied back that they knew about it and it had been discussed. He said that they were planning to put a new FAQ on their web page with the following text (plus a little more) -

"The stock EFI regulator was engineered with a mandate to hold fuel pressure for 30-minutes after engine shut-down. This is an government regulation with which new car manufacturers must comply. Unfortunately, the mechanism used to hold fuel pressure when the engine is off, has a derogatory affect on a high-flow, adjustable regulator’s ability to create and control fuel pressure when the engine is on. Aeromotive places fuel system performance when the engine is running, particularly running under high load, at the top of the priority board. Anything, such as a checking mechanism in the regulator valve, which compromises fuel flow and pressure control under high-load engine operation, is therefore eliminated. We believe this no-compromise approach to fuel system performance is one of the reasons Aeromotive fuel pressure regulators are universally preferred by enthusiasts and racing professionals alike, around the world."

He said they would also add some text to the instructions about the pressure leak-down.

I just checked their web page and don't see it yet, even though on the 24th of March, 2011 he said it would be up in a few days. It's one of those things that marketing people would hate so maybe it got quashed, or maybe they're just very busy.

[FastWoman]

Well, since the FPR comes after the injectors and before the return, I can't see how the design of the FPR would in any way impact flow to the injectors. There's no doubt the stock FPR is very tight, and it works very well. I've admittedly never tried an Aeromotive.

Eric, I've been having trouble with hot starts lately too -- the same as you describe. Unline on your car, my fuel rail IS insulated, albeit rather minimally (a layer a rubber hose, wrapped in silicone rubber). Also my OEM fuel pump is brand new. Other than that, everything is new except the FPR and cold start valve. I've checked both, and both are functioning correctly. Oh, I have the cheap Standard Ignition injectors, but I have no reason to believe they're acting up.

I don't honestly know how long my fuel rail holds pressure, but I'm 99% certain it has adequate pressure for a start after the 10 min or so that it takes for my car to have a rough start. To be clear, I seem to have a miss that results in an abnormally low idle and clears after I take off down the road.

BTW, like Zed, I do prime my fuel rail for a few seconds before every start, so it's not that I don't have pressure. I don't know if it's relevant, but if I leave my car sitting overnight, I'll sometimes hear a bubble or two pass through the fuel pump when I prime it. (The whine briefly changes pitch.) However, I have absolutely ZERO problem with a cold start. The engine fires up instantly and idles normally.

My own pet theory about the hard starting is that the CTS and air temp sender become heat-soaked, resulting in lean running for a short time. It would be interesting to see if jumpering the TPS connector for full throttle enrichment -- or unplugging the vacuum line to the FPR -- would compensate the rough running on warm restart. If so, it would support my heat-soaked-sensors theory.

[Blue]

Too bad you could not cobble the cold start valve outlet back to the tank and use it to pre-flow cool fuel through the rail.

OR

Connect one of these in parallel with your FPR and activate it to flow fuel through the rail

[Blue]

I also cut and reworked my "bugle" shaped rail into a straight line. I am not sure if it will help but it holds less fuel and has less surface area than the original thus it may clear faster and absorb less heat.

[cozye]

Blue, did this work for you? What are you going to use for an FPR ?

I have more information. I stayed up late last night digging through old threads etc.. It appears to me that a few years ago, the increased ethanol used in fuel along with reformulated gas lowered the boiling point enough that 280z owners started having heat soak issues. Lots of old owners who had cars running for years with no issue, suddenly started having hot start issues. I looked up the boiling points on gasoline, and my quick searches did not yield definitive hard facts but I did find that ethanol definitely lowers the boiling point. I also found that winter gas has a much lower boiling point than summer gas. My car still had winter gas in it, since I had a full tank early this spring and it's been getting painted since.

I filled up this morning and drove it running errands, is not as hot today but I did not have any hot start issues today.

I can't fathom that these cars were boiling fuel on the rail when new. The only thing that has changed since the mid 70's is the gas formula.

I wonder what fuel additives could be used to raise the boiling point ? Zman of washington in a thread suggested putting a quart of oil in the gas. I'm not a fan of that idea, although he is very knowledgable.

This theory works for me. I don't think there is anything mechanically wrong with my car, and I know for a fact that the fuel rail gets extremely hot and fuel is boiling.

I think I may try several approaches. I will find a way to either replace the fuel rail, or preferably insulate the one I have. I will install a 170 degree thermostat. I will look into fuel additives and see if there is anything that will raise the boiling point. Another idea is to scrounge up a ZX injector fan and install it.

[cozye]

link to old thread that has useful information

http://www.classiczcars.com/forums/s...ot+start+issue

Link to thread where ZMOW and another well known z car member (that I'm not a fan of personally) discuss theories. I like Zman's post #8. It makes a lot of sense. He works on a LOT of customer, street driven Z cars, so he has the experience. I've talked to him previously about other Z stuff and he's a hell of a nice guy and very helpful.

http://www.zcar.com/70-83_tech_discu..._883144.0.html

[Allen Rondeau]

Had the same problem with my stock 76 280z.The problem was traced to a faulty fuel pump check valve. ( no longer available ) I had to step up and purchase a new NOS Nissan fuel pump that came with the valve. Problem solved, she starts hot now. Thank you Doug at FairLady Z.

[mjr45]

Allen, if the fuel pump was good with just a bad check valve, you could have just done the check valve. There is a thread on how to do it.

[FastWoman]

To elaborate further, there are non-Nissan check valves that can be adapted. Someone started a great thread on it -- perhaps Captain Obvious? If you still have the pump, throw it in a box, and save it to sell/give to someone else with a failed pump, for the good of the Z universe and general good karma!