Need Help 77 280z hot start issue

[Captain Obvious]

Pump never ran? Any idea what was the threshold temp for the ZX fan switch?

What other supporting data might I be able to help with? I won't be in any position to further investigate until it warms up, but once that happens, I'd be glad to do what I can.

[Zed Head]

In looking back I think that there may have been times when I cranked my WTS resistance up (I have a Radio Shock volume potentiometer on mine) and still got some rough running. It helped a lot but didn't remove the issue. I think that's why I switched to the hot injector camp, since more duration wouldn't have a huge effect if the gas is boiling in the injector tip, or it seemed that not all of the injectors were affected the same.

I don't really know what data might tell the whole story, for me anyway. We'll know it when we see it. Maybe it's a combination of hot fuel in the rail, hot injectors, and an overly hot CTS. Two things that might cause premature vaporization and one that lowers open duration. Lean, lean and lean.

Instead of a fan to blow air, maybe a large electric water pump on a radiator hose. Can't go wrong cooling down the whole engine. I only had a tiny water pump pulling water through the AAR block, which then pulls from the thermostat housing (I said head earlier but it was only the T-stat housing). Depending on which way it goes, the water could have circulated through the radiator and open thermostat or through the block and head. But the flow would be low.

It's an aggravation, still worth serious thought.

Edit - never answered your question about the ZX switch. I know it worked because I heated it with a heat gun and measured resistance. I know the pump worked and was wired right because I shorted the switch leads and tested it. But even on a very hot day after a long drive, with the temperature gauge up on the high end, the switch never caused the pump to run. Somebody with a ZX can probably tell if they ever hear their fan run. I think I've read a few comments where ZX people never really hear it blowing. Maybe it's a poor design.

Edited February 24, 2013 by Zed Head

[Wade Nelson]

...found that the switch never closed under the conditions my engine saw even though I would still get the hot start problem. .

I found the same exact thing; the switch responsible for turning on the injector cooling fan never tripped even after a hot soak on a hot day that clearly caused vapor locking. I searched Standard Motor Parts and others looking for a 10 degree colder thermoswitch. None was available in the same thread pattern. I wasn't ready to have a machinist build an adaptor.

So I wired up a switch on the dash to manually run the fan after shutdown. What I really need is a "turbo timer" so it'll only run for X minutes after I trip it. Just haven't gotten around to it.

When I manually run the fan for a few minutes it actually does reduce vapor locking. But cracking the hood open / up works even better.

[bryand2]

Thank you for all the responses guys, I am fully aware that this is a common issue, especially with 77 and 78's. That being said I have spent the past 2 months reading every single forum post I could find on this issue and trying different anecdotal things that worked for different people. I have tested fuel pressure both with the car running and after shut down, pressure holds fine between 32-26 psi. New plugs, wires, cap, rotor. One thing that I should mention is that unlike many of you guys, I bought this car 2 years ago from an old lady who had invested $25,000 in this car. She spent roughly $12,000 on the engine rebuild alone, I know that sounds expensive but she had almost every wire, sensor, relay, hose replaced, on top of fuel injectors, fuel pump, valves and valve spring, pistons, connecting rods way too much work too list. This work was done in 2003. Since then i changed the thermostat and while I was in there, checked the WTS, Thermotime switch and gauge sensor, all three sensors were in perfect shape, still looked like new brass actually. To willoughby you are correct, it only happens when i turn the car off and leave it for 10-15 minutes. Now on to the testing ive done,, checked spark at cap, coil and wires, checked fuel pressure and its in spec, checked the injectors with a noid light, they are ok. As far as vapor lock, I understand that it is potentially possible for it to happen, albeit unlikely on FI, but when the car is doing this and the engine is hot, the fuel rails are the only thing in the engine that are actually cool to the touch. One more strange note, as soon as I start driving the car and the engine speed reaches 2,000 rpm it disapears...

[bryand2]

One thing I should mention, I am in northern california and we are still having 40-50 degree weather and my car is doing this, I am trying to get this figured out before it gets warm, once that happens the car will not restart at all when warm.

[bryand2]

I think I may have just answered my own question, if it stumbles but starts in cold weather, but wont warm start at all when the outside temp. is warm, possible faulty air temp. sensor?

[darom]

You should be able to test your sensors from the ECU plug, it will take you half an hour: at cold and once it is warm. I would even try to make it run really hot, shut down and immediately retest every sensor again (esp. the AFM) per Fuel Injection book. I am wondering if one of the sensor wires is close to the exhaust, and once hot, creates an infinite resistance.

Does your car have an aluminum shield under the intake? I have a '76 and without that shield the exhaust manifold would have cooked everything there. I also installed another shield from a later model to protect the master brake cylinder area.

To sum it up, it looks like our cars (smog results confirm that) run leaner than they should. Lean mixture makes cars run hot. My freshly rebuild Cardone AFM was set to be on a lean side, the WTS potentiometer mod fixed that problem without opening up the AFM for adjustments.

[bryand2]

Thank you everyone for your input, I have good news the problem is fixed. When I had the car smogged about a year ago the smog guy went into my afm and leaned the mixture to get it to pass. I went back in, adjusted the toothed wheel to richen the mixture up a bit and the problem is gone. I noticed an immediate difference in the temperatures coming out of my heater on my feet, not nearly as hot. The car was already running lean, generating more heat than it should and when the WTS and air temp sensor were adjusting fuel based on the temperature, it was dumping less fuel than it needed on an already lean running engine

[Wade Nelson]

My theory, which is mine...

I see now why you chose "Captain Obvious" as a NIC.

great post, btw. Nice to see someone willing to actually do some work, take some measurements and suggest a workable solution.

[Captain Obvious]

I see now why you chose "Captain Obvious" as a NIC.

Haha! The "which is mine" part is a quote from the famous paleontologist, Anne Elk. IIRC, she did a lot of work with Brontosauruses.

About the temp sensors... I did more, but figured that everyone here would gloss over and pass out so I didn't mention it. I wanted to determine what the temp sensor outputs would be at additional points, so I employed the Steinhart–Hart equation to model the thermistor outputs across the entire range.

Using the data points in the factory service manuals, I now have a graph of the sensor outputs full range. It was during this process that the math highlighted some mistakes in the FI "bible" and the pre-78 factory service manuals:

http://www.classiczcars.com/forums/fuel-injection-systems-s30/48782-water-temp-sensor-air-temp-sensor-resistance-charts-typos-manuals.html#post424758

[bryand2]

Fortunately, my afm has a white toothed wheel, there was a dark grey mark on one of the teeth, as well as a matching mark inside the casing so i knew where to set it

[rcb280z]

bryand2 I'm glad to see it worked for you also.