Hot Start Problem - a new clue and a work-around

[Zed Head]

My car's had the hot start stumbling problem more often recently so I decided to try something. In my case, the problem seems like a lean condition, based on the intake backfires and the lack of gasoline smell or smoke in the exhaust. I also noticed that if the temperature gauge went higher than usual, like up to the middle of the M, that the problem was likely to happen.

So I bought a 5 kohm Audio Taper Potentiometer and spliced it in to the water temperature circuit as a rheostat (two wires connected, one left open). Mounted it by the hood release handle in the cabin so that I could tune it when needed. It's basically the part behind the volume knob on an amplifier. The linear taper potentiometer changed resistance too fast (I had my ohm-meter in the Radio Shack store). It can only be used to add resistance so will only add more fuel.

When the engine is running normally, I can't turn the knob, increasing the resistance, at all without the RPM dropping as the injectors stay open longer. But today I had two "hot start" cases where turning it stopped the stumbling and smoothed the running. I still had to wait about 15 seconds before I could take off, but it seemed to clear up faster, probably because the water pump was pumping more water, cooling the WTS down to true engine temperature. Plus, at least it was running well, instead of jerking and popping.

My current theory is that the WTS overheats due to residual heat from the exhaust manifold (the only engine part that doesn't really get cooled by the radiator coolant) indicating a hotter engine, needing less gas. This may be why Nissan moved the sensor to the other side of the head on the P79 head. It's just a guess. Maybe the injectors also get hot or maybe it's just hot injectors and adding fuel still helps. Who knows. It really seems to work though.

It's worth a shot if anyone else wants to try it.

Edited February 16, 2012 by Zed Head

[Captain Obvious]

My current theory is that the WTS overheats due to residual heat from the exhaust manifold (the only engine part that doesn't really get cooled by the radiator coolant) indicating a hotter engine, needing less gas.

I like it. As good a theory as the others I've heard to date.

I picked up a 280 recently, but since I bought it in cold weather and haven't owned it through a summer yet, I haven't had the pleasure of dealing with hot starts after that heat soak. I'm assuming that this summer I'll be in the same boat as the rest of you guys. I've been paying more attention to the "hot start" threads than I used to.

You know, if the only thing you're using your pot for is the hot start, you could measure the resistance you need to fool the WTS on your hot start and then replace it with a switch an fixed resistor instead. Two positions on the switch: "Normal Start" and "Hot Start". Simply short around the resistor for "normal" and open the short for "hot".

That way, you wouldn't take the chance that you are messing with your mixture at other pot positions. (Unless, of course, you want the ability to do so. )

[FastWoman]

^^^ Not a bad idea, Obvious!

Zed, the only thing that makes me doubt your theory is that my stumbling problem is resolved immediately by my taking off down the road with my foot liberally on the gas. I only have to go a few hundred feet. I doubt that's enough time to change the CTS temperature much. OTOH, the stumbling will continue for at least a couple of min (?) if I just sit there with the car idling. I think the difference is gas flow through the injectors.

FAIW, your stumbling issue and mine might actually be different. I'm pretty sure my engine isn't running (as?) lean on a hot start. At least I'm not getting any intake pops.

[Willoughby Z]

My experience echoes Sarah's post #3. And I might add that the problem is more frequent, starting in the fall/winter than it is in the summer-lending some weight to seasonal fuel changes.

I recently replaced the CSV, injectors, FPR and check valve, in part because the injectors were due and also to chase the pressure leakdown (at rest) that would happen. Opinions regarding the pressure loss varied from, "who cares?", " doesn't matter...", "mine never did it", and that's a problem!".

Replacing those parts didn't solve the leakdown and it (leakdown) didn't really matter to funcionality, as the engine always cold-started, fuel pressure at the rail was instant as the starter cranked, so I never needed a switch to create pre-start pressure, as Fast Woman has done.

My point is that the "hot start stumble" was much better (or not as bad) after the new parts were installed.

I'm no longer frustrated with it, more curious to the solution.

[Zed Head]

Captain, I also pondered a switch, or moving the WTS (aka CTS), along with some other possibilities, but realized that I didn't know anything yet, hence the Radio Shack quick kludge.

So for ~$4 and some time I learned something. Putting it out here so, maybe, someone else can generate some results or at least stop the aggravation for a while.

The WTS theory could be rejected or supported by anyone with a P79 head, with a working CHTS instead of WTS. The CHTS shouldn't feel the residual heat of the exhaust manifold on the other side of the engine.

I also have an F54/P79 engine in the garage that might get installed eventually.

Good luck to all that suffer with it. I have an extra WTS that I might install somewhere else (maybe in a core plug to a water passage or that drain plug on the block) and switch in to the circuit. It's my rolling Nissan EFI theory laboratory.

Edited February 16, 2012 by Zed Head

[grantf]

This is food for thought. My car has this issue also, worse than Fastwoman's but not as bad as Zeds. It seems that a common solution is to trick the car into thinking that it is colder than it is (or did I get that backwards?).

[Captain Obvious]

Pure speculation of course, but I'm not so sure close proximity to the exhaust manifold is the problem. I would expect something sitting right on top of the manifold might get a little hotter than something screwed into the head on the plug side, but I'm skeptical as to how much difference there would be.

I suspect that the efficiency of conduction will mostly overrule even close proximity convection. By that, I mean... I bet that when you shut off the engine, the heat soak will raise the temperature of everything. But that big hunk of metal and the water that's inside it are excellent heat conductors. Way better than the air between the exhaust manifold and the thermostat housing. I wouldn't expect a big difference in temperature between something screwed into the thermostat housing and something screwed into the plug side of the head.

Speculation only. I'm not an ME and thermodynamics made me, well... :sick:

It's my rolling Nissan EFI theory laboratory.

Nice!

[Zed Head]

It's just my attempt to reason out where the extra heat comes from. When the engine is running and water is flowing, the heat is transferred to the water to the radiator to the air flowing by. When you turn off the engine, everything close to the water is at the temperature shown on the gauge. So where does the extra heat come from? It must be a large mass that is hotter than the water, since it is conducting in to everything around it. Anyway, that's my reasoning, plus the fact that Nissan put some effort in to moving the sensor. I wish one of the Nissan engineers would write his memoirs and expose the secrets from inside the company.

My new goose chase is to T in an auxiliary electric water pump on the AAR pad heating line. This will move hot water from the T stat housing to the lower intake hose. Bimmers, Audis, VWs, even the odd Dodge Durango have them. But, similar to some Z things, no one is really sure why they are there.

By the way, I had another hot start issue today, and twisting that potentiometer knob did the trick again. The reasoning may be off but the results are real.

This should be a solvable problem...

[Captain Obvious]

So where does the extra heat come from? It must be a large mass that is hotter than the water, since it is conducting in to everything around it.

My guess would be that the extra heat comes from a bunch of stuff: The pistons, the head, the cylinder walls, the valves, and yes... the exhaust manifold. When the engine is running, parts of those things are much hotter than the water (hence the fact that they are cooled by the water), and when you shut off the engine and stop the water flow, everything will stabilize to a higher temperature before decaying over time.

But my point about conduction is that I'm skeptical that the heat being conducted from all those sources would travel any faster to the thermostat housing than it would to something located near the spark plugs. I would expect that energy would conduct at pretty much the same rate in every direction away from the heat sources.

My new goose chase is to T in an auxiliary electric water pump on the AAR pad heating line. This will move hot water from the T stat housing to the lower intake hose. Bimmers, Audis, VWs, even the odd Dodge Durango have them. But, similar to some Z things, no one is really sure why they are there.

What's an AAR pad heating line? I've got a 77 and I don't have any water going to my AAR. Did they put that on in 78? Thanks to Blue, I've been messing with AAR's lately and he told me that later Nissans included the provision to heat the AAR with engine coolant, but I didn't think he was talking about Z's. I thought he was talking about newer stuff.

Just remember that the AAR might mess with your idle speed, but it shouldn't do anything to change your mixture ratios. Remember, you're already downstream of the AFM.

BTW - Something else I discovered recently while poking through the parts fische was that Canadian Z's included an insulating jacket on the outside of the AAR. Kind of a little "AAR cozy", eh?

[Zed Head]

I think that there might be a window of time where the WTS gets extra hot while the heat is equilibrating. That might be why the problem only occurs at ~15 -20 minutes after shut-down. Before then, the hot spot hasn't reached the WTS, after then, the heat has spread. The water temperature gauge is right next to it and shows as very hot whenever the problem occurs. You're right, it could be that the entire engine has extra heat and it doesn't matter where the WTS is. But the exhaust manifold is right next to the housing and has lots of mass and no cooling. It's a theory, just needs more data. Wish I had more equipment.

I did see in the FSM that Nissan explains the ZX cooling fans purpose as for cooling the fuel in the injectors and the fuel hoses. Which implies that they identified hot injectors and fuel as a problem. Maybe it would pay to put a timer on the fuel pump... Anything but the giant fan and air manifold hanging over the engine.

You should have a small water line running from the thermostat housing through an aluminum block under the AAR and then back down to a T in the return line from the heater core, by the water pump. It gets the AAR up to water temperature sooner, I assume, since it sits on the intake manifold, which would be cooled by flowing air while the engine is running.

Still haven't figured out how people live in Canada. Seems like one winter would be enough. No offense!

[Willoughby Z]

...(edited for brevity)...What's an AAR pad heating line? I've got a 77 and I don't have any water going to my AAR. Did they put that on in 78?

Jan/77 build: The lines run (metal and rubber) from the thermo housing to a block (heater housing assembly) underneath the AAR which also contain the thermal vacuum valve (controls vacuum between EGR and BCDD).

Edited February 17, 2012 by Willoughby Z
add'l detail

[grantf]

Just thinking out loud: What if there was an Air temp sensor that had the same resistance range as an inline resistore or pot people are putting inline from the WTS. Then if you mounted it near the fuel rail you would have automatic compinsation for the hot condition. Probably this would not work and there is no such atp in existance. Just thinking out loud.