Does your idle change with temperature?

[steve91tt]

Interesting, I did not know the idle could be effected by that system. I thought it was just for deceleration. I wonder if my 400-500 RPM low idle would be enough to kick in the system. If so then I will put it back on.

[Jetaway]

I have noticed lately that when my car is first warmed up the idle settles in at a nice steady 900RPM. However, if after it's warmed up I sit idling for a few minutes at a traffic light or at a drive through the idle will dip to 400-500RPM. The idle will stay low until I spend some time at higher speed (+50MPH?). When I stop from higher speed it will be back at 900RPM until I sit in one spot for a while and then I am back to low idle.

I was hoping that you'd have a more definitive answer as I have similar symptoms, which become more prominent as the weather warms up. Like yours, its a 240Z and idles nicely at 600-700 rpm when just warmed up (thermostat). Continued driving keeps the thermostat at the same level, but heat soaking creates an erratic idle. Sometimes -- 600 / 700 rpm, sometimes gets hung up around 1100 rpm, other times settles in at 400-500 rpm which is too low and will often fall further and stall out. I don't have the heat pipe hooked up and have by-passed the intake manifold coolant circuit. Still, I can't but help think that it has something to do with excessive heat in the fuel system.

I don't think its the throttle opener circuit. Course, could be as mine is a touchy little SOB. It didn't function at all until I opened it up and cleaned and lubed it (a foam ring, maybe, foam of some sort had dissolved from age inside it). Turn it up too far and it will loop in a positive feedback loop accelerating the engine until I pull the line on it. A little less and it still grips the throttle strongly and creates a hmmmwrrrp ... hmmmwrrrp ... hmmmmwrrrp sound as it drops the rpm maybe 200 per hmmmwrrrp. Not too much further and it has no effect at all. Anyway, currently if it is functioning, its grip is very weak and allows the rpm to drop far enough to make smooth acceleration through the gears difficult. And even if it did wake up, that wouldn't explain the drop to 400 / 500 rpm.

Any suggestions?

Chris

[240260280z]

Maybe the springs and linkage to the throttle valve are "inconsistent". Mechanical binding or mechanical-binding-with-heat or weak return spring could be the issue.

Try disassembling, cleaning, and lubing the complete linkage and throttle valves. Start at pedal and go all the way through. Lithium grease works great in the ball and socket joints.

Also check carb function. If float bowl level varies or piston return height varies then you could see this variance.

Also check choke function and see if any thermal related cable tension is dropping jets.

Maybe go to the hardware store and experiment with a couple of new supplemental springs to assist in throttle return.

Finally, check the oil in the carbs and maybe try heavier and lighter and none to see if it makes a difference.... maybe try this one first.

[steve91tt]

I ended up hooking a wideband up to the car and finding that the reason that I had poor idle when hot was that the car would become progressively more rich as it got hot. When heat soaked my air to fuel ratio was down in the high 9's at idle. The car does not seem to like anything less than mid 10's to be happy. If I adjusted the mixture nuts on the bottom of the SU's to lean it up a little at idle I found that I was overly lean on the highway which caused a surging issue. I assumed the rich idle condition is due to the boiling of fuel out of the nozzles so I extended the heat shield with some reflective insulation. I can't find the exact one that I used but it is similar to the following...

http://www.amazon.com/Thermo-Tec-14100-24-Cool-Mat/dp/B00029KCW0/ref=sr_1_1?ie=UTF8&s=automotive&qid=1302036108&sr=1-1

except with finished edges and meant for underhood use. Addition of the extra heat shield solved the problem. No more idle issues even in the Texas heat. I hope this helps.

[Jetaway]

Maybe the springs and linkage to the throttle valve are "inconsistent". Mechanical binding or mechanical-binding-with-heat or weak return spring could be the issue.

Try disassembling, cleaning, and lubing the complete linkage and throttle valves. Start at pedal and go all the way through. Lithium grease works great in the ball and socket joints.

Also check carb function. If float bowl level varies or piston return height varies then you could see this variance.

Also check choke function and see if any thermal related cable tension is dropping jets.

Maybe go to the hardware store and experiment with a couple of new supplemental springs to assist in throttle return.

Finally, check the oil in the carbs and maybe try heavier and lighter and none to see if it makes a difference.... maybe try this one first.

This will keep me busy for awhile.

The only one I'm confident about is carb functioning, the rest should be checked, if only on general principles. I do know that no oil in the carbs makes it run like crap, but I haven't tried anything other than 20 wt. oil. I've heard that ATF is 7 wt. (all of the different flavors?) and a straight 30 wt. is easy enough to find. Do I need to clean out the old oil or just let it run down and out and put the new stuff in?

Chris

[Jetaway]

I ended up hooking a wideband up to the car and finding that the reason that I had poor idle when hot was that the car would become progressively more rich as it got hot. When heat soaked my air to fuel ratio was down in the high 9's at idle. The car does not seem to like anything less than mid 10's to be happy. If I adjusted the mixture nuts on the bottom of the SU's to lean it up a little at idle I found that I was overly lean on the highway which caused a surging issue. I assumed the rich idle condition is due to the boiling of fuel out of the nozzles so I extended the heat shield with some reflective insulation. I can't find the exact one that I used but it is similar to the following...

http://www.amazon.com/Thermo-Tec-14100-24-Cool-Mat/dp/B00029KCW0/ref=sr_1_1?ie=UTF8&s=automotive&qid=1302036108&sr=1-1

except with finished edges and meant for underhood use. Addition of the extra heat shield solved the problem. No more idle issues even in the Texas heat. I hope this helps.

Dang it! I had a couple of square feet of HVAC air vent wrap laying around for half a year after installing a new furnace / ac unit until a week ago when, having removed the passenger door panel to work on window issues, decided to install it as a sound deadener / heat insulator on the passenger door. I'll definitely give this a shot. We had a freak June cold / rain front come through yesterday which will keep the high under 80 degrees today. The weather boffins are saying add 10 for Thursday, another 10 for Friday, and throw in some change for the weekend, so I should be able to tell pretty quick if this solves the problem.

Chris

[steve91tt]

I tried two different heat shield extensions before I found one that worked. The first was only a few inches bigger than the stock shield and that had little effect. The second one extended all the way to the inner fender and entirely separated the intake from the exhaust. I got the idea from Dave Rebello who told me a story of a guy who picked up 1 second per lap on the track by using this sort of heat shield. I'm not sure it helped my lap time but I do think it helped drivability. YMMV.

[Jetaway]

I never got around to trying the heat shield extension because I couldn't figure out how to do it. The heat shield, or what I think is the heat shield is a piece of fairly heavy sheet metal attached to the motor via four bolts on the intake manifold, extends down about 8 inches at it's deepest, is shaped to shield the engine compartment from radiant heat and also is the anchor point for the throttle return springs. So I think.

I don't see how the insulation you linked would be reliably attached to this thing. Drilling holes and bolting it in wouldn't seem to be that stable because of the construction of the insulation. Glue of some sort? Or am I looking at the wrong part?

Chris

[ozconnection]

I'm glad that someone decided to use an O2 sensor to determine what was happening at idle.

Not so long ago, the "top" tuners would have an educated guess as to what was needed and if they were right, he was the guy to go to.

Now average guys like us, can diagnose and get it tuned right.

Superb gentlemen!

[steve91tt]

I left the stock heat shield in place and drilled three small holes along the edge of it. I then poked holes in the heat blanket and zip tied it to the stock heat shield on one side and to the brake lines etc on the inner fender on the other side. I never meant this to be a permanent installation but more of a proof of concept. It held together without moving for 8 track sessions on a NASCAR oval and several weeks of daily driving. The heat blanket I am using has a finished edge. This most likely helps it to hold a zip tie better. I will eventually get around to building a more permanent, better looking heat shield but for now this one seems to be working great.

[StinkyCheese]

How about a picture?

[steve91tt]

I'd snap a pic but the intake and exhaust are off the car right now along with the transmission, diff and rear suspension. I'm stripping all the go fast bits off and transferring to my 71 track car and returning the 73 closer to stock. I'm guessing mid august till I get it put back together.