Running Hot: Correct Diagnosis and Treatment Plan?

[ConchZ]

Hi all,

As some of you may remember, last summer I improved my AC cooling by installing MSA's dual electric radiator fans and aluminum shroud. The side effect was that it made my car run hotter on extended freeway drives. It was really strange. The temps would drop as soon as i got off the freeway and into stop and go traffic. Thinking that the shroud was blocking too much airflow, I removed one of the fans. The summer heat let up last year before I really had time to decide if that made a difference, but I think it helped a bit. Well, it got worse this summer. The temps would get up as high as 210, as measured by an infrared thermometer pointed at the thermostat housing. This has been causing vapor locking, and the car will ping under acceleration when it gets that hot.

I mostly cured the vapor locking by bending the fuel rail back from the head. I think it was pushed towards the head somewhere in its life. I then tackled the pinging. The original owner replaced the points with a Crane Fireball 700 optical trigger system, and I could never get the initial advance set below 18 at 750 rpm, with it at 43 when at 3000 rpm. Except when it runs hot, there's never been any pinging. Since it recently got worse, I studied the optical trigger and noticed it was adjustable, so I moved it a bit and was then able to set the timing to about 8 initial and 32 fully advanced. The car seems to now run a little cooler, but it still creeps up to 200 degrees on a long hot drive. The exhaust temps now seem hotter, as evidenced by my warmer feet and butt. The infrared shows the manifold is around 350 degrees. Is that normal?

Thinking that the extra exhaust heat is related to lean fuel mixture, I remembered that I can never get the rear carb adjusted right. I sometimes get lean pops out of it on a cold start up, even with the mixture nut down a full 5 turns, and lifting the piston a bit always causes a stumble, indicating it's too lean. Using plastic tubes on the drain plugs of the 4 screw carbs (it's a 73 with older carbs), I have set the fuel level to 23mm below the rim of the float bowls. At that level(5 full turns down and 23mm fuel level), there's not any gas pooling on top of the jet, so maybe I need to raise the float level? I would think the top of the jet would be drowning in fuel. In any case, last night I decided to test the throttle shaft bushings by spraying carb cleaner on them with the engine running. Sure enough, I get a stumble. I assume my lean running is at least partly caused by worn throttle shaft bushings?

These carbs have been on the car for about 30,000 miles, and were not new when the original owner put them on the car. I have some really nice shiny SUs that I could swap on there, but I really want to use these as a learning experience first. Why am I not seeing fuel on the jet? Is the lean condition causing abnormal exhaust heat? Did the advanced timing contribute to the high heat on the highway?

Thanks,

Matt

[LeonV]

Retarded timing and/or a RICH mixture will both raise exhaust gas temperatures. With timing advanced enough to reach MBT (Max Brake Torque), you will be generating more heat in the cylinder since you will be generating the highest peak cylinder pressures. I think everything you see is normal.

I'd change the water pump if you don't have any history on it, same goes for the thermostat. You are robbing some power and fuel efficiency by retarding timing, although you're probably only a few degrees off. Aim for about 33-36 degrees at 3000rpm, depending on what piston/head combo you have.

Also sounds like your SUs are dead. They will not function properly with a throttle shaft leak.

[ConchZ]

Thanks Leon. I did change the water pump and thermostat when I did the fans and radiator, which was somehow dented by the PO on the bottom right and had a slight leak around the hose neck. As for timing, I set it to about factory. I couldn't tell you if I'm losing power over the previous too advanced setting, but it does seem to take less throtle input to cruise on the freeway, otherwise the performance is unchanged. I wanted to set it to specs until I diagnosed everything else, and may raise it back up a bit later. In any case, it's just nice to be able to adjust it, now that I figured out that optical trigger. I'm pretty sure my piston/head combo is stock 73. I don't think the head's ever been off. The car only has 105k well documented miles. The original owner kept every receipt and even logged his gas mileage...

Like I said, I have a really nice set of polished SUs and pretty intakes that I got from another member. I was planning to put them on this winter, when I had time to also pull the exhaust manifold and clean it up. I then want to sell the ones on the car, but I want to be able to honestly represent them to potential buyers, hence my recent hot weather testing/tweaking. They come with nice K&N filters with internal air horns, and they look decent. Guess the value drops significantly with a throttle shaft leak, though. Think the idea in this thread would patch them up for a while? http://www.classiczcars.com/forums/thread43184.html

So you say 200-210 degrees at the thermostat housing is normal on a long freeway cruise on a hot day? Think its normal for that to drop 20 degrees after idling in stop and go traffic with the AC running?

Let me close by saying the car seems to run perfectly, other than the highway cruise temps, and the recently increased exhaust temps. It starts first try, with a rock solid idle and revs to 6500 rpm under load just fine. Maybe I should just leave it alone until I have time to swap to the nice carbs.

[LeonV]

Thanks Leon. I did change the water pump and thermostat when I did the fans and radiator, which was somehow dented by the PO on the bottom right and had a slight leak around the hose neck. As for timing, I set it to about factory. I couldn't tell you if I'm losing power over the previous too advanced setting, but it does seem to take less throtle input to cruise on the freeway, otherwise the performance is unchanged. I wanted to set it to specs until I diagnosed everything else, and may raise it back up a bit later. In any case, it's just nice to be able to adjust it, now that I figured out that optical trigger. I'm pretty sure my piston/head combo is stock 73. I don't think the head's ever been off. The car only has 105k well documented miles. The original owner kept every receipt and even logged his gas mileage...

Like I said, I have a really nice set of polished SUs and pretty intakes that I got from another member. I was planning to put them on this winter, when I had time to also pull the exhaust manifold and clean it up. I then want to sell the ones on the car, but I want to be able to honestly represent them to potential buyers, hence my recent hot weather testing/tweaking. They come with nice K&N filters with internal air horns, and they look decent. Guess the value drops significantly with a throttle shaft leak, though. Think the idea in this thread would patch them up for a while? http://www.classiczcars.com/forums/thread43184.html

So you say 200-210 degrees at the thermostat housing is normal on a long freeway cruise on a hot day? Think its normal for that to drop 20 degrees after idling in stop and go traffic with the AC running?

Let me close by saying the car seems to run perfectly, other than the highway cruise temps, and the recently increased exhaust temps. It starts first try, with a rock solid idle and revs to 6500 rpm under load just fine. Maybe I should just leave it alone until I have time to swap to the nice carbs.

I should have said, "the consequences of your actions are normal", i.e. hotter exhaust temps with retarded timing and hotter engine with advanced timing.

Running 200 degrees is not normal, unless you're using a 200 degree thermostat. Did you flush your cooling system and make sure that the coolant level is adequate? Is the thermostat a Nissan unit?

Waitaminute, you didn't loop your heater hoses, did you?

The SUs are cores at this point, ZT values cores at $150. They will not ever run right with leaky shafts.

[ConchZ]

I should have said, "the consequences of your actions are normal", i.e. hotter exhaust temps with retarded timing and hotter engine with advanced timing.

Running 200 degrees is not normal, unless you're using a 200 degree thermostat. Did you flush your cooling system and make sure that the coolant level is adequate? Is the thermostat a Nissan unit?

Waitaminute, you didn't loop your heater hoses, did you?

The SUs are cores at this point, ZT values cores at $150. They will not ever run right with leaky shafts.

Thanks again Leon. Your explanation about the timing makes sense, and fits with articles I've read.

My heater hoses are a few years old, and I think I put them on right. I don't think they would have fit any other way, and the heater works when needed. The thermostat is a 180 Nissan unit that I tested in a pan on the stove. It starts opening around 180, and does open up fully.

Out of curiousity, can someone hit their thermostat housing with an infrared thermometer after a long drive and tell me what it reads? I wonder if the outside of the housing reads 200 on everyone's due to the exhaust manifold below it. I may be chasing something that isn't really a problem.

As for the carbs...well darn. At least I've got some other ones I will try.

[Jetaway]

Out of curiousity, can someone hit their thermostat housing with an infrared thermometer after a long drive and tell me what it reads? I wonder if the outside of the housing reads 200 on everyone's due to the exhaust manifold below it. I may be chasing something that isn't really a problem.

I don't have an infrared, but my old PYRO does do the job. 50+ miles, 95 -- 100 degrees, 60 -- 75 mph, 182 degrees with standard thermostat.

Chris

[Pomorza]

Out of curiousity, can someone hit their thermostat housing with an infrared thermometer after a long drive and tell me what it reads? I wonder if the outside of the housing reads 200 on everyone's due to the exhaust manifold below it. I may be chasing something that isn't really a problem.

When my Z was running I drove it up and back from Phoenix in 100+ degree weather. The thermostat housing read 190 when I pulled into my parking lot. (Don't ask why I was checking). I ended up swaping to a lower temp thermostat as it started creeping into the high 190's on mountain drives (that was from the sensor, I actually registered 197). I'm not a big fan of the head hitting temps above 200 as they're aluminum. Its a personal preference, it runs at about 170 right now after some mountain fun. I know I have a different car (a 280 in my case) but I lack hood vents and I live in Arizona, where it gets hot (right now its 105,).

I do find it weird that your car cooled 20 degrees when you idle in traffic. I would think that with the amount of air crossing that radiator at highway speed it would run cooler and not warmer.

Hope that helps

Jan

Edited August 10, 2012 by Pomorza

[ConchZ]

Thanks P and J.