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Heat Soak Question


zack_280

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I have a modified L28ET motor. It seems like the head gets very hot very quick under any driving conditions. For example, this weekend I drove my car for about 5-10 minutes. I pulled into the garage, popped the hood and the valve cover was almost too hot to touch. It was too hot to leave your finger on for longer than 1-2 seconds without pain. Is this within the bounds of normal for these motors?

The heat soak is leading to vapor lock issues. It will vapor lock while running even while driving under load. Obviously the vapor lock is worse with less fuel in the tank. My intended solution is to get a fuel cooler and install it after the fuel pressure regulator. I just didn't want to do that if it was only a Band-Aid for a potentially larger problem.

Here is my fuel system...
Tank->Low Press Pump->Surge Tank->High Press Pump->Fuel Rail->Fuel Press Regulator->Surge Tank->Tank

Thanks for any help suggestions,

Zack

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t will vapor lock while running even while driving under load. Obviously the vapor lock is worse with less fuel in the tank. 

Seems like this is close to your main symptom and you're making assumptions about what the cause is.  You might help your mission by just describing what happens and see if anyone has a suggestion about the cause.  Describe the "vapor lock" under load, for example.  EMS description will help also; stock, MS, etc.

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You shouldn't touch hot things...

 

Air circulation is your friend...but what you describe is nothing abnormal. Go to AutoZone and buy their little red and black I/R Heat Gun. Start shooting items under the hood during a drive and get baselines on what is normal for your can and what is not.

 

Realizing that your car likely has a 180F thermostat, and normal people can not hold their hands on anything over 140 degrees for more than a few seconds...it makes sense.

 

If you are boiling fuel it likely is not heat soak in the rail while running, it's hot fuel vapor flashing in the pump inlet causing pump cavitation and resulting in a pressure drop. It should not be happening in your fuel setup unless you have tapped the Surge Tank in the wrong place and are picking up return bubbles somehow. You might consider checking the surge tank pressure while you are at idle. This is 'maximum bypass' condition for your car. Then, if you are not running 1psi to 3psi in that tank, consider putting a small restrictor orifice in the line back to the tank. This will insure positive pressure to the HP Pump's inlet, and should take care of vapor flash cavitation when the fuel is 120-140+. And it WILL get that hot if you are running below half a tank and a 180F thermostat. I have not found the need to go any higher than 3-5psi in SoCal Desert but you have to watch that your restriction orifice at the tank does not hamper your FPR action at idle. 

 

One way to do that is with a needle valve, and have someone screw it in progressively until you see your idle fuel pressure start elevating (I have a 6" Mirrored Calibration Gauge that I can use to see this...) and then open it up a few cranks. I don't like needle valves because trash can stick in them easier than a fuel jet or drilled orifice in the return hose or return barb.

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Seems like this is close to your main symptom and you're making assumptions about what the cause is.  You might help your mission by just describing what happens and see if anyone has a suggestion about the cause.  Describe the "vapor lock" under load, for example.  EMS description will help also; stock, MS, etc.

I am definitely making assumptions about what the cause is, although I believe it is either cavitation as Tony mentioned above or heat soak.  I was going to buy one of the cheap I/R guns, but a friend of mine works at a place and said he could borrow a much better one.  That was 2 months ago.  So maybe I need to go and buy the cheap one.  I am running megasquirt injection, but I don't believe it is in any way related to my EMS.

 

Here is what happens: I start the car, it runs fine for a little while (less of a while with less than 3/8 of a tank of fuel).  Then my AFRs start getting lean spikes, like there is air in the fuel line.  Then it will basically completely lean out and die.  The engine usually only dies under idle at a stoplight or heavy load at an autox or track event.  I shut off the car, wait 10-15 minutes, start it and drive away.  I have felt the fuel boiling in the surge tank and in the pump and the fuel rail.

 

Tony,

I'll check the pressure in the surge tank, although I would be surprised if that was the problem.

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I'll let you know the difference between the $19.95 round fits in the palm of your hand IR gun, the $29.99 Auto Zone Pistol IR Gun, and the $757 Fluke IR Gun I have: at the range you use it (less than 6") less than 0.1 degree Celcius.

 

I stopped carrying my $700+ Fluke when I found this out since most of my measurements are less than 1m anyway, and I can adjust mine for the different emissivity characteristics of stainless steel---know what? Put a flat white spot of high temp bbq paint and it's just as accurate reading it with the AZ Gun! The only real difference is the spot size at a distance and the range of the thermal element. Some read higher than others. My AutoZone Gun was reading 345C Steam today, and 245C on the exhaust of a turbine. I got news.... 0.1 degree C isn't that big a deal.

 

And neither is it reading your thermostat housing. It's supposed to be 82C.... not 82.5 or 82.1.... 82. And its close enough for that as well. I carry those AZ guns because I can give them away to customers and they dig it. It's no loss for me, I'll just go get another one! But man....I'm SO glad I did that in New Caeldonia because they had a problem and I told them "go take a temperature profile" and they were able to get numbers for me...otherwise someone was going to Asbestos Island... Don't worry about a 'better' one---it's not anything you will ever notice!

 

Part of what you describe is likely the tank getting hot and the fuel density changing. I have seen it go from 11:1 with 21C fuel in the tank to 15 and 17:1 with 45-50C fuel and higher! The AutoX is the fuel sloshing around and the pickup coming uncovered---again something your surge tank should eliminate... if you feel 'boiling' in your surge tank, and you can put your hand on it to feel it---IT'S NOT BOILING! Likely it's air blowing in there from a hole in your pickup line being pushed in by your boost pump. Gas won't boil at only 120-140 generally...but 1-3PSI would definately mitigate the pressure boiling point. 

 

I'd call the technical services line for the fuel you use and ask them the pressure boiling characteristics of their fuel. They know it. See if you can get curves for 1 to 5 psi pressure... That will tell you how hot you will have to get to boil the fuel. At 30psi in the fuel rail...it's not going to boil. It will boil and flash-cavitate where it's hot and under low pressure (like at the pump inlet!)

 

You will note that the EFI cars have a MUCH HIGHER spring pressure on the Evaporative canister valve. When the fuel gets hot, and expands, it will make a blanket pressure in the tank helping combat the inlet cavitation of the pump. Going down the road at high speed...realize you don't recirculate so much fuel back, so even on a hot desert day you rarely exceed 120F in the tank due to air moving through the engine bay. It may be getting hotter...but once you're up to 30 psi it's not going to boil unless the pressure drops. Or the flow stops.

 

The stalling you have that goes away after 10-15 minutes should go away if you cycle fuel thorugh the rail for a few seconds before startup. Right at the head, the injectors are well up around 180 F and that will boil the fuel if the check valves are all working---the gas expands, the FPR dumps it...you try to start without a priming pulse and the pressure drops FLASH! To vapor.. Extreme cases the fuel does get to 190+ and the FPR dumps pressure and enough is lost that the fuel does boil in the rail. But this is a function of the pump check valve not holding pressure up in the rail as much as anything else. I run a 160 F thermostat and it greatly reduces this tendency. If it doesn't get that hot, it can't boil the fuel!

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I'll let you know the difference between the $19.95 round fits in the palm of your hand IR gun, the $29.99 Auto Zone Pistol IR Gun, and the $757 Fluke IR Gun I have: at the range you use it (less than 6") less than 0.1 degree Celcius.

 

I stopped carrying my $700+ Fluke when I found this out since most of my measurements are less than 1m anyway, and I can adjust mine for the different emissivity characteristics of stainless steel---know what? Put a flat white spot of high temp bbq paint and it's just as accurate reading it with the AZ Gun! The only real difference is the spot size at a distance and the range of the thermal element. Some read higher than others. My AutoZone Gun was reading 345C Steam today, and 245C on the exhaust of a turbine. I got news.... 0.1 degree C isn't that big a deal.

 

 The "better gun" I was referring to was an IR camera that could discern pretty high temps.  I'm not sure what the range is, but it's more than I would need to worry about.  Again, I see your point with the cheap one.  It could definitely provide some more info (not sure if it would be useful).

 

 

Part of what you describe is likely the tank getting hot and the fuel density changing. I have seen it go from 11:1 with 21C fuel in the tank to 15 and 17:1 with 45-50C fuel and higher! The AutoX is the fuel sloshing around and the pickup coming uncovered---again something your surge tank should eliminate... if you feel 'boiling' in your surge tank, and you can put your hand on it to feel it---IT'S NOT BOILING! Likely it's air blowing in there from a hole in your pickup line being pushed in by your boost pump. Gas won't boil at only 120-140 generally...but 1-3PSI would definately mitigate the pressure boiling point. 

I can feel the "boiling" with the car off and the high pressure pump sounds like it's trying to pump air rather than liquid when I turn it on.  However, the "boiling" is still there with no pump running. I'm not sure that it is boiling or if it could be fuel leaking down into the tank and air leaking up from the tank into the surge tank, but the surge tank and fuel lines get pretty hot and the "boiling" feels pretty violent.  I'll get a gauge and verify that there is positive pressure in the surge tank.

 

 

The stalling you have that goes away after 10-15 minutes should go away if you cycle fuel thorugh the rail for a few seconds before startup. Right at the head, the injectors are well up around 180 F and that will boil the fuel if the check valves are all working---the gas expands, the FPR dumps it...you try to start without a priming pulse and the pressure drops FLASH! To vapor.. Extreme cases the fuel does get to 190+ and the FPR dumps pressure and enough is lost that the fuel does boil in the rail. But this is a function of the pump check valve not holding pressure up in the rail as much as anything else. I run a 160 F thermostat and it greatly reduces this tendency. If it doesn't get that hot, it can't boil the fuel!

I thought the same thing about circulating the fuel, but that doesn't seem to work.  I have tried it multiple times, but it doesn't seem to have a noticeable effect.  The only thing that seems to work is to let the system cool off.

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Might be worthwhile to isolate the problem.  Bypass the surge tank and go back to just the high pressure pump feeding the fuel line directly from the big tank.  If the problem goes away it's coming from the surge tank system.  If it stays, at least the number of options will be smaller.

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Might be worthwhile to isolate the problem.  Bypass the surge tank and go back to just the high pressure pump feeding the fuel line directly from the big tank.  If the problem goes away it's coming from the surge tank system.  If it stays, at least the number of options will be smaller.

Not a bad idea, but it will take a little bit of effort to do that.  

 

This will get me to tinkering on the car again (and spending $50 here and $25 there and $200 there).  My wife will be thrilled.

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Insofar as you refer to a thermal imaging gun, SAME APPLIES, the FLIR guns that sell for under $1500 now give the same resolution that a $16,000 gun did 20 years ago...and prints higher resolution images. But no need for a thermal image. The little gun shoots what you need to know and you can work with it easier. Unless you are trying to diagnose a plugged radiator or something, then you need skills with an IR gun, but a quick snap with the FLIR and even Slingblade could tell you were the blockage was!

 

Yes, I have the Spicoli-Expressed "Totally Awesome Set of Tools"...

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