Air intake temperature SOAR when I step off the gas.

[supernova_6969]

Hi guys.

I have a 1983 280zx, modified by the guy who sold it to me.  Rebuilt engine, T3/T4 turbo, front mount intercooler, blow-off valve, megasquirt…  The air filter was located behind the driver side headlight, which is good for protecting it from dust and rocks (and it gets fresh air through holes in the radiator mount for intercooler piping) but it breathes gross hot air when stopped or doing city driving (upwards from 120 degrees Farenheght)).  See photo for details. 

 

 

It was supposed to need nothing but megasquirt tuning, but the more I play with it, the more I think there are other issues (I think not all the injectors work well amongst other things).  I’m also new to the turbo thing, only having had stock NA engines before. 

 

The issue that brings me here today is that I am embarking on a cold air intake build (I plan on building a scoop that goes under the rad and maybe back up in front of it (no more room to go through the rad supports as the weird intercooler has both in and out on the same side, that side), with the filter in an isolated box between the rad and the engine.  See upcoming post with air pressure and temperature tests...) and also am looking into isolating my intake piping (see other posts on that, with data too).  I don’t want to build things that are useless, so I started taking data before so I can compare before and after.  I’ve been measuring and comparing temperatures under various situations and came to the realisation that I might have a problem (not sure, thus the post).

 

If I cruise on the highway at steady speed (say 75mph), I get steady temperature: today at ambient 82 degrees, I got 90 degrees at the air filter, and 120-130 degrees at the air intake temperature (AIT) sensor (I don’t actually know where the guy put that sensor.   Any clues?).   I’m not certain how good this is, but it does not seems too too bad (still not happy that I’m gaining 65-85 degrees WITH an intercooler under very little load and high air flow.  Opinions?).

The potential problem occurs as soon as I lift of the gas.  If I lift off a little, say to slow down to 60mph(without de-clutching), the temperature climb real quick to 155-175 degrees AIT, while everything else stays the same (air filter temp, RPMs, air flow…).  If I completely let go of the gas (without declutching), the AIT skyrockets to 190-215 degrees.  In both cases, the temps stay high for a while and only then decrease slowly (unless I give it gas, at which point they go back down quick enough I guess).

 

If I let go of the gas but immediately de-clutch and stay in neutral, the temperature goes up to 150-165, but then comes back down on its own quick enough.

 

I thinking the really high temperature I see when I don’t de-clutch might be a combination of A) slower moving air that gets to take the time to absorb more heat and B ) the fact that, because the engine is working still, there is a significant amount of exhaust that might make the turbo work and thus heat up the air.   But how come the intercooler wouldn’t be making up for that?

 

So, is that temp spike normal(ish)?  Should I be worried?  Any explanations?

Edited May 27, 2015 by supernova_6969

[NewZed]

Where's the sensor and how is it mounted?  You might be measuring heat transferred from metal at low air flow.

[supernova_6969]

Thanks for the answer.

 

I don't know where the sensor is and how it's mounted ( I don't even know what to look for).  

 

 

Besides the turbo housing and the intake manifold (which is probably farther down the road from the sensor), is there anything else that is above 100 degrees that could heat up the air?  The pipes sure aren't THAT hot, I can touch them when the AIT is above 70 after a few minutes of idle.  

 

still is 100 degrees AIT acceptable or a cause for worries?

Edited May 27, 2015 by supernova_6969

[NewZed]

Your comment about the sensor location was buried, and in parens.  Missed it.  Follow the wires.

 

You should really do the conversion work, if you want more responses.  Most of the forum members work in Farenheit. 

 

(1.8 x Celsius) + 32 = Farenheit, for anyone who wants to do the math.

[supernova_6969]

I changed all the values to franheight and mph. 

 

thanks for the suggestion.

 

I'd love to followthe wires, but I have two issues: I don't know which one to start following (I guess I could take the plug to the megasquirt apart, but I did that once and i's not well made so I spent 7 days playing with it after to get the car working) AND all the stupid wires are the same Gdamn color...  

 

If it turns out ot be an issue, I definately will.

[SleeperZ]

With Megasquirt, there is only one sensor on the intake, that is the air temperature sensor.  That sensor should be somewhere between the intercooler outlet and the intake manifold.  The MAP sensor is built into the MS unit and sensed with a hose on the manifold.  If you are still not certain, post some pictures of your intake and plumbing.

[BluSkyLightning]

I'm no engineer, so I may be completely wrong here, but I would assume that as you let off the gas pedal the airflow through the intake is drastically reduced, which would result in air moving much slower through the intercooler, and causing the air to not be cooled efficiently.  Stagnant air in an intercooler doesn't get cooled as efficiently as properly moving air, so it makes sense to me that you see an increase in intake temps while everything else (Pipes) remain cool. Again, I'm no engineer but that's my two cents.

[supernova_6969]

Hi. Ill check the plumbing for the ait sensor. I'd not realised about it being the only one there. Thanks!

 

As for the intercooler, that's an interesting theory. I'd have thought the longer it stays there the more it can be cooled (unless the i/c is soaked), but i'm no engineer myself, so i really dont know...

Can anyone enlighten us?

Edited May 27, 2015 by supernova_6969

[SleeperZ]

If the temperature sensor is installed in a hard pipe or the manifold, I can see the indicated temperature rising with falling airflow, assuming the sensor is not insulated and the hard piping is warmer than the intercooled air.

[supernova_6969]

SleeperZ, if i understand correctly, you're saying that the limited air flow at closed throttle might not be enough to make up for the hot metal's radiant heat might surroung the probe..?

I like the idea, but wouldn't the same thing continue to happen when i de-clutch?

Really, the only variable that changes between the two situations is the engine rpm (as driven by the wheels) is high when in gear, and at iddle when on neutral.

This just points out i have to find the sensor location...

Still wondering if this is normal or worrisome...

Edited May 28, 2015 by supernova_6969

[SleeperZ]

Technically, it would be the conduction from the metal to the sensor that could be the issue.  I wouldn't be particularly worried about it, but I would try to fix it as it represents a tuning inaccuracy and may contribute to driveability issues.  Personally I would verify that it is the issue, then try to insulate the sensor body from the metal plumbing.

[supernova_6969]

So I checked some things out and the only sensor that makes sense for the AIT sensor is this one:

 

 

It's basically the only sensor anywhere on the intake, from filter to heads, si I'm guessing i'm right.

 

the sensor is screwed in straight into the manifold, so heat probably does transfer and could explain part of the spike, but again, if that was the main cause, I'd have the exact same issue when I de-clutch... 

 

anyone else (with a turbo) get spikes in heat when they let the gas pedal go?  if so, how high?

 

 

[NewZed]

Then intercooler only removes heat produced when the turbo pressurizes the system.  Otherwise there's no heat to remove, it's at equilibrium. ambient to ambient..  All of your measurements at low or no load (no boost), and with the throttle closed are the same that anybody with an NA engine would see.  At zero boost there is no way for the temperature sensor to get hotter than the surrounding metal.  The energy just isn't there.  It's phyicsally impossible (I left that S there on purpose).  So the turbo has little to do with what you're seeing.  

 

When you close the throttle but don't disengage the engine from the drivetrain, you might be pulling hot gases from the crankcase in to the intake manifold.  There's no air flow to purge them and the pistons are moving faster, compared to idle.  One possibility.  It might be telling you somersetting about your PCV system, or condition of your rings, but probably has little to do with much.  215 degrees is a pretty weak skyrocket.

[supernova_6969]

Hey New Zed

 

I'm with you on most of what you said.  I get that the sensor won't pick up temps higher than the surrounding metal (or else the surrounding metal itself would adsorb and diffuse it to the outside, I guess).  

I guess you might be right about the turbo not being involved..  I suppose that, even though the RPMs are higher when I let go of the pedal (compared to when I disengage the clutch), they are not high enough to create boost, therefore heat.

what keeps bugging me though is how the temp goes up when engages, but does not when on neutral.

 

I think your explanation is really interesting.  as far as I know, the engine has less than 5000kms on it (that's about 3000 miles) on it, so I'd be surprised that the rings are already shot..  but, you never know.  I'll try to see if there is still a pcv valve on it, and if so, disconect it and drive it like that. that's going to show me if that's the issue (the blow-by)

 

nice theory.  can't wait to check it out.  

[supernova_6969]

Hey guys.

 

so the crank is still connected to the intake via a PCV valve, but I haven't had a chance to disconnect them to see if that's what cause the heat spike....

 

I'll try to do it over the next week or so..

 

In case anyone is interested, i just started a thread on insulating air pipes.  I'm gathering before and after data to see if it's actually useful. 

http://forums.hybridz.org/topic/122561-insulated-intercooler-to-intake-pipes/

 

seb

[Six_Shooter]

I'm no engineer, so I may be completely wrong here, but I would assume that as you let off the gas pedal the airflow through the intake is drastically reduced, which would result in air moving much slower through the intercooler, and causing the air to not be cooled efficiently.  Stagnant air in an intercooler doesn't get cooled as efficiently as properly moving air, so it makes sense to me that you see an increase in intake temps while everything else (Pipes) remain cool. Again, I'm no engineer but that's my two cents.

 

This is basically what is happening.

 

At low throttle angles the air flow isn't high enough to pull in cold air before it gets heated by the engine bay heat, or the heat soaked intake manifold.

 

The temperatures you are reporting are excessive. Can you post the temps as they were in *C, I'm wondering if maybe your conversions are a bit off, or maybe the MS calibration is off for the MAT.

 

Back when I had my turbo L28, I would see cruise intake temps of around 105 to 115 *F with ambient temps between 75 and 90 *F. When stopped the temps would typically rise by about 5 to 10 degrees. When in boost my intake temps would drop to within a couple degrees of ambient.

 

Just FYI, my new engine a V6, using the same turbo and IC, reacts the same way, with similar temp readings.

 

My MAT (Manifold Temp Sensor, since technically an IAT would be pre-throttle body), was mounted just behind the throttle body, where the cold start injector used to be, which would be closer to the throttle body than where yours is mounted, but should make that much of a difference.

 

Just FYI, I did notice my cruising intake temps drop by about 20*F when I moved my filter from the engine bay out in front of the rad. I used a 3" tube and ran it under the left frame rail, and then below the IC. My car is an S30, but you might be able to do something similar on your car.

[supernova_6969]

Hi six shooter.

Thanks for the answer. The info confirms what i'd been reading happens with most people.

My numbers are right. Ambient these days are around 30-25 celsius, air filter temps around ambient+5 when cruising above 40mph, and the IAT (or MAT as you pointed out) is around 55-60 celsius (ambient + 25-30, filter +20-25) when i'm cruising, and goes all the way up to 90-100 celsius withing about 2-3 seconds when I let go of the gaz pedal while the transmission is engaged and I let her coast. If i declutch imnediately as i let the gaz go, it'll go to about 75 celsius and then slowly come back....

When i get boost the IAT/MAT temps actually go down a few degrees from cruising down at least 5-10 celsius (can't be precise i usually don't check the computer when under boost....). That tells me the intercooler works, i guess, although the temps go up once i stop boosting, also all the way up to 90-100C/95-210F. Heatsoak's a bitch.... Of i hit the gaz again (boost or not) it goes right back down at least 20-30 degrees so obviously the air flow volume is playing quite a role in the air temps....

 

What you mentionned makes me wonder if anyone else has a temp sensor right smack in the intake manifold. Most people seem to have a properly located IAT, so that might make a difference.

[(goldfish)]

Does it still have an EGR valve?  I suspect it has to do wiith the extra vacuum put on the intake manifold when you engine brake.

[supernova_6969]

Hi goldfish 

 

i still HAVE an EGR valve, but it's not plugged into anything; it's just blocking the intake manifold hole, and I'm only waiting till I get my hands on a plate to block the hole before I take it off. 

 

That said, I don't think there is a connection (bung?) on the exhaust for it to pull hot gases from.  The exhaust was made by the guy that rebuilt the whole car, and the only thing on it is a bung for the o2 sensor and a straight through muffler.

 

As far as I understand, if it was plugged in, the vacuum from the engine would be pulling hot gases, making the temperature higher in the manifold, yes? is that what you were talking about?  

 

In my case, that wouldn't apply...  

 

By the way, I really appreciate everyone who's trying to help.  Thanks a lot!

[Six_Shooter]

I was thinking about this the other day..

Pull the sensor out to see which type it is. There are two types of MAT sensors in this style, one is what is called a "closed element", the other is an "open element".

 

The closed element looks more like a coolant sensor, and is completely brass on the tip, with no openings, the open element has a plastic cap and has opening sin teh plastic cap to allow the air to pass directly across the sensor element.

 

I'm thinking that if you have a closed element it might start to explain the large jump in temperature, though I still think the sensor calibration is off.

 

I only ever use open element MAT sensors because I don't want heat soak of the sensor body to ever be a problem.