Heat soak altering driving performance.

[Q70sGuy]

Since I’ve had the q70 3.7, I’ve noticed it falls on its face at times. Couldn’t make much sense of it at first , but 15k miles later I’m starting to see the picture.

Heat, hot days, a bit of traffic, a few stop signs and red lights. All of a sudden the transmission wants to upshift so early im barely lugging forward from a light. Almost no response unless I mash the pedal 40% or more.

Heat soak is my hypothesis. Every drive starts off great. Super sharp throttle response, lots of power under low throttle, as expected. Eventually 20 minutes in, it feels like timing is retarded and the same throttle pressure won’t let you get to a cruising speed unless you double it.

Anyway, read through the manual and noticed these have a torque cut off if oil sensor detects high temps. Starting to wonder. Oil is fresh, car is very well maintained, no codes at all in any module.

Anyone have any ideas ? Does this car really only work well with an oil cooler installed? I find that hard to believe.

At the time of writing, I haven’t yet taken the oil temp measurements while experiencing the issue but will do so as soon as I can.

[VStar650CL]

There are other factors which can turn the engine into a hot slug, but the VQ37 is a humongous heat generator and VVEL's don't like running with thin oil, so there's good reason for the power reduction. However, it doesn't kick in until the oil gets really hot. Up to 220F is considered normal. The ECM will dial back the VVEL around 240F and go limp at 280F. A good scanner can stream what the ECM is seeing and tell you if you're hitting dangerous temperatures. If you are, for some reason Infiniti left the oil cooler off of the base 2WD's, I guess they didn't expect them to be driven like a racer. That said, I don't know anybody who ever raced a VQ37 without a cooler. If that does turn out to be your issue, first check the sensor resistance against the chart to make sure it's accurate, but if it is, fix the mistake and install a cooler. Personally, I'd fix it unconditionally.

It's also possible you have a MAF or IAT issue. Stream both readings and compare bank1 with bank2. A MAF reading low or an IAT reading high will both make the ECM underestimate the incoming air volume and add less fuel than the engine really wants, and both of those devices can experience temperature-induced non-linearities in old age. If a MAF is off you can usually resolve it by performing an IAVL, but if an IAT is off it means replacing the MAF assembly. Also check your intake ducting, major leaks will turn Infiniti's well-designed cold air intake into a hot air intake. That will definitely cause a noticeable HP loss. Lastly, make sure it isn't your transmission misbehaving and not the engine. If the shift points (in RPM and not throttle position) get weird as the car warms up, suspect a Torque Converter problem.

[Q70sGuy]

Appreciate your input vstar. Will continue and monitor all that. I have AWD with the integrated oil cooler and I drive like a grandma most of the time, so I’m confident something is wrong because it just wouldn’t make sense otherwise. Hard to diagnose because it mostly happens with higher outside temp but it will get warm soon enough.

[Q70sGuy]

I will say if any one from southern USA or Latin America wants to just let me know if you experience this, because if I am experiencing it in Canada then people in the south must know what I’m talking about!
Car only has about 75k miles too.

[VStar650CL]

I will say if any one from southern USA or Latin America wants to just let me know if you experience this, because if I am experiencing it in Canada then people in the south must know what I’m talking about!
Car only has about 75k miles too.

Honestly, I have to think something failed if it didn't do this out of the showroom. You really need to scan it and see what the ECM is seeing.

[Q70sGuy]

All right so finally hit some warm temps here.
I took some readings after a heat soaking and driving the car. Oil temps around 100c and coolant also around 100c. Doesn’t surprise me much and I’m pretty sure the car isn’t lowering power for that. Intake temps up to 50c though had me a bit shocked. I’m on stock intakes and outside temp is around 25c.
Any other members can corroborate that intake temp could be that different from ambient air?

Transmission temp 1 at 90 and temp 2 at 95. Not sure why there are two temps.

So to describe my issue better: power is there if I need it, but the car is just REALLY wanting to upshift a lot sooner than I’d expect. It’s drives me if I just put my foot down more but if I’m just trying to drive smoothly and decide I want to get a bit of speed I’ll find that instead of holding the gear it goes and upshift. It’s almost like it’s taunting me .

Anyone else experience this kind of thing?

[VStar650CL]

Maybe someone else with a Q70 can verify it, but a 25C differential between ambient and intake doesn't sound normal for a healthy induction path even with the engine fully warm. That sounds like air leaks in or before the airbox or a sensor that's drifted off-spec in old age. You can diagnose the IAT by checking it with the engine stopped and dead cold, it should read exactly ambient.

Vis the two transmission temps, T1 is the pan temperature, T2 is the ejection port temperature from the Torque Converter. Under normal circumstances they'll be close to one another, but after a heavy load or some spirited driving T2 may go much higher than T1. If there's a big difference even during normal driving, it usually means a problem in the TC.

[Q70sGuy]

It’s a bit weird. I’m not sure where the IAT sensor is though. I’ll note my car has a slightly misplaced front bumper which makes the big intake piece sit a bit higher up. I’m possibly sucking in a bunch of hot air because of that. The temp does cool down a bit.once I’m moving fast enough. Will check tomorrow before starting the car though I tend to believe the sensor is working correctly.


I’ll monitor the temp 1 and 2 of the transmission next time to see if any big gaps!

[VStar650CL]

It’s a bit weird. I’m not sure where the IAT sensor is though. I’ll note my car has a slightly misplaced front bumper which makes the big intake piece sit a bit higher up. I’m possibly sucking in a bunch of hot air because of that. The temp does cool down a bit.once I’m moving fast enough. Will check tomorrow before starting the car though I tend to believe the sensor is working correctly.


I’ll monitor the temp 1 and 2 of the transmission next time to see if any big gaps!

On an HR you should have two IAT's and two MAF's, one set on each intake. The IAT's are part of the MAF assemblies. Electrically, they usually share a common internal ground but don't share power. The MAF's have their own 5V or 12V supply, while the IAT's are biased to 5V from pullup resistors inside the ECM.